Dr Jean Weiss

Dr Richard Mattana

Jean-Christophe Lacroix

Van Quyen Nguyen

REDOX ACTIVE LAYERS IN SOLID STATE MOLECULAR ELECTRONIC DEVICES

Keywords:

Bisthienylbenzene

Co

First, I have studied the rectification behavior of single component asymmetric MJs by using several molecules including donor, acceptor molecules, donor-acceptor molecule. I have shown that the rectification ratio can be varied by 5 orders of magnitudes by changing the used molecule. Importantly, the polarity of the rectification can be reversed, and the change of the polarity was shown to come from variation of the electronic coupling at the top interface. Moreover, a new system showing nearly on resonant transport was developed.

Second, I have studied the transport in bilayer molecular junctions in which bilayer containing two different molecules, is deposited layer-by-layer by either an overgrafting approach or a diazonium approach. We found that the overgrafting approach generates a new layer in which a first layer is strongly electronically coupled with a second layer whereas the diazonium approach generate a bilayer in which a first layer is weakly coupled with the second layer. Using the first approach we developed another system in which transpot is dominated by on resonant tunneling whereas the second approach always leads to a decrease of the conductivity but to interesting rectification properties.

Third, I have also investigated the electrical properties of mixed component molecular junction in which a first molecular layer is deposited using a host/guest complex and allows after removal of the host to create free holes. A second deposition step generates the mixed layer. We found that below 5 nm, the transport in mixed molecular junction occurs by direct tunneling but that the current depends on the number of active channels in the layers.

Finally, I have studied the electrodeposition of organometallic compounds and charge transport through organometallic molecular junction. Above direct tunneling, we find that the conductivity of organometallic MJs is always higher that that observed with most molecular junction based on pi-conjugated organic molecules. We also observed a highly efficient longrange transport in Cobalt(II) terpyridyl MJs suggesting again transport by resonant tunneling. We evidenced light emission in Ru(II) bipyridyl MJs which is one of the first direct proof of redox events in solid state MJs and finally we partially explored the role of counter ion in organometallic MJs.

Résumé viii

RÉSUMÉ

Mon doctorat interdisciplinaire vise à étudier le transport de charge à travers les jonctions métal/molécules/métal (JMs) et s'attache à explorer diverses fonctionnalités, et en particulier les propriétés de rectification, dans des JMs dont l'épaisseur de la couche moléculaire est comprise entre 1-20 nm. Pour fabriquer de telles JMs, j'ai combiné des processus usuels de l'industrie microélectronique (CMOS : Complementary Metal-Oxide-Semiconductor) avec des modifications de surface par électroréduction de sel de diazonium. L'objectif principal est d'étudier l'effet des structures moléculaires sur le transport de charge à travers les couches moléculaire. Pour faciliter cet objectif, les jonctions sont fabriquées sans changer la nature des contacts métallique constituant les électrodes qui sont en Or et en Ti/Or et induisent donc une asymétrie vis-à-vis du transport électronique. J'ai étudié, tout d'abord, le comportement de JM asymétriques incorporant une seule structure moléculaire mais en utilisant des molécules soit donneuses d'électrons soit accepteuses d'électrons, et des molécules constituées d'une partie donneuse et d'une partie accepteuse. J'ai pu mettre en évidence que le taux de rectification de ces JMs peut varier de 5 ordres de grandeur en changeant de type de molécules, atteindre des valeurs supérieures à 1000 à faible tension, rarement obtenues dans la littérature et j'ai pu inverser la polarité de la rectification. Cette inversion a été attribuée à une modification du couplage électronique à l'interface supérieure. Par ailleurs, des jonctions dans lequel le transport s'effectue par tunnel résonant ont pu être développées.

Dans un deuxième temps, j'ai étudié le transport dans des jonctions moléculaires bicouches constituées de deux molécules différentes. Ces bicouches ont été déposée couche par couche en utilisant l'électroreduction de sels de diazonium pour la première couche, suivit, pour le dépôt de la seconde couche, soit par l'oxydation d'un monomère aromatique soit par la réduction d'un deuxième sels de diazonium. Nous avons trouvé que la première méthode conduit à une situation où les deux couches sont fortement électroniquement couplée et se comporte comme un nouveau matériau possédant de nouvelles propriétés alors que la deuxième génère une bicouche dans laquelle les couches sont faiblement couplées. La première approche a permis d'obtenir un deuxième système dans le lequel le transport à longue distance est très efficace et s'effectue par effet tunnel résonant, alors que la deuxième conduit à des diodes rectifiantes mais de conductivités plus faibles. Troisièmement, j'ai également étudié les propriétés électriques d'une jonction moléculaire basées sur une couche mixte. Celle-ci est obtenue par l'utilisation d'un complexe hôte/invité qui peut être greffé sur la surface via l'invité et qui permet ensuite d'enlever la molécule hôte générant ainsi une surface présentant des trous de tailles moléculaires. Un deuxième greffage peut s'effectuer dans ces trous ce qui permet l'obtention d'une couche mixte. J'ai pu alors mettre en évidence qu'en dessous de 5 nm d'épaisseur, le transport dans la jonction moléculaire mixte s'effectue par effet tunnel non résonant entre les deux électrodes mais le courant est proportionnel aux nombres de molécules active dans la jonction Enfin, j'ai étudié l'électrodéposition de composés organométalliques et le transport de charge à travers des jonctions moléculaires organométalliques. J'ai observé que la conductivité des JMs organométalliques est toujours supérieure à celle des JM organiques, mis en évidence un transport à longue distance très efficaces dans des JMs utilisant un complexe de cobalt (II) terpyridyle, une émission de lumière dans des JM basée sur un complexe Ru (II) bipyridyles. Ce résultat est une des rares preuves directes de l'existence d'évènements redox au sein de la jonction pendant le transport de charges. Enfin, nous avons partiellement exploré le rôle des contre-ions dans ces JMs organométalliques.

GENERAL INTRODUCTION

In the 1960s, Gordon Moore predicted that the number of transistors on a microprocessor chip would double every two years, this is known as Moore's empirical law. Since then, the size of the transistors has fallen from 90 nm in 2004 to 14 nm in 2014. In 2017, Samsung first released their version of a '10 nm' process in their newest products, Samsung S8 and S8 plus. Apple also released a new A11 Bionic chip which is manufactured by a 10 nm process and contains 4.3 billion transistors. Recently, Moore's law has started to be less effective, due to the heat issue that unavoidably arises when more and more silicon circuitry is merged into the same small area.

The question 'What is next beyond Moore' has been posed by many scientists. Several solutions have been proposed such as working on a 5 nm target or even lower; 3D integration; stacking to attain enormous parallelism or neuromorphic computing. In recent years, researchers have made efforts to create neuro-inspired computer chips. In 2014, IBM reported the new SyNAPSE chip consisting of 1 million neurons and 256 million synapses with 4096 neurosynaptic cores 1 and in general, it is based on a cross-bar structure.

An alternative solution could be molecular electronic devices where the molecules are sandwiched between two electrodes. Molecular electronics generally refers to studies which uses molecular building blocks as electronic components. The field emerged explosively in the late 1990s, accelerated by developments in scanning probe microscopy and nanofabrication techniques. Recently, molecular electronic has been proven as molecular rectifier, memristor, transistor, photo-switching. Another advantage of molecular electronics is that within a given device, several functionalities may be incorporated. Due to the prospect of size miniaturization in electronics and functional diversification, molecular electronic can provide a potential means to extend Moore's Law (more than Moore).

My PhD thesis is dedicated to understanding the transport in the cross-bar molecular junction (MJ) on a large scale as shown schematically in Figure I-1, which is the basic building block of molecular electronic circuits.

The Metal/molecules/Metal junctions with organic layers from 2-20 nm thickness are fabricated by combining CMOS compatible technology and diazonium electroreduction. Organic layers are deposited by electro-generated radical grafting processes. It is possible to evaporate metals onto the grafted organic layer directly. The charge transport mechanism is inferred by analyzing the dependence of current-voltage (IV curves), on the thickness of the molecular layer ( plot) and the temperature (Arrhenius plot). This thesis is based on the expertise of the Nanoelectrochemistry group (NEC) in surface science and nanoelectrochemistry. Metal/Molecules/Metal junctions are fabricated in a clean room at the Paris Diderot University. Transport measurements are made in both the NEC and the TELEM groups (Paris Diderot University). Some work in the thesis was done in collaboration with Professor Richard McCreery's group at the University of Alberta, Canada. The thesis focuses on several scientific challenges which will be introduced and exploited step by step in each chapter.

In the first chapter, I will introduce the basic theory and techniques of molecular electronics, including how to fabricate molecular junctions and general transport mechanisms. Then I will focus on molecular rectifiers and described different studies which have been devoted to their behavior.

In chapter 2, I will study the rectification behavior in molecular cross-bar junctions by using asymmetric contacts (Au as bottom and Ti/Au as top electrode). Several molecules will be tested in such devices to investigate the effect of the HOMO-LUMO levels on transport. Then a second parameter will be investigated by introducing a decoupling unit (alkyl chain) into the molecule. The decoupling unit will influence the strength of the coupling with the metallic electrode.

In chapter 3, I will study transport in bilayer MJs. I will describe the fabrication of such bilayer systems by electrochemical approaches. The bilayer can be deposited by two distinct strategies, overgrafting (or the bottom-up approach) and the diazonium approach. In the first part, I will study the bilayer deposited by overgrafting and investigate the correlation between the conductivity of the bilayer MJs and the energy level of the bilayer. Then I will investigate a bilayer based on the diazonium approach and compare electron transport in both approaches.

xi Thereafter I will investigate transport in mixed-layer molecular junctions (Chapter 4). Mixed-layer systems have been developed in the NEC group and consist of an organic layer based on two different components. I will use this approach to build new molecular junctions. The electrical properties of mixed MJs will be investigated and compared with those of single-component MJs. In addition, I will attempt to understand the cooperative effect with mixed layers or the correlation between the number of active molecules and the current density. Chapter 5 will be devoted to the study of electron transport through organometallic MJs. Organometallic molecules will be electrografted onto a surface by the diazonium approach. I will compare the electrical properties of several organometallic MJs and attempt to correlate the conductance and the electroactivity of the complexes. I will study the effect of the counter-ion on the conductivity of the MJs and the light emitted from complex MJs.

Finally, a summary on the PhD work will be given and I will suggest some perspectives.

1

MOLECULAR ELECTRONICS -STATE OF THE ART

Molecular electronics is the study and application of molecular building blocks for the fabrication of electronic components. 2 First, we will made an important distinction between "molecular electronics" and "organic electronics" is that of scale: at least one dimension of molecular electronic devices is in the range of 1-20 nm, compared to above 100 nm (up to µm) for organic films in organic field effect transistors, organic light-emitting diodes, and other "organic" electronic devices. Organic electronic usually uses π-conjugated materials deposited as relatively thick films on various substrates and exploits redox reactions for charge transport by "hopping" between molecules.

In both fields, knowledge of the interface between the metal or conductive material and the molecules and organic film is crucial. Depending on how the molecules contact the substrate, several effects are expected, such as modifications of the workfunction of the substrate and a modulation of the energy barrier heights, causing important variations in charge injection into the molecular film. A pinning of the molecular orbitals of the molecules at the Fermi level of the electrode is expected to be critical in the charge injection process.

Many aspects of molecular electronics have been described in recent reviews. [1][2][3][4][5] Some of them, useful for this PhD work, will be developed in this "state of the art" review. We will first briefly describe the main techniques used to fabricate a molecular junction (MJs), recall the main transport mechanism mentioned in MJs and then make a short review of results on molecular rectifiers reported in the literature.

1

Making a Molecular Junction.

Deposition of Molecular Layer

In this section, the techniques most often used to deposit a molecular layer will be briefly described and their advantages as well as disadvantages will be discussed.

Self-Assembled Monolayers

Self-assembled monolayer (SAM) formation is the main procedure used for fabricating monolayers. [6][7][8][9][10] SAMs can be formed on many substrates including metals, silicon and oxide. 6,9,10 Such a SAM consisting of a head group, a tail and a functional end group is depicted in Figure I-1a. The molecules in the SAM are not usually oriented perpendicular to the surface. Basically, a molecule with a head group, which has a strong affinity for a substrate, can form a covalent bond with the substrate. Several head groups can be used: for Source: https://en.wikipedia.org/wiki/Molecular_electronics 2 instance, thiol (SH), silane, phosphonate, etc. Moreover, the tail of the molecule is usually based on an alkyl chain but several others, such as π -conjugated molecules can also form monolayers. For large areas, the tails play a critical role in the net order of the monolayer. Generally, these tail interactions resulting from van der Waals interactions (alkanes) or πstacking (aromatics) are rather weak; therefore, it is extremely sensitive to the conditions of SAM formation including temperature, humidity and concentration. Usually, SAMs also contains terminal functional groups such as Fc, bipyridyl group, and the thickness of SAMs monolayer is usually limited to less than 3 nm. 6

Diazonium Approach

In 1992, Pinson and colleagues showed that the electroreduction of a diazonium salt led to the deposition of a molecular layer on a carbon electrode. 11 They proposed that the molecular layer is covalently bonded to the substrate. [11][12][13] This electrochemical reduction generates a highly active free radical and N2. [14][15][16] Subsequently, the free radical bonds irreversibly with the substrate, as illustrated in Figure I-1b.

Grafting by means of diazonium salts generally produces a multilayer. As illustrated in Figure I-1b, after electroreduction of diazonium occurs, the produced radicals attach to either the surface or the first grafted aryl group, which leads to the formation of a second or even a third layer. This process is responsible for the growth of the molecular layer. As the layer grows, the electron transfer through it becomes less efficient and finally stops (selflimiting process). As a consequence, compact, and pinhole free ultrathin layers, strongly adherent to the metal surface are generated. When grown from a flat surface their roughness remains low and similar to that of the initial surface. There is a possibility for the layer growing continuously if an electron relay (a reductive unit) is included in the molecular chain.

In such case, the diazonium reduction can be assisted by electron transfer to the electron relay and the layer can continue to grow up to µm. 17,18 In addition, some recent strategies have demonstrated the controlled formation of functional monolayers on substrates: use of aryldiazonium ions with cleavable protecting groups; [19][20][21] by the use of aryldiazonium ions with cleavable protecting groups, [19][20][21] aryldiazonium ions with steric constraints, 22 grafting in the presence of a radical scavenger; 23 and grafting from ionic liquids. 24,25 The electrografting by means of diazonium salt has been recognized as a powerful tool due to its high capacity for covalent surface modification with multiple functionalities and multicomponent layers. 12,14,16 For instance, diazonium reduction can be used to fabricate a mixed layer as depicted in Figure I-2. Bélanger et al and our group reported a multistep procedure using aryldiazonium cations and a nanosphere template to prepare heterogeneous structured surfaces with contrasted chemical functionalities at the nanometer scale (Figure I-2). 26,27,28 Santos et al. also reported a strategy using host/guest complexation combined with two successive electrochemical reductions of diazonium salts to generate a mixed organic layer. 27,28

Other Methods

Langmuir-Blodgett (LB), layer-by-layer (LbL) are other techniques that have been used for the formation of organic layers. The LB technique, often used 20 years ago, is based on the physical absorption of molecules on a substrate. 29,30 In contrast to SAMs, the LB technique suffers from the poor stability of the junctions, and the proportion of devices functioning correctly remains weak. 30 Nevertheless, Lee et al. recently reported a robust molecular junction 31 when strong π interactions exists between an aromatic terminal group (pyrene) and a graphene substrate.

LBL is an alternative method in which a layer is deposited step by step, as depicted in Figure I-3. [32][33][34][35][36] LBL has some advantages in that it is easy to perform, and the thickness can be controlled from monolayer up to a multiple layer (up to 40nm). However, its disadvantages are that the time needed to prepare the layer is usually long and the efficiency of the process on large areas is difficult to control.

Fabrication of Molecular Junctions

Molecular junctions are made with electrodes ranging in size from sub-nm to micrometer (Figure I-4). The size of the electrode will define the number of molecules in the MJ, for instance MJs formed by either STM or the break junction contain single molecule or only few molecules, while MJs formed by CP-AFM or nanopore techniques incorporate around 100 to 1000 molecules since the size of the electrode is 20 to 100 nm. Moreover, on a large scale, MJs fabricated by e-beam evaporation, nanotransfer printing (nTP) and mercury or EGaIn drop technique contains 10 6 to 10 10 molecules. In the following section, several techniques used to form MJs will be described.

Figure I-4 Schematic overview of the different test-beds used to electrically contact organic molecules. The scale correlates the number of the molecules contacted with electrodes and the size of the electrode. The techniques are: STM, metal deposition by FIB, crossed wires conducting AFM, nanotransfer printing(nTP), mercury drop (From right to left and lower part of the figure) and break junction, nanopore and large area MJs by e-beam evaporation. Figure adapted from

reference [ 37 ].

Single and Few Molecules Junctions

Scanning Probe Microscopy (SPM) is a powerful technique not only to image individual molecules on surfaces but also to map the conductance of a molecule. In such experiments, the two electrodes are the SPM tip and the surface, only one molecule or q few molecules are located between these electrodes (Figure I-4). The key advantage of SPM based junctions is that this technique allows to understand electron transport in many environments and the distance between the two electrodes can be controlled and varied with a high precision (~ 10 pm).

STM Break Junction

Scanning tunneling microscopy break junctions (STM-BJ) were firstly developed by Tao et al. 38,39 Briefly, they established individual molecular junctions by repeatedly moving a gold scanning tunneling microscope (STM) tip into and out of contact with a gold substrate in a solution containing 4,4′ bipyridine and N-alkanedithiols. During the initial stage of pulling the tip out of contact with the substrate, the conductance decreased in a stepwise fashion, with each step occurring at an integer multiple of the conductance quantum Go =

Molecular junctions formed by scanning tunneling microscopy break junctions technique. (A) Conductance of a gold contact formed between gold STM tip and gold substrate. (B) A corresponding conductance histogram constructed from 1000 conductance curves as shown in (A). (C) When the contact shown in (A) is completely broken, corresponding to the collapse of the last quantum step, a new series of conductance steps appears if molecules such as 4,4' bipyridine are present in the solution. (D) Conductance histogram obtained from 1000 measurements as shown in (C). Figure adapted from reference[ 38 ].

When the atomic chain was broken by continuously pulling the tip further away, a new sequence of steps in a lower conductance regime appears in the presence of 4,4′bipyridine (Figure I-5c). The corresponding histogram shows pronounced peaks near 0.01 × G0, 0.02× G0, and 0.03× G0 (Figure I-5d). They attribute the conductance steps that appeared after the breaking of the gold contact to the formation of stable MJs, and the corresponding conductance peaks at 1 ×, 2 ×, and 3 × 0.01 G0 to one, two, and three molecules, respectively, in the junctions. The symmetric molecules in the study usually have two end groups that can bind to the tip and the substrate electrode and the conductance depends on the selected molecule, especially on the anchoring groups binding it to the electrodes. 22,3,5,40 However, the study of MJs formed by STM-BJ is usually limited to determining the conductance at a fixed bias. This is because the lifetime of a single MJs is relatively short (less than 100 ms 41 ).

Conductive Tip Atomic Force Microscopy

Atomic force microscopy with conductive tip (CP-AFM) (Figure I-6a) has been widely used to investigate the electrical properties of organic films as well as molecules. [42][43][44][45][46][47][48][49][50] In CP-AFM, the size of the conductive tip usually ranges from 20 to 50 nm and, consequently, there are a few hundred to thousands of molecules sandwiched in MJs. This technique allows to investigate electron transport through the single-component layer whose thickness ranges from that of a monolayer to 10 nm. 45,[50][51][52]6,[53][54][55][56][57][58][59] In the contact mode, the conductive tip is contacted with the molecular layer by applying a certain force, this is fundamentally different from STM-BJ technique. As a result, the conductance of the molecule depends on the applied force (Figure I-6b). Recently Aradhya et al. reported that by using the tapping mode in which the tip vibrates, they are able to simultaneously measure force and conductance across singlemolecule junctions and use force signatures to obtain independent evidence of junction formation. 60

Figure I-6 (a) Schematic illustration of a molecular junction formed using CP-AFM (b) Conductance versus applied load data (unloading only) for monolayers of C8 (•), C10 (○), and C12 (▪) collected with a representative tip. Figure adapted from reference[ 61 ].

Fabrication of a Nanogap in a Conducting Material 62 The principle of this technique could be found in several reviews. 63,64 The main advantage of this setup is the ability to form well-defined atomic electrodes.

The nanogap between two electrodes can be controlled with sub-Angstrom precision and the experiment in liquid can be equipped with an electrochemical gate which would offer many interesting possibilities, for example, changing the redox state of the molecule. Like the STM technique, the conductance of a single molecule in a MCBJ is extremely sensitive to the exact geometry and chemistry of the anchoring group. E-beam lithography associated with oxygen plasma treatment ( Figure I-7b, d) and titled e-beam evaporation (Figure I-7c) are the other main techniques used to develop a nanogap between two electrodes. Molecules can be inserted between two electrodes by click chemistry. Recently, Guo et al. reported that single diarylethene molecules could covalently bond between graphene electrodes to form molecular junctions. 65 These single molecule junctions were highly stable (over a year), and gave rise to photo-switch with a high on/off ratio ( ~ 100) and more surprisingly, to a stochastic switching of the conductance (on/off ratio ~ 8), assumed to be due to a conformational change at low temperature (180 K).

Figure I-7. Molecular junctions fabricated by (a) Mechanical controlled break junction (b) nanogap generated between two graphene electrodes (c) nanogap fabricated by titled e-beam evaporation (d) nanogap in carbon nanotube by oxygen plasma irradiation. Figure adapted from

reference[ 1,2,66,67 ].

Large Area Molecular Junctions 'Soft' Top Electrode

To form a complete molecular junction on a large scale, several 'soft' top electrodes can be used (Figure I-8). The aim is to protect a molecular layer from direct e-beam evaporation which usually creates metallic wires between two contacts. Akkerman et al. reported 68 that the large-area molecular junction could be fabricated with high yield and reproducibility by using a conductive PEDOT:PSS layer (Figure I-8a). This conductive layer will protect the SAMs from direct e-beam evaporation (where the yield of the fabrication by direct e-beam evaporation is only 1.3%).{Hylke, 2008 #662;Jeong, 2017 #875} This technique seems promising, but the quality of PEDOT: PSS must be carefully considered and the nature of the molecules/PEDOT: PSS interface is yet not clearly defined.

Other techniques based on the transfer of the top electrode including nanotransfer printing (n-TP) by a stamp (Figure I-8b) or 'off lift' and 'on lift' steps (Figure I-8c) have also been used. 69,70 However, the fabrication of MJs requires a long time due to multiple fabrication steps and the top molecules/electrode interface is also not well-defined. Another soft electrode (Figure I-8d) is based on a liquid conductive layer such as Hg or EGaIn. Liquid Hg, often used previously, is now excluded due to its toxicity. 46 In contrast, an EGaIn top electrode, widely used by a few groups, can be formed easily in situ and yields good MJs. However, it must be noted that an oxide layer (Ga2O3) is usually formed at the interface between the SAM layer and the EGaIn electrode. Direct E-Beam Evaporated Top Electrode Apart from the above methods, direct e-beam evaporation can be used for the fabrication of a top electrode. This mean that the CMOS technology can be used to fabricate solid-state MJs. First attempts performed on SAMs were disappointing because of the low yields (less than 2 %), even when the temperature for e-beam evaporation was lowered.

In contrast, McCreery et al. reported that by combining the direct e-beam evaporation and diazonium approach, they were able to fabricate reproducible MJs in high yield when the molecular layer was deposited by electroreduction of a diazonium salt on a PPF substrate (Pyrolyzed Photoresist Film). [73][74][75] They reported also that several elements, including C, Au, Pt, Cu, Ti and Si, could be deposited directly on top of the molecular layer functionalized on PPF electrode. 73,[76][77][78][79][80][81][82][83][84][85][86]

Figure I-9. A) Schematic diagram of the different steps of the fabrication of metal/molecule/metal junctions. (B) Structure of the BTAB precursor and schematic representation of oligo(BTB) layer between two electrodes. Figure adapted from reference[ 87 ].

In 2012, we reported a process that is fully compatible with CMOS technology to fabricate a large area MJs (Figure I-9). Briefly, the Au stripe as bottom electrode can be fabricated by conventional photolithography (or e-beam lithography). 88 Then the molecular layer was grafted onto the bottom electrode by the diazonium reduction (see Section 1.1.1). The deposited molecular layer is compact and pinhole-free, which helps to prevent metal penetration through this layer during direct e-beam evaporation. To complete the MJs, the top electrode can be deposited using a second lithography followed by direct metal evaporation onto the grafted organic layer. MJs are obtained by this procedure in high yield, above 80%.

The strategy, which combines CMOS technology with diazonium salt electroreduction, will be mainly used in thi PhD due to its reproducibility, its robustness and its compatibility with processes of the electronic industry. In this thesis, the same MJ fabrication procedure will be used thoughtout and we will study electron transport in MJs by changing only the grafted molecules.

Electron Transport Mechanisms in Molecular Electronics

In this part, I will indicate the main electron transport (ET) mechanisms used in organic and molecular electronic. 3,30,45,[89][90][91][92][93][94][95][96][97] Table I-1 lists common mechanisms reported for ET in molecular electronics and gives some of their main characteristics in terms of temperature, voltage and thickness dependence. Several of these mechanisms are particularly relevant to MJs, and a brief description of these mechanisms is given in this section.

Direct Tunneling

Classical, or coherent, tunneling, also named off-resonance tunneling, is a purely quantum phenomena based on the possibility for electrons to penetrate a region where their total energy is below their potential energy. As the wavefunction describing the electron state in this region is not zero and when the thickness of this region is small, the probability of an electron crossing the junction is not zero. The tunneling barrier is characterized by its thickness and barrier height (Table I-1). The rate of electron transport based on coherent tunneling is independent of temperature, decreases exponentially with the thickness of the barrier, and is given in its simplest form by the relation.

𝐽 = 𝐽o × 𝑒 -𝛽𝑑

In the case of a MJs, d is the distance between the two electrodes or the length of the molecular layer, where β is a constant and has units of inverse Angstroms or inverse nanometers. We note that β is proportional to the square root of the barrier height 𝛽 = 2 ℏ √2𝑚𝛷 2 where Φ is the barrier energy. β in vacuum is 25 nm -1 but is smaller when molecules bridge two electrodes. β strongly depends on the molecule type and it is generally agreed, on the basis of many experimental observations, that β is 2-3 nm -1 for conjugated molecules and 8-10 nm -1 for saturated molecules. 2,3,5,64 A relatively small β of 0.5 Å -1 (5 nm -1 ) predicts a decrease in the ET rate by a factor of > 20000 for a 20 Å (2 nm) molecule, leading to the conclusion that direct tunneling or off-resonance tunneling is effective only for short distances.

Another tunneling mechanism is named resonant tunneling. It can occur when the energy barrier Φ is close to zero. In this situation, β is also close to zero and transport is still effective over a long distance. This on resonant tunneling can be achieved by using STM. This regime occurs when the energy level of the electrons at the STM tip coincides with that of one molecular orbital of the molecule deposited on a surface. Resonant tunneling is the basis of the STM spectroscopy mode familiar to physicists working at low temperature in very clean, ultra-high vacuum environments. 5,7

Fowler-Nordheim Tunneling

Fowler-Nordheim tunneling refers to a tunneling mechanism which occurs when high electric fields are applied to MJs. This usually means that the applied voltage changes the shape of the barrier, which can be considered triangular (Table I-1). As a consequence, the effective tunneling distance depends markedly on the electric field and the tunneling current do not vary linearly with the applied voltage. It is usually independent of temperature, but decreases exponentially with distance (Table I-1).

Hopping Mechanisms

The term "hopping" normally refers to thermally activated ET, which follows a classical Arrhenius relation.

𝐽 = 𝐴 × 𝑒 -𝐸𝑎 𝑅𝑇

Where Ea is the activation energy needed for the charge carrier to hop between two sites. According to the above equation, the slope of Ln J versus 1000/T (Arrhenius plot) can be used to estimate the activation energy of transport. The difference between tunneling and activated hopping is the involvement of nuclear motion before electron transfer. ET does not occur until thermal motion of nuclei results in a favorable molecular geometry. Hopping involves electron motion over the barrier but the molecule must rearrange for ET to occur, while tunneling involves ET through the barrier meaning there is a finite probability of finding the electron on the other side of the barrier, without requiring nuclear motion before ET.

Since hopping involves a series of transfers between sites, it does not exhibit the exponential distance dependence of coherent tunneling, but instead varies as J ∼ d -1 . It is also the basis of ohmic conduction, and follows a dependence on driving force predicted by Marcus theory. For a homologous series with varying length (e.g. polyphenylene oligomers), we expect coherent tunneling for small d, but hopping for larger d. For large d, the distance is too great for coherent tunneling, and then the electron can propagate more efficiently by a series of hopping sites. By using CP-AFM, Frisbie et al firstly reported charge transport in oligophenyleneimine (OPI) wire ranging in length from 1 to 7 nm. It clearly exhibits a transition in the length dependence of resistance near 4 nm (OPI 5). This indicates that the conduction mechanism is different in short and long OPI wires. In short wires, the linear fit in Figure I-10a indicates that β is around 3 nm -1 which can be attributed to non-resonant tunneling. For long OPI wires, the β is much smaller (β ~ 0.9 nm - 1 ). The small β (among the smallest at that time) suggests that the principal transport mechanism is hopping. The change in transport mechanism was confirmed by the temperature dependence (Figure I-10b). The resistance for OPI 4 is independent of temperature from 246 to 333 K, as expected for tunneling. However, both OPI 6 and OPI 10 display the strongly thermally activated transport that is characteristic of hopping and the activation energies determined from the slopes are identical at 0.28 eV for both OPI 6 and OPI 10.

Poole-Frenkel Effect

The Poole-Frenkel effect (PF), first described by Frenkel in 1938, is the thermal emission of charge carriers from coulombic traps enhanced by the application of an electric field. 99 The PF mechanism is driven by the electric field, which reduces the barrier height on one side of the trap, thereby increasing the probability of the electron escaping from it. The PF effect happens when an electron dissociates from a neutral site and leaves behind a positive site. The electron is then escaping from the coulombic potential of the positively charged site it left behind. The PF effect can also occur when a hole dissociates from a neutral site and leave behind a negatively charged site. 100 This process is illustrated in Figure I-11, where a coulombic potential well is shown in the presence of an electric field. In this picture, qΦ is the ionization potential in eV and is the amount of energy required for the trapped electron to escape the influence of the positive nucleus of the trapping center when no field is applied. β √ 𝜀 is the amount by which the trap barrier height is reduced by the applied electric field, ε. Poole-Frenkel conduction is related to hopping. Current or resistance varies exponentially with the square root of the electric field.

Schottky emission

Schottky emission or thermionic emission was investigated to explain ET at the interfaces of semiconductors with metals. A Schottky barrier usually arises when there is partial charge transfer from one to the other at an interface. As shown in Table I-1, the barrier height is influenced by the local electric field at the interface. We note here that the energy barriers for Schottky emission and Fowler-Nordheim tunneling are at the molecule/contact interface whereas that for Poole-Frenkel conduction is localized within the layer.

Molecular Rectifiers

Several electronic functions are expected to be realized with MJs, namely rectification, resistive memories and switches. In this work several new molecular rectifiers will be developed. Previous works on such devices are thus described below.

A rectifier is a diode that allows the flow of current only in one direction. It therefore shows asymmetric IV curves and basically converts alternating current into direct current. The performance of a diode is characterized by the rectification ratio (RR), i.e. the ratio of the electrical currents through the junction for two opposite bias voltages, V and -V (𝑅𝑅 = 𝐼(+𝑉)

𝐼(-𝑉)

). For a conventional inorganic diode, the rectification ratio is usually 10 6 at 1 V with a current around 10 -4 A. 101 Sometimes, the σ moiety is replaced by a conjugated group, which is often erroneously assumed to maintain the decoupling of A and D.

Figure I-12. Schematic diagram of the Aviram-Ratner conduction mechanism of a D-σ-A inserted between two metallic electrodes M 1 and M 2 (ф 1 and ф 2 , E F1 et E F2 .are the work functions and the Fermi levels of M 1 and M 2 , respectively). The alignement of HOMO (D) with E F1 and LUMO (A) with E F2 results from the applied bias V and allows the most favorable conduction by resonant tunneling. The molecule represented was imagined by Aviram and Ratner. Adapted from reference[ 102 ].

In the 40-years history of molecular electronics, there have been many studies on molecular rectifier based on single molecule to large area MJs. However, molecular rectifier exhibits lower rectification ratios and current densities than conventional diode.

Electron flow

Is there a room for molecular rectifiers? Molecular rectifiers certainly have a plenty of room for some practical applications. They are thus key elements of radiofrequency identification (RFID) tags. Operating speed is crucial since it defines the frequency range of operation. The frequency most used for RFID is 13.56 MHz which is already accessible to organic rectifying diodes 103 and allows shortdistance communications at low cost. However, for more demanding applications (cellphone to tag communication, for example) it is of great value to operate tags above 800 MHz. Generally, it is assumed that organic devices are limited to low-frequency operation either because of limited carrier mobility in polymer or molecular films or because of the intrinsically slow motion of ions as illustrated in Figure I-13b. However, when the transport distance is below 30 nm, it was predicted that the cut-off frequencies can be boosted up to GHz range (Figure 13b).

Figure I-13. (a) Experimental Vin and Vout signals versus time, illustrating the operation of the rectifying circuit at 15 MHz. (b) Rectifying circuit cut-off frequency versus diode thickness for various organic layer mobilities. Shadowed areas indicate either cut-off frequency too low or organic thickness too thin. Figure adapted from reference [104]

As an example, recently, Coppard et al. 104 provided a detail analysis of polymer diode cut-off frequency, showing the drastic impact of film thickness the reduction of which makes it possible to target the multi-GHz range, even at moderate carrier mobility (below 1 cm 2 /Vs). They proved that the cut-off frequency of an operating diode depends strongly on the thickness of TFB and theoretically predicted that a high frequency operating diode can be obtained when the thickness of the organic layer is below 20 nm, even with low charge mobility (see It can therefore be stated that it is interesting to develop molecular rectifiers using organic films less than 20 nm-thick for practical applications. It is also of high fundamental interest as it allows to probe electron transport at the nanoscale with transport distances that are below those used in the electronics industry.

In a general manner rectification in MJs results from an asymmetry of the junction which may come from the electrodes (two metals with different workfunctions), from the interface (Electrode/organic molecule), from the molecule itself or from a combination of these effects. Metzger distinguished three conduction mechanisms, classified as S, A and U: S indicates that a Schottky barrier is involved at one electrode/molecule interface; A denotes an asymmetric position of the conjugated group within a saturated chain; U (unimolecular) indicates that conduction depends solely on the position of the orbital levels of the molecular system interacting with the electrode. 30 In the next section, I will summarize the various mechanisms which are usually used to explain the rectification obtained in molecular rectifiers. We note here that in some cases, several mechanisms can occur simultaneously. We considered only molecular rectifiers which shows the rectification ratio above 20 at a given bias. Only a few examples will be described to illustrate each mechanism. This next section will therefore not be an exhaustive review of the results published on molecular rectifiers.

Donor-Bridge-Acceptor molecules

Several molecules following the Aviram-Ratner model (AR) were experimentally studied and their rectification ratios measured. 30,[105][106][107][108][109][110][111] A first set of work covering a period from the 1980s to the early 2000s was analyzed and described in detail by Metzger. 30,108 The rectification ratio in these cases usually remained relatively low (less than 50). 30,102 Many unimolecular Donor-bridge-Acceptor molecules have been investigated by using STM-based junction, 30,108,110 soft Hg top electrode or large-area MJs with top electrodes generated by direct e-beam evaporation. 30 The typical AR conduction mechanism (Figure I-12) shows the most favorable electron flow corresponds to resonant tunneling, resulting from the bias V which aligns the LUMO (Acceptor group) with the Fermi level EF2 (right electrode) and the HOMO (Donor group) with EF1 (Fermi level of the left electrode).

The opposite bias (not shown) will induce a wider barrier for the injection of electrons from M2 toward LUMO (A) and from HOMO (D) toward M1, and thus the current is greatly decreased, resulting in rectification. This rectification results from the dissymmetry of the molecule and implies that the LUMO (A) and HOMO (D) are not pinned to the Fermi levels. The preferential flow of electrons in this AR model is from the cathode to the LUMO of the acceptor group toward the HOMO of the donor moieties and finally to the anode (Figure I-14a).

This preferential direction of electron movement is not always observed, and an anti-AR mechanism has been proposed when the D-σ-A molecule first self-transforms into a zwitterion D+-σ-A-, resulting from an autoionization of the molecule induced by the electric field between the electrodes (this autoionization is facilitated when the σ group is replaced by a π-conjugated group). According to this mechanism, the high conductance corresponds to a flow of electrons from the positive electrode to the HOMO of the donor group toward the LUMO of the acceptor moieties (Figure I-14b).

Figure I-14. Schematic diagram of AR and anti-AR mechanisms. a) In the AR mechanism electrons move from the HOMO (D) to M2 and simultaneously electrons are injected from M1 to the LUMO(A) and then injected from the LUMO(-) (A) to the HOMO(+) (D) for neutralization. b) In the anti-AR mechanism, the electric field induces the autoionization of A-σ-D into A --σ-D + and, in contrast to the previous case, electrons flow from the positive electrode toward the HOMO+ (D). Figure adapted from reference [ 30 ].

Of all the systems that have been tested and recently reviewed by Metzger, it should be noted that, when the asymmetry results solely from the molecule, the rectification ratios are rather low (many diodes have RR ranging between 10-30) and rarely exceed 100-150. 30 In addition, some specific effects have been observed with conjugated dipolar molecules where the insulating σ group is replaced by an unsaturated one. Thus, Ashwell et al. have shown that an inversion of the dipole orientation of the molecule in the diode can reverse the rectification polarity (Figure I-15). 112 Two D-π-A-C16H33 (1) and A-π-D-C16H33 (2) molecules were deposited as Langmuir-Blodgett films on a gold substrate and probed by STM using a nano-PtIr tip. In both cases the hydrophobic tail pointed upwards and was probed by the PtIr tip. At ± 1V the rectification ratio ranges is 10-25 for (1) and 20-25 for (2) and, as expected, a mixed layer made of equal amounts of (1) and (2) gives symmetrical I/V curves with a RR of 1.

Figure I-15 I-V curves of molecular diodes deduced from STM measurements. Diodes are made with similar molecules 1 and 2 adsorbed on Au with dipole moments pointing in opposite directions. I/V curves 1 and 2 correspond to Donor (D) and Acceptor (A) fragments in contact with Au, respectively. In both cases the C 16 H 33 hydrophobic tail is probed by the PtIr tip and the polarity indicated in the graph is that of Au. Figure adapted from reference [ 112 ]

The fact that the whole molecule is conjugated, and does not have an insulating σ fragment between parts D and A is probably the cause of a low RR. Ashwell et al. confirmed this after testing a series of molecules in which the conjugation between fragments A and D is progressively reduced until it disappears, while RR increases sharply from 1 to 100-150. 113 This effect has been demonstrated by grafting bulky groups onto fragments A and D of molecule 1 (Figure I-16). This increases the dihedral angle θ between the planes formed by A and D and thus reduces the conjugation between A and D. The RR grows progressively from 1 (molecule 1), to 11-18 (molecule 2), then to 50-150 (molecule 3). This corresponds to an increase in θ from 10, 31 and 61 degrees, respectively, and to an enhancement in the insulating character of the middle vinylene group.

Figure I-16. Effect of the dihedral angle θ between fragments A and D on the rectification of the corresponding diodes formed with molecules 1, 2 and 3. Each molecule is adsorbed on Au and is probed by STM with a PtIr tip. In 1 conjugation extends over the whole molecule and RR is low while, in 3, the high dihedral angle breaks the conjugation and gives a much higher RR. Figure adapted from reference [ 113 ]

This last example confirms that the Aviram-Ratner model requires an isolating fragment between the donor and acceptor parts of the molecule to achieve the highest rectification ratios which, nevertheless, remain modest. Yee et al. reported the rectification of Donor-π-Acceptor MJs formed by CP-AFM, in which the molecule is covalently bond to both electrodes. However, the rectification ratio remained very low (~2). 53 An alternative approach developed by Frisbie et al. is to use click chemistry to build up Oligo(Donor)bridge-Oligo(Acceptor). 114 The MJs is formed by using CP-AFM. Frisbie et al. showed that the rectification ratio was 30 at 1V for Oligo(Donor)-bridge-Oligo(Acceptor) this junction, whereas removing the bridge led to a symmetric IV curve. The mechanism of rectification was considered to follow the Aviram-Ratner model. Overall, rectification is often observed in Donor-bridge-Acceptor MJs but the rectification ratios remain low and so far have never reached the performances of inorganic diodes.

Bilayer Molecular Rectifiers

Bilayer molecular junctions uses two chemically different molecular layers which are deposited on top of each other. Their interactions are usually weak since, in most published studies, the second layer is physisorbed on the first layer.

Ashwell et al. reported the rectification of a bilayer MJs. 30,115 It had a high rectification ratio at 1V (up to 3000), but was unstable after a few potential sweeps. Smerdon They attributed the rectification to the fact that there is strong electronic coupling between C60 molecule and Cu (111) substrate but a weak electronic coupling at the other interfaces (Figure I-17c). Later on, Smerdon et al. also reported that C60/C60 (Acceptor/Acceptor) bilayer MJs also show the rectification behavior (RR up to 1000 at 1V) and again assigned it to strong electronic coupling at the bottom interface between C60 and Cu(111), and weak coupling with the STM tip. 117 Recently, McCreery et al. and our group showed that a bilayer can be deposited by using two successive diazonium electroreductions. 118 They fabricated bilayer MJs in which the bilayer system include the oligo(Donor) in one layer and an oligo(Acceptor) in the other; the bilayer is sandwiched between symmetric e-C contacts (Figure I-18a). 118 The order of the bilayer system can be changed by the diazonium reduction conditions. The MJs was very stable in a large potential window (-5 to 5 V). Rectification was observed, and RR depended on the thickness of each layer. When each layer is less than 5 nm-thick RR is relatively low, below 5, whereas above 5 nm RR at 4.5 V increased to 400 at 300 K and 150 at 7 K. Interestingly, the polarity of rectification can be reversed by changing the order of the two layers.

Single-Component Molecular Rectifier

Instead of the Aviram-Ratner model, Kornilovitch et al., then Larade and Bratkovsky theoretically proposed that asymmetric molecules comprising a conjugated group X sandwiched between two aliphatic chains, such as [HS-(CH2)m-X-(CH2)n-SH], are also likely to produce dissymmetrical IV curves. The system was first studied with X a phenylene group (C6H4), 119 and at the same time, by with X a naphthalenyl group (C10H6). 120 The benzene or the naphthalene core is the electroactive part of the molecule and the two thiolated aliphatic chains of different lengths (m and n) are the insulating parts. The SH end-moieties are used as anchoring groups and the electrodes are either identical or made of metals with different workfunctions (WFs). A detailed calculation of the IV curves was carried out for the molecules HS-(CH2)2-C6H4-(CH2)n-SH and HS-(CH2)2-C10H6-(CH2)n-SH with different values of n. As expected, no rectification occurs when the junction is symmetrical (n = 2) and the greatest RR is reached for the greatest asymmetry of the molecule, corresponding to n = 10. Moreover, it should be noted that when n = 10 the resistance of the junction is maximum and leads to a significant decrease in the intensity, which drops from a few μA to a fraction of nA when n goes from 2 (symmetrical structure, no rectification) to 10.

Whiteside et al and Nijhuis et al. experimentally reported that when the top electrode was EGaIn, MJs based on a SAMs layer incorporating a redox center show significant rectification properties. The RR was up to 100 at 1V. [121][122][123][124][125] When ferrocene (Fc) was used as redox center, (Figure I-19a), RR was from 100 to 1000 at low potential even though the current magnitude, especially for large-area MJs, was relatively low (mA/cm -2 ). When the redox center is removed, MJs exhibit the symmetric IV curve.

Figure I-19 (a) schematic of molecular junction Ag TS -SC 11 Fc//Ga 2 O 3 /EGaIn (b) Energy level diagram and mechanism of charge transport across Ag TS -SC 11 Fc//Ga 2 O 3 /EGaIn junctions. At positive bias, the arrows suggest that the charge traversing the junctions has to tunnel across the whole length of the molecule (C 11 alkyl chain and Fc group; a distance of 2 nm), because the HOMO level of the Fc moiety does not overlap with the Fermi levels of the electrodes. At negative bias, the HOMO level of the Fc moiety can participate in charge transport, and the charge only has to tunnel across the C 11 moiety over a distance of 1.3 nm. In a second step, the charge can hop across the Fc moiety. Figure adapted from reference [ 126 ]

They attributed the rectification to the fact that the interaction at the top interface between EGaIn and Fc is stronger than that at bottom electrode. This leads to different tunneling distances for forward bias and reversed bias (Figure I-19b). This mechanism is consistent with that proposed by Pomerantz et al. in 1992 where single-molecule junctions were studied by STM 11 and also with the theoretical model proposed by Kornilovitch. They also reported that RR of MJs based on SAMs can be adjusted by either the length of the alkyl chain or the macroscopic organization of the redox centers.

The rectification polarity can be reversed by changing the redox center from easily oxidized moieties such as Fc and Fc2 to one that can be reduced (bipyridyl unit). 71,[127][128][129][130] Additionally, Nijhuis et al. proved that the rectification polarity depends strongly on the location of Fc redox group 131 and can be reversed by moving it from the top interface (with the EGaIn electrode) to the bottom interface (with the Au electrodes). In this case, electronic coupling between the bottom electrode and the Fc redox moieties is controlled by varying the number of carbon in the alkyl chain backbone. When there are 10 to 13, RR reaches up to 100 at 1 V and decreases to 1 when there are from 9 to 5. The rectification polarity is reversed if the number of carbons is changed from 10 to 3. Interestingly, when there are only 0 to 2, no rectification is observed.

More recently, Nijhuis et al. showed that similar SAMs terminated by a di-ferrocenyl ethynyl (Fc-C≡C-Fc) group formed the molecular diode, Pt/(SH(CH2)15 Fc-C≡C-Fc)/EGaIn which has an extremely high rectification ratio of 6.3 10 5 taken at 2.7 V due to a specific interaction of the Fc group with EGaIn top electrode. 132 A high current intensity is observed at forward bias, when the top electrode is negatively polarized (12 mA.cm -2 at -2V and 300 K) whereas the current remains low at reverse bias (≈ 2.10 -5 mA.cm -2 ) This result was attributed to a change of the interaction of the -Fc-C≡C-Fc group with the Ga2O3 monolayer (resulting from EGaIn surface oxidation). When EGaIn is negative -Fc-C≡C-Fc is doubly charged (2+) and the interaction with Ga2O3 is electrostatic, whereas at positive polarization Fc-C≡C-Fc is neutral and a much weaker interaction results from van der Waals forces. In this latter case the number of connected molecules is lower than for the electrostatic interaction, which explains the higher current observed for the forward polarization.

It is worth noting that at forward bias, the HOMO level of the ethynyl ferrocenyl group is favorably aligned with the Pt Fermi level, providing a sequential tunneling effect: one from the HOMO to Pt across the C15 insulating chain, and another from EGaIn electrode to the HOMO, whose effect is to neutralize the positive charge created in the HOMO. On the contrary, for reverse bias, the position of the HOMO level is unfavorable, and there is only direct tunneling through the whole thickness of the layer. Consequently, this results in a lower current intensity than with forward bias (

For forward bias voltage (Pt grounded, EGaIn negative) and in contrast to reverse bias (Pt grounded, EGaIn positive) two effects contribute to the current: i) electrostatic attraction between (Fc-C≡C-Fc) 2+ and Ga 2 O 3 which increases the number of molecules in contact with the EGaIn electrode;this does not occur for reverse bias where Fc-C≡C-Fc remains neutral; ii) sequencial tunneling which makes that the part of the current the most limiting occurs through the SC15 chain instead of the whole thicknessof the molecule for reverse bias. Figure adapted from reference [ 132 ].

Recently, Lee et al. reported that replacing the EGaIn electrode by PEDOT: PSS in MJs leads to 100-fold decrease in RR. 133 Clément et al. also reported that MJs based on the same molecule formed by CP-AFM show a small RR (below 5 at the same bias, 200 times less). 42 We note here that the polarity of MJs formed by CP-AFM is the opposite of that with the EGaIn top electrodes. These apparent contradictions indicate that rectification is rarely due only to the molecule and that rectification in MJs can only be understood in terms of the interplay between molecular and contact effects.

Overall, single-component molecular rectifiers have been theoretically proposed and experimentally proven. When the top electrode is EGaIn, these molecular rectifiers usually show high rectification ratio. This strategy, based on single component, is a promising approach to practical molecular rectifier.

Asymmetry induced by Electrode contacts

Kroemer (Nobel prize, 2001) said that 'the interface is the device'. Hence, the interfaces should be carefully considered in organic and molecular electronics to control and understand new functionalities, such as current rectification or photovoltaic, induced by breaking the symmetry in the devices.

Rectification behavior is related to several effects, some being due to the molecular layer and others to the different electrodes or interfaces. Some of the examples reported in section 3.3 already illustrate the interplay between molecular and contact effects. Indeed, Nijhuis et al. reported that Ag/SC13-Fc/Ga2O3/EGaIn MJs exhibit high rectification, (RR ~ 1000 at 1 V). However, replacing Ga2O3/EGaIn by Pt conductive probe decreased RR by a factor of 100 and more importantly, changed the rectification polarity. The bilayer C60/Pn, (Acceptor/Donor) and C60/C60 (acceptor/acceptor) systems studied by Smerdon et al. 116 , showing RR up to 1000 at 1 V, were discussed in term of coupling strength with the bottom and top electrodes.

These examples show that the molecules/metal interface plays a crucial role in controlling rectification.

Three major effects at the molecule/metal interface must be considered: electronic coupling between the metal and the orbitals of the molecule, the different work functions of the metal electrodes and the presence of a dipole layer at the interface, which modifies the work function of the metals. All these parameters are significant and can strongly influence the rectification process.

Electrode-molecule coupling and pinning effect

The energy levels of an isolated molecule are discrete. Each discrete energy level with respect to a vacuum energy can take two electrons, and the frontier energy levels, highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), are particularly important as they define electron affinity and ionization potential. In contrast, the electrode contains many electronic states than a molecule. These states are filled up to the Fermi level, and the minimum energy required to remove an electron from a metal is known as the work function (Figure I-21a). In molecules, there is no 'realistic' Fermi level, so that sometimes the Fermi level in molecules is calculated by a formula: EF = (EHOMO + ELUMO)/2.

Figure I-21 a) The energy lineup of an isolated contact and an isolated molecule. (b) Energy alignment following the Schottky Mott rule (weak coupling). Figure adapted from reference [ 3,134,135 ]

When a molecule is brought into interaction with the surface of an electrode either by physi or chemisorption, coupling between the molecular and the electrode states can be categorized as weak, intermediate or strong. These interactions induce a wide variety of effects on both the molecule and the electrode such as a change in the vacuum level, a shift and a broadening of the molecular energy levels, a change of the electrode work function and a modification of the energy barrier between the Fermi level of the electrode and the molecular level. More details of these effects can be found in well documented reviews. 6,136,137 Shortly, for negligible interactions, the Schottky-Mott rule of thumb states that vacuum level alignment occurs, the vacuum level is constant over both contact and molecules (Figure I-21b).

This rule is often broken even in the weak electronic coupling regime due to the intrinsic dipole of the molecule (Figure I-22a). For instance, dipolar dithiocarbamate (DTC) monolayers covalently bonded on a metal substrate can increase (up to 2.1 eV) the work function due to a shift of the relative vacuum level. 138 In the case of strong electronic coupling, hybridization between molecular orbitals and the substrate wave function occurs (Figure I-22b). Such interfaces are typically formed when molecules are deposited as (SAMs) or by the diazonium electroreduction approach. Hybridization may involve a net transfer of charge and the molecular energy level is broadened as the life time of the electron in this level is decreased. Broadening is usually characterized by a coupling parameter (Г) and these hybrid levels can be pinned to the Fermi energy of the electrode, meaning that the energy difference relative to the Fermi level of the electrode is difficult to change. Changing the strength of the coupling parameter (Г), which expresses the mixing between molecular electronic and electrode levels, usually leads to a change in the conductivity of the molecule in MJs. For instance, Berke et al. reported that the conductance of a molecule increased 1000 times when an Au-C bond was changed to Au-S bond. 139 More generally, it can explain the many effects of anchoring groups in single molecule junctions. Strong electronic coupling associated with pinning effects have been evidenced when molecules are deposited on carbon electrodes by diazonium electroreduction. McCreery et al. reported that Carbon/molecules/Carbon MJs for seven aromatic molecules exhibited the same IV behavior. UPS and transport measurements showed that the tunneling barrier is compressed relative to that predicted from the HOMO or LUMO energies in the Schottky-Mott limit. 140 This behavior was clearly due to the fact that the strong electronic coupling between the molecules and carbon electrode leads to a partial charge transfer.

Pinning in the strong coupling regime was recently used by Van Dyck and Ratner who proposed a new scheme for developing molecular rectifiers based on molecules similar to that used in the Aviram Ratner model. Note that in the original AR model, coupling between the molecular level of the donor or the acceptor with the electrodes is implicitly supposed to be weak, so that applying a bias makes it possible to reach resonance between the HOMO or LUMO level and the energy level of the electrons in the contact. In the new proposal the molecule is terminated at its ends by two SH and CN anchoring groups (Figure I-23). 141,142 A π-decoupling unit positioned between the two conjugated fragments makes these two parts of the molecule independent, but each part is subjected to the electrostatic field of the corresponding anchor group. SH on the left, considered as a donor anchoring group, will promote the alignment of the HOMO near the Fermi level of the left electrode while the CN acceptor group, on the right, will induce the LUMO alignment of the right fragment near the Fermi level of the right electrode as illustrated in Figure I-24a. These pinning effects of SH and CN bonded to Au electrodes and other metals (Ag, Cu) have been already described and confirmed by several authors. [143][144][145][146]

Figure I-23. Typical molecular structure proposed by Van Dyck and Ratner

Such molecules, anchored to Au electrodes by a donating (SH) and an accepting (CN) group are predicted to lead to rectifiers with much higher rectification ratios than those obtained with the same molecules anchored to Au electrodes by two SH groups. The pinning effect of the SH and CN groups on the HOMO (left) and LUMO (right) orbitals of both fragments induces resonant and non-resonant tunneling at opposite potentials, resulting in rectification.

Figure I-24 Rectification mechanism of the diode Au/SH-(C=C)n-(CH2)m-(C=C)n -C≡N/Au. a) Position of the HOMO (left conjugated fragment) and LUMO (right conjugated fragment) levels before and after contact of the molecule with the Au electrodes. After contact the HOMO and the LUMO are pinned to the electrodes and shift up and down, respectively, b) The negative bias applied to the left electrode brings the HOMO level closer to that of the LUMO causing a compression of the gap (ΔEgap) between both levels, and then makes the tunneling resonant. For the opposite bias, (ΔEgap) is large and the tunneling between the LUMO and the HOMO is no longer resonant and consequently its intensity is weaker than previously. For the negative bias, the HOMO and LUMO levels of both fragments are within the window defined by the Fermi levels of the electrodes while, for the positive bias, they are outside the same window. Figure adapted from reference [ 141 ].

Compared to the classical AR mechanism (in which no pinning is assumed to occur), the double pinning effect, which aligns the HOMO and LUMO close to the Fermi levels of both electrodes, contributes to improving RR. The scheme of Figure I-24b clearly shows that when the right electrode is biased positive resonant tunneling between the LUMO (right) and the HOMO (left) is favored because they are close to each other. On the contrary, the opposite bias induces a large gap between the HOMO and the LUMO, so tunneling is no longer resonant and is consequently weaker than before. Note also that this new mechanism predicts that the preferential electron flow will be from the HOMO toward the LUMO and is thus in the opposite direction to that observed in AR initial proposal.

3.4.2

Rectification induced by dissymmetric coupling at two interfaces.

Another way to induce rectification is to play on the coupling strength of the two interfaces. Indeed, for similar electrode material and symmetric single component systems, asymmetry in transport can be generated if the coupling at one interface in strong while that at the other is weak.

Recently, Siya et al. reported 147 rectification in single polyoxometalate ([Dy5P5W30O110] 12 (POM) molecular junction. The POM was adsorbed on a conductive substrate and observed by an STM tip. They proved that by controlling the distance between the tip and the molecule, RR could be changed from 100 to 1. They proposed that this effect was related to the change of the coupling strength (from weak to strong) between the tip and the POM induced by the distance variation. Additionally, the rectification polarity was changed by reversing the electronic coupling at the bottom interface (substrate/molecule) (Figure I-25). It is suggested that the electron transport is mediated by the LUMO, since the POM cluster is an electron acceptor. The previous mechanism can be applied to explain the rectification of POM MJs. However, when POM is replaced by C60 molecule, RR decreases dramatically from 600 to less than 5 at 1.5 V. They attributed this huge difference to the fact that, since the LUMO of the POM is essentially localized on the metal ions, its coupling with the substrate is expected to be relatively weak. This is in contrast with the case of fullerene where the LUMO is relatively large and delocalized over the molecular structure, implying greater coupling with substrate.

Figure I-25. Inversion of rectification as the molecule is transferred from the substrate to the tip. (a) STM image before approach experiment. (b) Tunneling I-V characteristics on the molecule in the center of the red circle. (c) Tunneling I-V characteristics after performing an approach experiment. The inversion of the I-V characteristics is due to transfer of the molecule to the tip. (d) STM image after approach experiment. The target molecule has been transferred to the tip and then lost during scanning. Figure adapted from reference [ 147 ].

Apart from STM-based junction technique, rectification behavior is rarely found by either STB-BJ or MCBJ techniques in a solvent. Usually the RR achieved by these techniques is relatively low (less than 10) and is attributed to the difference in electronic coupling at the two interfaces, 148,149 molecular dipoles 150 or the difference between the electrodes. 151 Capozzi et al. reported 152 that the RR of symmetric single molecule junction can be enhanced by breaking the symmetry of junction by exposing different electrode areas to an ionic or polar solution. The RR at 370mV, when the substrate is grounded, is up to 200, which is the highest ratio reported at such small bias.

Rectification induced by dissymmetric electrodes

The symmetry of molecular devices can be broken by using metals of different work function. On a large scale, "organic p-n diodes" composed of > 100 nm thick films of organic donor and acceptor molecules, [153][154][155][156][157][158][159] including the wide range of OLEDS, show strong rectification behavior, but the focus of this work is on much thinner molecular layers, i.e. < 20 nm, which despite their ultrathin thickness exhibit high RR. In 2003, one of the first example was MJs based on SAM with Titanium and platinum electrodes (direct e-beam evaporation was used for top electrode fabrication). 160 The reported rectification ratio was10 5 but the fabrication yield of such junction was low. The mechanism of rectification was partially assigned to the difference in the work functions of Ti and Pt electrodes. Later on, McCreery et al. reported that by changing the conditions of pressure during the evaporation, the rectification of the 3 nm molecular junctions can be varied from 400 to less than 5 when the pressure decreases from 10 -5 mbar to 10 -7 mbar. They assigned the observed rectification to the formation of a TiO2 layer.

More recently hybrid single molecule junction, using a semiconductor as n-GaAs 161 or silicon. 162 as the bottom electrode have been investigated. Vezzoli et al. reported that junctions with Au STM/molecules/n-GaAs show rectification, but RR is low (less than 20). 161 Aragonès et al reported that Si/molecules/Au junctions with low doping level Si show rectification with RR exceeding 4000 at 1.5V. but when high doping level Si electrodes were used, no rectification was obtained. 162

Conclusions

In this chapter, several aspects of molecular junctions have been introduced: their fabrication methods, some of the various transport mechanisms and the different ways of inducing rectification. Such important backgrounds is essential to the understand the rest of the PhD work.

During my PhD thesis I will study large-area molecular junction including roughly 10 10 molecules. Molecular layers, 2 to 20 nm thick, will be deposited by in situ diazonium salt reduction, while the electrodes will be fabricated using CMOS technology. Asymmetric electrode, Au & Ti/Au, will be used mainly. I will try to understand transport and rectification in these devices and then use this knowledge to tune and if possible enhance rectification properties. Many different compounds will be investigated. Many approaches will be used. Single-component, bilayer and lastly mixed layer molecular junctions will be investigated. In doing so we hope to obtain a large overview of the many parameters that can impact transport and rectification in MJs. Several important scientific questions, will be listed in each Chapter and we expect to answer to some of them.

RECTIFICATION IN LARGE-AREA SINGLE-COMPONENT MOLECULAR JUNCTIONS 1 General Introduction and Objectives

As mentioned in section 2, Chapter 1, rectification behavior is usually observed in molecular junctions by breaking their symmetry. This can be done by using dissymmetric molecules, dissymmetric electrodes or by bilayers. Usually, dissymmetry originating from both molecules and electrodes exists simultaneously in MJs. For instance, Nijhuis et al. reported that Ag/SC13-Fc/Ga2O3/EGaIn MJs exhibit rectification, the rectification ratio (RR) reaching 1000 at 1 V (see Figure I-15). In such MJs, the symmetry is broken by using different electrodes (Ag and Ga2O3/EGaIn as bottom and top electrodes, respectively) and dissymmetric molecules (HSC13-Fc). In many cases, it seems likely that the best rectification is obtained by using both asymmetric electrodes and dissymmetric molecules.

This chapter aims at searching and investigating rectification in single-component MJs formed by using asymmetric electrodes on a large scale. Several molecules will be sandwiched in between the gold bottom electrode (Au) and the titanium-gold top electrode(Ti/Au). The electrodes are fabricated by using direct electron beam (e-beam) evaporation and the molecular layer is deposited by electrochemical reduction of diazonium salts (see General Appendix). This work may provide answers to several scientific questions.

• Do single-component MJs rectify? If so, what is RR?

• Can RR be tuned? and is there a molecule which does not rectify?

• What is the origin of rectification? asymmetry due to contacts (Au & Ti/Au) or molecular effect? These questions have been partially studied in an article published in JACS (DOI: 10.1021/jacs.7b05732) during this work. The first part of this chapter is essentially an adaptation of this publication.

In the second part of the chapter, several attempts have been made in order to answer the unexpected result obtained in part 1.

Introduction

The molecular junction (MJ) is the basic component of molecular electronics, and consists of a single molecule or an ensemble of many molecules between two conducting electrodes. [1][2][3][4] For molecular layers with thickness, d, below 5 nm and in many single-molecule devices, direct tunneling through the molecule acting as a barrier is the dominant transport mechanism and an exponential decrease in junction current with increasing distance is observed. [4][5][6][7] The attenuation factor, β, describing this exponential decay is in the range of 7-9 nm -1 for alkane MJs, and close to 3 nm -1 3,4,8-10 for conjugated molecules. When the thickness of the layer increases, transport is no longer consistent with direct non-resonant tunneling between the contacts, and a change in β is often observed. It was attributed to alternative mechanism to tunneling such as activated redox "hopping". [11][12][13][14] Using bisthienylbenzene (BTB)-based molecular layers up to d = 22 nm, activationless transport was observed with a β ≈ 1 nm -1 at temperatures below 100 K, with high current densities (> 10 A/cm 2 ). 15 These results clearly indicate unusual transport mechanisms for conjugated MJs with d in the range of 5-22 nm, above the direct tunneling limit. 3 Rectification controlled by molecular structure in MJs has been one of the founding proposals of Molecular Electronics. 16 When MJs contain a single molecular structure, either as a monolayer or as oligomers, and uses symmetric contacts, the current-density vs. voltage (JV) behavior is generally symmetric with respect to polarity; for example, in Au/oligophenylimine/Au MJs 17,18 and a variety of aromatic structures between conducting carbon electrodes. 8,15,19 Rectification must necessarily include an asymmetry in the transport direction. 20 This can be done either by using contacts based on conductive materials of different workfunctions h unidirectional charge transport through the frontier molecular orbitals is favored. [21][22][23] A new design based on asymmetric anchoring moieties was also recently proposed. 24 Using such systems to create rectifiers has been widely reported and recently reviewed. 2 Early experimental results were reported on devices incorporating hexadecylquinolinium tricyanoquinodimethanide, which gave rectification ratios (RRs) of 30. 25,26 Rectification in MJs incorporating alkane moities bearing redox units was reported [27][28][29][30][31][32] but transport is limited by the high β value. A--D and more rarely A--D molecules were widely used with the preferred direction of electron flow from A to D (Aviram-Ratner direction) or from D to A (reverse Aviram-Ratner direction) 33,34 and RR rarely exceeded 100, [33][34][35][36][37] especially for single-molecule devices. 21,[38][39][40][41] High rectification ratios were obtained using liquid mercury as a top electrode on porphyrin/fullerene bilayer devices. 42 High RRs were also observed in self-assembled alkyl-ferrocene monolayers, contacted by chemisorption to a bottom electrode and physisorption to a top electrode, and were shown to be strongly dependent upon the position of the Fc between the two conducting surfaces. 27,29,43,44 These devices are among the best molecular rectifiers to date but the techniques used to fabricate them suffer from the intrinsic stability limitation of the SAM-based molecular layer and the EGaIn contact. Rectification as a result of asymmetric contacts, 24,[45][46][47] ionic screening, 48,49 contact roughness differences 50 and molecular layers on silicon [51][52][53][54] was also reported but in each case RR was low (i.e. below 100) or a clear molecular structural effect on rectification was not demonstrated. On a larger scale, "organic p-n diodes" composed of > 100 nm-thick films of organic donor and acceptor molecules, [55][56][57][58][59][60][61] including the wide range of OLEDS, show strong rectification behavior but the results reported here focus on much thinner molecular layers, i.e. < 10 nm, which despite such ultrathin-ness exhibit high RRs depending both on asymmetric contacts and molecular structure.

We have used an Au/molecule/Ti-Au MJ structure to examine molecular layers produced by diazonium electroreduction. Rectifiers were obtained when BTB was used with thicknesses above 5 nm 62 and a 7 nm BTB film showed an RR over 50 while no rectification was observed with thicknesses below 5 nm. 63 The effect was attributed to hole injection in the BTB layer decreasing the effective tunneling barrier distance when the Au bottom electrode was positively polarized, which is similar to the mechanism proposed by Nijhius et al. for SAM-based devices incorporating ferrocene units. 44 The same layers when contacted by Carbon show symmetric I(V) curves for film thicknesses between 2 and 22 nm. 15 Moreover, when bilayers with overall thicknesses above 5 nm but below 22 nm were used, control of electronic symmetry and rectification through energy level variations was recently demonstrated. 64 The change in transport mechanism observed above 5 nm in Carbon-based BTB junctions stimulated consideration of molecular layers in the Au/molecule/Ti-Au configuration with thicknesses above 5 nm, in order to develop reliable rectifiers based on a robust device design. Diazonium-based MJs have proven suitable for commercial applications in electronic music, and are compatible with massively parallel fabrication techniques. 65 In this thickness range with d > 5 nm we also hope to demonstrate a strong "molecular signature" for the electronic response of the devices, compared to that observed when direct tunneling between the contacts is the dominant transport mechanism.

Figure 1. (a) Molecular unit of the oligomers used in this report with their frontier orbitals in vacuum (HOMO calculated by B3LYP/6-31G* and LUMO deduced from UV-VIS calculation (TD-DFT B3LYP/6-31G(d)) and the HOMO level. (b) Molecular orbital levels in vacuum of the various molecules and Fermi levels of the contact. Oligomer length was set to six aromatic units (phenyl or thiophene) for calculations of BTB, NB and FB, while NDI levels are for single NDI molecules since oligo(NDI) is weakly conjugated. 66 Note that the frontier orbital energies in vacuum neglect the mutual interaction of the oligomers as well as their interaction with the electrode.

The current report describes single-component MJs made by electrochemical reduction of different diazonium reagents, 19,64,67,68 yielding layers 7 to 9 nm-thick. The contacts were Au and Ti/Au, so structural asymmetry from contacts with different work functions was always present in the devices studied. The molecular layers included donor molecules, namely bisthienylbenzene (BTB), oligomers with relatively high-energy HOMO, molecules with high HOMO-LUMO gaps (tetrafluorobenzene (FB) and nitrobenzene (NB) oligomers, and acceptor molecules, namely n-hexyl-naphthalenediimide (NDI) oligomers, with low-energy LUMO and alkyl moieties, as shown in Figure 1a. The contacts induce asymmetry in the junction, and the effects of molecular structures on JV symmetry were observed by varying only the composition of the layers. The rectification mechanism is discussed in terms of Fermi level pinning and electronic coupling between molecules and contacts.

Experimental Methods.

Unless noted otherwise, the MJs were fabricated using a gold stripe substrate on SiO2/Si wafers with width of 20 µm and several millimeters long. The gold thickness was 45 nm on a Ti adhesion layer (2 nm). After electrochemical deposition of the organic layer, an electron-beam Ti/Au top contact was deposited as described in detail previously. 62,63 Each sample allowed the fabrication of 16 molecular junctions with lateral dimensions of 20 by 20 µm (area = 4 x 10 -6 cm 2 ). Careful control of the back pressure during Ti deposition was important, as this very reductive metal can easily be oxidized to various conductive titanium oxides. [69][70][71] A pressure of 10 -8 torr was used to reduce oxidation from the residual oxygen and water during metal deposition but, since reaction of Ti with the organic layer cannot be excluded, the molecular/Ti interface was carefully characterized using X-ray photoelectron spectroscopy (XPS). For all junctions using different molecules the Ti/Au top contact was deposited under identical conditions and all junctions were completed using lithography and shadow mask for top-electrode deposition. No significant differences were observed when either shadow mask or direct lithography on the molecular layer was used, which shows that the fabrication techniques here are fully compatible with standard microfabrication techniques.

1-(2-Bisthienyl)-4-aminobenzene and N-(4-aminophenyl)-N'-hexylnaphthalene-1,8:4,5-tetracarboxydiimide were synthesized using procedures adapted from the literature. 66,67,72 Since the latter molecule was synthesized for the first time its characteristics (NMR and High Resolution Mass Spectroscopy) are given in the Supplementary Information (SI files). Tetrafluoroaniline and para-aminonitrobenzene were purchased from Aldrich. Their corresponding diazonium salts were generated in situ by using tert-butyl nitrite (Aldrich). This procedure has been described in detail. 19,67,68,73,74 Organic layer thicknesses were determined by AFM measurements. The thickness of the gold stripe deposited on SiO2 was first measured using an AFM line profile. The thickness of the gold stripe covered by the grafted layer was then measured and the thickness of the organic layer was deduced by subtracting the gold thickness 75 using a statistical procedure shown in Figure S1. Standard deviations of thickness were in the range of 0.4 to 0.8 nm (Table S1). Junction designations include subscripts indicating layer thicknesses in nanometers, with all devices using the same bottom and top contact electrodes. For example, Au45/BTB9/Ti2/Au45 indicates a layer of 9 nm of BTB formed electrochemically on 45 nm or Au followed by 2 nm of Ti and 45 nm of Au top contact deposited in vacuum.

Current density vs. bias voltage (JV) electrical characteristics of MJs were measured in air with a Keithley 2602b source-meter at 2 V.s -1 or by DC polarizing the junction while measuring the current with a low-noise current amplifier. In each case a two-probe set-up configuration was used with the top electrode grounded while a bias was applied to the bottom electrode. The resulting JV curves exhibited no hysteresis when acquired in air, or in vacuum (<10 -5 torr for several hours, Figure S2-a Low temperature measurements were performed using a variable-temperature insert at a base temperature of 5 K while keeping the sample in a He atmosphere. The device was allowed to heat from 5 K to room temperature while successive JV curves were recorded. XPS analysis was performed using Kratos AXIS XPS with Al Kα radiation and depth profiling with a 4 kV Ar + beam at 5 x 10 -10 torr. "Blanket" samples on 1.8 x 1.3 cm silicon surfaces covered with a pyrolyzed photoresist film. 64 Deposition of the BTB layer (d = 9 nm) was achieved using the same procedure as that for molecular junctions, and 2 nm of Ti and 20 nm of Au were deposited at <2 x 10 -7 torr pressure. The thinner Au layer was required by subsequent depth profiling. Survey and high resolution spectra for C, S, O, and Ti were obtained initially and during Ar + etching at intervals of 120 seconds. CasaXPS software was used for deconvolution of the XPS spectra using a Shirley baseline and Gaussian-Lorentzian line shapes.

Results

The bottom metal/BTB structure shown in Figure 2a has been described in detail previously, and consists of oligothiophene-based molecules bonded by covalent bonds to the gold bottom contact. 68 Such layers exhibit unique on/off switching of their transport properties in an electrochemical environment. Insulating behavior is observed below 0.65 V/SCE, whereas BTB films become conductive above this potential. As a consequence, such systems combine the advantages of conductive oligomers with those of diazonium-based layers, i.e. fine control of the film thickness in the 2 to 20 nm range with robust covalent bonding. The electroactive layers with two redox states of different conductivity can be considered switchable organic electrodes.

JV curves for an Au/BTB9/Ti2/Au MJ are shown in Figure 2a, for the case of ~ 9 nm thick films of BTB (an overlay of several JV curves collected on various junctions is given in Figure S3). As reported previously, JV curves for BTB layers between Au and Ti/Au are not symmetric with respect to polarity provided the thickness, d, of the molecular layer, is greater than 5 nm. Large currents are observed when the bottom electrode is positively polarized at moderate bias whereas when it is negatively polarized the current through the junction is much smaller. Figure S4 shows that RR varies with applied potential up to V = 2.7 V, and Figure 2b shows histograms of RR at +2.7 V (RR2.7V) obtained with 63 different devices produced on different samples (of the 64 junctions tested, 63 showed behavior similar to Figure 2a, for a yield of 98% when 9 nm BTB layers were used). Average RR2.7V is 1200, with a few devices showing RR2.7V of 300 and some devices as high as 3000. The association of such RRs to current densities as high as 2.5 A.cm -2 is, to the best of our knowledge, unprecedented for a single-component MJ. These results are reproducible and devices show long term stability (over a year). Minor dependence on scan rate from 0.1 to 100 V.s -1 was observed (see Figure S5). Temperature-dependent measurements showed that rectification was still observed at 7 K (see Figure S2c-d, RR for BTB decreases with decreasing temperature, by a factor of 10 between 300 and 7 K, but is still large at 7 K) and transport was activation-less in the 7 to 100 K temperature range, with an Arrhenius slope < 5 meV (Figure S2-c), and an activation barrier of 150 meV was measured from 200 to 300 K, in agreement with previous results. 15 These findings indicate that activation or reorganization before electron transfer is not important for transport and rectification at low temperature, and exclude redox events involving titanium dioxide 69,76 as being the origin of the observed rectification properties.

Since similar BTB layers sandwiched between two carbon electrodes show symmetric JV curves 15 we can attribute at first glance the large RR observed here to the use as contacts of two different metals with different workfunctions.

Figure 2: a) Average JV curves for Au/BTB 9 /Ti 2 /Au MJs. Inset: Schematic illustration of a MJ with a 9 nm-thick layer of bis-thienylbenzene (BTB) grafted on a gold electrode followed by the direct deposition of a Ti(2 nm)/Au(45 nm) top electrode. Note that the number of BTB units involved in the device is more than 2. b) Rectification Ratio (J(+2.7 V)/J(-2.7 V)) histogram for 63

Au/BTB 9 /Ti 2 /Au junctions.

Ti metal can be easily oxidized to TiO2 during the fabrication process, and has been previously reported to be the origin of rectification in carbon-based MJs. 69,76 To rule out the involvement of TiO2 in the rectification mechanism, we acquired XPS depth profiles during Ar + etching to obtain the elemental composition of Si/SiO2/PPF/BTB9/Ti2/Au20 made in exactly the same way as the Au/BTB9/Ti2/Au MJs but on a 1.8 x 1.3 cm area. Figure 3a shows the atomic percentages (%) vs. etching time (s) of this structure before and during Ar-ion etching in the XPS chamber. At the beginning of the etching process only the Au signal is observed, as expected, but after 360 s of etching the Au signal decreases and simultaneously carbon and titanium atomic percentages start to increase. Note that the titanium and carbon signals increase before the oxygen signal, and the atomic percentage for O1s is lower than that of Ti for all etching depths. Note also that the sulfur signal increases only after 700 s and that the C/S ratio reaches the theoretical value of 7 expected for a BTB layer when the etching time is between 800 and 960 s. Similar experiments were performed on a large-area Si/SiO2/PPF/Ti2/Au20 sample and are shown in Figure S6. In this case Au decreases and simultaneous carbon, titanium and oxygen signals increase (but no sulfur) are also observed with the atomic percentage of titanium again always higher than that of oxygen. This excludes the formation of a 2 nm-thick TiO2 layer in the device, and indicates that the Ti layer is mainly Ti II and Ti III oxides, presumably mixed and disordered. Such "sub-oxides" TiO and Ti2O3 contain mobile conduction band electrons and are considered metallic. 69,[77][78][79] There is no evidence that the TiOx layer contains significant TiO2, and it therefore behaves like a metal rather than a large bandgap semiconductor. Figures 3b andc show the high-resolution XPS spectra of the titanium and carbon regions at 720 seconds etching time, at which point the oxygen signal is at its maximum. No Ti 0 signal at 454.0 eV is apparent in the spectra, which implies nearly total reaction of the metallic Ti during deposition. No signal for the Ti 2p3/2 and Ti 2p1/2 peaks of TiO2 at 459.0 eV and 464.7 eV are observed, but two types of titanium are present. The first one shows Ti 2p3/2 and Ti 2p1/2 at 455.5 and 460.7 eV and can be assigned to TiC or TiO species with an area of more than 60% of the total high-resolution Ti 2p signal whereas the second one at 457.1 and 462.5 eV can be attributed to Ti2O3. [80][81][82][83] The presence of Ti-C bonds is also confirmed by the C 1s signal observed at 282.1 eV (see figure 5c) while that of carbon of the BTB moieties are seen at 284.3 eV.

Overall, this detailed XPS analysis clearly indicates that the Ti layer used in the BTB devices is not transformed into TiO2, and that some titanium atoms bind to the molecules and create covalent C-Ti bonds at the molecule/top electrode interface. The completed MJ is electrically equivalent to a BTB layer between two metallic electrodes, with at least partial covalent bonding between the molecule and each electrode. The rectification observed in this work cannot be attributed to redox events involving TiO2.

The dependence of rectification on molecular structure was investigated by first replacing BTB with NB and FB (structures in Figure 1) layers of similar thicknesses. Oligo(NB) is less easily oxidized than oligo(BTB), with a free-molecule HOMO at -7.0 eV while that of BTB is -5.2 eV (see Figure 1). Oligo(FB) is a system with a large HOMO-LUMO gap, as it is based on phenylene moieties bearing fluorine substituents. When grafted onto gold by diazonium electroreduction fluorinated oligo(p-phenylene)s are generated. Poly(p-phenylene) is known to have a bandgap of 3 eV 84 and the presence of four fluorine groups on each benzene rings in FB will increase this further by increasing inter-ring torsion between adjacent phenyl units. It is thus likely that the HOMO-LUMO gap of this layer is close to that of a single AF unit and significantly above that of BTB and NB. DFT calculations for sexi(FB) oligomer predict a HOMO at -7.5 eV and a HOMO-LUMO gap close to 4.3 eV.

Figures 4a andb compare the JV curves of Au/NB8/Ti2/Au (green) and of Au/FB8/Ti2/Au (blue) junctions with that of Au/BTB9/Ti2/Au (red). The same data are displayed as LnJ vs. V in Figure S7. NB and FB devices are consistently much less conductive than BTB devices as the measured currents in the -2.7 V, +2.7 V potential range are smaller at all voltages. FB devices are also clearly less conductive than NB devices; the current scale is magnified in Figure 4b for comparison with BTB junctions. Moreover, Figure 4a shows that NB devices also rectify with a RR (J(+2.7 V)/J(-2.7 V) between 50 and 100, that is clearly lower that those for BTB devices. On the contrary, FB devices show weak rectification behavior (RR between 1 and 3) in the same bias range. These clear variations of the current densities and RRs with molecular structure are a strong indication of a molecular signature relating structure to conductance and electronic behavior when 8 to 9 nm-thick aromatic films are used between Au and Ti/Au electrodes.

/Au (red) MJ; d) Rectification Ratio (J(+2.7 V)/J(-2.7 V)) histograms for 63 BTB (red), 21 NB (green), 42 FB (blue) and 26 NDI (gray) junctions

As BTB layers consists of easily p-dopable oligomers and are conductive above 0.6 V/SCE in an electrochemical environment, we turned to layers based on NDI which is easily reduced (reduction potential at -0.5 and -0.7 V/SCE see Figure S8) and is an n-dopable material widely used in organic electronics. This molecule also incorporates a terminal alkyl chain that may change the electronic coupling with the top Ti/Au electrode despite the covalent Ti-C bonds. Details on the grafting procedure and the characterization of the Au/NDI modified electrode used here are given in Figure S8. As shown in Figure 1, the HOMO and LUMO energies for BTB and NDI in vacuum differ significantly, with the low LUMO energy of NDI (-4.0 eV) making it an acceptor with an energy close to the contact Fermi level of Ti or TiC 85 .

Figure 4c compares the JV curve of Au/NDI7/Ti2/Au (gray) with that of Au/BTB9/Ti2/Au (red). NDI devices are again consistently less conductive than BTB devices over the entire -2.7 V to +2.7 V potential range. Importantly, the direction of rectification in the NDI device is reversed with respect to that for BTB devices; larger J is observed when the substrate bias is negative, i.e. when the potential of the Ti/Au layer is more positive than that of the Au/NDI bottom contact.

Figure 4d shows the RR (J(+2.7 V)/J(-2.7 V) histograms of more than 150 different devices (63 BTB, 21 NB and 42 FB and 25 NDI) while Figure S7-d overlays the results obtained with the four different molecular layers plotted as lnJ vs. V. The electronic behavior of the devices clearly depends on the molecular structure of the layers, with a high level of statistical confidence. Figure 4d also makes it possible to draw several intermediate conclusions about the effect of molecular structure on JV response in single-component layers contacted by Au and Ti/Au. First, single-component layers can support electron transport across thicknesses of 9 nm, too thick to be explained by coherent tunneling between the contacts. Second, there are large differences in conductance between NDI7, NB8, FB8, and BTB9 (Figures 4a, b andc), whereas no such differences were observed for different aromatic structures with thicknesses below 5 nm. 19 The weak effect of orbital energies on JV response when d < 5 nm was attributed to strong electronic coupling between the contacts and the aromatic molecules, which causes deviations from classical Mott-Schottky behavior. This coupling induces a "leveling" effect (or "vacuum level shift") which is known to occur at metal/organic interfaces. [86][87][88] However, above 5 nm, current densities for single-component layers of NDI, NB, FB and BTB differ by orders of magnitude. This result clearly indicates that molecular structure and orbital energies have pronounced effects on JV behavior for aromatic MJs with d > 5 nm and reveals that the "leveling" effect is limited to ~ 5 nm from the conducting surface in aromatic devices. Third, in all reported cases of single-component molecular layers between symmetric carbon contacts the JV response is nearly symmetric, with RR < 2 8,15,89 , whereas NDI7 and BTB9 layers between Au and Ti/Au contacts are strongly asymmetric, with RR statistically below 0.01 and above 1000, respectively, i.e. with RR changing by five orders of magnitude. Such variations of RR with molecular structure are significantly larger than those reported in the literature. Some mechanistic conclusions are provided below, but the experimental evidence clearly indicates that when the organic layer thickness between Au and Ti/Au contacts is large enough to avoid direct tunneling (i.e. d > 5 nm) rectification which depends strongly on molecular structure is possible for low bias voltages. Rectifying MJs can be reproducibly fabricated and their behaviors are robust due to the diazonium grafting process which bonds covalently to the bottom electrode. The rectification behavior clearly originates from the asymmetric contacts but also shows that both RR and polarity depend markedly on molecular structure.

As already stated, rectification must include an asymmetric feature along the transport direction. Reported examples include contacts based on conductive materials of different workfunctions, bilayers with a donor and an acceptor part in which unidirectional charge transport is favored (Aviram-Ratner or reverse Aviram-Ratner current flow), redox groups in the layer, or layers contacted by chemisorption to the bottom electrode and physisorption to a top electrode. Since the MJ structure in the present case differs from that is most previous studies, several factors might contribute to the rectification mechanism. However, the present results clearly demonstrate that both contacts and molecular structure can affect rectification; we consider several of these factors below.

The offset between the molecular orbitals and the contact Fermi level is often used to estimate the tunneling barriers for MJs and to identify the main charge carrier. The Fermi level of Au 90 is -5.1 to -5.2 eV vs. vacuum while that of Ti and TiC is -4.3 eV 85 . The BTB HOMO is close to the Au Fermi level and is separated from that of Ti or TiC by ~1 eV, while the barrier between the BTB LUMO and the Fermi levels of either contact is much higher. This suggests that transport in BTB involves holes and agrees with previous reports based on transport 15 and photocurrents. 88,91 Moreover, when BTB layers are used and covalent bond formation at both bottom and top electrodes is indicated experimentally, we propose that the difference in barriers for hole injection, associated with strong coupling at both electrodes, is the origin of the large rectification (RR >1000) behavior observed for these devices.

Figure 5 shows a plausible scheme explaining rectification in BTB devices. It shows only the oligo(BTB) HOMO levels, as BTB acts as a hole-transporting layer and p-dopable species in electrochemical experiments. 68 Four HOMO levels have been arbitrarily represented as molecular relays for intrachain hopping across the 9 nm-thick layer. Note that the exact number of hopping steps remains unknown, but it must be between 2 and 7 since the size of a hole in an oligo(BTB) layer is 4-5 nm and that of one BTB unit is 1.3 nm. 92 We assume strong coupling occurs at the Au/BTB interface due to the presence of Au-C bonds and the proximity of the BTB HOMO to the Au Fermi level; therefore, hole injection at this interface is near-resonance with a small barrier. In other words, we can assume that part of the BTB layer acts as an extended electrode with almost no barrier for hole injection at the Au/BTB interface. 62 The presence of TiC at the BTB/Ti/Au top interface also leads to strong coupling but the initial barrier for hole injection at this interface is much higher. Consequently, Fermi level pinning is expected at both interfaces and the difference in the workfunctions between Au and TiC generates the energy scheme depicted in Figure 5a at zero bias, with the four BTB HOMO levels decreasing in energy from the gold to the titanium electrode. The barrier between the BTB HOMO and Ti, noted 1 in Figure 5 is significantly larger than that between BTB and Au. Since the barrier for hole formation in BTB at the Au/BTB interface is smaller than the barrier at the TiC electrode, we expect holes to form in BTB HOMO levels under positive bias. Once these holes are created it is possible for the TiC electrode to compensate the resulting positive charge by injecting electrons into the emptied HOMOs of the oligomers. Note that transport can be obtained by tunneling from a molecular state in resonance with the Fermi level of the electrode (straight arrows) but also by activated processes depicted in Figure 5b by curved arrows. When the bias is reversed, as in Figure 5c, hole injection from the BTB layer into Ti is energetically much less favorable for two reasons.

First, the applied bias has to compensate for the initial (1) barrier located at the BTB/TiC interface. Second, tunneling from a molecular state of an oligoBTB moiety in resonance with an empty level in the electrode will be slower because of a larger tunneling distance. As a consequence, at -2.7 V little current crosses the devices, as depicted in Figure 5c. As Fermi level pinning occurs at the BTB/TiC interface, decreasing 1 is difficult and needs to reach bias above -2.7 V to allow the coupled HOMO of oligo(BTB) at the BTB/TiC interface to rise close to the Fermi level of titanium and to participate in charge transport. Moreover, a bias above 2.7 V will also be needed to allow a molecular state with a tunneling distance small enough to reach resonance with the Fermi level of the titanium electrode.

represents the initial barrier between coupled oligo(BTB) HOMO and the Fermi level of TiC whereas that between coupled oligo(BTB) HOMO and the Au Fermi level is taken as close to zero for clarity. Note that the exact number of hopping steps is unknown.

This proposed mechanism, involving strong electronic coupling and Fermi level pinning at both interfaces, can reasonably rationalise the large rectification observed for BTB. Note that this mechanism also easily explains the temperature dependence of the current at positive or negative bias (see Figure S2). Indeed, for positive bias, the thermally activated current observed between 100 and 290 K with a low activation energy of 150 meV can be attributed to the small barrier at the Au/BTB interface. For negative bias no observable activation in the same temperature range was observed, as a result of the large barrier at the BTB/TiC interface. It also predicts that the RR ratio in BTB devices is mainly related to the initial barrier and to Fermi level pinning at the BTB/TiC interface of the top contact. Decreasing this barrier or Fermi level pinning at this interface, by any means, should decrease RR. Figure S9 shows an extreme case in which electronic coupling is negligible at the top electrode, as if the layer were studied using STM with Ti tip. In this case, it is easier to compensate for the initial (1) barrier at the weakly coupled BTB/Ti interface by applying negative bias which decreases sharply RR. The situation shown in Figure 5b is also consistent with a preferential current flow for positive bias with NB, i.e. in the same direction as that observed with BTB (see Figure S10) and a smaller RR observed for FB. In the former case, photocurrent experiments indicates that transport involves holes but the lower energy HOMOs increase the barrier for hole injection at both electrodes. Consequently, NB devices are less conductive and the differences in barriers for hole injection have less effect on RR.

(a) V = 0V (b) V = +2.7V (c) V = -2.7V f 1 f 1 f 1 Part 1 -Control of
In the latter case (FB), the high HOMO-LUMO gap creates significant barriers at both interfaces, thus decreasing again J and RR.

It is, however, not possible using this mechanism to explain the reversal of rectification polarity for NDI-based devices. The NDI LUMO is only 0.3 V above the Fermi level of Ti and TiC while its HOMO is 2 V below that of gold; this suggests that transport mainly involves electron injection in NDI MJs with molecular layer thicknesses above the direct tunneling limit between the two electrodes. This is confirmed by photocurrent experiments on NDI junctions which are consistent with LUMO mediation. 91 This injection will be much easier at the NDI/Ti interface, as depicted in Figure 1b, so that if electronic coupling is similar at both electrodes, current will be higher for positive bias, i.e. in the same direction as that observed in BTB devices. This is clearly not observed here, as electrons are more easily injected into the NDI layer at negative bias, i.e. when the Au bottom electrode is negative and the Ti/Au top electrode is positive. We, therefore, propose that the direction of the rectification observed with NDI-based devices can be explained if the couplings of the molecular layer with the contacts are not the same as those demonstrated in BTB-based devices, despite the identical conditions used for fabrication of the devices. One plausible explanation is that weak coupling occurs at the bottom Au/NDI interface as a result of a LUMO localized (see Figure S11) on the central part of the NDI molecule and a large energy difference between the NDI LUMO and the Fermi level of Au. It is also possible that NDI is less strongly coupled at the Ti interface than BTB due to the long alkyl moiety, which is known to induce a decoupling effect. 93 If such a situation is assumed, higher current can be obtained when the device is negatively biased depending on the relative coupling strength at each electrode, as recently demonstrated in single-molecule junctions. 94 More complex scenarios can also be considered as molecular resonance between HOMO and LUMO orbitals inside a molecular chain leading to bipolar transport. 95 Full explanation of the mechanism behind rectification in NDI-based devices is beyond the scope of this work and experiments including XPS depth profiling and varying the length of the alkyls moieties will be reported elsewhere. Moreover, a full understanding of the rectification mechanism discussed in this study may be achieved by state of the art theoretical modeling that includes the entire system using a non-equilibrium Green's function (NEGF) approach. 46,96

Conclusions

Molecular junctions with thicknesses between 7 and 9 nm using two contacts with different workfunctions exhibit significant changes in electronic behavior with variation of the structure of the molecular layer. Using oligo(bisthienylbenzene), a molecule whose HOMO energy level in vacuum is close to the Fermi level of the gold bottom electrode, the devices exhibit robust and highly reproducible rectification ratios above 1000 at low voltage (2.7 V). Higher current is observed when the bottom gold electrode is biased positively, and Ti/Au is negative. When the molecular layer is based on a molecule with a high HOMO-LUMO gap, i.e. tetrafluororobenzene, no rectification is observed, while the direction of rectification is reversed if the molecular layer consists of molecules with low LUMO energy level, i.e. naphthalene diimides with alkyl moieties. Rectification persists at very low temperature (7 K), and is activation-less between 7 and 100 K. The results show that rectification is induced by the asymmetric contacts but is also directly affected by the orbital energies of the molecular layer. A "molecular signature" relating structure to transport through layers with thicknesses above those used when direct tunneling dominates is thus clearly observed, and RR variation by five orders of magnitude is demonstrated in this regime. The rectification mechanism is discussed in terms of electronic coupling between molecules and contacts, and can be understood if one considers that this coupling depends on the molecular layer structure, since the same fabrication process was used for all junctions. These observations clearly demonstrate that a rectifying molecular junction can be "designed" by controlling the HOMO and LUMO energies as well as the electronic coupling at the interfaces.

Supporting Information a, Synthesis of NDI Scheme S1: Synthetic pathway to N-(4-aminophenyl)-N'-hexylnaphthalene-1,8:4,5tetracarboxydiimide (NDI).

N-Hexylnaphthalenetetracarboxylic-1,8-anhydride-4,5-acid monoimide (1). 1,4:5,8-Naphthalenetetracarboxylic dianhydride (4.36 g, 16.3 mmol) was dissolved in dry dimethylformamide (40 mL) and the solution was heated to 140 °C under argon. Then, n-hexylamine (1.72 mL, 13.0 mmol) in dry dimethylformamide (3 mL) was added dropwise, and the solution was further heated for 16 h. The reaction Part 1 -Control of Rectification in Molecular Junctions: Contact Effects and Molecular Signature 52 mixture was cooled in a fridge for 2h and the resulting precipitate was filtered. The filtrate was evaporated, dissolved in 50 ml of acetone, cooled in a fridge overnight and the resulting precipitate was filtered. The filtrate was evaporated and the residue was purified by silica gel chromatography eluted with dichloromethane. Yield, 40% (1.83 g). 1

N-(4-tert-Butyl

carbamate-aminophenyl)-N'-hexylnaphthalene-1,8:4,5-tetracarboxydiimide (2). A solution of 1 (560 mg, 1.6 mmol) and tert-butyl carbamate-protected p-phenylenediamine (334 mg, 1.6 mmol) in dry dimethylformamide (30 mL) was heated to 120 °C under argon for 18h. The reaction mixture was cooled to room temperature, washed with water and extracted with dichloromethane. Organic layers were collected, dried over MgSO 4 and the solvent evaporated. The residue was purified by silica gel chromatography eluted with dichloromethane/ethanol (99/1). Yield, 57% (494 mg). 1

b, Electrochemical reduction conditions

Before electrochemical deposition, 30 equivalents of tert-butyl nitrite were added to 10 mL acetonitrile of 0.5 mM amine precursor in 0.1 M tetrabutylammonium tetrafluoroborate (TBABF 4 ). After 5 minutes' wait, the parameters of Table S1 were used to form organic films on Au stripe electrodes.

Figure S1. AFM image of (a) Au stripe, (b) Au stripe functionalized by oligo(NB), (c) Height histogram with the Gaussian fitting profile of Au stripe (black) and Au stripe functionalized by oligo(NB) (dark yellow). (d) Image (5×5 µm) of stripe functionalized by oligo(NB) with

Figure S2. (a) JV curves obtained under different conditions. (b) JV curves obtained at temperatures from 7 K to 290 K. (c) Ln J vs. 1000/T with some Arrhenius slopes indicated. (d) RR

ratio vs. temperature. l, Ln J vs. V curves of BTB, NDI, AF, NB MJs.

e, Overlay of JV curves of several MJs

Figure S7. Ln J vs. V (a) BTB (red) vs. NB (green), (b) BTB (red) vs. FB (blue), (c) BTB (red) vs. NDI (black). (d) Overlay of all molecules.

m, Electroactivity of NDI-modified carbon electrode

Figure S9. Plausible mechanism with weak coupling and no Fermi level pinning at the top electrode. The initial potential barrier at the BTB/Ti interface can be easily modified by applying a bias, explaining a smaller large rectification for the Au/BTB/Ti/Au devices compared to Figure 5. i.e. Energy diagram at different applied biases: (a) zero, (b) positive +2.7 V, (c) negative -2.7 V. represents the initial barrier between uncoupled oligo(BTB) HOMO and Fermi level of Ti whereas that between coupled oligo(BTB) HOMO and Au Fermi level remains taken

Part 2 -Effect of Alkyl chain on rectification

We reported in the first part that molecular junctions (MJs) using two different contacts exhibit significant changes in the rectification ratio (RR) with variation of the structure of the molecular layer. When oligo(bisthienylbenzene) (BTB), a molecule whose HOMO energy level in vacuum is close to the Fermi level of the gold bottom electrode, is used the devices exhibit robust and highly reproducible RRs above 1000 at low voltage (2.7 V). When one goes from BTB to nitrobenzene (NB) oligomers, where the HOMO energy level is 1.8 eV lower than that of BTB, RR changes from 1000 to 100 at the same bias. When the molecular layer is based on a molecule with a high HOMO-LUMO gap, i.e. tetrafluororobenzene, no rectification (RR ~ 1 at 2.7 V) is observed and, finally, the direction of rectification is reversed if the molecular layer consists of molecules with low LUMO energy, i.e. naphthalene diimides with terminal alkyl moieties. Rectification was attributed to the asymmetric contacts and the orbital energies of the molecular layer. The rectification mechanism was discussed in terms of electronic coupling between molecules and contacts. However, this could not explain the reversed rectification of NDI MJs.

In this section, we intend to go further to understand the behavior of NDI MJs. We note here that NDI consists of a naphthalene diimide core with a C6H13 alkyl chain as terminal group. Since it is known that alkyl chains can act as electronic decoupling units in molecular electronics, the first question raised is: what is the impact of this chain on the electronic behavior of NDI-based MJs ?

In order to answer to this question, we first attempted to synthesize a compound based on the naphthalene core with a terminal methyl group. However, due to its low solubility, this compound could not be synthesized. Studies on a compound based on the NDI core and bearing a terminal phenyl group with two alkyl chains in the meta position are being performed and will be reported elsewhere. In parallel, we designed and synthesized alternative acceptor molecules with the viologen core. These compounds were easily synthesized and have no solubility issues (see SI a). The chemical structures of these compounds are presented in Scheme II-1. Both compounds have an aniline group used for in situ diazonium generation. The main difference between them is that one (named VIO-C1) is terminated by a methyl group whereas the other (named VIO-C6) bears a six-carbon (C6H13) terminal chain. Figure II-1a compares the electroactivities of the two compounds in solution. Like most viologen-based compounds, two reversible redox signals, assigned to the reduction of VIO 2+ to VIO + and of VIO + to VIO are observed in the negative potential region. The first reversible reduction peak is at -0.4 V/SCE and the second at -0.8 V/SCE. The two compounds have similar electrochemical properties in solution, though electron transfer seems to be slower in VIO-C6 + /VIO-C6 than in VIO-C1 + /VIO-C1, as indicated by the recorded Ep.

We first studied the grafting of the two viologens on Glassy Carbon (GC) electrodes using in situ diazonium generation. [1][2][3][4][5][6][7][8] Briefly, 10 equivalents of tert-butyl nitrite were added to the solution containing the compound (5 x 10 -4 M), then incubated for 5 minutes under argon flow, and finally cyclic voltammetry (CV) was used to deposit the molecular layer on the substrate. The electroactivities of VIO-C1 in solution as well as that of the VIO-C1modified substrate are displayed in Figure II-1b. After VIO-C1 diazonium electroreduction, the GC electrode shows two reversible redox signals with shapes typical of surface-bonded electroactive groups (Ep close to zero); the first reduction wave is observed at -0.2 V/SCE and the second at -0.7 V/SCE. These values are significantly higher (0.2 V higher) than that of VIO-C1 in solution. For VIO-C6 with a C6H13 chain, this difference is only 0.1 V (Figure S II-2a). These observations indicate that both molecules can be easily grafted onto GC electrodes by this approach, and confirms that there is strong electronic coupling between the GC electrode and the surface-grafted VIO-based moieties. Next, the thickness of the deposited layer was estimated by AFM. 6,7 Briefly, blank studies was performed first on bare Au stripes and then the height histogram was performed by SPIP software as shown ( compared to that of Au bare (black). The thickness of the VIO-C1 layer is estimated to be 3 nm. By changing the electrochemical conditions for film deposition (from 5 cycles between 0.4 and -0.25V/SCE to 10 cycles between 0.4 and -0.3 V/SCE, the thickness was increased from 3 nm to 9 nm (Figure II-2d and Figure S II-3). The root means square roughness (Sq) of the molecular layers can also be estimated from AFM measurements and is around 0.8 nm, which is close to that of a bare Au stripe electrode (see Figure S II-3). The thickness of the VIO-C6 layer can also be easily controlled by changing the electrochemical conditions (Figure S II-4).

In the following part, we will study the electrical properties of MJs based on VIO-C1 and VIO-C6. Large-area molecular junctions with molecular layers of various thicknesses were fabricated using Au & Ti/Au electrodes, as reported before. 6,7 Briefly, the molecular layer is deposited on the Au stripe bottom electrode fabricated by direct e-beam evaporation of 45 nm of Au. In a second step the molecular layer is grafted electrochemically. Then 2 nm of Ti followed by 45 nm of Au is deposited on top of the molecular layer in order to complete the MJ. Junction designations include subscripts indicating layer thicknesses in nanometers, with all devices using the same bottom and top electrodes. For example, VIO-C15 indicates a 5 nm layer of VIO-C1 on 45 nm of Au, followed by 2 nm of Ti and 45 nm of Au top contact. The fabrication yield of these devices (with 3 to 13 nm VIO-C1 layer) is high (above 90%; 45 out of 48 junctions worked).

Figure II-3. JV curves of several VIO-C1 MJs of different thicknesses (a) 3nm (b) 5nm (c) 7nm (d) 10nm.

The JV curves of several MJs for each thickness are presented in Figure II-3. They show low standard deviation from one device to another one, which signifies that the molecular films are homogeneous and reproducible. A first important result is that the JV curves of all VIO-C1 MJs are symmetric with no preferred current direction flow; RR is below 5. This is in marked contrast with the NDI-based MJs studied in Part 1 of this Chapter and will be commented later. reported 9 that in single-molecule MJs based on extended viologen compounds, the β measured by STM-BJ is 0.06 nm -1 for thicknesses from 2 to 11 nm, which they attributed to the delocalized LUMO orbital and to phase-coherent tunneling through the MJ.

Figure II-4. (a) JV curves for Au/VIO-C1/Ti/Au MJs with different thicknesses (subscript). (b) LnJ vs. thickness (nm) plot (0.5 and 1 V) in VIO-C1 MJs. (c) LnJ vs. thickness (nm) plot taken at 1 V for VIO-C1 MJs (red) and in BTB MJs (geren).

Figure II-4c compares Ln J versus thickness at 1 V for VIO-C1 and BTB MJs. The marked differences clearly indicate a strong molecular signature on transport.

First, the current density in VIO-C1 MJs is much higher than in BTB MJs, which were among the most conductive large-area MJs reported before this work. Indeed, at 1V the current density through VIO-C17nm MJs is 0.018 A.cm -2 , which is 55 times that for BTB7nm MJs. At this point it is difficult to attribute this effect but we note that VIO-C1-based MJs incorporate a large amount of ionic species; this suggests there may be a counter-ion (PF6 -) effect on the electrostatic field inside the junction, which may enhance the electron injection rate at the interfaces.

Secondly, we observe that in the 1 to 5 nm thickness range, the attenuation value for VIO-C1 MJs is much smaller than in BTB MJs, which is ~ 1.8 nm -1 and is consistent with several previous studies on conjugated molecules. [10][11][12][13][14][15] This value generally indicates that transport occurs by direct non-resonant tunneling between the two electrodes. In this tunneling regime, β depends on the type of organic molecule incorporated in the MJs, and a general consensus from numerous experimental results is that β values for saturated molecules are between 8 and 10 nm -1 and between 2 and 3 nm -1 for most -conjugated molecules. [10][11][12][13][14][15] Among -conjugated molecules deposited by diazonium electroreduction, a variety of molecular structures have already been investigated. For a selection of eight different molecules, which have calculated gas-phase molecular orbital energies (i.e. HOMOs and LUMOs) that vary by >2 eV, it was demonstrated that all MJs showed similar attenuation plots with β close to 2.8 nm -1 . This effect was attributed to a compression of the molecular tunnel barrier in the strong coupling regime. 12 An important result is that VIO-C1 MJs do not behave, in the 1 to 5 nm range, in the same way as these molecules, and exhibit a clearly different and unique electronic behavior not yet reported for large-area MJs, with a much lower current attenuation factor.

Thirdly, variation of the slopes in the attenuation plot has been used to unravel the underlying transport mechanism. 11,13,14,16 As non-resonant direct tunneling cannot be efficient for transport above distances of 5 nm, a transition in the attenuation plot has often been reported and attributed to a change in the dominant transport mechanism. 10,11,13,17 This is clearly observed in BTB MJs: in the tunneling regime the β value is around 2 nm -1 whereas if the layer is thicker than 5 nm, it falls to 0.35 nm -1 (see Figure II-4c). This variation has been attributed to the transition from direct tunneling 12,13 to hopping 11,14 or intrachain hopping (activationless). 13 It is important to note that the transition between two transport regimes (direct tunneling and hopping), usually observed for -conjugated molecular junctions, does not occur in VIO-C1 MJs (Figure II-4c). In other words, there is no evidence for a change in transport mechanism. On the basis of this evidence, we can exclude direct tunneling as the dominant transport mechanism in VIO-C1 MJs with 3 to 13 nm-thick layers. However, to understand exactly the effective long-range charge-transport mechanism in these MJs, temperature dependence measurements are required.

Next, MJs based on VIO-C6 were fabricated and studied. Figure II-5a presents the overlay of JV curves for several Au/VIO-C65nm/Ti/Au MJs, where the molecular layer is 5 nm-thick. These devices were fabricated in high yield (> 90%, 50 out of 56 junctions worked) and are also highly reproducible. In all cases, the JV curves are now asymmetric, with higher current obtained when negative potential is applied to the bottom electrode. The RRs at -2. The current density through VIO-C15nm MJs at -1 V is 0.03 A.cm -2 which is 60 times that of VIO-C65nm. Moreover, VIO-C6 MJs rectify whereas VIO-C1 MJs do not. Introducing a terminal C6H13 alkyl chain on the electroactive viologen moieties has thus a tremendous impact on the electronic properties of the MJs

(a) Overlay of JV curves of several VIO-C6 5nm MJs (a) Log RR (J(-2.5 V)/J(2.5 V) histograms for VIO-C6 5nm (red), VIO-C1 5nm /AlOx 2nm (blue) junctions.

In order to investigate further the role of the alkyl chain, we introduced an insulating layer consisting of 2 nm of AlOx on top of the VIO-C15nm layer. The JV curves of VIO-C15nm/AlOx2nm devices are shown in Figure S II-6. There are several differences from VIO-C1 MJs: (i) the current density is dramatically decreased by the AlOx2nm film; this current level is consistent with that observed in VIO-C6 MJs; (ii) the JV curves of VIO-C15nm/AlOx2nm MJs are asymmetric; the polarity of rectification is the same as that of VIO-C6 MJs and RR at -2.5V is 30 to 50 (Figure II-5b). These experiments are similar to those of Arktar et al. 18 on BTB and NDI MJs using e-C electrodes. Indeed, these MJs show almost symmetric JV curves, but when 1.5 nm of AlOx is deposited on top of these layers, the curves of NDI/AlOx or BTB/AlOx MJs are asymmetric. The polarity of rectification can be reversed by changing from a NDI layer (LUMO-dominated transport) to a BTB layer (HOMOdominated transport).

At this point, several important conclusions can be drawn. First, viologen derivatives with two different alkyl chains as end-groups can be used to fabricate viologen-based MJs in high yield. Secondly, MJs based on VIO-C1, with a methyl substituent, exhibit symmetric JV curves, small β values and no transition in the dominant transport mechanism when the layer thickness goes from 3 to 13 nm. Thirdly, all MJs based on VIO-C6, which has a C6H13 terminal chain, rectify current with RR as high as 200; the higher current is obtained when a negative potential is applied to the bottom electrode. The same behavior is observed in VIO-C1/AlOx MJs. Finally, the current densities for VIO-C1 MJs are much higher than that for VIO-C6 or VIO-C1/AlOx MJs.

We have demonstrated 6 that asymmetric Au & Ti/Au electrodes can induce rectification but that the RR can be varied by 5 orders of magnitude by changing the molecular layer from BTB to NDI. In VIO-C1 MJs, we observed also a strong molecular signature on transport with even higher current densities than in NDI or BTB MJs at the same bias, but no rectification. Scheme II-2 presents a plausible energy diagram for VIO-C1 MJs explaining both effects. We assume here that charge transport mainly involves electron injection, since the LUMO energy of the VIO-C1 layer is close to the Fermi level of the electrodes. It was estimated from electrochemical data to be around -4.5 eV (from the electroactivity of the grafted layer) while the Fermi levels of Au and Ti (TiC) electrodes are -5.1 eV and -4.1 eV. The barriers between the LUMO level and the Fermi levels of Au and Ti (TiC) are 0.6 and -0.4 eV, respectively and, consequently, VIO-C1 MJs are the first devices reported to date in which the LUMO level lies between the Fermi level of the two electrodes (see Scheme II-2a).

Scheme II-2. Energy diagram of Au/VIO-C1/Ti/Au MJs in (a) free state, (b) connected state and zero bias. Numbers on the orbital energy levels of the VIO-C1 layer are arbitrary.

Moreover, due to the formation of a covalent bond, we expect strong electronic coupling at both interfaces and that the LUMO level of the VIO-C1 layer is pinned to the electrodes. In the connected state and at zero bias, the LUMO level of the layer cannot remain 0.43 eV below the Fermi level of the Ti electrode (as described in the free state) and some electrochemical equilibrium must occur so that the LUMO level of the VIO-C1 layer and electron energy in Ti are nearly aligned, as illustrated in Scheme II-2b. When a bias is applied, and independently of its polarization, there will always be a molecular level of the layer in resonance with the Fermi level of the gold electrode, so that there will be a high current density between the two electrodes with no preferred direction. Furthermore, in this situation the dominant transport mechanism does not depend on thickness (Figure II-4b).

Scheme II-3 shows a possible scheme explaining the rectification of VIO-C6 and VIO-C1/AlOx MJs.

Scheme II-3. Possible energy diagram for Au/VIO-C1/AlOx/Ti/Au and Au/VIO-C6/Ti/Au devices under different conditions: a) Free state; b) connected state and zero bias; (c) at -2.5 V; (d) at 2.5 V: top electrode is grounded and potential applied to bottom electrode.

The top interfaces of VIO-C6 and VIO-C1 /AlOx MJs are totally different from those of VIO-C1 MJs, as electroactive viologen moieties are now separated from the top electrode by an intermediate layer of AlOx or the C6H13 alkyl chain (Scheme II-3a). We assume here again that charge transport mainly involves electron injection into the LUMO level. Three LUMO levels have been arbitrarily represented for intrachain hopping across the 5 nm-thick layer(the length of the compound being roughly 1.5 nm, estimated by Gaussian09 calculations).

We also assume that there is strong electronic coupling at the Au/VIO-C6 bottom interface and that it is similar to that of Au/VIO-C1 bottom interface. Consequently, electron injection at this interface is not a limiting step, since in Au/VIO-C1 MJs current density is much higher than in Au/VIO-C6 MJs. On the contrary, at the VIO-C6/Ti/Au top interface, the presence of the intermediate layer (AlOx or C6H13) acts as an insulator, which decreases the current densities and decouples the viologen unit from the top electrode. In other words, electronic coupling between the viologen units and top electrode is now weak and the viologen LUMO is not pinned to the Ti Fermi level. Therefore, Fermi level pinning is occurring only at the bottom interface, whereas the energy of the LUMO level close to the top electrode is slightly affected when the top electrode potential is varied. This is illustrated in Scheme II-3b which shows the energy scheme at zero bias, where the two Fermi levels of the two metallic electrodes are equal. Note that the viologen LUMOs are now above the Fermi levels of both electrodes.

Since electronic coupling at the bottom electrode is much stronger than that at the top electrode, we expect that when a bias is applied, the barrier between the LUMO and Au at the bottom will shift slightly, whereas that between the LUMO and the top electrode will change considerably. In other words, the applied bias will be mainly localized at the top electrode interface. As depicted in Scheme II-3c, when negative potential (-2.5 V) is applied to the bottom electrode, the energy barrier (ΦR) gets smaller and may go into resonance with the LUMO of the viologen unit. In other words, at this stage we can assume that part of the VIO-C6 layer acts as an extended electrode (dextended) with almost no barrier for electron injection at the Au/VIO-C6 interface. Consequently, the 'effective' thickness (de = d -dextended) for electron transport is shorter and may even be that of the AlOx or the hexyl chain. On the other hand, Scheme II-3d illustrates the situation when a positive potential is applied to the Au bottom electrode. Due to the weak electronic coupling at the top interface, the energy of this electrode is above that of the viologen LUMOs but electron injection will be difficult, because of the AlOx or the hexyl chain, and the effective thickness for electron transport (d) may be unchanged, which may induce rectification.

Nevertheless, both bias polarizations indicate that transport is probably limited by electron transport through the terminal hexyl or AlOx layer, and the preferred current direction may also be due to differences in the electron-transfer rate across this barrier (Scheme II-3c and d). Indeed, the electron transmission probability from a continuum of filled states from the top electrode into a discrete empty state of a viologen unit (Scheme II-3d) is much smaller than that from a discrete filled state to a continuum of empty states in the top electrode. 19 As a result, rectification with high current for negative potential is observed. This mechanism also predicts that the polarity of rectification can be reversed by replacing an n-dopable molecule by a p-dopable one. This proposed mechanism, involving strong electronic coupling at the bottom interface and very weak coupling at the top interface, can reasonably rationalize the rectification observed in this study and in Arktar's PhD, where AlOx is deposited on top of the BTB or NDI layer in MJs based on carbon electrodes. It also explains the preferred current direction of BTB/AlOx and NDI/AlOx MJs. Note here that, since there is very weak electronic coupling at the top interface, rectification is expected to be weakly dependent on the workfunction or the Fermi level of the top electrode. Williams et al. reported 20 that Pt/Molecules/Ti MJs in which SAMs made of alkoxynaphthalene-thiol(I) did rectify the current with higher current for positive potential applied to the bottom electrode. This is the opposite direction to that of the VIO-C6 MJs studied here. However, the difference in the polarity can be explained by the fact that the major carrier in VIO-C6 MJs is the electron whereas holes are supposed to be the main carriers in the Pt/Molecules/Ti MJs.

To summarize, in the case of VIO-C1 MJs there is no rectification and the current density is high. We attribute this to the fact that electronic coupling at both interfaces is strong. The VIO-C1 MJs are the first devices reported to date in which the LUMO level lies between the Fermi levels of the two electrodes. When an intermediate insulating layer (AlOx or C6H13 terminal chain for VIO-C6) is inserted, the JV curves become asymmetric and the RR of VIO-C65nm MJs can reach 200 at -2.5 V with relatively low current density (~ 0.02 A.cm -2 ). The proposed mechanism is based on the assumption that electronic coupling at the bottom interface is strong whereas that at the top interface is very weak. This assumption leads to the idea that the effective thickness for electron transport at negative bias may be shorter than that at positive bias, and that the electron transmission probability from a continuum of filled states into a discrete empty molecular state is much smaller than the same probability from a discrete filled molecular state to a continuum of empty states. These observations clearly demonstrate that electronic coupling at interfaces plays a crucial role in determining rectification. This can also partially explain the rectification properties of NDI based MJs studied in Part 1. However, we notice that NDI and viologen based MJs are intrinsically different as NDI MJs do not contain the same amount of ionic spices. To fully explain NDI MJs, the ongoing study is to control the ionic concentration in VIO MJs.

Supporting Information a, General Synthesis

Reagent-grade solvents and reagents were used as received from commercial sources unless specified otherwise. The syntheses of 1-(4-aminophenyl)-1'-methyl-4,4'-bipyridinedinium bis-hexafluorophosphate 13 and 1-(2,4-dinitrophenyl)-1'-hexyl-4,4'-bipyridinedinium chloride iodide A were already described. 4 Reactions were monitored by thin-layer chromatography (silica gel 60 F 254 ). Column chromatography was performed using 40-63 μm silica gel and eluant mixtures are reported as volume ratio (v/v). 1 H and 13 C NMR spectra were recorded on a Bruker AC400. 1 H and 13 C chemical shifts are reported in parts per million (ppm) referenced to the residual non-deuterated solvent peak. This work has benefitted from the facilities and expertise of the Small Molecule Mass Spectrometry platform of ICSN (Centre de Recherche de Gif -www.icsn.cnrs-gif.fr) for HRMS analyses.

Viologen 2. To a degassed solution of A (300 mg, 0.57 mmol) in MeOH/H 2 O (1/1, 10 mL) was added p-phenylendiamine (75 mg, 0.684 mmol). The mixture was stirred at 90 °C for 22h. After cooling to room temperature, the precipitate was removed and the filtrate was evaporated under vacuum. The residue was diluted in the minimum amount of methanol and was added to a saturated solution of KPF 6 . The precipitate was washed with water and diethyl ether. Brown solid. Yield: 88% (311 mg). 1

Conclusions

In this Chapter, we investigated MJs which have been fabricated by combining CMOS technology for electrode fabrication and the diazonium approach for grafting the molecular layer. In our case, asymmetric electrodes, Au & Ti/Au, were used to look for rectification and the effects of the molecule on rectification. Finally, several scientific questions listed at the beginning have been answered.

In the first part, the electronic behavior of MJs with thicknesses above direct tunneling, using two contacts with different workfunctions, depend on the structure of the molecular layer. When oligo(bisthienylbenzene), a layer whose HOMO energy in vacuum is close to the Fermi level of the gold bottom electrode, is used the devices exhibit robust and highly reproducible RRs above 1000 at low voltage (2.7 V); higher current is observed when the bottom gold electrode is biased positively, and Ti/Au is negative. When the molecular layer is based on tetrafluororobenzene which has a high HOMO-LUMO gap, no rectification is observed, but the direction of rectification is reversed when a naphthalene-diimide with low LUMO energy is used. Rectification remains at very low temperature (7 K), and is activation-less between 7 and 100 K. The results show that rectification is induced by the asymmetric contacts and is also directly affected by the orbital energies of the molecular layer.

In the second part of the Chapter, we synthesized compounds based on viologen and clearly demonstrated the effect of an alkyl chain on rectification. In VIO-C1 MJs, there is no rectification and the current density is high. This is attributed to the fact that the electronic coupling strengths and the energy barriers at the two interfaces are similar. When an intermediate insulating layer (AlOx or C6H13 terminal chain for VIO-C6) is introduced into the top interface, the JV curves are asymmetric and the RR of VIO-C65nm MJs at -2.5 V can reach 200 with relatively low current density (~0.02 A.cm -2 ). The rectification mechanism is discussed in terms of the assumption that the effective thickness of the molecular layer when negative bias is applied to the bottom electrode is smaller than in a reversed bias. These observations clearly demonstrate that the rectifier has a molecular signature. We note here that the origin of rectification in NDI MJs may be not the same as in VIO-C6 MJs, since electronic coupling at the bottom interface may be different and because counter-ions inside VIO-C6 MJs may play a role. This Chapter also opens few important scientific questions. The first concern the operating frequency of molecular rectifiers: how can one enhance the performance of such molecular diodes and rectifiers. Other important questions are about the role of the counterion inside MJs and if it helps to create a new functionality.

Molecular Signature In Metal/Bilayer Component/Metal Junctions 1 Introduction

In general, bilayer systems consisting of two single components play an essential role in life sciences and the modern electronics industry. 1 In biology, a lipid bilayer acts as the barrier that controls the transport of ions, protein or other molecules. 1 Hence, the lipid bilayer is an building block of almost all living organisms. A bilayer system is also an essential building block in all modern electronic devices; for example, p-n junctions are at the heart of all such devices. Furthermore, bilayer organic electronics is extensively used to convert solar light into current and electrical power into light. 2 Bilayer systems have rarely been investigated in molecular electronics due to the difficulty of fabricating bilayer molecular junctions (MJs).

Whitesides et al. reported that bilayer MJs could be fabricated by making a direct mechanical contact between two conductive electrodes which have both been functionalized by SAMs as depicted in Figure III-1a. 3,4 Bilayer MJs were designated as Metal/SAM-1//SAM-2/Hg where '/' represents an interface formed by a covalent bond between a SAM and an electrode, and '//' indicates an interface between the two SAMs in van der Waals contact. [3][4][5][6] Bilayer MJs, where SAM-1 contains tetracyanoquinodimethane (TCNQ) 3 as head group covalently bonded on silver (or gold) and SAM-2 consists of an alkanethiol on mercury, acted as rectifiers with electron flow from the silver electrode through the LUMO level of TCNQ to the Hg electrode (Aviram-Ratner-preferred electron flow), but the rectification ratio (RR) at 1 V was relatively small (from 2 to 10). The origin of the rectification was attributed to the TCNQ redox group, but similar devices, incorporating another redox group such as ferrocene, were not rectifiers. 3 Later, other compounds such as conjugated molecules 5 or SAMs containing redox-active head-groups such as [pyRu(NH3)5 +2 ](PF6 -)2 5,6 and hexa-perihexabenzocoronene (HBC) 4 derivatives were developed but these bilayer MJs did not show rectification either.

Ashwell et al. also studied bilayer MJs fabricated by Langmuir-Blodgett methods and Scanning Tunneling Microscope (STM) (see Figure III-1b).

They reported that bilayers, consisting of self-assembled monolayers of cationic donor-(π-bridge)-acceptor dyes coupled with an anionic donor, rectify current and the reported RR at 1 V could be as high as 3000, i.e. the highest value at that time, when the cationic moiety was 5-(4dimethylaminobenzylidene)-5,6,7,8-tetrahydro-isoquinolinium and the counter-ion was copper phthalocyanine-3,4',4'',4'''-tetrasulfonate 7 . Recently, Smerdon et al. also reported bilayer MJs of C60/Pn, Acceptor/Donor, where Pn is pentacene, behaving as a donor and C60 acts as an acceptor (Figure III-1c). 8 The IV curves of single-component MJs (only pentacene) are almost symmetric but those curves of the bilayer MJs are asymmetric and the RR at 1.2 V reached 1000. 8 (Figure III-1c). Later, the same group reported that a bilayer consisting of C60/C60 (Acceptor/Acceptor) junctions also shows rectification with RR up to 1000 at 1.3 V. 9 This behavior was attributed to the different electronic coupling strengths at the bottom (Cu(111)) and top electrodes. During this PhD, Akhtar et al., in collaboration with our group, reported that the in situ electroreduction of a diazonium salt provides a robust and versatile procedure for fabricating bilayer MJs. 10 They reported that a symmetric device containing a single component did not rectify whereas bilayer devices including two distinguishable components showed medium rectification at high bias (RR = 400 at 4.5 V). They associated the electrontransport mechanism with the molecular energy levels of each layer, and the preferred current is again in accordance with the Aviram-Ratner-predicted flow. However, the effects of the electronic coupling between each layer were not investigated.

Strategies and Objectives

The chapter is aimed at building up bilayers illustrated in Scheme III-1 and studying transport therein. These are fabricated by the same procedure used for single-component MJs described in Chapter 2, described in the General Appendix.

The bilayers in our study consist of two layers which are chemically different and are deposited on top of each other. In these systems, there are three possibilities, as shown in Scheme III-1, depending on the strength of the electronic coupling between the two layers. A first possibility (Scheme III-1a) is that the two layers are only physiosorbed on each other. This leads to weak electronic coupling and was not studied in this PhD. In the two other cases, covalent bonds are generated between the two layers. In "intermediate electronic coupling" the two different layers do not influence each other much. In other words, each layer keeps its own electronic properties, the two layers remaining physically and electronically separated, as illustrated in Scheme III-1b. In "strong electronic coupling" (Scheme III-1c) the two layers affect each other and, despite the fact that they are physically separated, they constitute a new single layer with new electronic properties that are not the superposition of those of the two layers. In this chapter, only cases shown in Scheme III-1b and c will be studied.

Scheme III-1 . Scheme of bilayer system in which (a) two layers are separated from each other (b) two layers are Intermediate electronic coupling (c) two layers form a new single layer with new electronic properties.

In order to create the bilayers, two distinct strategies will be used.

The first strategy is based on previous work in our group, [11][12][13] referred to as the overgrafting approach or the bottom-up approach. The strategy consists of two steps: the first is to generate an organic electrode which can switch from an insulating state to a conducting state; the second is to perform an irreversible electrochemical oxidation of an aromatic monomer on the switchable organic electrode at a potential where this electrode is conductive. More precisely, we will first generate an electrode functionalized by an oligo(bisthienylbenzene) (BTB) layer. The BTB-modified electrode can be switched from an insulating state to a conducting state at 0.6-0.7 V/SCE. For the second layer we will use 3,4ethylenedioxythiophene (EDOT) or 4'-(3,4-ethylenedioxythiophene)-2,2':6',2''-terpyridine (EtPy) (Scheme III-2). Both monomers can be irreversibly oxidized in solution to create unstable radical-cations and deposit oligo(EDOT) or EtPy dimers on a bare electrode. Oligo(EDOT) is known to be a donor while EtPy, containing a terpyridine unit, is easily protonated and can then be considered as an electron-withdrawing moiety.

The second strategy is based on the successive electroreduction of two different diazonium salts and has been developed in parallel through a collaboration with McCreery's group. The several compounds (Scheme III-3) which will be used can be classified into two groups: one includes BTB and Co(tpy2) which act as donors; the other includes NDI, AQ and VIO-C1 which act as acceptors.

Scheme III-3. Chemical structures of compounds used in the diazonium approach.

We expect to address the following scientific questions.

• What are the electrochemical properties of such bilayers? What is the difference between the two strategies? What will the coupling strength between the two layers be and what types of bilayer can be formed by the two strategies?

• What are the effects on the electrical properties of bilayer MJs of adding a second layer on top of the first layer? Does this affect current densities, i.e. conductance or the rectification properties of the MJs? And if so, can we understand these effects?

3

Results and Discussion

Bilayers Based on Overgrafting Approach

Electrochemical deposition

The overgrafting or bottom-up approach is based on the irreversible electrochemical oxidation of aromatic monomers on an already modified electrode. More details can be found in SI 5.1.1, Stockhausen's thesis, 12 Fillaud's thesis and a recent publication. 11,13 Briefly, an electrode is first functionalized by an oligo(bisthienylbenzene) (BTB) layer using the diazonium approach. The BTB layer electrografted on the Au stripe electrode switches from insulating to conducting at 0.7 V/SCE, and then behaves, above this switching potential, as a new organic electrode. This is evidenced by the shift of the ferrocene (Fc) oxidation signal on the BTB-modified electrode, illustrated by the red curve in Figure III-2a. Indeed ferrocene, which is oxidized at 0.3 V/SCE (black curve) on a bare electrode, is not oxidized at this potential on a BTB-modified electrode, since the BTB layer is insulating, but at an higher potential where it has become conductive This electrochemical behavior of the switchable organic electrode has been well documented by our group. [14][15][16]

Figure III-2 . (a) Response of ferrocene (1 mM) in acetonitrile, LiClO4 (0.1 M) on bare (black), BTB-modified (red) and BTB-EDOT-modified (green) Au stripe electrodes. Scan rate 0.1V.s -1 (b) Response of decamethylferrocene (1 mM) in acetonitrile, LiClO4 (0.1 M) on bare (black), BTBmodified (red) and BTB-EDOT-modified (green) Au stripe electrodes. Scan rate 0.1V.s -1 (c)

Height histograms for bare (black), BTB-modified (red) and BTB-EDOT-modified (green) Au electrodes.

Next, we electrooxidized EDOT on top of the BTB layer. EDOT is a monomer whose oxidation (~1 V/SCE) is irreversible, gives unstable radical-cations and leads to PEDOT deposition on a bare electrode. As EDOT oxidizes at a higher potential than the switching potential of the grafted oligo(BTB) (~0.6-0.7 V/SCE), it will take place on a pdoped organic conductive electrode, i.e. a layer in which oligo(BTB) radical-cations are already generated. Thus, two types of reactive radical-cations, i.e. EDOT and oligo(BTB) radical-cations (Scheme III-4), will be created in the vicinity of the electrode. Scheme III-4 displays the two possible scenarios and materials that can be expected, depending on the concentration of the EDOT radical-cations generated in the vicinity of the modified electrode and their reactivity towards oligo(BTB) radical-cations.

First, at low EDOT radical-cation concentration (i.e. low EDOT oxidation current density), the radical-cations can couple covalently with the grafted layer, and thus extend the aromatic chain and create new block oligomers (Scenario 1) which corresponds to the bilayer illustrated in Scheme III-1b or Scheme III-1c. In the latter case (strong electronic coupling at the interface) a new material is generated; its electrochemical and interfacial properties may differ markedly from those of the initial BTB layer.

Alternatively, at high current density, EDOT radical-cations may prefer to react with each other and form oligomers in solution or even a polymer. PEDOT can then precipitate on the first BTB layer, yielding a double-layer structure by physisorption (Scenario 2). In this case, two different materials are deposited in a bilayer structure with a specific interface separating them (Scheme III-1a).

Scheme III-4. Scenarios for oxidation of EDOT on an oligo(BTB) layer. In scenario 1, a new material is generated by covalent bonding of EDOT to oligo(BTB). In scenario 2, the oligo(BTB) layer is covered by PEDOT and there is an interface between the two materials.

We have investigated carefully the first scenario. In order to have a low EDOT radical-cation concentration, EDOT was oxidized using CV cycles at a current below 0.3 mA.cm -2 which is only 5% of the EDOT oxidation diffusion-limited current. After each oxidation cycle, the new modified electrode was thoroughly rinsed with ACN and electrochemically characterized with Fc redox probes in an electrolyte solution (0.1 M LiClO4 in ACN). The Fc and decamethylferrocene (DmFc) probe signals change after each overgrafting cycle and, consequently, the switching potential of the EDOT-modified BTB electrode can be adjusted. As shown in Figure III-2a, the ferrocene oxidation signal on the Au stripe electrode shifts back to a value close to that observed on a bare Au electrode (green curve), but in DmFc, the switching potential of the same electrode was changed from 0.7 V to 0.3 V.

Figure III-2c

displays the statistical height analysis of the electrode estimated by AFM measurement. It indicates that when only 2 nm of oligo(EDOT), i.e. 3 to 4 EDOT units, is added on top of the 7 nm-thick oligo(BTB) layer, the switching potential of the layer changes from 0.7 V (for BTB7nm) to 0.3 V for (BTB7nm-EDOT2nm). Note that the switching potential of the new organic electrode can be shifted to below 0.3 V by changing the conditions of EDOT oxidation. When these low current densities are used for EDOT oxidation, short EDOT oligomers are generated and covalently grafted onto the oligo(BTB). This overgrafting process results in elongated oligomers with an oligo(BTB)-oligo(EDOT) block structure which has new electrochemical properties. The shift of the switching potential indicates that a new material is generated by this overgrafting procedure and demonstrates strong coupling between the oligo(BTB) layer and the grafted EDOT moieties. More details can be found in our recent ACS Applied Material and Interfaces (DOI: 10.1021/acsami.6b08754) publication. 11 Likewise, EtPy monomer was also used for the overgrafting process. Thanks to the EDOT unit, EtPy can be irreversibly oxidized. The apparent oxidation wave of EtPy is at 1.2 V, slightly above that of EDOT, as the terpyridyl group is electron-withdrawing. Using a similar procedure, we were able to graft EtPy onto oligo(BTB) layers of various thicknesses and to generate new organic electrodes. Figure III-3a shows the response of Fc probe on this modified electrode. The Fc oxidation signal is now shifted to higher potential, which indicates that the switching potential of the new electrode (BTB-EtPy) has increased to 1.2 V/SCE, i.e. 0.6 V higher than that of the BTB-modified electrode. Figure III-3b shows one example of such layers in which the thickness of the organic layer increases from 7 nm (for the initial BTB layer) to 9 nm (for BTB-EtPy layer). An important result is that when only 2 nm of EtPy is added the switching potential of the layer shifts considerably. Since the gold stripes are unstable above 1.2 V (Figure S III-1). We optimized the EtPy deposition process to get BTB-EtPy bilayers switching at this potential. In summary, the overgrafting approach based on the irreversible electrochemical oxidation of EDOT-based aromatic monomers (EDOT or EtPy) allows to deposit an ultrathin second organic layer (2 nm) on top of the first layer. This process generates a new layer or material with a new switching potential. This clearly indicates strong electronic coupling between the two layers. Despite the fact that they are physically separated they form a new single layer with new electronic properties that are not the superposition of those of the two layers. The switching potential of the new electrode can be changed from -0.3 V/SCE for BTB-EDOT to 1.2 V/SCE for BTB-EtPy. The changes in the switching potential can be attributed to the donor character of EDOT and to the acceptor character of EtPy, possibly enhanced by protonation.

BTB-EtPy Bilayer Junctions

The fabrication of bilayer devices is similar to that of single-component devices, which is detailed in the Appendix. Figure III-4a illustrates a BTB-EtPy bilayer MJ using Au45nm and Ti2nm/Au45nm bottom and top electrodes. For all BTB-EtPy bilayer devices, the switching potential of the bilayer is optimized at around 1.2 V/SCE, and there is approximately 2 nm of EtPy on top of the original BTB layer. Figure III-4b shows the HOMO energy levels of the oligo(BTB) and BTB-EtPy layers calculated from electrochemical data. The HOMO level of BTB-EtPy is -5.9 eV which is 0.6 eV lower than that of BTB (-5.3 eV). Molecular modeling indicates that this HOMO decrease is probably due to partial protonation of one pyridine unit of the terminal terpyridine. Figure III-6 compares the JV curves of single-component BTB and BTB-EtPy bilayer MJs with different thicknesses. As depicted in Figure III-6a, when 2 nm of EtPy is added on top of the BTB4nm layer, the JV curve for BTB4nm-EtPy2nm is shifted to the right (at positive bias) compared to that of BTB4nm. At the same potential, the current density of the bilayer MJ is smaller than that of the single-component BTB MJ for positive and negative bias. In other words, the conductance of (BTB-EtPy)6nm is smaller than that of BTB4nm. This 'thickness' dependence of the current is usually observed in molecular electronics. [17][18][19][20][21][22][23][24][25][26] Figure III-6b compares the JV curves of BTB and BTB-EtPy layers of similar thickness (10 nm). The current density at 2 V of BTB10nm is roughly 30 times that of the BTB-EtPy10nm layer. A similar tendency is observed for negative potential. The same experiments were performed on a bilayer with BTB as the first layer and 4'-(3,4-ethylenedioxy-5,2'bithiophen-2-yl)-2,2':6',2''-terpyridine (TEtPy) as the second layer (i.e. a monomer with one additional thiophene unit). Adding 2 nm of TEtPy on top of the BTB layer also decreases the conductance (see Figure S III-2).

The rectification ratio is therefore different in single-component BTB MJs and BTB-EtPy bilayer MJs. It is much less in bilayer than in single-component BTB MJs; for example, the RR of BTB9nm at 2.7 V is around 1000, but that of a BTB8nm-EtPy2nm (total 10 nm) is only 100 at the same bias. This may be attributed to the fact that for positive bias the current through the MJs involves hole transport and redox events which are more easily triggered by positive polarization, as the HOMO of BTB is higher than that of BTB-EtPy. It correlates with the fact that the current in BTB MJs is much higher than in BTB-EtPy MJs. On the contrary, the current at negative bias is mainly attributed to tunneling (See Chapter 2) and thus it depends strongly only on the thickness of the layer. In other words, the decrease in the conductance of BTB-EtPy MJs for positive bias leads to a reduction of the rectification.

Figure III-6. Representative JV curves of (a) BTB 4nm and BTB-EtPy 6nm and (b) BTB 10nm and BTB-EtPy 10nm MJs.

To correlate the current density and the thickness of the layer in MJs, [26][27][28][29][30] we will use the simplified Simmon's equation: 𝐽 = 𝐽𝑜 𝑒 -𝛽𝑑 , (J is the current density and Jo the pre-exponential factor; β is the attenuation (or decay) factor), used to estimate how fast the current changes with the thickness, d, of the layer.

Figure III-7 compares the plot of LnJ taken at 1 V versus the thickness of the BTB-EtPy bilayer (blue points) and the single-component BTB MJs (black points). The plot could be made at smaller bias, but for good accuracy at high thickness the plot is preferably done at 1 V. In the thickness range from 1 to 5 nm (tunneling regime), the β value of singlecomponent BTB MJs is ~1.8 nm -1 , which is in good agreement with several studies on conjugated molecular junctions. 22,31,32 As mentioned in the literature, the β value strongly depends on the type of molecule, and β values for -conjugated molecules are between 2 and 3 nm -1 . 22,29,32 We note here that when the layer is less than 5 nm-thick, the JV curves of BTB and (BTB-EtPy) MJs are almost identical. As a result, below 5 nm, the β values of BTB and BTB-EtPy MJs are quite similar. Moreover, in the direct tunneling regime the Jo of MJs, estimated by extrapolation of the plot, is independent of the molecular structure of the layer (LnJ intercepts at d = 0 are similar for BTB and BTB-EtPy devices).

Using PPF & e-C as bottom and top electrodes, McCreery et al. reported that in the

tunneling transport regime, 22,32 the β value is 2 nm -1 and is the same for all the -conjugated molecules tested. The latter point also supports our observation that, when the total thickness of the BTB-EtPy bilayer is less than 5 nm, the slope of the LnJ vs. d plot is probably the same for both single-component and bilayer MJs.

Figure III-7. Plot of Ln J versus layer thickness, taken at 1 V: (a) single component BTB layer (black); (b) BTB-EtPy bilayer (blue).

Figure III-7 also shows a change in the Ln J vs. thickness (nm) plot for BTB and BTB-EtPy MJs. This clearly indicates that when the layer is more than 5 nm-thick, electron transport can no longer occur by direct tunneling and a new transport mechanism appears. In this regime the conductance of the MJs depends strongly on the molecule, and single-component BTB and BTB-EtPy bilayer MJs behave differently. This behavior is consistent with previous work on MJs using e-C substrates. 33,34 More precisely, the β value of BTB-EtPy bilayer MJs is 0.83 nm -1 with Au & Ti/Au electrodes whereas that of single-component BTB MJs is 0.35 nm -1 ( Figure III-7). This is less than half the size and indicates that molecular structure strongly affects transport properties. Note that the thickness of EtPy was optimized and kept the same (2 nm) for all BTB-EtPy MJs, so the thickness variations here are only due to the oligo(BTB) part of the layer.

In other words, the attenuation factor of the BTB-EtPy bilayer MJs is clearly different from that of the single-component BTB MJs despite the fact that in both cases only the thickness of the BTB layer was changed. This clearly demonstrates that the 2 nm EtPy block changes the whole behavior of the MJs and suggests that it is strongly coupled with the oligo(BTB) units. Indeed, if the two layers were weakly coupled the β values of BTB and BTB-EtPy MJs would be similar, but the Jo,5nm value in the hopping regime (Jo at d = 5 nm will now be used as the pre-exponential factor, since the mechanism has changed and the intercept of LnJ at d = 0 is meaningless) will be different by 2×βEtPy and will be smaller for the BTB-EtPy MJs. This is clearly not observed here, and the decrease in conductance cannot be attributed to a decrease in Jo,5nm in the hopping regime. Consequently, the change in β cannot be understood if the EtPy units are weakly coupled with the BTB layer. The change in β also supports the electrochemical observation that the overgrafting process generates a new, uniform layer with a HOMO level lower than that of BTB. This difference may be explained by using a simplified formula 2 2 m

, which shows the relationship between β and the energy barrier (φ) between the molecular level and the workfunction of the electrode. 35 Note that this formula is derived from Simmon's model for the tunneling transport regime and may not be fully valid in the present case. Since the workfunction of the Au electrode is -5.1 eV and oligo(BTB) is oxidized at 0.6-0.7 V/SCE, the barrier between the HOMO of the BTB layer and the Au Fermi level is approximately 0.2 eV while the energy barrier for the BTB-EtPy layer is around 0.8 eV (BTB-EtPy layers switch at 1.2 V/SCE). By using the above formula and assuming the effective masses of the major carriers are the same, we can calculate a theoretical β for the BTB-EtPy device of 0.7 nm-1, which is very close to the experimental value (0.83 nm-1). Despite the limited validity of this model in the present case, this agreement clearly shows that the barrier between the HOMO level of the BTB-EtPy layer and the Au Fermi level is higher than observed in singlecomponent BTB MJs. This is again consistent with the electrochemical data and with Figure III-4. As shown in the diagram, when the energy barrier at the bottom interface increases from 0.2 eV to 0.8 eV, we expect the conductance of the MJs to decrease, while the polarity of rectification will remain unchanged and RR will decrease. This situation is similar to that observed in Chapter 2 when BTB-MJs were compared with NB-MJs.

Figure III-8. Energy diagram shows the barrier of BTB 9nm (blue line) BTB-EtPy 10nm (red line) layers with Au electrode at free state. The numbers on the HOMO levels inside the MJs are merely for illustration.

At this stage, we can draw several important conclusions.

(i) When the layer is less than 5 nm-thick, then the transport properties in BTB and BTB-EtPy MJs are identical: the JV curves, β and Jo at d = 0 nm are similar; no or little rectification is observed. β is around 2 nm -1 and Jo is 3 A.cm -2 .

(ii) Above 5 nm, the transport mechanism changes from direct tunneling to intrachain hopping. The decay constant in this new transport regime is smaller than in the direct tunneling regime but BTB-EtPy bilayer MJs are much less conductive than single-component BTB MJs. The β value of BTB-EtPy MJs is 0.83 nm -1 and is twice that of BTB MJs. However, Jo at d = 5 nm is similar for the two devices.

(iii) Above 5 nm, rectification is observed in BTB-EtPy MJs, but the RR is smaller than that for BTB MJs, though the polarity of the rectification is unchanged.

(iv) These results can be understood within the framework of very strong electronic coupling between the BTB layer and the overgrafted EtPyr moities.

BTB-EDOT Bilayer junctions

As mentioned above, when the HOMO of the layer is decreased, the conductance of BTB-EtPy bilayer is much lower than that of BTB MJs. In this section we will study the electrical properties of BTB-EDOT MJs, where the HOMO of the BTB-EDOT layer is higher than that of BTB layer.

We have demonstrated that just by grafting 2 nm of oligo(EDOT) on top of the BTB layer, the switching potential is decreased from 0.7/SCE to 0.3 V/SCE for the BTB-EDOT layer. Therefore, in order to study the electrical properties of BTB-EDOT MJs we tried to keep the switching potential of the BTB-EDOT layer at around 0.3 V/SCE, and the thickness of EDOT layer was again carefully optimized at about 2 nm. Figure III-9a shows the MJ scheme for the bilayer device where the BTB-EDOT layer is sandwiched between two conductive electrodes. From electrochemical data the HOMO energy level of this bilayer is calculated to be around -4.8 eV.

Figure III-9. (a) Schematic representation of BTB-EDOT bilayer devices based on overgrafting approach. (b) HOMO energy level of the layer.

Several MJ devices were elaborated using a procedure similar to that reported above for Au & Ti/Au electrodes. Carbon electrodes (e-C) produced by direct e-beam evaporation were also used to investigate the transport properties of the BTB-EDOT bilayer. These (ii) The deviation of the current level in the JV curves is relatively small. This high yield associated with small variations in the current density suggest that the overgrafting approach generates uniform homogeneous layers which prevent penetration of metal during the evaporation of the top electrode. We also obtained high yield in the device fabrication and small deviation in JV curves for BTB-EDOT MJs based on e-C substrates (see Figure S III-3).

(iii) The conductance of BTB-EDOT MJs depends on the thickness of the bilayer. For example, the current density at 1V of BTB-EDOT9nm MJs is around 0.25 A.cm -2 , which is 20 times that measured for (BTB-EDOT)16nn.

(iv) The JV curves are symmetric; no rectification is observed at any bilayer thickness. Using the Au & Ti/Au configuration, we reported in Chapter 2 that a high RR of over 1000 is observed at 2.5 V for BTB9nm MJs. On the contrary, for BTB-EDOT bilayer MJs, the RR for Au/BTB-EDOT/Ti/Au MJs falls from 1000 to less than 5 for BTB7nm-EDOT2nm (a total of 9 nm).

Figure III-10. Several JV curves with standard deviation in BTB-EDOT bilayer devices: (a) (BTB-EDOT) 9nm ; (b) (BTB-EDOT) 16nm .

Figure III-11 compares the JV curves of single-component BTB MJs with those of BTB-EDOT MJs. BTB-EDOT MJs obtained by adding 2 nm of EDOT on top of the BTB layer conduct much better than BTB MJs. For instance, as shown in Figure III-11a, the current density of BTB-EDOT9nm MJs at 1 V is 0.25 A.cm -2 , which is 600 times higher than that of BTB7nm. This unusual behavior has never been reported in the literature. Usually, when the thickness of MJs increases, the conductance decreases.

Similar behavior is also obtained for BTB-EDOT based on e-C substrates (Figure III-11b).

For example, the current density of BTB-EDOT13.5nm MJs at 1 V is 0.36 A.cm -2 , which is 250 times higher than that of BTB11.5nm. This suggests that the increase in the current density is due to the intrinsic properties of the molecular layer. At first glance, we can attribute the increases in current density to the increase in HOMO level of the bilayer. We notice here thqt this change is consistent with the experimental observation being reported in Section 2, Chapter 2 on viologen MJs and in Section 3.1, Chapter 5. As illustrated, when the molecular layer is more than 5 nm-thick, the β values of BTB and BTB-EDOT MJs are similar at 0.36 nm -1 and 0.32 nm -1 , respectively. The current densities through BTB-EDOT devices are much higher (400 times) than those for single-component BTB devices. Similar behavior is seen in BTB-EDOT MJs based on e-C electrodes (Figure S III-4). Let us recall here that we have attributed the difference in β for BTB (0.36 nm -1 ) and BTB-EtPy (0.83 nm -1 ) to the higher energy barrier between the HOMO level and the Fermi level of the Au electrode, and considered that the charge transport mechanisms in both MJs, above 5 nm, are similar and mainly involve electric-field-induced ionization associated with activationless intrachain hopping mechanism. 31

Figure III-11. JV curves of representative (a) BTB and BTB-EDOT MJs using Au & Ti/Au electrodes (b) BTB 11nm and BTB-EDOT 13.5nm MJs using e-C electrodes

Figure III-12. Plot of Ln J vs. layer thickness, taken at 1 V for BTB (black points) and BTB-EDOT (red points) MJs.

To better understand charge transport in BTB-EDOT MJs, temperature dependence measurements were performed on (BTB-EDOT)16nm MJs using Au & Ti/Au electrodes. Several MJs were bonded onto a chip, and their electrical properties were measured from 5.8 to 275 K. All MJs are stable under cryogenic process (cooling down or heating up) and the process has a little effect on their electrical properties. Below 100 K, the activation energy is just a few meV. Note first that this activation energy is below that of single-component BTB MJs (150 meV between 100 and 280 K). This very small activation energy may be attributed to the barrier between the Fermi level of gold and the BTB-EDOT HOMO. It suggests that this barrier is smaller than that in single-component BTB MJs, in agreement with the electrochemical data (switching potential of the BTB-EDOT layer lower than that of BTB). It also clearly indicates strong electronic coupling between the two layers so that, despite the fact that BTB and EDOT moities are physically separated, they form a new single layer with new electronic properties that are not the superposition of those of each layer. Moreover, due to the very low temperature dependence, we can exclude several transport mechanisms, including interchain 'activated' hopping or the Schottky mechanism (usually for such mechanisms, the activation energy is around 0.3 to a few eV). In addition, due to the thickness of the device, we can also exclude direct non-resonant tunneling between the two electrodes through the BTB-EDOT MJs. Consequently, two other mechanisms remain to explain the long-range transport behavior observed, namely, electric-field-induced ionization associated with activationless intrachain hopping 31 or on-resonant tunneling; both mechanisms should be considered further and will be discussed below.

We mentioned above that a change in the β value with layer thickness indicates a transition in the transport mechanism. In a joint study, between Alberta and Paris Diderot Universities, we have reported that in BTB MJs using e-C substrates, the β value in tunneling regime is around 3 nm -1 (for thicknesses below 8 nm) and decreases to 0.8 nm -1 above 8 nm when the charge transport mechanism changes from direct tunneling to electric-field-induced ionization and the activationless intrachain hopping. We notice also that this change in transport mechanism leads to a change in the Jo value. Very similar behavior is also demonstrated in single-component BTB MJs based on Au & Ti/Au electrodes and reported in chapter 5. As shown in Figure III-12, a change in the β value with layer thickness is observed for 5 nm BTB layers, and Jo in the tunneling regime is 3 A.cm -2 while Jo in the intrachain hopping regime is 9.12×10 -4 , i.e. 3×10 3 times smaller. Note also that a change in β with layer thickness is also observed for BTB-EtPy layers (Figure III-12).

On the contrary, in BTB-EDOT MJs there is no change in the β value when the thickness of the layer increases, i.e. the decay constant remains the same for all bilayer thicknesses. This clearly suggests that there is no transition in the transport mechanism when the thickness of the layer increases and that the long-range transport behavior observed with BTB-EDOT has to be explained within the framework of a single mechanism. This result also is very close to that observed with viologen based MJs in Chapter 2.

In BTB-EDOT MJs, if the charge transport mechanism were electric-field-induced ionization associated with activationless intrachain hopping, as it is in BTB and BTB-EtPy MJs above 5 nm, we expect that Jo at 5 nm in the intrachain hopping regime will be almost the same as in BTB and BTB-EtPy, since the electrodes are identical in all MJs. However, Jo in BTB-EDOT MJs obtained from the extrapolation in the Ln J vs. d plot is 6 A.cm -2 at d = 0 which is very close to Jo (3 A.cm -2 ) in the tunneling regime for single-component BTB MJs. A similar tendency also appears in BTB-EDOT MJs based on e-C substrates (Figure S III-4). We note that Jo in the tunneling regime usually depends on the electrodes, but that if the electrodes are kept the same, then Jo is unchanged.

Based on these observations, we propose that the mechanism of charge transport in BTB-EDOT MJs is tunneling at all bilayer thicknesses but that this tunneling is nearly onresonance. We also propose the energy diagram for BTB-EDOT MJs in Figure III-14. In the free state, the workfunctions of Au and Ti electrodes are -5.1 and -4.3 eV, respectively, while the HOMO energy of BTB-EDOT was estimated to be -4.8 eV from the electrochemical data. In the free state, the HOMO energy level is above the Fermi level of the Au bottom electrode but below that of Ti electrode. In other words, BTB-EDOT MJs is the first examples of MJs with HOMO level lies between the Fermi level of the electrodes, which is close to that in viologen based MJs reported in Chapter 2.

When the MJs are connected, and zero bias is applied (Figure III-14b) we expect that electronic coupling between the Au bottom electrode and the HOMO level of BTB-EDOT will be strong, and that the HOMO level of the BTB will be in resonance with the Fermi level of the bottom electrode. We also expect strong electronic coupling at the top electrode. As a result, the charge transport mechanism in BTB-EDOT MJs is on-resonance tunneling, there is no rectification and β value is small.

Figure III-14. Energy diagrams (a) in free state, no connection (b) in connected state at 0 V bias.

To summarize, several conclusions can be drawn:

(i) Firstly, we have demonstrated that a bilayer based on the overgrafting approach acts as a new single layer and that the energy level of this new layer can be tuned from 0.3 V/SCE (BTB-EDOT) to 1.3 V/SCE (BTB-EtPy) Adding only 2 nm of a second layer completely changes the electronic properties of the whole film.

(ii) We are able to fabricate bilayer MJs based on the overgrafting approach in high yield; MJs can be fabricated with Au & Ti/Au electrodes or e-C contacts.

(iii) In the intrachain hopping regime, adding 2 nm of EtPy on top of the BTB layer decreases current density through the MJs and increases β (from 0.36 nm -1 to 0.85 nm - 1 ), associated with a decrease in RR. These effects are attributed to the decrease in the HOMO energy from -5.3 eV for the BTB layer to -5.9 eV for BTB-EtPy.

(iv) On the contrary, adding 2 nm of EDOT on top of the BTB layer has the opposite effect, and BTB-EDOT MJs behave differently from BTB-EtPy MJs. BTB-EDOT MJs are more conductive that single-component BTB or BTB-EtPy bilayer MJs. The β value decreases slightly to 0.32 nm -1 while Jo suddenly increases, and no rectification is observed at any thickness. These effects are attributed to the increase in the HOMO energy from -5.3 eV for the BTB layer to -4.8 eV for BTB-EtPy. Consequently, this orbital is now very close to and almost resonant with the Fermi level of gold, and the most plausible mechanism for charge transport is on-resonance tunneling.

3.2

Bilayer based on Diazonium Approach

Electrochemical deposition

The second strategy for depositing a bilayer is to use only the electroreduction of a diazonium salt (see Section 5.1.2 for experimental details). In a first step a first molecular layer is electrografted onto an electrode by cyclic voltammetry. This first layer can consist of various molecules, some of them being already used in single-component MJs (Chapter 2) and includes donors, namely bisthienylbenzene (BTB), oligomers with relatively high HOMOs, and acceptors, namely n-hexyl-naphthalenediimide (NDI) oligomers, with lowenergy LUMOs and alkyl groups.

If the first layer is electroactive at negative potential, it can act as an electronic shuttle or as a redox mediator for the electrochemical reduction of species in solution. A second diazonium salt can then be easily reduced on the first layer, and a bilayer is generated with an interface between the two layers that probably involves covalent bonds (the bilayer resists an ultrasonic bath). This redox mediation by the the first-deposited layer explains why it is possible to grow thick (above 5 nm) films with NDI-and viologen-based systems If the first layer is insulating at negative potential, then it is more difficult to deposit a second layer. To do so, we need to apply a 'more' negative potential during cyclic voltammetry, and this proved to be possible in some cases already published in collaboration with McCreery's group. 10 However, in this case the second layer does not grow very well and the interface between the two layers is badly characterized.

In both cases, the thickness of each layer can be controlled by the electrochemical conditions used for deposition and monitored by AFM. The number of voltammetry cycles and the potential window can be adapted to get the desired thickness of each layer. [36][37][38] As a first example, we will first describe how to deposit a bilayer with NDI and BTB as first and second layers, respectively. NDI in solution can be reduced at -0.5 and -0.8 V/SCE (See Chapter 2). Figure III-15a presents the cyclic voltammograms (CV) of NDI diazonium salts when the potential is scanned from 0.0 to -0.4 V/SCE. In the first cycle, there is an irreversible broad wave with a peak potential around -0.2 V/SCE which can be ascribed to an irreversible reaction (i.e. the cleavage of the C-N bond, elimination of dinitrogen and grafting of NDI onto the electrode). Then in the subsequent scans, a new irreversible reduction peak at -0.25 V/SCE is observed. The intensity of this peak gradually increases and its potential decreases slightly from -0.25 V to -0.3 V/SCE after several scans. The new reduction peak can be attributed to the reduction of the electroactive NDI film deposited on the electrode (It is also partially due to the reduction of NDI still in solution.). Note that the current is not zero, which indicates that the NDI layer deposited on the electrode in not insulating in this potential range.

Figure III-15 (a) 10 cyclic voltammograms of NDI diazonium on glassy carbon electrode. (b) NDI -Gc (green) and bare (black) electrodes in Fc probe solution, scan rate 0.1 V.s -1 (c) Height histograms for bare Au stripe (black) and NDI film deposited on Au stripe (green).

Figure III-15b

shows the cyclic voltammogram of the NDI film deposited on a glassy carbon (GC) electrode in an acetonitrile solution of Fc, under Argon flow. At positive potential, the probe signal at 0.3 V/SCE has almost disappeared from the NDI-modified GC electrode while at negative potential two reversible reduction peaks at -0.5 V and -0.75 V/SCE and ΔE ~0.15 V are observed (corresponding oxidation peaks at -0.35 V and -0.6 V/SCE, respectively). The reversible signal can be assigned to the electroactivity of the NDI layer. 37,38 Integration of the reductive peaks gives a surface concentration of 2 x 10 -9 mol cm -2 . If the surface concentration of the monolayer is taken as 3 x 10 -10 mol cm -2 , observed for closepacked monolayers of aromatics such as nitrobenzene, 39 and the length of the NDI molecule is 1.1 nm (computed with Gaussian09 software), then the thickness of the NDI layer, deduced from the electrochemical data, is roughly 8 nm. AFM analysis (Figure III-15c) was performed on bare Au stripe and Au stripe functionalized by NDI under similar electrochemical conditions. The thickness of the NDI layer is found to be around 7 nm, which is consistent with the value estimated from the electrochemical data.

Figure III-16a

presents the electroreduction of BTB diazonium salt on top of the NDI-modified GC electrode using a potential from 0.4 to -0.4 V for a first scan then from 0 to -0.4 V for the others. For the second diazonium reduction (Figure III-16a) there is no irreversible wave around 0.1 V/SCE which is usually seen during BTB grafting on a bare carbon electrode. Instead, there is a new reduction peak again at around -0.35 V/SCE in the first scan and this peak gradually disappears after a few scans. The new reduction peak is very close to the first reduction peak of the NDI. In other words, in the first scan, at -0.35 V/SCE the NDI core is reduced, then reduced NDI film acts as a redox mediator for BTB diazonium salt electroreduction. As a result, the grafting of a BTB layer on the NDI layer occurs in a similar way to the grafting of BTB on the carbon electrode.

Next, the same experiments were performed on an Au stripe electrode for AFM measurement. Figure III-16b presents the height analyses for bare, NDI-modified and NDI/ BTB-modified Au stripe electrodes. The second diazonium deposition increases the thickness of the molecular layer from 6 nm to 10 nm, indicating that 4 nm of BTB has been deposited on top of the 6 nm NDI layer. The width of the peak (see Figure III-16b) is similar for all samples, which suggests that the roughness of the molecular layer is not changed by the second diazonium step.

Figure III-16. (a) Cyclic voltammograms of BTB diazonium reduction on NDI-modified glassy carbon electrode (NDI-GC). (b) Height histograms for NDI film deposited on Au stripe (green) and BTB/NDI film deposited on Au stripe (red). (c) NDI/BTB-GC (red); NDI-GC (green) and bare carbon (black) in Fc probe solution, scan rate 0.1 V.s -1 .

Figure III-16c compares the electrochemical response of Fc on the NDI/BTB bilayer film (red curve), single-component NDI film (green) and on a bare electrode (black curve). The current level of the NDI reduction signals in the bilayer is similar to that obtained for a single-component NDI-modified GC electrode, which indicates that after the second diazonium deposition the first layer is chemically stable and can still be reduced. The NDI reduction in the bilayer occurs at -0.7 V/SCE and -1.0V/SCE, which is -0.25 V lower than that in NDI film and the ΔE of this peak is around 0.3 V. This indicates that the second BTB layer deposited on top of the NDI layer is partially insulating and slows down NDI reduction.

In addition, at positive potential the signal of Fc at 0.3 V/SCE is still absent from the NDI/BTB-modified GC electrode and the irreversible oxidative peak at 0.7 V/SCE, which is usually observed in single BTB layers, [14][15][16]21 does not appear. This once again suggests that the second BTB layer is grafted on top of the NDI layer, that the electroactivity of this layer is not significantly changed by adding BTB, and that the BTB layer cannot be switched to a conductive state.

Finally, bilayers, designated as NDI6nm/BTB4nm (6 nm of NDI film is deposited as first layer, then 4 nm of BTB film as second layer) can be easily generated. Similar experiments were performed to create an AQ/BTB bilayer (see Figure S III-6) but in this case interpenetration of the two layers leading to mixed layers was evidenced, and these devices will not be presented in this chapter.

It is possible to reverse the order of the bilayers and to deposit the BTB layer first, followed by the NDI layer. The deposition curves for the creation of the BTB/NDI bilayer are shown in Figure III-17. The crucial difference between BTB/NDI and NDI/BTB bilayers arises from the first layer. In contrast to NDI, the BTB layer is insulating at negative potential and probably can only be reduced at below -2 V/SCE. 40 This means that in the 0 to -1 V/SCE potential range, the BTB layer is insulating (see Figure III-17a). Hence, the efficiency of the second diazonium deposition is expected to be relatively low. Consequently, in order to deposit NDI on top of BTB, the applied potential needs to be more negative than that used on a bare electrode (see Figure III-17b). Despite this, NDI growth on BTB is still difficult and we could only deposit 2 nm of NDI on top of BTB layers of various thicknesses. More importantly, when NDI is deposited on top of BTB, the Fc probe signal is shifted to values close to that observed for a BTB-modified electrode, which indicates that the NDI does not completely change the electronic behavior of the BTB electrode Figure III-17c.

Figure III-17(a) Response of ferrocene (1 mM) in acetonitrile, LiClO4 (0.1 M) on BTB-modified (red) GC stripe electrodes. (b) Second NDI deposition on top of BTB-modified GC electrode (BTB-GC) at 3 different conditions. (c) CV of NDI/BTB-GC in Fc probe solution after each second NDI depostion, scan rate 0.1 V.s -1 .

Note also that trilayer systems including three distinguishable layers can be fabricated by successive diazonium reductions (Figure S III-7). For instance, a trilayer including NDI/APC/BTB, Acceptor-(π-bridge)-Donor, where APC is 4-aminophenylacetic acid, can be easily deposited. Such trilayer devices may provide ways to generate multifunctional molecular junctions, and first steps in this direction were recently reported. 41 They were not fully investigated within the framework of this PhD.

To conclude, a strategy based on two successive diazonium electroreductions provides a versatile, powerful, robust way to generate bilayers, and more generally multilayer systems. The order of each layer can be changed, and the thickness of each layer can also be controlled to some extent. Importantly, adding the second layer on top of the first layer does not significantly change the electroactivity of the first layer. This is in marked contrast with the overgrafting approach developed in the first part of this chapter, and indicates much weaker electronic coupling between the two layers, despite the fact that covalent bonds are probably created at the interface. This weak coupling between layers was evidenced by electrochemistry (this work) as well as by analysis of UPS and UV spectra in Akhtar et al. 10 and Akhtar's thesis 42 in McCreery's group.

Electrical properties in Bilayer Molecular Junctions

In this section, we will study the electrical properties of bilayer devices where the bilayers were deposited by the diazonium approach. We will focus only on the bilayers based on NDI, BTB, VIO-C1 and Co(tpy2) since the electrical properties of each single-component MJ was extensively studied in Chapter 2; Co(tpy2) will be studied in Chapter 5.

Bilayer devices were fabricated by applying the previous procedure. Junction designations include subscripts indicating layer thicknesses in nanometers. For example, Au45/NDI7/BTB4/Ti2/Au45 indicates a first 7 nm NDI layer on a Au stripe electrode and then a second 4 nm BTB layer on top of the NDI7nm followed by 2 nm of Ti and 45 nm of Au top contact deposited under vacuum; the device name will be abbreviated to NDI7nm/BTB4nm. These NDI7nm/BTB4nm MJs were fabricated in high yield (above 90%; 44/48 MJs tested worked and 10% behaved as open-circuit devices (no measurable current)).

Figure III-18a

shows JV curves for several NDI7nm/BTB4nm devices. They are asymmetric: the current is higher when the negative potential is applied on the bottom Au stripe. The deviation of the current level in these devices is low, which suggest that the bilayers are homogeneous and uniform. RR at -2.7 V is around 100 and the polarization of the rectification is similar to that of single-component NDI MJs reported in Chapter 2. With the reverse order of the layers, BTB7nm/NDI2nm devices were also fabricated at high yield (> 90%). Figure III-18b shows JV curves of several MJs. BTB7nm/NDI2nm devices rectified the current and the current was higher when the positive potential was applied on the bottom electrode. RR was 100 at 2.7 V, and the polarity of the rectification is the same as that of single-component BTB devices.

Figure III-18. Electrical measurement of bilayer devices based on (a) NDI 7nm /BTB 4nm (b) BTB 7nm /NDI 2nm . The top electrode is grounded, and the potential is applied to the bottom electrode.

Several important conclusions can be drawn.

(i)

First of all, in all cases, the currents measured for bilayer MJs are dramatically less than those for single-component MJs. For example, the current density through the BTB9nm device at 2.7 V is around 2 A.cm -2 , whereas that for BTB7nm/NDI2nm devices (9 nm in total) at 2.7 V is 0.04 A.cm -2 , almost 50 times less. The current at negative bias (-2.7 V) for BTB7nm/NDI2nm MJs decreases less, and consequently the RR of BTB7nm/NDI2nm is only 100, which is roughly 10 times less than for BTB9nm devices.

(ii) Secondly, the polarity of the rectification of these bilayer devices can be tuned by changing the order of bilayer from Donor-Acceptor (BTB/NDI) to Acceptor-Donor (NDI/BTB) but it stays disappointingly the same as that observed for single-component MJs. Adding the second layer on top of the first layer does not change the preferred direction of electron transport, already observed with BTB or NDI MJs and partially attributed to the difference in work functions between the Au and Ti/Au electrodes. Furthermore, this bilayer strategy does not increase RR.

We recall here that using symmetric e-C electrodes, Akhtar et al. 10 reported that BTB/NDI and NDI/BTB bilayer devices act as molecular rectifiers and that the polarity of the rectification can be tuned by changing the order of the layers, whereas the singlecomponent MJs do not rectify. In this case, the preferred electron flow corresponds to the Aviram-Ratner theoretical model. These results are the same as those observed here with bilayer devices based on Au & Ti/Au electrodes. It clearly suggests that the origin of the rectification obtained in bilayer molecular junctions is not only due to the difference in the work functions of the bottom and top electrodes, and is probably mostly due to the molecular structure of the layers in the MJs.

Overall, the second layer seems to act as an energy barrier that decreases the total current though the MJs and, even though the polarity of the rectification of these bilayer devices can be tuned by changing the bilayer order, this effect may also be partially due to the difference in contact workfunctions. Consequently, these systems were not investigated further, and we turned our attention towards new bilayer systems using molecules that do not induce rectification when used to fabricate single-component MJs with Au and Ti/Au contacts.

To further investigate the origin of the rectification observed in our bilayer devices, we will now study the electrical properties of bilayer MJs based on VIO-C1 and Co(tpy2). The structures of these compounds are presented in Figure III-19a. VIO-C1, which includes a viologen unit, is reduced at -0.2 V/SCE and as expected acts as an acceptor whereas Co(tpy2) oxidizes at 0.2 V/SCE and can be considered as a donor.

As mentioned previously in Chapter 2, single-component VIO-C1 MJs exhibit symmetric JV curves and allow a larger number of electrons to pass, although asymmetric Au & Ti/Au contacts were used. The electrical properties of single-component Co(tpy2) MJs will be reported in more detail in Chapter 5. Briefly, Co(tpy2) MJs also show symmetric JV curves, and there is no rectification, which is similar to the behavior of VIO-C1 MJs. MJs based on these single-component layers show a low thickness dependence on the electrical properties and do not show rectification; this is remarkably different from the singlecomponent MJs based on NDI or BTB. The LUMO energy of VIO-C1 and the HOMO energy of Co(tpy2) calculated from the electroactivity of the films are around -4.5 and -4.85 eV, respectively. In the free state, this energy level lies between the workfunctions of the Au (-5.1 eV) and Ti/Au (-4.3 eV) electrodes. In the connected state and at zero bias, electronic coupling at the two interfaces is expected to be strong, and the barrier for electron (or hole) injection is very small, so the charge transport rates at the two interfaces may be quite high and similar. Consequently, the current level in single-component devices using these two molecules is high and no rectification is obtained. More details will be presented in Chapter 5.

In other words, using two different molecules, showing symmetric JV curves when engaged in single-component MJs with Au & Ti/Au contacts, we seek to see if building bilayer MJs from such molecules will lead to rectifiers. If so, the origin of rectification in these bilayer devices with asymmetric contacts will be clearly attributable to the molecular structure of the bilayer devices and not to the asymmetry of the contacts.

A VIO-C1/Co(tpy2) bilayer was deposited on an Au stripe electrode by the above procedure. Figure III-19b presents JV curves of several VIO-C16nm/Co(tpy2)6nm devices. A few junctions showing small hysteresis at 2 V.s -1 scan rate are presented in Figure S III-9, but this hysteresis seems insignificant, as the yield of these devices is relatively low (5%) and the hysteresis disappears after a few potential sweeps.

We first observed that the current level measured is unexpectedly very much lower than for a single-component device. For instance, the current through each single-component device is quite high: the current density for VIO-C112nm at -1 V is approximately 10 -3 A.cm -2 while for VIO-C16nm/Co(tpy2)6nm (12 nm in total), the current density at -1 V is roughly 1×10 - 5 A.cm -2 , which is 100 times smaller. This dramatic decrease in the current is in a good agreement with the above bilayer devices (NDI/BTB, for example) but the interpretation of this effect remains unclear.

Secondly, the JV curves become interestingly asymmetric, RR remains relatively low (~ 50 at -2.5 V) and the preferred current flow is from the VIO-C16nm towards Co(tpy2) and is similar to that of NDI/BTB MJs, i.e. from the acceptor to the donor fragment. We recall here that in the case of NDI/BTB and BTB/NDI MJs, rectification is always observed, and its polarization is the same as that of the single-layer MJs. Hence, the origin of the rectification in these bilayer MJs is not clear. A very important result is that in VIO-C1/Co(tyr2) MJs rectification is clearly observed although the single-component MJs do not rectify. Therefore, we can exclude the idea that rectification is due the asymmetry of the Au & Ti/Au contacts.

These results clearly demonstrate that bilayer MJs have different properties from single-component MJs and, despite a relatively low RR, the rectification is clearly due to the molecular structure of the layer used in the devices.

Figure III-20 presents the possible energy diagram for the VIO-

C16nm/Co(tpy2)6nm (acceptor/donor) devices at zero bias with the Fermi level of the Au and Ti/Au electrodes equal due to the vacuum shift at the equilibrium state. The LUMO energy of VIO-C1 and the HOMO of Co(tpy2) were inferred from the electrochemical data; they are separated by a low gap ΔE of only 0.35 eV which makes this bilayer like a low-bandgap semiconductor material but with physically separated HOMOs and LUMOs. Note also that the LUMO of Co(tpy2) and the HOMO of VIO-C1 do not participate in transport, so VIO-C1 can be considered as an electron-transporting layer and Co(tPy2) as a holetransporting layer. In other words, bipolar transport may occur in these devices but at this point we have no clear experimental evidence for this possibility. Since adding the second layer on top of the first layer dramatically decreases the current through MJs, we can only suggest that there is an effective barrier which decreases the current and which do not exist in the single-component devices. Moreover, as the interfaces at the bottom and top electrodes are identical to those in single-component MJs, this effective barrier should be located at the The preferred mode of electron transport is for electrons to move from the Au bottom electrode to the LUMO energy level of the first layer (VIO-C1) and then either directly towards the top electrode by tunneling across the 6 nm Co(tpy2) layer or by hole transport through the HOMO level of this layer. This is similar to what is proposed by the Aviram-Ratner mechanism 43 where the first step is to move electrons from the bottom electrode into the LUMO of the acceptor unit and simultaneously electrons from the HOMO level of the donor unit move into the top electrode. This Aviram-Ratner predicted current flow was also observed in NDI/BTB bilayer devices using symmetric e-C electrodes. 10 Recently, Van Dyck et al. 44 proposed a new molecular design based on asymmetric anchoring moieties which are strongly electronically coupled with the Fermi level of the electrode and with the HOMO and LUMO of the molecule pinned to the Fermi level of each electrode and completely decoupled in the center of the devices. A new rectification mechanism was proposed where the electron flow is controlled by ΔEgap; this situation was predicted to lead to preferred current flow in the anti-Aviram-Ratner direction (see Figure I-24, Chapter 1).

Here we studied the VIO-C16nm/Co(tpy2)6nm bilayer system, where the electronic coupling at both interfaces is strong and covalent coupling is likely to induce a strong pinning effect between the frontier orbitals and the energy levels in the contacts. The current was higher when negative potential was applied to the bottom Au electrode, which corresponds to positive bias in the model of Van Dyck and Ratner (Figure I-24, Chapter 1) and a situation in which ΔE at the interface increases and drives the HOMO and LUMO out of the transmission window. This experimental observation is in clear contradiction with the theoretical mechanism proposed by Van Dyck et al. In other words, one of the three rules used to build this model in not fulfilled and it is likely that decoupling between the two layers is not good enough to make the two layers of our VIO-C16nm/Co(tpy2)6nm independent.

Hence, in order to go further and to understand the origin of the rectification in these bilayers, we really need to explore the effect of the order of the VIO-C1/Co(tpy2) bilayers on the rectification polarity.

Let us nevertheless recall here that by using the overgrafting approach, the conductivity of the bilayer can be increased or decreased, and RR can also be tuned from 1200 to less than 5. We have attributed these effects to the fact that overgrafting generates a new unique layer that has a different HOMO energy level thanks to the strong coupling between the two components of the bilayer. On the other hand, when the diazonium approach is used, the bilayer seems to be electronically separated, although there are probably covalent bonds between the two layers. Adding the second layer always decreases the current through MJs as compared to single-component MJs, which indicates that transport is blocked by one or by both layers.

In summary, we can draw several important conclusions regarding bilayer devices based on the diazonium approach:

• Firstly, the diazonium approach provides a versatile and robust way to deposit ultrathin bilayers as well as multiple layers. The total thickness can be easily tuned from a few nanometers to 20 nm by the electrochemical conditions used in the successive deposition processes.

•

Secondly, electronic coupling between the two layers is weaker than with the overgrafting process, as the electrochemical properties of the first layer are only slightly affected by the second layer. (iii) Adding the second layer dramatically reduces the conductivity of bilayer MJs compared to singlecomponent MJs and, in all cases, the current density decrease in bilayer MJs can be attributed to a new effective barrier at the interfaces between the two layers, which probably reveals weak electronic coupling between the two layers (or slow charge recombination if bipolar transport occurs).

• Rectification always appears in bilayer systems and does not depend on the electrode used or on the fact that the single-component devices rectify or not. An important result is that rectification can be obtained starting with molecules that do not show rectification in single-component MJs with Au & Au/Ti contacts; this result indicates a clear molecular signature on the transport properties in bilayer MJs. Rectification always follows the Aviram-Ratner-preferred electron flow and cannot be explained by a strong decoupling effect at the interface between the two layers. The polarity of rectification can be tuned by changing the order of the bilayer (from Donor-Acceptor to Acceptor-Donor), is always of Aviram-Ratner type and RR is unfortunately relatively low.

Conclusions

In this chapter, we studied how to fabricate bilayers and also investigated the electrical properties of bilayer MJs fabricated completely by CMOS technology. The bicomponent layer was deposited by two strategies: the diazonium and overgrafting approaches. Several important remarks can be made.

The overgrafting approach based on the irreversible electrochemical oxidation of monomers allows to deposit an ultrathin second organic layer on top of the first layer. This approach generates a new uniform layer having a new switching potential. The switching potential of the new electrode can be changed from 0.3 V/SCE for BTB-EDOT to 1.3 V/SCE for BTB-EtPy. The change in the switching potential can be attributed to the donor or acceptor property of the EDOT or EtPy units, which in turn change the HOMO level of the layer. The diazonium strategy also provides a versatile, powerful and robust way to generate bilayers and more generally multi-layer systems; the order of the layers can be easily changed. The electroactivity of these new bilayers is probably just a sum of the two layers; this is a crucial difference between the two approaches. In other words, the electronic coupling strength of the two layers is smaller when the diazonium approach is used. In all cases, the fabrication yield of MJs devices is almost 90%, which is relatively high. The addition of a second layer on top of the BTB layer by overgrafting varies the electrical conductivity of the new layer compared to the single-component BTB MJs. Adding 2 nm of EtPy on top of a BTB layer decreases the current density of MJs, increases the β value and also decreases RR. This is attributed to the decrease in the HOMO energy from -5.3 eV for BTB to -5.9 eV for BTB-EtPy. However, when 2 nm of EDOT is added on top of BTB, Jo suddenly increases and rectification is completely suppressed. We attribute this to the fact that the HOMO is raised and becomes on-resonance with the Fermi level of the electrode. Transport in 16 nm-thick BTB-EDOT MJs is activationless, is associated with a small decay constant of 0.3 nm -1 and a very high Jo value of 3A.cm -2 . This set of data strongly suggests on-resonant tunneling as the dominant mechanism.

For the bilayer devices based on the diazonium approach, adding the second layer always dramatically reduces the conductance compared to single-component MJs. In all cases, the current density decrease can be attributed to the weak electronic coupling between the two layers. Rectification always appears and does not depend on the electrode used or on the fact that the single-component devices rectify or not. The rectification polarity can be tuned by changing the order of the layers (from Donor-Acceptor to Acceptor-Donor), and is always of the Aviram-Ratner type but RR is still disappointingly small. By using the overgrafting approach, the conductivity of the bilayer can be increased or decreased, and RR can also be tuned from 1200 to less than 5. On the contrary, when the diazonium approach is used, adding the second layer always decreases the current through the MJs. Rectification behavior always appears. These differences can be related to the fact that overgrafting generates a new, unique layer whereas the diazonium approach generates a bilayer which is the sum of two layers.

Overall, the current report demonstrates that such strategies can provide a verstatile and robust tool for building bilayer MJs and even multi-layer MJs. These strategies can also be extended to design new functional MJs, for instance memristors, photo-emisson molecular electronic devices and molecular iontronic devices.

5

Supporting Information

Electrochemical deposition

For electrochemical experiments, a conventional three-electrode cell and a CHI 760C potentiostat (CH Instruments, Austin, TX) were employed. The auxiliary electrode was a platinum grid, and a SCE (3 M KCl) was used as reference electrode. Working electrodes were glassy carbon (GC) disks (area = 0.07 cm 2 ) purchased from CH Instruments or Au stripe electrodes (20 µm x 1 cm) which are fabricated by e-beam evaporation. Before use, carbon electrodes were polished with 3 µm then 1 µm diamond paste (Struers, Denmark), ultrasonicated in analytical grade acetone for 3 min and rinsed with acetonitrile.

Overgrafting approach

Electrochemical deposition of the oligo(BTB) layer: the switchable BTB layer is deposited by the above procedure. The BTB layer was deposited by cyclic voltammetry between 0.4 V/SCE and -0.4 V/SCE on glassy carbon electrodes as well as on gold stripes (20 µm x 5 mm) at 100 mV.s -1 . Each electrode was then characterized in acetonitrile containing 1 mM ferrocene and 0.1 M LiClO 4 .

Electrochemical oxidation on the oligo(BTB) organic electrode: Electrodes previously functionalized by oligo(BTB) were sonicated in ACN for 10 minutes and dipped into a solution containing 2.10 -2 M EDOT and 0.1 M LiClO 4 . Cyclic voltammograms were performed while the potential window was adjusted in order to obtain oxidation charges around 0.3 mC cm -2 , corresponding to a maximal theoretical deposition thickness of 2 nm for each EDOT oxidation cycle (or around 3-4 moieties, considering a size of 0.6 nm per EDOT unit). After each oxidation cycle, the glassy carbon electrode was thoroughly rinsed with ACN and electrochemically characterized in an electrolyte solution (0.1 M LiClO 4 in ACN). Then, voltammograms in the presence of Fc were recorded using the same modified electrode. This procedure (EDOT deposition on the organic electrode, rinsing, characterization in Fc and DmFc, and in probe-free solution) was repeated several times. This procedure can be also applied to EtPy monomer. For carbon substrates fabricated by e-beam evaporation, aluminium foil was used to create the electrical contact between the electrode and the cable for CHI connection. Note that the mechanical contact between the e-C substrate and CHI equipment should be controlled carefully in order to have a homogeneous film for all the e-C line.

Diazonium approach

Donor units including 1-(2-bisthienyl)-4-aminobenzene (BTB), cobalt complex (Co(tpy2)) and acceptor units including N-(4-aminophenyl)-N'-hexylnaphthalene-1,8:4,5-tetracarboxydiimide (NDI), viologen (VIO-C1) were synthesized using procedures adapted from the literature. [1][2][3] Their corresponding diazonium salts are well-documented [2][3][4] (see Chapter 4 for cobalt complex) and were generated in situ using tert-butyl nitrite (Aldrich).

To perform electrochemical deposition, an equivalent of tert-butyl nitrite was added to 10 mL of 0.5 mM amine precursor in 0.1 M of electrolyte under argon gas flow (for more detail see Table III-1). After waiting 5 minutes, cyclic voltammetry is used to generate radical species which electrograft onto an electrode. To form the second layer, cyclic voltammetry was performed on the electrode modified by the first layer in a solution of the in-situ-generated diazonium salt of the second material under Ar gas. The modified electrodes were characterized in Fc probe solution (LiClO 4 ) under Ar flow and the thicknesses of the layers were measured by AFM. To control the thickness of each layer, the electrochemical conditions of cyclic voltammetry were carefully controlled.

Table III-1. Electrochemical conditions to generate a radical from an amine precusor.

Solution

Equivalent of tert-butyl nitrile Electrochemical conditions

Stability of BTB-EtPy deposited on Au stripe electrodes

Figure S III-1. Response of ferrocene (1 mM) on bare (red), BTB-modified, BTB-EtPy-modified Au stripe electrodes and BTB-EtPy-modified

Electrical properties of BTB-TEtPy MJs

Figure S III-2. representative JV curves of (a) BTB 10nm (black) and BTB 10nm -TEtPy 2nm (red) MJs.

Electrical properties of BTB-EDOT bilayer MJs using e-C substrate.

Figure S III-3. Overlayer Of Several Curves, and Standard deviation Of Current in BTB-EDOT bilayer

Devices using e-C substrate.

Plot Ln J vs. thickness of BTB and BTB-EDOT MJs using e-Carbon electrodes.

Figure S III-4. Ln J vs. thickness of BTB (orange points) and BTB-EDOT( blue points) MJs using e-C electrodes for both contacts.

Temperature dependence of BTB-EDOT molecular junctions

Figure S III-5 . Arrhenius plots of Ln J vs. 1000/T at different potentials as indicated for BTB-EDOT MJs.

Deposition of AQ/BTB bilayer

Figure S III-6. (a) 10 cyclic voltammograms of AQ diazonium on GC electrode. (b) AQ -Gc (green) and bare electrode (black) in Fc probe solution, scan rate 0.1 V.s -1 . (c) 15 Cyclic voltammograms of AQ-modified GC electrode (AQ-GC). (d) AQ/BTB-GC in Fc probe solution,

scan rate 0.1 V.s -1 .

5.8

Trilayer based on diazonium approach electrochemistry, AFM and electrical measurement. with d. Values of β depend on the nature of the molecules and on the bias voltage used to measure J. For saturated molecules, [3][4][5][6] β taken at low bias is around 8 to 10 nm -1 while it is between 2 and 3 nm -1 for -conjugated molecules. 2,[7][8][9][10][11] The value of β seems to be independent of the technique used to form the molecular junctions (see Figure IV-1a) and does not seem to change with the anchoring groups. For example, in STM-, 10 CP-AFM-, 8,11 SAMs 2 -based MJs or in large-area MJs, using -conjugated molecular junctions, 7 β remains in the 2 to 3 nm -1 range, which clearly indicates that it is independent of the number of molecules involved in the junction.

Figure S III-7. Cyclic voltammograms for (a) NDI deposition; (b) APC deposition; (c) BTB deposition; (d) JV curves of NDI/APC/BTB devices.

VIO-C1/Co(tpy2) electrochemistry and AFM measurement

J is the current density passing through the MJs while Jo can be obtained through the extrapolation to d = 0 in the plot of Ln J vs. d. The meaning of Jo in MJs is only partially understood, and its value is related to the molecular energy level and to the electronic coupling between the anchoring group and the contacts. Since J and Jo are current densities, they should be independent of the area of the MJs; this is often observed when the technique used to generate the junction is kept the same while the electrode area is scaled. 12,13 As an example, we studied single-component BTB junctions with Au and Ti/Au contacts of areas from 25 µm 2 to 10000 µm 2 ; in these experiments Jo was independent of the MJ area. 12,13 Unfortunately, J and Jo vary considerably with the technique used to generate the MJs. Efforts have been made to compare Jo obtained from different techniques, but their dependance on the number of molecules involved in the MJ remain unclear. In 2008, Akkerman et al. and, later on, Amdursky et al. summarized the dependence of Jo on the size of molecular junctions (and thus indirectly with the number of molecules sandwiched) fabricated by different techniques from single-molecule to large area MJs. 14,15 As illustrated in Figure IV-1a, this comparison suggests that there is a non-linear relationship between Jo and the number of molecules between the two electrodes. For example, the Jo measured at 0.1 V from CP-AFM is 10 -6 A nm -2 , which is 10 6 times larger than that measured for macroscopic MJs (Jo ~ 10 -12 A nm -2 ) at the same bias potential.

Figure IV-1. a) Dependence of J o on size of molecular junction (Akkerman et al. 14 or Amdursky et al. 15 ) b) Effect of intermolecular coupling on transport properties of large-area MJs (Ratner et al. 16 )

Moreover, the electronic coupling in single-molecule MJs may differ from that in large-area MJs. Indeed, in large-area molecular junctions, intermolecular coupling exists and could be important for the transport properties of the device. This effect, illustrated in Figure IV-1b, was studied theoretically by Ratner et al. 16 and it was suggested that optimal adlayer density and interwire distance could enhance the conduction and thus affect Jo. Recently, Clément et al. reported the first experimental evidence that the π-π electronic coupling (τ) between ferrocene units in MJs depends on the size of the Au nanoparticles making up the electrode and that this τ can be assessed from a statistical analysis of current histograms. 17 Overall, it is now well documented that Jo depends on the fabrication technique and may depend on intermolecular coupling, but there is no direct way to probe the relationship between the number of molecules and Jo without changing the electrodes. Developing molecular junctions based on mixed layers may be a solution to this problem. This will be the main objective of this chapter.

Strategy

In this chapter, we will use a host-guest strategy in order to deposit a mixed molecular layer. This strategy has been previously reported by our group. 18,19 In short, a hostguest complex between 1-(2-bisthienyl)-4-aminobenzene (BTB) and β-cyclodextrin (β-CD) is generated in water. Gold stripe electrodes are then electrochemically functionalized by the diazonium salt of the host-guest complex (see Step 1 in Scheme IV-2). Electrochemical investigation shows that the water-based modified surface is similar to that generated in acetonitrile without β-CD, but the modified surfaces consist of grafted bisthienylbenzene (BTB) and cyclodextrins that can be removed from the surface. This modified electrode can thus be seen as a π-conjugated BTB-based layer with oligo(BTB) moieties separated from each other by saturated cyclodextrins acting as insulating molecules with respect to charge transport. It will be referred to as BTB@β-CD in what follows (see Scheme IV-2a: the thickness of this mixed layer will be restricted to 5 nm since thicker BTB@ β-CD layers are difficult to obtain. The transport mechanism in BTB@ β-CD is therefore likely to be direct tunneling.)

Scheme IV-2. (a) Deposition of BTB@ β-CD and (b) modified BTB@holes layers. Note that the schemes are just for illustration.

Strategy 120 β-Cyclodextrins can be removed from a BTB@β-CD surface by treatment with THF.

Their removal creates pinholes and the electrochemical behavior of BTB@β-CD is lost. This procedure may thus be considered as a new means of creating nanometric holes in an organic semiconducting layer. Such a layer will be referred to as BTB@holes, as shown in Scheme IV-2b.

Following this, a second diazonium can be used to graft a second molecule inside the pinholes. Electrochemical study shows, for instance, that upon grafting nitrophenyl moieties (NB) onto a BTB@holes layer, the electrochemical behavior of the BTB layer is fully restored. This demonstrates that NB is grafted predominately into the empty spaces generated by β-CD desorption. As a result, a mixed BTB@NB organic layer covalently attached to a Carbon electrode was obtained using the stepwise electrochemical reduction of two diazonium compounds. 19 In general, this strategy allows to create mixed layers incorporating two different π-conjugated molecules (one being easily p-dopable (high HOMO) and the other being easily n-dopable (low LUMO)). Such layers will be designated BTB@M2 where M2 is the molecule grafted by the second electrochemical reduction step, as shown in Scheme IV-3b.

Scheme IV-3. (a) Modified BTB@holes layers (b) and fabrication of BTB@M2 modified layer where M2 is deposited from the second diazonium reduction and (c) fabrication of a BTB@BTB mixed layer from BTB@holes layer by BTB as second diazonium reduction .

In particular, BTB@BTB denotes a layer in which the pinholes between oligo(BTB) wires have been filled by oligo(BTB) wire deposited by a second electroreduction after CD desorption, thus reconstructing a single-component electrode, as illustrated in Scheme IV-3c. The electronic properties of MJs based on a BTB@BTB layer should be similar to those of MJs using a single-component BTB layer, whereas those of BTB@β-CD should show some effects related to the lower number of π-conjugated oligomers in the junction.

Overall, by using this strategy several MJs based on single-component BTB, mixed BTB@β-CD, BTB@holes or BTB@BTB molecular layers have been fabricated, studied and compared. All MJs in this chapter were fabricated using the same procedure, where the bottom electrode is Au and the top electrode is Ti2nm/Au45nm. The different electronic behaviors will thus be attributed to the effect of the molecular layer composition.

More precisely, this chapter is divided in four parts:

• The electrochemical deposition of mixed layers and their properties are first described.

• Next, the β and Jo values of single-component BTB MJs, where the BTB layer is produced by the one-step diazonium reaction described in Chapter 2, has been studied. These measurements constitute the reference behavior to which the MJs based on mixed layers will be compared.

• BTB@β-CD molecular junctions of various thicknesses have been fabricated, and the β and Jo values of these mixed layers measured and compared with those of singlecomponent MJs, one of the main questions being: Does β-CD affect β and Jo ?

• Finally, the β-CD molecules were removed and we have tried to create MJs with BTB@holes layers or with BTB@BTB layers obtained using a second diazonium grafting step to fill the free holes. The important scientific question here is: Do mixed BTB@BTB MJs behave like single-component BTB MJs made by a one-step deposition process ?

3

Results and Discussion

Electrochemical deposition of the mixed layer

In this section, we will study the electrochemical deposition of mixed layer on Au stripe electrodes. The BTAB/β-CD host-guest complex was first formed in aqueous solution, then HClO4 was added, followed by NaNO2 to generate in situ the corresponding diazonium salt, which was electrochemically reduced on gold stripes, as reported previously 18,19 (for more details see Appendix 5.1). The mixed layer obtained consists of covalently bonded bithiophene phenyl oligomers and adsorbed cyclodextrin. An average composition close to one β-CD molecule for four to five grafted BTB molecules and/or one β-CD molecule encapsulating an oligomer consisting of four to five BTB molecules was evidenced by XPS measurements. 18,19 Figure IV-2a compares the electrochemical response of ferrocene-methanol (FcMeOH) on a bare Au stripe and on a BTB@β-CD-modified stripe, before and after β-CD desorption. For the BTB@β-CD-modified Au stripe electrode, there is no FcMeOH signal at the expected potential (red curve) where FcMeOH is usually oxidized on a bare Au stripe electrode (black curve). However, an irreversible wave appears with an oxidation peak at 0.8 V/SCE. This behavior is related to the electroactivity of the grafted layer, which acts as a dopable organic semiconductive material and switches from an insulating state to a conductive state above a potential threshold at around 0.6 V/SCE (see dark cyan curve in Figure IV-2a). The adsorbed cyclodextrin can be easily desorbed by immersing the modified electrode in THF for 15 minutes, creating nanoholes in the film. After β-CD desorption, the FcMeOH signal is similar to that recorded on a bare electrode (blue curve). This change in the electrochemical response is due to the free space left after β-CD desorption. 20

Figure IV-2. Cyclic voltammograms in 1 mM FcMeOH + 0.1 M LiClO 4 in water on Au stripe electrodes: (a) bare (black); BTB-modified (dark cyan); BTB@β-CD-modified before (red) and after β-CD desorption (blue). Scan rate: 100 mV s -1 .(b) Height histograms for Au stripes: bare (black); BTB@β-CD-modified (red) and BTB@holes-modified (blue).

AFM measurements were made on the BTB@β-CD-modified and BTB@holesmodified Au stripe electrodes in order to investigate the mixed layer surface before and after β-CD removal. The height histograms of these Au stripe electrodes are shown in Figure

IV-2b.

The height histograms contain two peaks: the first at around 0 nm corresponds to the SiO2 support, while the second, between 40 and 45 nm, represents the thickness of the Au stripe electrode and any other material that has been deposited on it. The thickness of this layer is given by the difference between the height of the Au stripe electrode (40 nm) and that of the modified electrode. After removal of β-CD, the thickness of BTB@holes layer (3.7±0.6 nm) is slightly decreased compared to the BTB@β-CD layer (4.0±0.6 nm). Using Gaussian fitting the roughness of the BTB@holes layer is found to slightly increase compared to that of the BTB@β-CD layer. By combining the AFM measurement and electrochemical characterization, we establish that removing β-CD molecules generates the free space between the oligo(BTB) wires, as depicted in Scheme IV-2b. The morphology and thickness of the mixed layer before and after β-CD removal are similar. At this stage, the surface is partially modified by separate oligo(BTB) wires and can be used to perform the second diazonium step, as sketched in Scheme IV-3.

Next, a second BTB diazonium was reduced on the BTB@holes-modified Au stripe. Figure

IV-3a

shows and compares the cyclic voltammograms of FcMeOH on the BTB@BTB-(dark cyan) and BTB@holes (blue curve) surfaces. Before the second BTB grafting, the CV of FcMeOH shows a reversible signal at the expected potential, due to electron transfer from the electrode to the solution through the pinholes. After the second BTB grafting, the expected electrochemical signal of the FcMeOH redox process at ∼0.2 V/SCE has completely disappeared. The absence of this signal can be considered as a proof that a new insulating organic layer has been generated (at this applied potential). 20

Figure IV-3 . Cyclic voltammograms in 1 mM FcMeOH + 0.1 M LiClO 4 in water on (a) BTB@holes (blue) and BTB@BTB (maroon) Au stripes. Scan rate: 100 mV s -1 .(b) Height histograms for BTB@β-CD (red) and BTB@BTB (maroon) Au stripes.

In addition, an irreversible oxidation wave appears around 0.8 V/SCE. This behavior is similar or close to that found for FcMeOH on BTB@β-CD or single-component BTB layers. This wave is attributed to the oxidation of FcMeOH on the BTB@BTB layer. In other words, the second BTB molecules are probably electrografted inside the free spaces left by β-CD desorption. AFM measurements were also performed on the BTB@BTB-modified Au stripe electrode and compared to the mixed BTB@β-CD layer. When the second BTB reduction was performed by scanning two cycles from 0.4 to -0.4 V, the thickness of the new BTB@BTB layer was slightly increased (from 4 nm to 4.5 nm), as shown in Figure IV-3b.

The thickness of the BTB@BTB layer can be increased to 7 nm by scanning 4 cycles in the same potential range.

Electrical properties of BTB-based and BTB@-CD-based MJs

All molecular junctions (surface area = 400 µm 2 ) were fabricated by the previously described method. Briefly, molecular layers were electrografted onto Au stripe bottom electrodes (45 nm thick) and the thickness of the molecular layer was controlled by the diazonium electro-reduction conditions (Table S .1) and checked by AFM. The top electrode was patterned by a second photolithography step, followed by direct e-beam evaporation of Ti (2 nm) and finally Au (45 nm). The molecular junction is denoted in the previous chapter with a subscript indicating the thickness of the layers: Au45nm/BTB2nm/Ti2nm/Au45nm will be abbreviated to BTB2nm MJs where the BTB layer is 2 nm-thick.

BTB Molecular Junctions

We will first focus on single-component BTB MJs, where the BTB layer is electrografted onto Au stripe in one step. BTB2nm MJs were fabricated in low yield (7 out of 24 junctions worked). The MJ fabrication yield increased from 30% to 90% when the thickness of the BTB layer was increased from 2 to 5 nm.

Figure IV-4a

presents the overlay of the logarithm of the current density vs. voltage (JV) curve obtained from single-component BTB MJs with thicknesses ranging from 2 to 5 nm. We observe: (i) J varies linearly with V at low bias potential but is non-linear at higher potential; (ii) JV curves are symmetric at low potential, and become slightly asymmetric at high potential (> 1.5 V) (the rectification ratio at 2 V is less than 5) which can be attributed to the use of asymmetric electrodes; (iii) the electrical properties of MJs strongly depend on the thickness of the BTB layer.

Figure IV-4b

presents the current density on a logarithmic scale as a function of the thickness of the BTB layer (Ln J vs. d) at different bias voltages. The slope of this plot gives the β value of the single-component BTB junction at a given bias voltage. The β value of single-component BTB devices using asymmetric electrodes, taken at low bias (0.1 V), is 2.58 nm -1 , which is in a good agreement with the previous report on -conjugated MJs 3,7,10,11,21 and BTB MJs based on carbon electrodes. 22 Note that this value also indicates that the dominant transport mechanism is direct tunneling between the electrodes. 8,[21][22][23] We also observe here that the β values depend on the applied bias voltage. Indeed, they decrease from 2.58 nm -1 at 0.1 V to 2.1 nm -1 at 0.5 V, to 0.94 nm -1 at 1.5 V and 0.5 nm -1 at 2V. This dependence, predicted by Simmon's theoretical model, is rarely demonstrated in the literature, which may be due to the fact that the electrical properties (or conductance) of MJs are usually measured at low bias. Simmon's theoretical model states 1 that β correlates with the energy barrier and the bias voltage following:

𝛽 = 𝛽𝑜 √1 - 𝑒𝑉 2Φ𝐵 1/2
where βo is the value at low bias, ΦB the effective energy barrier of the tunneling junction, and V the applied bias. The experimental values fit this square root dependence, as illustrated in Figure IV-4c, and the best fit is obtained with an energy barrier ΦB of 1.05 eV. 5 This value is in good agreement with that between the BTB HOMO level and the Fermi energy level of PPF electrodes estimated by UPS (1.2 eV). 23 The pre-exponential factor Jo can be extracted from the extrapolation of the Ln J vs. d plots. The Jo of the single-component BTB devices using Au or Ti/Au electrodes is 1 A cm - 2 at 0.5 V, which is smaller than that measured from CP-AFM and STM (see Figure IV-1).

However, for large-area MJs, Jo measured from Au/BTB/Ti/Au MJs is 3 orders of magnitude less than that measured for BTB MJs using e-C and PPF electrodes, as depicted in Figure IV-4d. These results clearly confirm that Jo depends on the electrode type. Similar effects have already been reported and are usually attributed to differences in the electronic coupling strength between the molecule and the contacts. As an example, using CP-AFM, Xie et al. have shown that changing both electrodes from Ag to Pt decreases the resistance Ro (Jo) by 2 orders of magnitude whereas β did not change much. 11,24 Schwarz et al. reported for single organometallic junctions generated using MCBJ that changing the anchoring group from NCS (forming Au-S-C) to C≡CSnMe3 (forming Au-C≡C) induces a conductance increase of 3 orders of magnitude. 25 They attribute this effect to strong hybridization of molecular and metal states enabling the delocalized electronic system of the organometallic backbone to be extended over the molecule-metal interfaces to the metal electrodes to establish highconductive molecular wires. This interpretation can be used here to explain the remarkable difference of Jo in Au/BTB/Ti/Au and PPF/BTB/eC MJs. This is also supported by the fact that tunneling transport occurs for BTB layers up to 8 nm-thick in PPF/BTB/eC MJs, whereas the transition from direct tunneling to filed induced ionization activationless intrachain hopping is observed at 5 nm in Au/BTB/Ti/Au MJs (see Chapter 3 for a full β plot for BTB junctions in the 2 to 15 nm range).

Mixed Layer BTB@β-CD Molecular Junctions

Next, we studied the electrical properties of mixed-layer BTB@β-CD MJs. Mixed BTB@β-CD organic layers were electrografted onto Au stripe as stated above, 18,19 and the thickness of the organic layer was controlled up to 5 nm by changing the electroreduction conditions (more details are given in Table S1). In the mixed layer, π-conjugated oligo(BTB) molecular wires are expected to be separated from each other by saturated β-CD molecules. BTB@β-CD MJs were fabricated in low yield (40%) when the thickness of the BTB@β-CD layer was only 2 nm. This yield is similar to (and slightly above) that obtained with singlecomponent BTB2nm junctions. The fabrication yield increased to 90% when the thickness was increased to 4 nm.

Figure IV-5a

displays JV curves measured for Au/BTB@β-CD/Ti/Au junctions with BTB@β-CD thicknesses from 2 to 4 nm. For mixed-layer junctions, we observe: (i) J is linear with V at low potential for a thin layer and becomes non-linear at higher bias; (ii) J is slightly asymmetric (Rectification ratio RR, below 5); (iii) the electrical properties of the MJs depend markedly on the thickness of the BTB@β-CD layer. These results are similar to those obtained for BTB MJs. 21 This low temperature dependence clearly demonstrates that charge transport in mixed BTB@β-CD4nm MJs occurs through a direct tunneling mechanism, in agreement with other MJs based on -conjugated molecular layers less than 5 nmthick. 7,8,21,22

Figure IV-6. JV Comparison of (a) BTB2nm (red) and BTB@β-CD 2nm (purple) (inset is in Ln scale). (b) BTB 4nm (green) and BTB@β-CD 4nm (orange).

Figure IV-6 compares the electrical properties of BTB@β-CD to those of BTB MJs. When the layer thicknesses are similar, BTB MJs is much more conductive than BTB@β-CD MJs. For example, as illustrated in Figure IV-6a, the current density through BTB2nm at 0.5 V is 0.0085 A cm -2 which is 20 times higher than for BTB@β-CD2nm MJs. There is similar difference between BTB4nm and BTB@β-CD4nm, as shown in Figure IV-6b. At this stage, we conclude that inserting the β-CD molecule into MJs decreases drastically the total current density through them.

Figure IV-7a

and b present the plots of Ln J vs. d (nm) for mixed-layer BTB@β-CD junctions at two bias voltages and compare them with that of BTB MJs at the same bias voltage. The β value for BTB@β-CD MJs is around 2.1 nm -1 at 0.5 V and decreases slightly at 1 V. These values are very similar to those for single-component MJs incorporating conjugated molecules. 7,8,10,21 , inserting an alkyl chain (saturated chain) into the backbone of oligophenyl groups (i.e. in a bilayer configuration) changes β from 3 nm -1 to 6.6 nm -1 . 3 In our case, for mixed BTB@β-CD junctions (-conjugated and saturated molecules deposited in parallel channels), the value of β does not change compared to that obtained in single-component BTB MJs. It is important to note that, as shown in Figure IV-7, the two Ln J vs. d (nm) plots for mixed-layer BTB@β-CD junctions and BTB MJs at the same bias voltage are parallel, and the situation is similar at two different bias voltages.

Figure IV-7. Attenuation plots for single-component BTB and mixed-component BTB@β-CD taken at bias (a) 0.5 V (b) 1 V.

Overall several important conclusions can be drawn:

✓ when the layer thickness is below 5 nm, off-resonance tunneling transport remains the main mechanism in both MJs (single-component BTB and mixed BTB@β-CD MJs).

✓ The values of β at low bias for the two MJs are similar and consistent with other single-component MJs based on -conjugated molecules. An important conclusion is that the difference in conductance between single-component and mixed-layer MJs cannot be attributed to a variation of the attenuation factor. This clearly demonstrates that all the current goes through the -conjugated molecules of the devices.

✓ MJs based on single-component BTB layers are more conductive than those based on mixed-layer BTB@β-CD (for the same layer thickness). Inserting β-CD molecules into MJs, thus creating parallel channels of -conjugated and saturated molecules, decreases the total current density through the MJs. This decrease is independent of the layer thickness.

✓ The decrease in the total current density through the mixed-layer MJs can be clearly attributed to a decrease in the pre-exponential factor Jo which is 20 times less in BTB@β-CD than in single-component BTB MJs. We thus report here the first direct evidence for relationship between the Jo and the number of BTB molecules in the device.

Another important question is whether the current decrease is proportional to or is a non-linear function of the number of BTB molecules. To discuss this point we have calculated the surface area occupied by β-CD molecules and by oligo(BTB) using the hypothesis that only one oligo(BTB) wire is inserted inside the cavity of one cyclodextrin and that two oligo(BTB) wires are thus separated by a distance close to the diameter of β-CD. As shown in Figure IV-8a, we assume that a β-CD molecule is a sphere 1.66 nm in diameter. Then the surface area of one β-CD molecule is around 2.2 nm 2 , which leads to the surface concentration of β-CD monolayers of 7.69×10 -11 mol cm -2 , which is close to the experimental surface concentration (4-6×10 -11 mol cm -2 ). 26 Using the above assumption, the number of oligo(BTB) wires sandwiched in mixed BTB@β-CD MJs (400 μm 2 ) is around 1.8×10 8 . In order to calculate the number of oligo(BTB) wires in single-component BTB MJs, we consider an oligo(BTB4) wire in which there are four BTB units connected to each other, as shown in Figure IV-8b (see side view). Then, to calculate the surface area, we consider the top view of an oligo(BTB4) wire as a 0.61×0.19 nm rectangle. The surface area of one wire is then 0.116 nm 2 , hence the surface concentration of oligo(BTB4) wires is around 1.43×10 -9 mol cm - 2 , and the number of wires sandwiched in mixed BTB@β-CD MJs (400 μm 2 ) is around 3.5×10 9 . Note that the theoretical surface concentration is higher than that found by the electrochemistry of aromatic compounds (3-4×10 -10 mol cm -2 ). 27 Using the calculated number of oligo(BTB) wires sandwiched in MJs, we find that the number of wires in single-component MJs is 20 times higher than in mixed BTB@β-CD MJs, whereas Jo is 20 times higher than that estimated for mixed BTB@β-CD MJs. In other words, the current density through one oligo(BTB) wire in single-component BTB MJs is quite the same as that through one oligo(BTB) wire in mixed BTB@β-CD MJs. This simplified calculation may suggest that the conductance through separated, well-structured BTB molecules is the same as that through a disordered single-component BTB layer. However, in order to probe clearly this relation, we still need to know exactly the number of the molecular wire inside MJs.

MJs based on two successive diazonium salt reductions

In this last part, we will study BTB@BTB MJs and compare them with singlecomponent BTB MJs. In the latter case, the BTB layer was electrografted onto an Au stripe by one-step deposition whereas in the former the BTB@BTB layer was electrografted by two successive electrochemical reductions, as described in Scheme IV-4.

β-CD molecules physically absorbed on the electrode can be easily removed. The fact that this generates free pinholes is supported by the electrochemical reponse of FeMeOH on the BTB@holes electrode, which is similar to that of the bare electrode, as shown in Figure IV-3. It is therefore expected that this layer will not allow direct metal deposition as the top electrode by e-beam evaporation and that metallic wires will probably go through the free space and make direct contact with the bottom electrode. MJ devices, based on the BTB@holes electrode, were fabricated; these devices show ohmic JV curves indicating direct electrical contact between the Au and Ti/Au top electrodes (short-circuits) and are similar to those of metallic Au/Ti/Au junctions, as shown in Figure IV-9a (black line).

Figure IV-9. (a) JV comparison of BTB@holes (black line), BTB@BTB4.5nm (red), BTB@ β-CD4nm (orange) and single-component BTB4nm (green) MJs. (b) comparision of plot of Ln J vs d (nm) for single component BTB (green), BTB@β-CD (blue) and BTB@BTB (red) MJs.

Next a second BTAB diazonium salt solution was used to electrograft BTB into the pinholes generated after removing the β-CD. The electrochemical responses of the FcMeOH redox probe on BTB@β-CD, BTB@holes and BTB@BTB electrodes are compared in Figure IV-3. They are close to that of a single-component BTB, i.e. shifted to an unusually high potential (above 0.7 V). This clearly demonstrates that the BTB@BTB layer performs as a BTB switch which controls FeMeOH oxidation on the modified electrode, and strongly suggests that the second diazonium is mainly electrografted into the pinholes left by β-CD desorption, as already observed with NB. 19 The thickness of the BTB@BTB mixed layer was carefully controlled by the electrochemical conditions and measured by AFM. Figure IV-3b compares the thicknesses of the initial BTB@β-CD layer and of the BTB@BTB layer obtained after the second BTB deposition step. The mean thickness of the layer increases by only 0.5 nm while its roughness is unchanged. BTB@BTB MJs were then fabricated in high yield (above 90%) using Au and Ti/Au as bottom and top electrodes, respectively. Figure IV-9a compares the JV curves for BTB4nm, BTB@β-CD4nm and BTB@BTB4.5nm MJs. Several important observations are made: i.

BTB@β-CD4nm is less conductive than BTB4nm, as mentioned above.

ii. BTB@BTB4.5nm is much more conductive than BTB@β-CD4nm. Filling the pinholes with new oligo(BTB) wires drastically increases the current density through the MJs despite the slight increase in thickness. To the best of our knowledge, this is the first time that such an effect has been demonstrated.

iii. The standard variation of the current density in BTB@BTB4.5nm MJs (28 out of 32 junctions tested worked) is very low (see Figure S 10), which shows that the mixed BTB@BTB layers are homogeneous and uniform. This fact is also supported by AFM analysis, since the square root roughness of the BTB@BTB surface and the singlecomponent BTB layer are the same.

iv. BTB@BTB4.5nm conductivity is similar to that observed in single-component BTB4nm.

This clearly supports the idea that the pre-exponential factors of the BTB@BTB4.5nm and BTB4nm MJs are similar.

The plot of Ln J vs. d (nm) in Figure IV-9b shows that after the free holes are filled by a second BTB diazonium salt reduction, the total current density of BTB@BTB (red point) comes back to the level of single-component BTB junctions, demonstrating that β and Jo for BTB@BTB junctions are the same as for BTB junctions.

Next, we continued to deposit BTB@BTB7nm by two successive diazonium salt reductions. However, as mentioned above, the deposition of a mixed BTB@β-CD where the mixed layer is thicker than 5 nm is still difficult. Hence, in order to deposit BTB@BTB7nm, we first deposited BTB@β-CD4nm; then, for the second BTB reduction, we scanned from 0.4 to -0.4 V (8 cycles). Under these conditions, the BTB@BTB layer increased to 7 nm, as shown in Figure IV-10a. The roughness of the BTB@BTB7nm layer is slightly different from that of BTB@β-CD4nm. Figure IV-10b shows that the JV curves of BTB@BTB7nm and BTB7nm MJs are almost identical. After the free holes are filled with a second BTB diazonium, the total current density of BTB@BTB7nm (red point) comes back to the level of single-component BTB7nm junctions, demonstrating again that BTB@BTB junctions behave as single component BTB junctions. The same behavior was observed for BTB@BTB4.5nm and BTB4nm MJs, as illustrated in Figure IV-9a. Note also that JV curves of these BTB@BTB7nm MJs are now asymmetric, which is consistent with a previous report, 13,28 i.e. when the thickness of the BTB layer is above 5 nm, rectification appears and the rectification ratio can reach 1200 at 2.7 V for BTB9nm (see Chapter 2).

Finally, instead of using BTB as a second diazonium, we used p-nitrophenyl diazonium (NB) in order to generate a mixed-layer BTB@NB MJ. Figure S 11 shows that the total current density is lower for BTB@NB5nm than for BTB@β-CD4nm MJs. This is unexpected, since oligo(NB) is still a -conjugated molecule. This decrease in current suggests that the NB molecules may covalently bond to the BTB wire, and change its electronic properties. This seems likely to happen, since the NO2 group occupying the para position may prevent the formation of oligo(NB) in the second diazonium salt reduction.

To summarize, we reported here that we are able to fill the free holes with a second molecule. If the second component (BTB) is the same as the first component (BTB), then the electrical properties of the mixed-component BTB@BTB MJs are identical to those of BTB MJs. If the second component differs from the first component, then the electrical properties of the mixed-layer and single-layer devices are also different.

Conclusions

In this chapter we have fabricated and investigated mixed molecular junctions and compared their behavior to that of single-component BTB MJs. All the MJs were fabricated using similar procedures. By doing so, we are able to answer several questions listed in Section 1.

We have shown that the value of β for BTB MJs in the tunneling regime (thickness below 5 nm) is 2.58 nm -1 at 0.1 V and depends on the bias voltage; it changes to 1.8 at 1 V and 0.57 nm -1 at 2 V. The energy barrier estimated by fitting with Simmon's model is 1.05 eV.

We were also able to fabricate MJs based on mixed BTB@β-CD layers with thicknesses from 2 to 5 nm. The temperature dependence shows that charge transport occurs by a direct tunneling mechanism. The β value of mixed BTB@β-CD MJs at 0.5 V is 2.1 nm - 1 which is similar to that obtained for BTB MJs. Interestingly, the Jo estimated for BTB@β-CD MJs is 20 times lower than that for BTB MJs. This clearly indicates that inserting β-CD into the BTB layer reduces the number of 'active' channels for charge transport.

We also investigated the effect of filling the free holes left by the removal of β-CD with a second molecule. This free space allows the formation of direct metallic contact between the two electrodes. When the second component (BTB) is the same as the first one (BTB), then the electrical properties of the mixed-component BTB@BTB and BTB MJs are identical. If the second component is an NB molecule, then the electrical properties of the mixed and single layers are different.

Based on the knowledge gained in this chapter, we would like to raise a few scientific questions. The first question is: how can control cooperative effects in molecular junctions? Inserting a β-CD molecule reduces the total or effective current density through MJs, and we proposed that the current through each channel in mixed layer is enhanced. And in order to explore further, we could fill inside the free hole with several compounds with different Jo.

Second question: can we modulate Jo by filling the free holes with other molecules or not? Last but not least, imagine that we can incorporate other molecules, such as VIO-C1 containing the viologen unit, into the free holes. VIO-C1 acts as an acceptor whereas BTB wire is a donor, so in the end we will have two channels, one p-dopable and the other ndopable, in parallel. By this means, can we create a new functionality using mixed molecular junctions?

5

Supporting Information

Electrochemical reduction condition

1-(2-Bisthienyl)-4-aminobenzene was synthesized and purified according to previously published procedures. Ferrocene-methanol (FcMeOH) was supplied by Alfa Aesar, acetonitrile (HPLC grade) by Prolabo, and NaNO 2 by Merck. Hydrated β-cyclodextrin (β-CD) and perchloric acid (HClO 4, 17 mol.•L -1 ) were from Acros Organics, tetrabutylamonium tetrafluoroborate [(Bu 4 N)BF 4 ] and lithium perchlorate (LiClO 4 ) from Aldrich.

Basic electrochemical experiments were carried out in a single-compartment three-electrode cell using a CHI 440A potentiostat (CH Instruments, Austin, TX). The auxiliary electrode was a platinum grid; a saturated calomel electrode (SCE) was used as reference. Working electrodes were homemade glassy carbon (GC) disks (area = 0.2 cm 2 ) fabricated from carbon rods (Tokai, Japan) embedded in Teflon, and gold disk electrodes (area = 0.1 cm 2 ) from CH Instrument (Austin, TX). An aqueous solution of 2 × 10 -3 mol•L -1 BTAB, 0.1 mol•L -1 HClO 4 , and 4 × 10 -2 mol•L -1 β-CD was degassed for 5 minutes, then 2 × 10 -3 mol•L -1 NaNO 2 was added to the solution and incubated for 5 minutes before electrografting. Electrografting was performed on glassy carbon and Au stripe electrodes. The deposited film was then electrochemically characterized by the FcMeOH probe. β-CD molecules were de-absorbed in THF for 15 minutes. The second diazonium step was performed in acetonitrile solution with t-BuNO 2 as a reagent to generate the diazonium salt in situ.

ORGANOMETALLIC-BASED MOLECULAR JUNCTIONS 1 Introduction

In molecular electronics, several types of molecules have been extensively investigated, including purely organic compounds, 1-3 proteins, 3 and more recently complex clusters based on silicon 4,5 or germanium 5 as well as organometallic compounds. [6][7][8][9][10][11][12] Usually, in the tunneling regime, the rate of charge transport depends very much on the length of the organic molecule: for example, the β values of saturated molecules are from 5-10 nm -1 whereas β ~ 2-3 nm -1 for -conjugated molecules. A few exceptions have been found in molecular junctions: for instance, nearly length-independent conductance (β value ~ 0.01 nm - 1 ) is observed in polyporphyrin molecular wires, 13 porphyrin nanorods 14 and extended viologen molecules, 15 and there is no exponential length dependence of conductance in iodide-terminal oligothiophene single-molecule tunneling junctions. 16 We have also seen in the previous chapters that β can vary considerably when the dominant transport mechanism is not direct tunneling between the contacts.

In addition to organics, organometallic compounds can be seen as multifunctional molecules with many interesting electrochemical and photophysical properties that can be used in MJs to add new functionalities, such as light emission, 6 photovoltaic effect, 10 conductance switching 9 for resistive memory 17,18 and coulomb blockade. 19 In 2003, using a cross-wire tunnel junction, Shashidhar et al. first showed that the conductance of MJs incorporating a platinum(II) complex was higher than that of oligo(phenylene ethylene) (OPE) MJs. They attributed this effect to the thickness of the film. 20 Since then the conductance of some other organometallic-based MJs has been investigated by several techniques, such as CP-AFM, 11,21 Hg liquid drop 12 and MBCJ. 22 For example, using CP-AFM, Frisbie et al. 11,21 found the first evidence that the β value of an organometallic (platinum(II)) wire junction is much smaller than that of an organic molecule in the same range of thickness, and also that the effective resistance (Ro) of MJs depends markedly on the anchoring group. Rampi et al. 12 also reported that the β values of organometallic wire junctions are much smaller than those of organic compounds and showed that they depend on the type of metallic ion. An important result from this group is that β for a cobalt complex is 0.01 nm -1 whereas it is 0.28 nm -1 for an iron complex. Small β values were also found in organometallic MJs containing porphyrin units. [23][24][25][26] Several efforts have been made to compare the conductance of organic molecular MJs and organometallic MJs directly and to establish the relationship between redox event of organometallic compounds and the conductance. 7,8,[27][28][29][30] However, the change in the conductance is usually attributed to the difference in the length of the molecule or to the strength of the electronic coupling between the contact and the molecule. It is rarely related to the redox events that may occur in such organometallic-based MJs. 8,27,28 For example, Davidson et al. reported that the conductance of a single organometallic complex based on terpyridine is independent of the redox potential of the metallic core. 29 Let us also remark that studies on transport in organometallic MJs are usually limited to systems where the distance between the two electrodes is small (below 3 nm) and, consequently, understanding the long-range transport regime (thicknesses from 5 to 20 nm) is still a challenge.

Objectives

This chapter aims at investigating charge transport through MJs based on organometallic compounds. We synthesized several organometallic compounds that have different redox potentials. They will be covalently bonded to the electrodes by the electroreduction of diazonium salts, and the thickness of the layer will be controlled by the electrochemical conditions and measured by AFM. Large-area organometallic molecular junctions with various thicknesses will be fabricated using the procedure already described in the previous Chapter and detailed in the Appendix. By means of these organometallic compounds we wish to address several important questions:

• Is it possible to functionalize the electrode with organometallic molecules, to generate various films of different thicknesses and to fabricate organometallic-based MJs in high yield using fully CMOS-compatible processes?

• What is the difference between organometallic-and organic-based MJs? and can we understand it in terms of transport mechanisms? Is there a relationship between the conductance of the MJs and the redox potential of the organometallic compound used? In terms of transport, is there rectification behavior or some specific behavior due to the metallic center?

• Due to the ionic nature of these organometallic compounds, the derived MJs incorporate many ionic species and counter-ions (including counter-ions such as PF6 -). Then we also want to understand whether counter-ions play a role in the transport properties of the MJs. Can we understand these effects, and can we vary the type or the number of counterions to tune the transport properties?

• As some organometallic complexes based on ruthenium show electrochemiluminescence properties (thick Ru(II)(by)3 films have been used for light emission [31][32][33] ), can we use this property in ultrathin MJs to emit light and to report on the transport mechanism and the possible redox events occurring during transport ?

3

Results and Discussion

Electron transport in Organometallic Molecular Junctions

In this section, we will describe the electrodeposition of three organometallic molecules and then the transport through the corresponding MJs. The chemical structures of these compounds and their redox potentials in solution are presented in and -1.8 V/SCE which can be attributed to the reduction of the bipyridyl ligands. The irreversible oxidation peak at 1 V/SCE can be attributed to the oxidation of the NH2 group and the reversible peak at 1.25 V/SCE is due to the oxidation of Ru(II) to Ru(III). For the Ru-C complex, the oxidation peak appears reversibly at 0.5 V/SCE which is 0.7 V lower than that for Ru(by3). This lower oxidation potential can be attributed to the oxidation of Ru(I) to Ru(II). The reduction peak of the Ru-C complex at -1.6 V/SCE and -1.8 V/SCE can be attributed to the reduction of the ligands. The CV of Co(tpyr2) shows a peak at 0.25 V/SCE which we assign to the oxidation of Co(II) to Co(III). We observed a first reduction peak at -0.8 V/SCE which can be assigned to the reduction of Co(II) into Co(I) and reduction of terpyridyl ligand appears at -1.8 V/SCE. The difference between Co(tpyr2) and the others is that the Co(II) core can be oxidized at 0.25 V/SCE and reduced at -0.8 V/SCE whereas the Ru(II) and Ru(I) cores oxidize at 0.5 V/SCE, but cannot be reduced in the potential window used; the reduction peaks appearing for those compounds are assigned to the reduction of the ligands.

The organometallic compounds were grafted onto a substrate by in situ diazonium salt generation and electroreduction. In general, a certain amount of tert-butyl nitrite (a few equivalents) was added to the solution of the complex and then, the solution was incubated The survey and high-resolution XPS spectra of Co and N are shown in Figure V-2c, d andf. The single well-defined N 1s peak at 400.1 eV in the high-resolution spectra rules out the possibility of azo-bridged grafting and is assigned to the signal of nitrogen in an aromatic ring (pyridyl). The photoelectron spectrum of Co 2p is shown in Figure V-2c. The binding energies of Co2p3/2 at 780 eV and Co2p1/2 at 796 eV and Co2p peak has significant split spin-orbit component (ΔE) found at 15.7 eV, they all are in good agreement with the literature. 34,35 Experimental atomic ratios for P, N and Co are consistent with the expected composition of the layer. The N/Co, P/F and P/Co ratios are 5.8, 5.3 and 1.8 which are very close to the theoretical values (6, 6 and 2), respectively. This XPS evidence concludes that the Co(tpyr2) functionalized on substrate are chemically stable; no azo groups are formed when the compound is electrografted.

Similar experiments were performed for the other complexes; the electroactivity and XPS data are given in Appendix 5.1 and 5.2. These results confirm that the molecular layers are electrografted onto the substrate, and that the film composition and the redox states of the metals correspond to what is expected. AFM measurements (see Appendix 5.3) also show that the thickness of the molecular layer can be changed from 2 nm to 20 nm by modifying the electrochemical conditions.

Next, in order to study the electrical properties of MJs based on these molecules, we fabricated organometallic junctions using the CMOS-compatible procedures described in previous chapters and given in the General Appendix. Several thicknesses of each organometallic layer were electrografted onto Au stripes. Complete organometallic MJs (Au/Molecules/Ti/Au) will be designated by the molecular layers, including subscripts for layer thickness in nanometers. For example, Au/Co(tpyr2)7nm/Ti/Au means a 7nm layer of Co(tpyr2) electrografted onto a bottom Au electrode (45 nm) with a top electrode of 2 nm of Ti followed by 45 nm of Au deposited directly by e-beam evaporation. All organometallic MJs were fabricated in high yield (above 90%; 36 over 40 tested junctions worked) and were highly reproducible. The JV curves of these devices are almost symmetric, and the standard deviation of the current density is very small, which indicates that the Co(tpyr2) layer is homogeneous and uniform. In addition, the current density in these MJs (1.2 A.cm -2 at 1 V) is just 4 times that of Co(tpyr2)7nm (0.25 A.cm -2 ). This observation is very interesting since usually in MJs based on -conjugated organic compounds and for transport in the tunneling regime (thickness below 5 nm), if the thickness changes from 2 to 5 nm, then the current density decreases by two orders of magnitude (assuming that β is around 2 nm -1 ). The standard deviation of the current is very small, which suggests again that the organometallic films are homogeneous, uniform and reproducibly deposited on the various gold stripes used. An important result is that the conductivity of Co(tpyr2)7nm is much higher than that of Ru-C8nm, which is in turn higher than that of Ru(by3)7nm. For instance, at 1 V the current density of Co(tpyr2)7nm (~ 0.25 A.cm -2 ) is 40 times that of Ru-C8nm (0.006 A.cm -2 ) while the current density of Ru(by3)7nm (0.001 A.cm -2 ) is 6 times less than for Ru-C8nm MJs. We note that JV curves are always almost symmetric although the electrodes are asymmetric (Au & Ti/Au).

The JV curves of Co(tpyr2), Ru-C and Ru(by3) MJs with different thicknesses are presented in Appendix 5. 4 and summarized in Figure V-4. Figure V-4a compares representative curves of Co(tpyr2) MJs with several thicknesses. They are almost symmetric and depend slightly on the thickness of the layer. For example, when the thickness goes from 2 nm to 7 nm and finally to 13 nm, the current density decreases by factors of only 4 from 2 nm to 7 nm, and by roughly 2 from 7 nm to 13 nm.

applied voltage (JV) curves of Co(tpyr2) MJs for several thickness. (b) Attenuation plots taken at 1 V of MJs for different molecular units and slopes (β value).

To directly visualize the effect of the thickness, we plotted Ln J at 1 V vs. thickness for several compounds including Co(tpyr2), Ru-C, Ru(by3) and BTB (Figure V-4b). The β value estimated from the slope of the attenuation plot usually gives information on the dominant transport mechanism. For the molecule unit including BTB, Ru(by3) and Ru-C, there are two distinguishable slopes in the attenuation plot. For instance, BTB MJs have two  values, 1.8 nm -1 for thickness from 2 to 5 nm and 0.36 nm -1 from 5 to 15 nm. The β value below 5 nm agrees well with those reported for aromatic molecules and assigned to the 'offresonance tunneling' transport regime. 6,36 Above 5 nm, the small  can be attributed to intrachain hopping. 37 Similar results were reported for BTB MJs fabricated by e-C contacts, where  is 2.9 nm -1 for thicknesses below 8 nm and 0.8 nm -1 for 8 to 16 nm. The difference in the transition thickness of MJs based on BTB can be attributed to the very strong electronic coupling at the e-C interface, which in turn may change the effective thickness of the molecular layer. Similar behavior is found for Ru(by3) and Ru-C MJs, i.e. there are two distinguishable slopes in the attenuation plot, indicating a change in the dominant transport mechanism at a transition thickness, dtrans, which depends on the molecule used. That for Ru-C MJs is around 2.8 nm while that of Ru(by3) MJs is around 3.5 nm. Both values are smaller than that measured for BTB (5 nm). Interestingly, for thicknesses below dtrans, all the points lie on the same line with the same slope, whatever the molecule used (Figure V-4b). These results demonstrate that the dominant mechanism for thicknesses below dtrans is non-resonant tunneling and that when the thickness increases, a new transport mechanism, more efficient than non-resonant direct tunneling, is operative. Above dtrans (in this new long-range transport regime), the  values of Ru(by3) and Ru-C MJs are indistinguishable at around 0.3 nm -1 , which is slightly smaller than that of BTB MJs (0.36 nm -1 ).

The absolute current density (Jo) for each transport regime extracted from the attenuation plot give insights on the transport. In the tunneling transport regime (below dtrans), we found that the Jo of all Ru(by3), Ru-C and BTB MJs are similar and equal to 3 A.cm -2 which is in good agreement with another report 38 and with results obtained in Chapter 3. However, above the direct tunneling regime, Jo at dtrans (Jo,dtrans) strongly depends on the molecule. For example, Jo,dtrans of Ru(by3) is 12 times higher than that of BTB MJs. This indicates that Jo,dtrans is not correlated with the energy barrier between the HOMO level and the Fermi level of the Au electrode, since this barrier is smaller for BTB (0.2 eV) than for Ru(by3) MJs (0.7 eV). Note that in Chapter 3 we found, in MJs based on -conjugated organic molecules, that decreasing the HOMO energy of the molecular layer (increasing the energy barrier) decrease the conductivity, but also that in BTB-based MJs transport above 5 nm can be attributed to field-induced ionization associated with activationless interchain hopping. In other words, redox events, with or without reorganization before or during electron transport, are occurring. 37 Following this idea, we suspect and propose that above the transition thicknesses, redox events inside the junctions are turned on and transport is due to electron transfer between ruthenium redox centers and intrachain hopping. Within this framework, the smaller transition thicknesses for ruthenium-based MJs than in -conjugated-based MJs can be understood in terms of the fact that the charge carriers in Ru-based mixed-valence compounds are smaller than in oligothiophene (5 nm) and that there is a large number of counter-ions in ruthenium-based MJs. The increase in the current in this case can be due to the presence of counter-ions (PF6 -) in these ionic compounds that may modify the electrostatic field, concentrate the potential drop at the interface and, consequently, increase the injection rate at both interfaces. Moreover, as the Jo of Ru-C MJs is 0.08 A.cm -2 , which is 13 times that of Ru(by3) MJs, counter-ion effects alone cannot explain the difference between these two materials, which could be better attributed to the difference in the redox potentials of the two molecules.

More interestingly, we found that Co(tpyr2) MJs show unique electrical properties. First, the current density measured for Co(tpyr2) MJs is always higher than that for BTB, Ru-C and Ru(by3) MJs at a given bias. For instance, the current density of 7 nm Co(tpyr2) MJs at 1 V is 0.25 A.cm -2 , which is 1000 times that of 7 nm BTB MJs. Such a huge increase in the current density was already observed when 2 nm of oligo(EDOT) was added on top of the BTB layer; we have attributed this to the increase in the HOMO level from -5.3 eV (BTB layer) to -4.8 eV (BTB-EDOT layer). Secondly, as shown in Figure V-4b, we found that there is no transition point at which we observe a change in the dominant transport mechanism. The β value of Co(tpyr2) MJs is very small 0.17 nm -1 and stays much the same at different bias. This unique property has rarely been reported. 12,24 Such small  values have been observed in MJs based on porphyrin units: for instance, Bruce et al. reported a small  for conjugated (porphinato)-metal MJs. 24 Such small  depend slightly on the metallic core, whereas the resistance of MJs depends very much on the thickness of the molecular layer. 24 Note also that Tuccitto et al. were the first to report small values for cobalt-based MJs whose thickness was restricted to 40 nm. To explore in more detail the transport mechanism, temperature dependence measurements (from 7 K to 290 K) were performed on 7 nm Co(tpyr2) MJs, a thickness above that of the direct tunneling regime. These MJs are stable during the cooling or heating process and several ups and downs were performed. shows the Arrhenius plot Ln J vs. 1000/T from which we can calculate the activation energy of charge transport. As shown in the plot, there are two distinguishable regimes: one regime at very low temperature from 7 K to 100 K, where no activation is needed for electron transport, and a second, from 100 to 300 K, where transport is weakly activated with an activation energy of 80 meV calculated from the slope of the plot. This small activation energy is close to that reported for BTB MJs with e-C contacts and attributed to field ionization associated with intrachain transfer. Since the activation energy is so small, we can reasonably exclude several electron-transport mechanisms, including thermionic emission and activated hopping (interchain hopping), for which the activation energy is close or above 300 meV. Such a small activation energy has rarely been reported for film thickness above 5 nm and it is only recently that the activation energies in BTB 37,39 and fluorene molecular junctions with thicker films were found to be less than 100 meV. In the latter case, a multi-step tunneling mechanism controlling electron transfer was proposed. 40 We reported in Chapter 2 and Chapter 3 that VIO-C1 and BTB-EDOT MJs show a small  value, that there was no transition point, and that transport was activation-less (for BTB-EDOT MJs). We attributed this to an "on-resonance tunneling mechanism". Co(tpyr2) MJs behave in a very similar way. The HOMO energy levels of BTB-EDOT and Co(tpyr2) estimated from electrochemical data are very close (roughly -4.8 eV). Hence, we can reasonably assume that the dominant transport mechanism of Co(tpyr2) MJs is long-range resonance tunneling, but that another mechanism such as phase-coherent electron tunneling 23 or multi-step tunneling 40 could also be considered.

At this point it is difficult to propose a general framework to explain all these results, as some apparent contradictions must be underlined. Indeed, it is, for instance, not easy to explain why the Jo of Ru(by3) MJs is higher than that of BTB MJs, even though the HOMO energy of BTB (-5.3eV) is higher than that of Ru(by3) (-5.9 eV), whereas the Jo of Ru-C is higher than that of Ru(by3) MJs, which seems to correlate with the HOMO energies of these two molecules. However, the current densities in all the MJs thicker than 5 nm clearly depend on the electronic structures of the molecules involved.

Figure V-6. Ln J for V = 1 V and d = 8 nm for the seven molecules plotted vs. the energy of the molecular layer (a) LUMO energy (b) HOMO energy and (c) HOMO-LUMO gap. The energies of Ru-C, Co(tpyr2) and Ru(by3) were estimated from electrochemical data while the others were calculated by DFT and TDDFT.

In order to explore the relationship between J and the electronic properties of the molecular layer, we plotted LnJ vs. the various energy levels of the molecules (Figure V-6). The LnJ was taken at 1 V from MJs where the molecular layer is 8 nm-thick (above the direct tunneling regime). The HOMO, LUMO and bandgap energies of Co(tpyr2), Ru-C and Ru(by3) layers were estimated from electrochemical data, while the values for BTB, NB, FB and VIO-C1 were calculated from DFT(B3LYP/6-31G*) and TDDFT (B3LYP/6-31G(d)). However, it seems to correlate very well with the HOMO-LUMO gap with R 2 ~ 0.97. This unique correlation is very significant and applies to a large range of gap energies (from 1 V to 4.5 V). This correlation was already observed for a smaller number compounds and was the basis of the recent proposal of a multi-step tunneling mechanism controlling electron transfer. 40 This correlation works also for MJs based on e-C substrate obtained in Alberta in collaboration with McCreery's group (unpublished data). Although there are large differences between e-C substrate (symmetric device) and Au & Ti/Au substrate (asymmetric device), the correlation between the current density and the HOMO-LUMO gap is clearly observed with both contacts. This very good correlation means that the rate of charge transport is controlled by the HOMO-LUMO gap, when the thickness of the layer is above the transition thickness and suggests that the injection rate at the interface does not limit current. Currently several experiments are being performed in order to better understand the underlying meaning of this correlation.

To summarize, several important remarks can be made. First, this study has allowed for the first time to compare -conjugated and organometallic-based MJs with exactly similar contacts. It clearly shows that the latter have several advantages in MJs capable of withstanding high current densities. In particular, the Co(tpyr2) MJs, where the complex consists of Co(II) and terpyridinyl ligands, shows unique properties including a very low  (0.17 nm -1 ), high current density, and no transition thickness (dtrans) indicating a change of mechanism with film thickness. Such unique properties can be attributed to the fact that the HOMO level of Co(tpyr2) is almost in resonance with the Fermi level of the electrode.

Second, we found that for molecules whose HOMO levels are not in resonance with the gold Fermi level the attenuation plot shows two different regions corresponding to two dominant transport mechanisms. Below dtrans, non-resonant direct tunneling dominates and the conductances of most MJs are similar. In other words, in the direct tunneling regime, the conductances of MJs are almost independent of the molecule (or redox potential), which is in good agreement with the study of single molecules using STM-BJ. 7,8,28,41 Third, above the tunneling regime, i.e. for thicknesses above dtrans, a new mechanism is operative and overcomes direct tunneling. In this transport regime, the current densities (or conductivity) through the MJs depend strongly on the molecules. Jo of Co(tpyr2) MJs is higher than Jo at dtrans (Jo,dtrans) of Ru-C and Ru(by3) and higher than observed in most MJs based on -conjugated molecule. The value of dtrans also depends very much on the molecule.

For example, the transition point in BTB MJs is around 5 nm, while it is 3.5 and 2.8 nm for Ru(by3) and Ru-C MJs. We propose that this transition is the trace of redox events occurring in the MJs despite the fact that they are solid-state. We also propose that the difference in dtrans between organometallic-based MJs and -conjugated MJs can be attributed to the size of the polaron formed after injecting the charge into or from the molecules. Above the transition point, the energy gap between the HOMO and LUMO plays a crucial role in determining on the conductance of the MJs. We conclude that transport within the layer limits the current density whereas charge injection at the interfaces does not.

Rectification with Organometallic Molecular Junctions.

Several examples of molecular rectifiers based on organic -conjugated layers have been reported in the literature and were reviewed in Chapter 1 of this thesis. We also studied several rectifiers in Chapter 2 and reported that when asymmetric contacts (Au & Ti/Au) are used, BTB MJs show high rectification (1300) at low potential (2.7 V) with high current density (2 to 10 A.cm -2 ) and that the polarity of the rectification can also be changed by going from BTB to NDI. Rectification has rarely been observed in organometallic MJs but the best rectifiers to date incorporate organometallic redox groups. 42 In this part of the Chapter, we will study rectification in organometallic MJs fabricated using asymmetric contacts (Au & Ti/Au). The structures of the organometallic compounds used are presented in Figure V-7a and their molecular energy levels are extracted from electrochemical data. Briefly, the HOMO of (Ru(by3)) (-5.9 eV) is as the same as that of Ru(tpy2) but is 0.8 eV higher than that of the Ru-C layer containing a Ru-C bond. The HOMO and LUMO of the Co(tyr2) layer are around -4.9 and -3.9 eV, respectively.

As mentioned in Section 3.1, MJs based on Ru-C and Co(tpyr2) do not rectify, and have almost symmetric JV curves and small RRs (below 5). We have already observed in Chapter 2 and Chapter 3, that rectification is not observed when the molecular energy level (either LUMO or HOMO level) lies between the Fermi energy of the bottom and top electrodes in the free state, even though asymmetric electrodes are used. As shown in Figure V-7b, the HOMO energies of Co(tpyr2) and Ru-C are between the Fermi energies of the two electrodes, and this situation leads to symmetric JV curves, in agreement with results found in previous Chapters.

Figure V-7. (a) Chemical structures of organometallic compounds. (b) Molecular orbital levels in vacuum of Ru-C (blue line), Co(tpyr2) (violet line) and Fermi levels of contact in the free state.

On the contrary, the Ru(by3)-and Ru(tpy2)-based MJs rectify. Figure V-8a presents the JV overlay of Ru(tpy2)7nm MJs on the semilog scale. The yield of MJ devices is quite high (above 90%) which is the same as the others. The JV curves of Ru(tpy2) junctions are asymmetric, with higher current density (up to 2 A.cm -2 ) observed when the negative potential is applied to the bottom Au electrode. The rectification ratio of Ru(tpy2)7nm junctions at 2.7 V is around 150±10, but that of some junctions reached 300 (Figure V-8b -blue). JV curves of Ru(by3)12nm MJs (Figure S V-11) also appear asymmetric; RR is relatively low, around 30 at -2.5 V (Figure V-8b -red). Note that the difference in the RRs for Ru(tpy2) MJs and Ru(by3) MJs is statistically significant but, since they are not very large, we did not try to compare them at similar thicknesses.

Figure V-8. (a) JV overlay of organometallic Ru(tpy2) 7nm MJs. (b) Rectification ratio histogram for Co(tpyr2) 7nm MJs (blue), Ru(by3) 13nm MJs (red) and Ru(tpy2) 7nm MJs (green) taken at -2.5 V.

To summarize, we found that the rectification is not observed in Co(tpyr2) or Ru-C MJs but Ru(by3)12nm MJs show small rectification whereas Ru(tpy2)7nm MJs have a significant RR around 150 at -2.5 V. The absence of the rectification in Co(tpyr2) and Ru-C MJs could be attributed to the fact that the HOMO energy lies between the Fermi energy levels of the two contacts while rectification is observed when the Frontier Orbitals of the organometallic compound are below or above the Fermi levels of the contacts.

Ionic Effects on Molecular Rectification.

Donor-bridge-Acceptor chemical structures have been proposed as the stereotypical building units for molecular rectifiers as well as switches, memories, transistors and photoemission MJs. Considering the poor RRs observed in Section 3.2 we decided to construct a new Donor-bridge-Acceptor structure incorporating organometallic moieties. A new structure, Ru-NDI, was thus designed with an NDI electron-accepting unit covalently bonded to a ruthenium bisterpyridine complex (see Figure V-9a); a hexyl chain was introduced to enhance solubility. The structure also features a phenylamine terminal group in order to enable later in situ preparation of a diazonium cation and subsequent grafting onto various substrates. Moreover, MJs based on this compound will make it possible to study the effect of the number of counter-ions on their electrical properties.

Electrochemical deposition of Ru-NDI molecule

In this section, we will study the electroactivity of the molecule in solution as well as the electroactivity of the Ru-NDI functionalized substrates. The electrochemical behavior of a solution of Ru-NDI (0.5 mM in ACN + 0.1 M TBAPF6) was studied by cyclic voltammetry (CV) on a glassy carbon (GC) electrode as shown by the black curves in Figure V-9b and c. The CV curve displays two waves in the positive potential range, corresponding to the irreversible aniline oxidation at Epa = 1.08 V/SCE and the quasi-reversible oxidation of the Ru metal center at E1/2 = 1.32 V/SCE. In the negative region, there are four partially reversible one-electron waves at E1/2 = -0.52 V/SCE and 0.95 V/SCE, corresponding to the reduction of the NDI fragment, and at E1/2 = -1.30 V/SCE and -1.49 V/SCE, corresponding to the reduction of the terpyridine ligands. The diazonium cation derived from Ru-NDI was generated in situ by addition of 10 equivalents of tert-butyl nitrite. Ten successive cycles of electrochemical reduction between 0 and -0.5 V/SCE were run on the resulting solution (see Appendix 5.1). The electrochemical behavior of the modified GC electrode was studied by CV (red curve in Figure V-9b and cError! Reference source not found.). These CV curves are stable upon further cycling (Figure S V-3). Reversible oxidation of ruthenium occurs at E1/2 = 1.32 V/SCE. The region of negative potential displays four reversible waves corresponding to the reduction of the NDI fragment at E1/2 = -0.50 V/SCE and -0.93 V/SCE, and for the reduction of the terpyridine ligands at E1/2 = -1.24 V/SCE and -1.53 V/SCE. These potentials are slightly shifted to less negative potential compared to that of monomer in solution. Similar behavior is observed for a Ru-NDI-modified gold electrode (Appendix 5.1. By changing the electrochemical window during grafting, the thickness of Ru-NDI, measured by AFM, can be tuned from a monolayer (around 3 nm, see

3).

Scheme V-1. PF6-concentration controlled by polarization at different potentials.

As we mentioned above, Ru-NDI deposited on a substrate has four reduction waves, where two at -0.5 and -0.95 V/SCE belong to the NDI unit and the others belong to the terpyridyl unit. Hence, in the next part we will study the effect of NDI reduction on the number of PF6 -counter-ions in the Ru-NDI films. As illustrated in Scheme V-1, when the NDI unit is reduced first at -0.5 V/SCE, it becomes negatively charged: NDI -. Then, to compensate the charge of the Ru-NDI molecule, we expect to lose some of the PF6 -. The theoretical atomic ratio of P/Ru at the beginning is 2 and after reduction at -0.5 V/SCE, the atomic ratio of P/Ru is expected to decrease to 1. When the Ru-NDI film is reduced at -0.95 V/SCE, the NDI unit is reduced to NDI 2-, which may lead to the decrease in the P/Ru atomic ratio from 2 to 0.

In order to explore the above assumption, three Ru-NDI-functionalized Au substrates were prepared with the same potential range [0:-0.3 V vs. SCE] and then two of them were polarized at different negative potentials in solution by using chronoamperometry.

One sample was polarized at -0.5 V for 30 seconds, which is close to the first reduction potential of NDI, and another at -0.95 V for 30 seconds (close to the second reduction potential of NDI). Then XPS was performed on all 3 samples, including a fresh one without polarization. The high-resolution spectra of Ru 3d, N 1s and P 2p are shown in Figure V-11a.

Figure V-11. (a) High-resolution spectra of C 1s-Ru 3d; N 1s and P 2p of Ru-NDI functionalized on Au substrate at different condition (b) Atomic ratio estimated from XPS after polarization at different potentials.

When the NDI unit is reduced the Ru and N signals are stable. However, the P 2p signal decreases and finally disappears after reduction at -0.95 V/SCE. The Table in Figure V-11b presents the atomic ratios of F, P, N, and Ru. The crucial observation is that P/Ru can be varied from 2.17 to 1.03 and eventually to zero, when no PF6 -is detected by XPS. The XPS data clearly indicate that the amount of PF6 -counter-ion in the molecular layer can be easily controlled by changing the potential applied to the modified electrode and reducing NDI, as illustrated in Scheme V-1.

To conclude this section, we found that Ru-NDI can be electrografted onto substrates, and that the electroactivity of the modified substrate is similar to that of Ru-NDI in solution. More interestingly, we are able to control the concentration of PF6 -in the Ru-NDI film by reducing the NDI unit at different potentials. The P/Ru ratio changed from 2 to 1 when it was reduced at -0.5 V/SCE and finally to zero if it was reduced at -0.95 V/SCE.

Transport controlled by redox gating or PF6 -concentration

In this second part, we will study the electrical properties of Ru-NDI MJs and also explore the effect of RU-NDI redox state and PF6 -concentration on these properties. Largearea Ru-NDI MJs (Figure V

(b) JV curves of Ru-NDI 3nm and (c) Ru-NDI 8nm MJs with P/Ru ratio ~ 2. (d) Rectification ratio Histogram of Ru-NDI 3nm (red) and Ru-NDI 8nm MJs (blue) taken at -2.5 V.

Current density vs. applied voltage (JV) curves are presented in Figure V-12b-c. when the thickness (2.9 nm measured by AFM) of the MJs is close to the length of the molecule, suggesting the formation of a monolayer of Ru-NDI. A good yield in operating junctions is reached (80%, 32 out of 40 junctions worked) showing that the monolayer is robust enough to enable MJ creation despite the fact that the top electrode is directly e-beam evaporated. JV curves are asymmetric with a higher current density (up to 10 A.cm -2 ) when the bottom electrode is negatively polarized at a moderate potential of -2.5 V. Moreover, RRs of 300 were measured but decreased as the Ru-NDI thickness increased (Figure V-12b andc, d). More precisely, RR decreased from 300 for Ru-NDI3nm to 30 for Ru-NDI8nm. This behavior is the opposite of that observed in organic -conjugated molecules, where high RRs were obtained with thick films (above 5 nm) while at tunneling distance rectification is very low.

Note that MJs based on SAM consisting of an alkyl chain and a ferrocene unit with a EGaIn top contact show giant rectification of 10 5 at 3 V with low current densities of 0.1 A.cm -2 . 42 In our present study RR is only 300 but the current density is as high as 10 A.cm -2 at -2.7 V. Such a high conductance may be important in applications.

Overall, large-area solid-state devices based on Ru-NDI monolayers associate robust RR with high conductance, and are among the best molecular rectifiers fabricated by fully CMOS-compatible technology. These results were recently published (DOI: 10.1039/c7cc04972b).

Next, we explored the effect of the Ru-NDI redox state and of the counter-ion on the electrical properties of Ru-NDI MJs. To do so, we fabricated MJs with different PF6 -contents. We first deposited Ru-NDI on Au stripes using similar conditions to the above, then the modified stripes were polarized at two different potentials, -0.5 and -0.95 V/SCE, as mentioned above. The P/Ru atomic ratio is close to 2 for the fresh Ru-NDI Au stripe while it is around 1 for the modified stripe polarized at -0.5 V/SCE, and that polarized at -0.95 V/SCE did not show any signal for phosphorus.

First of all, Ru-NDI MJs with P/Ru ratios of 2 and 1 were both fabricated in high yield (above 80%) where the same MJs without PF6 -(when the film was polarized at -0.95 V) had a very low fabrication yield, as almost all junctions showed short-circuit behavior. This shows clearly that this polarization changes the morphology of the layer and probably creates holes inside it. As a consequence, metallic filaments are created during e-beam top electrode deposition. The current densities of the curves depend significantly on the PF6 -/Ru(II) ratio. For example, at -2.5 V, the current density of Ru-NDI3nm MJs with a ratio of 2 is 10 A.cm -2 which is roughly 30 times that of the same MJs with a ratio of 1. In other words, the conductance decreases considerably when PF6 -is removed. A similar tendency is observed in Ru-NDI8nm MJs before and after polarization at -0.5V/SCE. The current density goes from 0.6 A.cm -2 to 0.0125 A.cm -2 (roughly 50 times lower). These results seem contradictory since, after polarization at -0.5V/SCE, we expect the Ru-NDI film to be less compact. This might induce metallic penetration in the film and thus higher conductance or even a short circuit (as found when NDI was reduced at -0.95 V/SCE). Therefore, the current decrease suggests that after polarizing at -0.52 V/SCE, the Ru-NDI film is still well organized, and the change in the PF6 - concentration, associated with the open shell structure of the reduced Ru(II)-NDI -, is responsible for the current decrease. Moreover, we also found that the RR of Ru-NDI3nm MJs can be varied by changing the PF6 -/Ru(II) ratio inside the molecular layers: RR falls from 300 to 20 when PF6 -/Ru(II) goes from 2 to 1.

Figure V-13. JV overlay of several Ru-NDI 3nm MJs with (a) P/Ru ratio ~ 2, (b) P/Ru ratio ~ 1 and JV overlay of several Ru-NDI 8nm MJs with (c) P/Ru ratio ~ 2, (d) P/Ru ratio ~ 1.

To conclude, we found that MJs incorporating a monolayer of Ru-NDI can be fabricated in high yield and show medium rectification with RR around 300 at -2.5 V and a high current density of 10 A.cm -2 . We also note that rectification is observed with films as thin as 3 nm and that RR decreases as the thickness of the layer increases. Moreover, RR can be varied by just changing the redox state of the NDI moieties and, consequently, the PF6 -/Ru(II) ratio in the molecular layer. The decrease in the PF6 -concentration can lead to a change of the electrostatic field inside the MJs, which can then decrease the injection rate of the major carrier. We emphasize that this is the first evidence that the conductance of MJs can be modified by ex situ redox gating of the molecular layer. Temperature measurements, the study of solvent and environmental effects, and theoretical calculations are still needed for us to understand the transport mechanisms more thoroughly.

Light Emission Based on Ru(by3) Molecular Layer

Finally, some experiments on Ru(bpy)3(PF6)2, performed in Alberta, shed some light of a new aspect of Ru(bpy)3 MJs and are reported here at the end of this thesis. We notice here that the Ru(bpy)3 compound is exactly the same as Ru(by3) molecules. Ru(bpy)3(PF6)2 is known to have interesting photochemical properties and is widely used in electrochemiluninescent systems. In general, light emission of Ru(bpy)3(PF6)2-based electrochemical systems and solid-state devices 20,32,33,43,44 with thicknesses > 70 nm is due to the generation of Ru +3 and Ru +1 centers followed by recombination to form excited Ru(bpy)3 +2 * which emits light (Scheme V-2). In other words, electrochemiluminescence in such systems is due to redox events in the electrochemical cell or in thick-film solid-state devices incorporating these species.

Scheme V-2. Light emission induced by electrochemical reaction of Ru(by3).

In this section, we will investigate the photoemission properties of solid-state Ru(by3) MJs. We wish to probe redox events occurring in thin MJs and to establish whether the transition thicknesses observed in these MJs (Figure V-4b) is indeed the thickness at which redox events start to occur. Note that emission is weak for d < 4 nm and a clear threshold for emission occurs at V = 2.7 V. A similar threshold has been reported for light emission in asymmetric solid-state Ru(bpy)3 devices with d > 80 nm. [31][32][33]44,48 Light emission is weak for d < 4 nm, and attributed to hot electrons crossing the molecular layer and emitting light at the top contact. However, when d > 4 nm, Ru(by3) devices exhibit very different behavior, both in the magnitude and attenuation of the current density with thickness (Figure V-15c) and by a dramatic increase in light emission. The clear threshold for light emission from Ru(by3) MJs at V = 2.7 V (Figure V-15c) occurs when the bias exceeds both the difference in the standard electrochemical potentials for Ru(bpy)3 +3/+2 and Ru(bpy)3 +2/+1 (2.62 V) and the optical gap determined from UV-Vis absorption of the Ru(by3) film on e-C (2.68 V). The electrochemical mechanism previously presented in Scheme V-2 may apply to the present devices. Light emission is slightly stronger in vacuum (compared to air) and is visible at 7 K, thus providing evidence against ionic motion required for conventional electrochemistry. The much shorter response time of the current devices (<5msec) compared to reported Ru(bpy)3 light-emitting electrochemical 43,49,50 and solid-state 33,44 cells (sec to min) may result from the absence of required ionic motion to establish a double layer. In a formal sense, electron injection into the Ru(by3) LUMO produces a Ru+1 center and hole injection produces a Ru+3 center analogous to what occurs in electrochemical cells, but such designations implicitly assume that reorganization occurs to complete a redox event. Preliminary experiments on temperature dependence indicate that light emission for d = 10.8 nm is visible at 7 K, and the Arrhenius slope of LnJ vs. 1/T is < 1 meV in the range of 7 to 70 K. Therefore, reorganization is not required prior to transport, but may occur during or after electron transfer.

An alternative mechanism for light emission, which was presented in detail in Ushula's paper, 6 may be considered. In short, strong light emission occurs when the bias exceeds 2.7 V; this indicates that there must be injection from both electrodes into the Ru(by3) film. Note that the high electric field across the molecular layer (4 MV.cm -1 ) makes resonant injection possible for both electron and hole that can tunnel. This high electric field can also drive transport of the resulting eˉ and h+ to the approximate center of the Ru(by3) film, where recombination can occur with light emission.

In conclusion, light emission in symmetric eC/Ru(by3)/e-C molecular junctions provide direct evidence for a bipolar charge-injection mechanism which occurs when the molecular layer thickness exceeds 4 nm and the bias exceeds 2.7 V, and demonstrates that redox events occur in such MJs. In the context of this PHD, this last result is highly significant as it is the first direct experimental indication that the transitions in the attenuation plots obtained within this work are linked to redox events inside the junctions which are turned above the transition thicknesses.

Robust light emission for at least several hours may become a useful feature for MJs, for the diagnosis of mechanisms and practical applications. To get a better understanding of the mechanism of light emission, several compounds are being investigated, for example, an Ir(III) complex and Ru(tpy2), consisting of Ru 2+ and terpyridinyl ligands.

Conclusions

In this chapter, we studied the electrografting of the organometallic molecules onto gold substrates and investigated the electrical properties of organometallic MJs fully fabricated by CMOS technology. Several important results have been obtained. First, we have shown that the electroreduction of organometallic diazonium salts does not affect the structure of the molecule and allows the modification of a gold electrode surface. XPS measurement and the electroactivity of films grafted onto substrates confirm that such organometallic compounds are chemically stable and retain their properties.

Second, we have shown that organometallic MJs can be fabricated in high yield (above 90%) with CMOS-compatible procedures. We found that Co(tpyr2) MJs exhibit a very interesting behavior including a very low  value (~ 0.17 nm -1 ), high current density and also no transition thickness. We propose that the mechanism of charge transport in Co(tpyr2) MJs is, at low temperature, almost on-resonance tunneling at long distance (from 2 nm to 13 nm). We also found that for Ru(by3) and Ru-C MJs two different transport regimes can be observed as the thickness increases. The value of the transition thickness is a clear molecular signature on transport and the conductance of the MJs are strongly dependent on the molecule when the thickness of the layer is above the transition thickness. However, the attenuation factors in this regime are rather small (β ~ 0.3 nm -1 ) and do not depend strongly on the molecular structure of the layer. Above the transition point, we also showed that the conductance of the MJs is strongly correlated with the energy gap between the HOMO and LUMO levels. In other words, when the gap is decreased, then the MJs become more conductive, which clearly indicates that current is not limited by charge injection at the interfaces. Interestingly, for thicknesses below the transition point, the conductance of MJs is similar, and no molecular signature is found.

Third, we have found that Co(tpyr2) and Ru-C MJs do not show rectification whereas Ru(by3) MJs show a small RR and Ru(tpy2) MJs have a significant RR, around 150 at -2.5 V. The absence of rectification in Co(tpyr2) and Ru-C MJs can be attributed to the fact that the HOMO lies between the Fermi levels of the two contacts. This observation is consistent with the results of Chapter 2 and Chapter 3.

Fourth, we have developed new Donor-bridge-Acceptor chemical structure incorporating organometallic moieties. A Ru-NDI structure was designed with an electronaccepting unit covalently bonded to a ruthenium bis-terpyridine complex. The structure also features a phenylamine terminal group in order to enable later in situ preparation of a diazonium cation and subsequent grafting onto various substrates. We have been able to graft monolayers and thicker films onto gold. Monolayer-based MJs were fabricated in high yield and were shown to withstand high current density and to rectify current (RR around 300 at -2.5 V with high current density of 10 A.cm -2 ). Moreover, we have shown for the first time that the conductance and the RR of MJs can be adjusted by changing the redox state and the ionic strength in the molecular layer. The decrease in the PF6 -concentration associated with NDI reduction at -0.5V/SCE leads to a dramatic decrease in conductance. This can be attributed to the change of the electrostatic field in the MJs or to a change in the electronic structure of Ru-NDI.

Lastly, we have shown that Ru(by3) MJs emit light when the layer thickness exceeds 4 nm and the bias exceeds 2.7 V. This observation provides direct evidence for a bipolar charge injection mechanism and demonstrates that, above the transition thickness of the attenuation plot, redox events occurs in Ru(by3) MJs at 2.7 V.

All in all, this Chapter has demonstrated that these organometallic molecules have a huge potential in the field of molecular electronics and constitute a type of system that can be used to explore new functionalities in MJs.

5

Supporting Information

Electrochemical deposition

The corresponding diazonium salts of 1-(2-bisthienyl)-4-aminobenzene (BTB), cobalt complex (Co(tpy2)) and Ru-C, Ru(by3), Ru(tpy2) and Ru-NDI were generated in situ by adding tert-butyl nitrite (Aldrich). The number of equivalents of tBuNO 2 depends on the working molecule, as shown in Table S 2. To perform electrochemical deposition, t-butyl nitrite was added to 10 mL of 0.5 mM amine precursor in 0.1 M of electrolyte (TBAPF 6 ) under argon flow. After 5-minute incubation, cyclic voltammetry was used to generate the radical species that electrografts onto the electrode. The modified electrodes were characterized in ACN solution containing only electrolyte 0.1 M TBAPF 6 under argon flow. The thickness of each layer was measured by AFM. To control the thickness of each layer, the electrochemical conditions of cyclic voltammetry were carefully adjusted.

Table S 2. Electrochemical conditions to generate a radical from an amine precusor.

Solution (5×10 -

AFM measurements on Ru-NDI film

Electrical measurements on Co(tpyr2) MJs

CONCLUSIONS AND PERSPECTIVES 1 General Conclusions

In this PhD, I have explored charge transport in large area molecular junction with Au & Ti/Au contacts and more precisely "the room in the middle" which occurs when the thickness of a molecular layer is in between 5 and 20 nm. Do we have strong effects of the molecular structure of the layer on the transport properties of the MJs? can we generate new functionalities using specific molecules? can we use redox events and ion motion within these layers to generate such functionalities? were the main initial questions of this work.

In Chapter 2, I have reported that the electronic behavior of single component MJs with thicknesses above direct tunneling, using two contacts with different workfunctions, strongly depends on the structure of the molecular layer. This is in marked contrast with that observed when off resonance direct tunneling is the dominant transport mechanism and underlines the much stronger effect of the molecular structure of the used layers when their thicknesses in above 5 nm. A large panel of electronic behaviors was observed. When oligo(bisthienylbenzene) is used the devices exhibit robust and highly reproducible RRs above 1000 at low voltage (2.7 V) and high current densities. This set of characteristics, at the beginning of the PhD, was not reported in the literature and remains among the best results reported to date. Moreover, we showed that the rectification ratio can be adjusted by 5 orders of magnitude by just changing the molecules and observed with viologen-based MJs unprecedented (for organic based MJs) highly efficient long-range transport which we can likely be attributed to resonant tunneling. The effect of electronic strength at the top interface was also investigated and we showed that it has a strong impact on the transport properties of the MJs. When weakened by inserting an intermediate insulating layer (AlOx or C6H13 terminal chain for VIO-C6) the JV curves became asymmetric and the RR of VIO-C65nm MJs at -2.5 V reaching 200 with relatively low current density (~0.02 A.cm -2 ).

In Chapter 3, I have studied bilayer molecular junctions. The bilayer was deposited by two strategies: the diazonium and overgrafting approach. The overgrafting approach based on the irreversible electrochemical oxidation of aromatic monomers on a first BTB layer generates a new uniform layer with a new switching potential. The two layers are thus in an extremely high electronic coupling situation and behave as a new material. The diazonium strategy also provides a versatile, powerful and robust way to generate bilayers and more generally multi-layer systems; the order of the layers can be easily changed, and they are only weakly coupled. This is a crucial difference compared to the results obtained using the overgrafting approach. The addition of a second layer on top of the BTB layer by overgrafting changes the frontier orbitals of the new layer. Adding few EDOTs moieties on BTB oligomers raises the HOMO energy of the new material and decrease the energy barrier with the fermi level of the contacts. This has a dramatic impact on transport in BTB-EDOT MJs. It is activationless associated with a small decay constant of 0.3 nm -1 and a very high Jo value of 3A.cm -2 . As an exciting result, transport in such device appears to be close to on-resonant tunneling but no rectification is observed despite the use of asymmetric contacts. On the contrary, for the bilayer devices based on the diazonium approach, adding the second layer always dramatically reduces the conductance compared to single-component MJs. In all cases, the current density decrease can be attributed to the weak electronic coupling between the two layers. On the other hand, rectification always appears when using such systems, does not depend on the electrode used or on the fact that corresponding single-component devices rectify or not.

In Chapter 4, I have investigated mixed molecular junctions with thicknesses from 2 to 5 nm and compared their behavior to that of single-component BTB MJs. The temperature dependence shows that charge transport occurs by a direct tunneling mechanism. We found that the β value of mixed BTB@β-CD MJs at 0.5 V is 2.1 nm -1 which is the same as that obtained for BTB MJs and interestingly, the Jo estimated for BTB@β-CD MJs is 20 times lower than that for BTB MJs, that clearly indicates that inserting β-CD into the BTB layer reduces the number of 'active' channels for charge transport. Then, we investigated the effect of filling the free holes left by the removal of β-CD with second component (BTB). We found that the electrical properties of the mixed-component BTB@BTB and BTB MJs are identical.

In Chapter 5, I have studied the electrografting of various organometallic molecules onto gold substrates and the electrical properties of organometallic-based MJs. Then, I have compared its electrical properties to those observed in π-conjugated based MJs with the same configuration. Many important results, never or rarely observed in literature were obtained.

Firstly, I have found that Co(tpyr2) MJs exhibit a very interesting behavior including a very low β value (~ 0.17 nm -1 ), high current density and no transition between transport mechanisms as the thickness increases. We propose that the mechanism of charge transport in Co(tpyr2) MJs is, at low temperature, almost on-resonance tunneling at long distance (from 2 nm to 13 nm). Note that on-resonant tunneling, rarely observed in large area MJs before, but within this PhD, it is observed with three different molecules including Viologen, BTB-EDOT and Co(tpyr2). I also found that for Ru(by3) and Ru-C MJs, two different transport regimes can be observed as the thickness increases. The value of the transition thickness is a clear molecular signature on transport and the conductance of the MJs are strongly dependent on the molecule when the thickness of the layer is above the transition thickness. Above the transition point, we also showed that the conductance of the MJs is strongly correlated with the energy gap between the HOMO and LUMO levels. We also showed in collaboration with McCreery group that Ru(by3) MJs emit light when the layer thickness exceeds 4 nm and the bias exceeds 2.7 V. This observation provides direct evidence for a bipolar charge injection mechanism and demonstrates for the first time that, above the transition thickness of the attenuation plot, redox events occurs in Ru(by3) MJs.

Thirdly, we have developed new Donor-bridge-Acceptor chemical structure incorporating organometallic moieties, Ru-NDI molecule. I have been able to graft monolayers and thicker films onto gold. Ru-NDI Monolayer-based MJs were fabricated in high yield and were shown to withstand high current density and to rectify current (RR around 300 at 2.5 V with high current density of 10 A.cm -2 ). Moreover, we have shown for the first time that the conductance and the RR of organometallic MJs can be adjusted by changing the redox state and the ionic strength in the molecular layer. The decrease in the PF6 - concentration associated with NDI reduction at 0.5V/SCE leads to a dramatic decrease in conductance and a clear tuning of the rectification properties. This can be attributed to the change of the electrostatic field in the MJs or to a change in the electronic structure of Ru-NDI but remains to be fully understood.

Overall, most of the initial questions raised at the beginning of the PHD have been answered or partially answered. The "room in the middle" has been largely explored and molecular effects in 5-20 nm thick MJs are extremely important and makes it possible to generating new functionalities. Redox events and ion motion within these layers have been clearly demonstrated and are among the key parameters to generate new functionalities. I remain slightly frustrated by the fact that these redox events and ion motion in the MJs remains not quantitively investigated at the end of this work.

Perspectives -Toward Functional Molecular Junction

The results of the PhD thesis clearly show the 'molecular signature' in 5 to 20 nm thick molecular junction. The understanding in asymmetric molecular junction gained in the PhD thesis give us opportunities to go toward functional molecular junction for the continuation of this work. Several possibilities can be considered as discussed below.

Low Cost Rectifying Molecular Devices

Rectifying devices is used to transform AC signals into DC ones as defined in Chapter 1. It is generally assumed that organic rectifying devices are limited to low frequency operation. As mentioned in Chapter 1, molecular rectifier with the thickness below 20 nm is very promising to obtain the cut-offs frequency up to GHz. The molecular rectifiers observed in Chapter 2 and Chapter 3, in principle, can be used to fabricate rectifying molecular devices where the thickness of such rectifying molecular devices is below 20 nm. We could predict that such rectifying molecular devices have a high cutoffs frequency. In addition, the molecular rectifiers formed in Chapter 2 and Chapter 3, can be used to fabricate a flexible molecular rectifier which could be incorporated into wearable electronic devices. These two points needs to be investigated soon.

Ionic Molecular Junction

Incorporating ions inside MJs was one of the main objective of the PhD work and we have studied it at certain extent in Chapter 5. Recently it was demonstrated that introducing Li+ ions into MJs produces a multifunctional MJs with rectification and memory effect. Introducing ion motion accompanying redox reactions, is clearly a new concept in molecular junctions that needs to be developed. 6 The success of two step diazonium reduction for mixed molecular junction fabrication in Chapter 4 and the counter ion effect investigated in Chapter 5 are a very good strategy to go toward multifunctional molecular junctions. A first example that could be developed is shown in Figure P-1a, a mixed layer contains electroactive channels (BTB unit) and adjacent electro-inactive channels with ionic groups (for instant SO3 -Na + ). In doing so, we may clearly see the effect of ion motion on the redox event occurring in active channel without changing the electronic state of the active layer. Second example is presented in Figure P-1b, a mixed layer consists of p-dopable channels (BTB unit) and n-dopable channels (VIO-C1 unit), these channels are in parallel with each other. During biasing, each layer can be simultaneously oxidized and reduced, hence light emission or large photocurrents may be observed. Bilayer strategy in Chapter 3 could be used to investigate memory or even battery in such ultrathin layer. For instance, one systems containing two successive p-dopable and n dopable layers, clearly separated by an intermediate layer (alkyl chain or PEG chain) could be used for this purpose.

Planar molecular junction

Planar molecular junction opens many new possibilities, not investigated in this PhD.

Planar molecular junctions can be fabricated by combining e-beam lithography and plasmon induced diazonium reduction. By using e-beam lithography, the width of an electrode can be sinking down to 100 nm as seen in Figure P-2a. In this case, a nanoelectrode with several µm long will exhibit localized surface plasmon in one direction perpendicular to the nanoelectrode. Our groups have investigated surface grafting from diazonium solutions triggered by localized surface plasmon. 4 We show that an organic layer, 20-40 nm thick, is easily grafted onto nanoobject by light illumination (Figure P-2b). Imagine two nanoelectrodes in parallel with each other with a gap between these nanoelectrodes less than 50 nm, then localized surface plasmon of the nanoelectrode can induce reduction of diazonium salt, graft molecules in the nanogap and bridge both electrodes. In doing so, we will be able to fabricate planar molecular junction using a new technique A first possibility is to study the effect of hot carriers generated by plasmon on the conductance of planar MJs. By changing the nature of molecules, from p dopable into n dopable molecule, we may expect to address several important scientific questions such as What is the effect of molecules on the conductance of MJs when illuminating light? Is it hot hole or hot electron? when hot carriers are generated?

A second possibility is to study the magnetoresistance of MJs when using paramagnetic molecular systems such as cobalt complexes. A third possibility is to study practical molecular transistor with a 50nm width between source and drain. In this case, a third electrode as a gate can be used to modulate a redox state of a molecular layer and to study dynamically the effect of the redox state of the molecular layer on mobility of major carriers.

GENERAL APPENDIX -EXPERIMENTAL PROCEDURE

In this part, I will describe the fabrication of molecular devices and the transport measurement. The devices fabrication process was performed in the clean room of the MPQ laboratory, except for the deposition of a molecular layer. The deposition of the molecular layer was done by cyclic voltammetry at the ITODYS laboratory. The transport measurement was performed in our lab and also in TELEM's group (especially for temperature dependence measurement).

1

Au/ Molecules/Ti/Au Device Fabrications

In order to probe electronic transport properties at the molecular scale, we build solid state planar junctions in a cross-bar geometry where a thin layer of molecules is contacted to two metallic electrodes (see FigureA-1). By applying a voltage bias, this configuration allows to generate a current flow through the device and gives access to the I(V) or JV.

Figure A-1. Schematic profile view of a solid state planar junction reprensented with a voltage drop applied between its extremities

The fabrication of the molecular junctions requires three successive steps: the fabrication of the bottom electrode, the electrografting of the molecular layer and the deposition of the top electrode. I describe step-by-step below the optical lithography and the electron beam metallic evaporation processes used to build the electrodes.

Fabrication of the bottom electrode

The fabrication of the bottom electrode requires three main steps including cleaning SiO2 substrate, optical lithography to generate the pattern and finally metal deposition and liff-off as illustrated in FigureA-2.

Step 1: Cleaning SiO2 substrate

The samples are all designed on Si/SiO2 wafers where the 500 nm thick Si oxide layer makes the substrate completely insulating. We clean and rinse them in an acetone bath (5 min) and an isopropanol solution (5 min) before drying and inserting them in the plasma O2 (1atm) machine for 2 min. These cleanings enable to get rid of every grease stains and dusts that could remain on the surface of the substrate. Step 2: Optical lithography Optical lithography process consists in recovering the substrate with a thick layer of photoresist exposed then to a UV beams (λ = 365 nm) through an optical mask where the pattern of the electrodes is drawn with chromed and un-chromed zones. All the samples are made using image reversal resists in a negative way. Then, samples are developed a few seconds (17 s) in base solutions (AZ400) to generate the pattern.

Step 3: Metal deposition and liff-off Metallic evaporation process was done by an electron beam evaporator (Plassys MEB 550S). A filament is strongly heated until electrons are given enough kinetic energy to be expelled and accelerated (10 keV) towards a crucible full of the metal bar. Local heating allows the crucible to reach its sublimation temperature and to evaporate metal atoms towards the sample's surface. The sample is pumped in a base pressure (10 -8 mbar) to enhance the purity of the deposition chamber. We first deposit a very thin (2 nm) Ti adhesion layer on the substrate (rate 0.02 nm/s) and then deposit the 45 nm Au layer (rate 0.05 nm/s). Deposition rates are chosen to limit the kinetic energy of the metallic atoms and thus their possible diffusion inside the molecular layer that could favor the formation of metallic filaments short cutting the device. After evaporation process, liff-off was done in acetone to remove all the resist. The figure of Au stripe electrode as bottom electrode is in

Electrochemical Deposition

For electrochemical experiments, a conventional three-electrode cell and a CHI 760C potentiostat (CH Instruments, Austin, TX) was employed. The auxiliary electrode was a platinum grid, and a SCE (3 M KCl) was used as reference electrode. Working electrodes were glassy carbon (GC) discs (area = 0.07 cm2) purchased from CH Instruments or Au stripe electrodes (20 µm width and 1cm length) which are fabricated by e-beam evaporation. Before use, glassy electrodes were polished with 3 µm then 1 µm diamond paste (Struers, Denmark), ultrasonicated in analytical grade acetone for 3 min and rinsed with acetonitrile. For Au stripe electrode, big square pad is used to connect to the potentiostat.

Au/molecules/Ti/Au Devices

The cross-bar MJs (20*20 µm 2 ) were fabricated using the same procedure for the bottom electrode. Firstly, a first photolithography and evaporation are used to fabricate Au stripe bottom electrode, a width of 20 µm and roughly 1cm long as shown in Figure A-3a. After an electrochemical deposition of an organic layer, a second photolithography and then a second evaporation were performed to generate the top electrode, Ti/Au top electrode, to complete the MJs as shown in Figure A-3b. This procedure was also reported details in Chapter 2 as well as several publications in our group. Careful control of the back pressure during Ti deposition is very important, pressure of 10 -8 mbar was used to reduce oxidation from the residual oxygen and water during metal deposition.

Figure I- 1 .

 1 Figure I-1. Scheme of cross-bar Metal/Molecular/Metal junction.

Figure I- 1 .

 1 Figure I-1. (a) Self-Assembly Monolayer Approach. Figure adapted from reference [ 6 ] (b) Diazonium electroreduction.

Figure I- 2

 2 Figure I-2 Mixed layer based on nanosphere strategy and diazonium reduction. Figure adapted from reference[ 26 ]

Figure I- 3

 3 Figure I-3 a, Assembly on the gold surface of the MPTP/MB 1:1 'platform' SAM. b, Coordination of the MC to the MPTP ligand. c, Coordination of the TPT ligand. Processes b and c are repeated iteratively to increase the length of the MCMWs by one metal-ligand unit up to the desired dimension (up to more than 40nm). Figure adapted from reference [ 34 ].

2e 2 /

 2 h (Figure I-5a) and a histogram constructed from ∼1000 conductance curves shows pronounced peaks at 1Go, 2Go, and 3Go (Figure I-5b). These values are obtained when the contact is reduced to a chain of Au atoms (inset Figure I-5a).

Figure I- 5 .

 5 Figure I-5. Molecular junctions formed by scanning tunneling microscopy break junctions technique. (A) Conductance of a gold contact formed between gold STM tip and gold substrate. (B) A corresponding conductance histogram constructed from 1000 conductance curves as shown in (A). (C) When the contact shown in (A) is completely broken, corresponding to the collapse of the last quantum step, a new series of conductance steps appears if molecules such as 4,4' bipyridine are present in the solution. (D) Conductance histogram obtained from 1000 measurements as shown in (C). Figure adapted from reference[ 38 ].

  Among several techniques used to fabricate molecular junctions (Figure I-7), the mechanically controlled break junction (MCBJ) technique (Figure I-7a) was initially developed by Moreland and Ekin in 1985, then first used by Reed et al. to investigate the electrical properties of single-molecule junction in 1997.

71

 71

Figure I- 8 .

 8 Figure I-8. Large Area MJs fabricated by: (a) PEDOT:PSS top electrode, (b) transfer printing of a top electrode by a stamp, (c) transferred top electrode, (d) EGaIn liquid electrode. Figure adapted from reference [ 46,69,70,72 ]

  Figure I-10 shows the semi-log plot of resistance (R) versus molecular length.

Figure I- 10 .

 10 Figure I-10. Measurements of molecular wire resistance with CP-AFM. A gold-coated tip was brought into contact with an OPI monolayer on a gold substrate. The I-V traces were obtained over ±1.5 V for OPI 3 to 10 and ±1.0 V for OPI 1 and 2 at a load of 2 nN on the tip contact. (A) Semi-log plot of R versus L for the gold/wire/gold junctions. Each data point is the average differential resistance obtained from 10 I-V traces in the range -0.3 to +0.3 V. Error bars, 1 SD. Straight lines are linear fits to the data according to Eq. 1. (Inset) A linear plot of R versus L, demonstrating linear scaling of resistance with length for the long OPI wires. (B) Arrhenius plot for OPI 4, OPI 6, and OPI 10. Each data point is the average differential resistance obtained at six different locations on samples inthe range -0.2 to +0.2 V. Error bars, 1 SD. Straight lines are linear fits to the data. Figure adapted from reference [ 98 ].

Figure I- 11 .

 11 Figure I-11. Coulombic potential well in the presence of an electric field illustrating the Poole-Frenkel effect. Figure adapted from reference[ 100 ]

  The concept of a molecular rectifier was first introduced by Aviram and Ratner in 1974. 101 It concerns molecules of the form D-σ-A inserted between two metal electrodes, where D and A are Donor and Acceptor conjugated groups separated by a non-conjugated (σ) insulating group whose effect is to decouple D and A orbitals (Figure I-12).

First,

  McCreery et al recently shown that molecular rectifiers with low rectification ratio (30 at 4V with ) can be used to change the harmonic distribution of an input waveform, providing a useful sound in electronic music. Secondly, molecular rectifiers can be used to transform AC signals into DC ones (see Figure I-13a) and to power devices at distance by converting radiofrequency energy.

  Figure I-13d). Recently, Clement et al. demonstrated 42 that a molecular diode can operate up to 17.8 GHz even though the rectification ratio remained low (~ 5).

  et al. investigated an Acceptor/Donor bilayer C60/Pn, where Pn is pentacene, a donor unit. 116 The layers were physically deposited layer by layer. The bottom electrode was Cu (111) where the STM tip is Cu atom. IV curves of the single-component junction are almost symmetric whereas those of the bilayer junction are asymmetric and RR can reach 1000 at 1 V (Figure I-17a and b).

Figure I- 17 .

 17 Figure I-17. (a) IV curve for C60 and C60/Pn MJs (b) RR vs bias in C60/Pn MJs. (c) Mechanism proposed for Molecular rectifier in C60/Pn MJs. Figure adapted from reference [ 117 ]

Figure I- 18

 18 Figure I-18. (a) Schematic diagram of eC/AQ//BTB/eC MJs (b) Energy level diagram at zero bias (c) at -4 V (d) at + 4 V. Figure adapted from reference [ 118 ]

  Figure I-20).

Figure I- 20 .

 20 Figure I-20. Interpretation of the rectification mechanism for Pt/(SC15 Fc-C≡C-Fc)/EGaIn MJs. For forward bias voltage (Pt grounded, EGaIn negative) and in contrast to reverse bias (Pt grounded, EGaIn positive) two effects contribute to the current: i) electrostatic attraction between (Fc-C≡C-Fc) 2+ and Ga 2 O 3 which increases the number of molecules in contact with the EGaIn electrode;this does not occur for reverse bias where Fc-C≡C-Fc remains neutral; ii) sequencial tunneling which makes that the part of the current the most limiting occurs through the SC15 chain instead of the whole thicknessof the molecule for reverse bias. Figure adapted from reference [ 132 ].

Figure I- 22 .

 22 Figure I-22. Energy alignment (a) with vacuum shift due to dipole moment of molecule (b) strong electronic coupling between HOMO level and the electrode induced Fermi pinning.Figure adapted from reference [3,134,135 ]

  ). All JV curves in the Figures were obtained in air, and each curve presented in the Figures is a representative curve of JV characterization obtained with the MJs fabricated on each sample.

Figure 3 :

 3 Figure 3: a) Atomic percentages (%) vs. Etching time (s) for a Si/SiO 2 /PPF/BTB 9 /Ti 2 /Au 20 . b) High-resolution titanium XPS signal. c) High-resolution carbon XPS signal.

Figure 4 :

 4 Figure 4: a) JV curves for: a) Au/NB 8 /Ti 2 /Au (green) and Au/BTB 9 /Ti 2 /Au (red) MJ; b) for Au/FB 8 /Ti 2 /Au (blue) and Au/BTB 9 /Ti 2 /Au (red); c) for Au/NDI 7 /Ti 2 /Au (gray) and Au/BTB 9 /Ti 2 /Au (red) MJ; d) Rectification Ratio (J(+2.7 V)/J(-2.7 V)) histograms for 63 BTB (red), 21 NB (green), 42 FB (blue) and 26 NDI (gray) junctions

Figure 5 :

 5 Figure 5: Plausible mechanism explaining the large rectification for the Au/BTB/Ti/Au devices, i.e. energy diagram at different applied biases: a) zero, b) positive + 2.7 V: c) negative -2.7 V. Workfunctions of 5.1 eV and 4.3 eV are assumed for a polycrystalline Au surface and a Ti surface, respectively.  1 represents the initial barrier between coupled oligo(BTB) HOMO and

  AFM measurements were performed on a Keysight system (Picoscan 5500). AFM images are recorded in the tapping mode with a tip from Budgetsensor (AFM tap 300 G, resonance frequency 300 kHz, force constant 40 nN.m -1 ). Firstly, blank studies were performed on bare Au stripes (the topography of an Au stripe shown in Figure S-1a), then the statistical height of the image is calculated by SPIP 5.1.3 software, shown in Figure S-1C (black curve) with the Gaussian fitting profile to get a statistical estimate of the organic film thickness on the Au stripe. For example, the topography of the Au stripe functionalized by oligo(NB) is illustrated in Figure S-1b and the height histogram is shown in Figure S-1C (dark yellow curve). In Figure S-1C the estimated NB thickness is 8 nm. Figure S-1D shows the top-view image (5×5 µm) of the Au stripe functionalized by oligo(NB) with the root mean square (Sq) of around 0.8 nm.

  Figure S1. AFM image of (a) Au stripe, (b) Au stripe functionalized by oligo(NB), (c) Height histogram with the Gaussian fitting profile of Au stripe (black) and Au stripe functionalized by oligo(NB) (dark yellow). (d) Image (5×5 µm) of stripe functionalized by oligo(NB) with the rootmean square roughness (Sq) of around 0.8 nm.

Figure S3 .

 S3 Figure S3. JV curve overlays of several MJs: (a) BTB 9nm , (b) NB 8nm , (c) FB 8nm , (d) NDI 7nm .

Figure S8 .

 S8 Figure S8. Electroactivity of NDI-modified carbon electrode in LiClO 4 solution in ACN, scan rate 0.1 V.s -1 .

58 Figure

 58 Figure S9. Plausible mechanism with weak coupling and no Fermi level pinning at the top electrode. The initial potential barrier at the BTB/Ti interface can be easily modified by applying a bias, explaining a smaller large rectification for the Au/BTB/Ti/Au devices compared to Figure 5. i.e. Energy diagram at different applied biases: (a) zero, (b) positive +2.7 V, (c) negative -2.7 V. represents the initial barrier between uncoupled oligo(BTB) HOMO and Fermi level of Ti whereas that between coupled oligo(BTB) HOMO and Au Fermi level remains taken as close to zero.

Figure II- 2 .

 2 Figure II-2. AFM image and statistical height of (a) Au stripe, (b) Au stripe functionalized by VIO-C1 at [0.4:-0.25V] (c) Au stripe functionalized by VIO-C1 at [0.4:-0.3V].

  Figure II-2a, b). In Figure II-2a, the height histograms contain two peaks: the first at around 0 nm corresponds to the silica support, while the second at 40 nm represents the thickness of the Au stripe electrode. AFM was continuously performed on the functionalized Au stripes. The thickness of the molecular layer grafted onto the Au stripe can be extracted by comparing the heights histogram of the Au layer and the functionalized Au layer. As an example, the height histogram of the Au stripe functionalized by VIO-C1 (the corresponding deposition CV is given in Figure S II-3) are shown in red in Figure II-2c,

Figure

  Figure II-4a overlays representative current density vs. applied voltage (JV) curves for each thickness of the VIO-C1 layer. Figure II-4a also reveals a clear influence of the thickness on the electrical properties of the MJs. This dependence is better seen in Figure II-4b which presents the plot of Ln J (at 0.5 and 1 V) versus the thickness of the VIO-C1 layer. Ln J varies linearly with thickness from 3 to 13 nm, which indicates that 𝐽 = 𝐽𝑜 𝑒 -𝛽𝑑 applies, where J and Jo are the current density through the devices and the pre-exponential factor, and β is the attenuation factor. The β value of VIO-C1 MJs is between 0.3 nm -1 at 1 V and 0.25 nm -1 at 0.5 V, as indicated in Figure II-4b. This small β is among the smallest values obtained in large-area MJs based on -conjugated organic molecules. Kolivoska et al.

  5 V are between 50 and 350 with a mean value of 200 (Figure II-5b). The RRs of VIO-C6 MJs also depend on the layer thickness (Figure S II-5), with RRs for MJs with 3 nm and 8 nm layers around 30.

Figure II- 5 .

 5 Figure II-5. (a) Overlay of JV curves of several VIO-C6 5nm MJs (a) Log RR (J(-2.5 V)/J(2.5 V) histograms for VIO-C6 5nm (red), VIO-C1 5nm /AlOx 2nm (blue) junctions.

  Figure S II-1. VIO-C1 calculated by DFT (B3LYP/6-31G*): (a) optimized structure; (b) LUMOorbital.

Figure S II- 3 .

 3 Figure S II-3. CV for VIO-C1 deposition at (a) 5 cycles at [0.4: -0.25]V/SCE, (b) 10 cycles at [0.4: -0.3]V/SCE. (c) root mean square roughness of VIO-C1 9nm film.

Figure III- 1

 1 Figure III-1 Bilayer MJs mentioned in a) Whitesides et al. b) Aswall et al. c) Smerdon. Figure adapted from references [3,7,8]

Scheme III- 2 .

 2 Scheme III-2. Chemical structure of (a) 3,4-ethylenedioxythiophene (EDOT) and (b) 4'-(3,4ethylenedioxythiophene)-2,2':6',2''-terpyridine (EtPy).

Figure III- 3 .

 3 Figure III-3. (a) Response of ferrocene (1 mM) on bare (black), BTB-modified (red) and BTB-EtPy-modified (green) Au stripe electrodes in acetonitrile, LiClO 4 (0.1 M), scan rate 0.1 Vs -1 . (b)Height histograms for bare (black), BTB-modified (red) and BTB-EtPy-modified (green) Au electrodes.

Figure III- 4 .

 4 Figure III-4. (a) Scheme presenting BTB-EtPy bilayer device based on overgrafting procedure. (b) HOMO energy levels of BTB and BTB-EtPy layers calculated from electrochemical data.

Figure III- 5

 5 Figure III-5 shows the JV curves of several BTB-EtPy MJs with different thicknesses. Several important points can be drawn: (i) The yield of the BTB-EtPy bilayer is quite high, 90% junctions working and 10% junctions open-circuit (meaning that no current is measured). (ii) The standard deviation of the current level for these devices is relatively low. This high device yield and small standard deviation suggest that the bilayer is homogeneous and uniform, and that no pinholes are generated during the oxidative step. (iii) The JV curves are asymmetric, and RR varies with the thickness.

Figure III- 5 .

 5 Figure III-5. JV curves of several BTB-EtPy bilayer MJs of different thicknesses; note that the oligo(EtPy) thickness is kept at around 2 nm.

Figure III- 8

 8 Figure III-8 illustrates a possible energy diagram for BTB9nm and BTB-EtPy10nm MJs in the free state, where the workfunctions of Au and Ti/Au electrodes are -5.1 and -4.3 eV,

1 Figure

 1 Figure III-10 presents JV plots obtained with several BTB-EDOT MJs with the same thickness using Au & Ti/Au electrodes. Several observations can be summarized.

Figure III- 12

 12 Figure III-12 presents the LnJ vs. thickness plot of these bilayer devices taken at 1 V using the Au & Ti/Au configuration. The plot for BTB-EDOT MJs based on e-C configuration is shown in Figure S III-4.As illustrated, when the molecular layer is more than 5 nm-thick, the β values of BTB and BTB-EDOT MJs are similar at 0.36 nm -1 and 0.32 nm -1 , respectively. The current densities through BTB-EDOT devices are much higher (400 times) than those for single-component BTB devices. Similar behavior is seen in BTB-EDOT MJs based on e-C electrodes (FigureS III-4). Let us recall here that we have attributed the difference in β for BTB (0.36 nm -1 ) and BTB-EtPy (0.83 nm -1 ) to the higher energy barrier

Figure

  Figure III-13a presents representative JV curves. They are slightly asymmetric and also weakly dependent on the temperature. Figure III-13b presents the LnJ vs. 1000/T plot (Arrhenius plot). The activation energy calculated from this plot at 1 V is around 55 meV (between 100 and 280 K) and is slightly dependent on the potential (Figure S III-5).Below 100 K, the activation energy is just a few meV. Note first that this activation energy is below that of single-component BTB MJs (150 meV between 100 and 280 K). This very small activation energy may be attributed to the barrier between the Fermi level of gold and the BTB-EDOT HOMO. It suggests that this barrier is smaller than that in single-component BTB MJs, in agreement with the electrochemical data (switching potential of the BTB-EDOT layer lower than that of BTB). It also clearly indicates strong electronic coupling between the two layers so that, despite the fact that BTB and EDOT moities are physically separated, they form a new single layer with new electronic properties that are not the superposition of those of each layer.

Figure III- 13 .

 13 Figure III-13. (a) Electrical measurement on BTB-EDOT 16nm with temperature dependence from 5.8 to 280 K (b) Arrhenius plot: Ln J at 1 V vs. 1000/T.

  Figure III-19a depicts a bilayer sandwiched between Au & Ti/Au electrodes, where the first layer is 6 nm of the viologen component (VIO-C1) and the second layer is 6 nm of Co(tpy2) (see for Figure S III-8 the electrochemical condition used). The yield of these bilayer devices is relatively high 90% (61/72 devices tested work).

Figure III- 19

 19 Figure III-19. (a) Scheme of bilayer devices and (b) representative JV curve based on VIO-C1 6nm /Co(tpy2) 6nm when the potential is applied on the bottom electrode.

  interface between Co(tpy2) and VIO-C1. It is illustrated as the pink section in Figure III-20 but its origin remains unclear.

Figure III- 20 .

 20 Figure III-20 . Possible energy diagram at connected state for electron transport through VIO-C1 6nm /Co(tpy2) 6nm bilayer.

  Figure S III-1. Response of ferrocene (1 mM) on bare (red), BTB-modified, BTB-EtPy-modified Au stripe electrodes and BTB-EtPy-modifiedAu stripe electrodes scaned to 1.4 V with LiClO 4 (0.1 M) scan rate 0.1 Vs -1 .

Figure S III- 8 . 8

 88 Figure S III-8. 8 Cyclic voltammograms of VIO-C1 diazonium on Au stripe electrode. (b) 7 cyclic voltammograms of Co(tpy2) at scan rate 0.1 V.s -1 . (c) AFM measurement after each diazonium grafting.

Figure IV- 4 .

 4 Figure IV-4. (a) JV curves for Au45nm/BTB/Ti2/Au45 junctions with molecular layer thickness indicated for each curve; (b) plot of Ln J vs. d (nm) at different bias voltages; (c) the relation between β and the bias voltage. (d) plot of Ln J vs. d (nm) taken at 0.5 V in Au/BTB/Ti/Au and PPF/BTB/Ti/Au MJs.

Figure IV- 5 .

 5 Figure IV-5 . (a) JV curves for Au 50nm /BTB@β-CD/Ti 2 /Au 50 junctions with layer thickness indicated for each curve. (b) Temperature dependence of JV curves for Au/BTB@b-CD 4nm /Ti/Au MJs with insert Arrhenius plot Ln J vs. 1000/T taken at 1 V.

  They are also very close to those measured for singlecomponent BTB MJs, shown in Figure IV-4d and Figure IV-7. As shown by Bowers et al.

Figure IV- 8 .

 8 Figure IV-8. Simplified model to calculate the surface area and the surface concentration of (a) β-CD molecules and (b) oligo(BTB 4 ) wires.

Figure IV- 10 .

 10 Figure IV-10. (a) Height histograms for bare (green) and BTB@BTB-modified (red) Au stripes. (b) JV comparison of BTB@BTB 7nm (blue) and BTB 7nm (green) MJs.

Figure V- 1 .

 1 All the compounds include a terminal aniline group which allows to form the diazonium salt in situ. Ru(by3) consists of a Ru(II) core and bipyridyl ligands while Ru-C contains a new ligand which forms a Ru-C bond and hence the oxidation state of the Ru is formally +1. The Co(tpyr2) complex is based on Co 2+ and the terpyridine ligand; there are two aniline groups since the synthesis method used only allows symmetric compounds when cobalt complexes are generated.

Figure V- 1 .

 1 Figure V-1. (a) Chemical structures of organometallic compounds and (b) cyclic voltammograms of these compounds in solution (5.10 -4 M in ACN with 0.1 M TBAPF 6 ) at scan rate 0.1 V/s.

Figure V-1b

  Figure V-1bpresents the electroactivity of these compounds in ACN. The cyclic voltammogram (CV) of Ru(by3) exhibits three reversible reductive waves at -1.3, -1.6 and -1.8 V/SCE which can be attributed to the reduction of the bipyridyl ligands. The irreversible oxidation peak at 1 V/SCE can be attributed to the oxidation of the NH2 group and the reversible peak at 1.25 V/SCE is due to the oxidation of Ru(II) to Ru(III). For the Ru-C complex, the oxidation peak appears reversibly at 0.5 V/SCE which is 0.7 V lower than that for Ru(by3). This lower oxidation potential can be attributed to the oxidation of Ru(I) to Ru(II). The reduction peak of the Ru-C complex at -1.6 V/SCE and -1.8 V/SCE can be attributed to the reduction of the ligands. The CV of Co(tpyr2) shows a peak at 0.25 V/SCE which we assign to the oxidation of Co(II) to Co(III). We observed a first reduction peak at -0.8 V/SCE which can be assigned to the reduction of Co(II) into Co(I) and reduction of terpyridyl ligand appears at -1.8 V/SCE. The difference between Co(tpyr2) and the others is

  for 5 minutes in an inert atmosphere. The color of the solution changes slightly during this time. The details of the in situ diazonium generation and the electroactivity of each molecule are given in Appendix 5.1. For example, when 10 equivalents of tert-butyl nitrite are added to Co(tpyr2) solution, the solution changes from red to green. The electroactivity of a Co(tpyr2) -functionalized carbon electrode is shown in Figure V-2a and b. The oxidation peak of Co(II) to Co(III) appears at 0.3 V/SCE, which is slightly higher than that in solution, and the reduction peak of Co(II) to Co(I) appears at -0.7 V/SCE, which is slightly lower than in solution. To get information of the chemical composition of Co(tpyr2)-based film, the modified Au substrate was analyzed by XPS.

Figure V- 2 .

 2 Figure V-2 . Cyclic voltammogram (a) at negative potential of Co(tpyr2) in solution (pink) and grafted on carbon electrode in ACN (blue) (b) at positive potential of Co(tpyr2) in solution (pink) and grafted on carbon electrode in ACN (green), scan rate 0.1 V.s -1 . (c) survey XPS of Co(typ2)functionalized gold electrodes and high-resolution XPS spectra of (d) Co 2p and (f) N 1s.

Figure V-3a

  Figure V-3a and b show the JV overlay of 20 different Co(tpyr2) MJs with the thickness noted in the Figure.The JV curves of these devices are almost symmetric, and the standard deviation of the current density is very small, which indicates that the Co(tpyr2) layer is homogeneous and uniform. In addition, the current density in these MJs (1.2 A.cm -2 at 1 V) is just 4 times that of Co(tpyr2)7nm (0.25 A.cm -2 ). This observation is very interesting

Figure V- 3 .

 3 Figure V-3. JV Overlay of several (a) Co(tpyr2)2nm (red), (b) Co(tpyr2)7nm (pink) MJs. Comparison of JV curves of (c) Co(tpyr2)7nm (pink) and Ru-C8nm (blue), (d) Ru-C8nm (blue) and Ru(by3)7nm MJs (purple).

Figure V-3c

  Figure V-3c and d compare the JV curves of organometallic MJs based on different molecules with similar thickness (~ 7 nm). Those of Ru-C8nm and Ru(by3)7nm are overlays of JV curves from 20 different MJs.The standard deviation of the current is very small, which suggests again that the organometallic films are homogeneous, uniform and reproducibly deposited on the various gold stripes used. An important result is that the conductivity of Co(tpyr2)7nm is much higher than that of Ru-C8nm, which is in turn higher than that of Ru(by3)7nm. For instance, at 1 V the current density of Co(tpyr2)7nm (~ 0.25 A.cm -2 ) is 40 times that of Ru-C8nm (0.006 A.cm -2 ) while the current density of Ru(by3)7nm (0.001 A.cm -2 ) is 6 times less than for Ru-C8nm MJs. We note that JV curves are always almost symmetric although the electrodes are asymmetric (Au & Ti/Au).

Figure V- 4

 4 Figure V-4. a) Current-density vs. applied voltage (JV) curves of Co(tpyr2) MJs for several thickness. (b) Attenuation plots taken at 1 V of MJs for different molecular units and slopes (β value).

12

 12

Figure V- 5 .

 5 Figure V-5. (a) JV overlay of Co(tpyr2) 7nm MJs at different temperature dependence. (b)Arrhenius plot of Ln J vs. 1000/T taken at 0.5 V (blue) and 1V(red).

  Figure V-5a shows a dependence of the transport properties on temperature; JV curves are almost symmetric at all temperatures. Figure V-5b

Figure V -

 V Figure V-6a and b show a poor correlation of LnJ with individual orbital energies.However, it seems to correlate very well with the HOMO-LUMO gap with R 2 ~ 0.97. This unique correlation is very significant and applies to a large range of gap energies (from 1 V to 4.5 V). This correlation was already observed for a smaller number compounds and was the basis of the recent proposal of a multi-step tunneling mechanism controlling electron transfer.40 This correlation works also for MJs based on e-C substrate obtained in Alberta in collaboration with McCreery's group (unpublished data). Although there are large differences between e-C substrate (symmetric device) and Au & Ti/Au substrate (asymmetric device), the correlation between the current density and the HOMO-LUMO gap is clearly observed with both contacts. This very good correlation means that the rate of charge transport is controlled by the HOMO-LUMO gap, when the thickness of the layer is above the transition thickness and suggests that the injection rate at the interface does not limit current. Currently several experiments are being performed in order to better understand the underlying meaning of this correlation.

Figure V- 9 .

 9 Figure V-9. (a) Chemical structure of Ru-NDI and (b) Cyclic voltammogram of Ru-NDI in solution (black) and Ru-NDI grafted on carbon electrode at negative potential (red) (b) at negative potential and (c) at positive potential, in ACN with 0.1 M TBAPF 6 scan rate 0.1 V.s -1 .

Figure V- 10 .

 10 Figure V-10 . Characterization of Ru-NDI modified gold electrodes: (a) Survey XPS. (b) C 1s-Ru 3d high-resolution XPS spectra. (c) N 1s High-resolution XPS spectra. (d) Atomic ratio of the layer determined by XPS. (e) AFM topography of the surface 5x5 μm2 (top) and cross-section (bottom) of the modified nanostructured electrode corresponding to the thickness measurement. (f) AFM statistical analysis of the thickness, fitted with gaussian. (g) Schematic representation of the monolayer.

  Figure V-10f) to 8 nm, which indicates multiple layers (Appendix 5.3.2). The survey and high-resolution XPS spectra of Ru and N were measured and are shown in Figure V-10. A characteristic peak at 281.4 eV can be assigned to Ru(II) 3d5/2. The corresponding Ru(II) 3d 3/2 peak at 285.4 eV is masked by the C 1s signal (Figure V-10b). On top of the main C 1s peak at 285.1 eV, several other signals can be attributed to terpyridine ligand CN bonds at 286.3 eV, NDI imide CO bonds at 288.7 eV and a π-π* satellite due to aromatic nuclei at 292.4 eV. The single narrow N1s peak at 400.1 eV in the high-resolution spectra rules out the possibility of azo-bridged grafting (Figure V-10c). Experimental atomic ratios for C, N and Ru reproduce fairly well the expected composition of the layer. The P/Ru ratio (2.17) is very close to the theoretical value (2).

  -12a) were fabricated with Au & Ti/Au electrodes, and Ru-NDI layers of several thicknesses were deposited by controlling the electrochemical conditions used.First of all, we will study several Ru-NDI MJs generated with the same potential range [0:-0.3 V vs. SCE]. Two samples (Au stripes) with different layers of Ru-NDI were fabricated. The Ru-NDI film thickness was measured by AFM and XPS analysis confirmed that the number of PF6 -counter-ions per Ru incorporated in the layer is very close to the theoretical ratio of 2.

Figure V- 12 .

 12 Figure V-12. (a) Device scheme. (b) JV curves of Ru-NDI 3nm and (c) Ru-NDI 8nm MJs with P/Ru ratio ~ 2. (d) Rectification ratio Histogram of Ru-NDI 3nm (red) and Ru-NDI 8nm MJs (blue) takenat -2.5 V.

Figure V- 13

 13 Figure V-13 presents the overlay of several JV curves of Ru-NDI MJs (at least 20 MJs each) with PF6 -/Ru(II) ratios of approximately 2 or 1, indicated in Figure V-13.The current densities of the curves depend significantly on the PF6 -/Ru(II) ratio. For example, at -2.5 V, the current density of Ru-NDI3nm MJs with a ratio of 2 is 10 A.cm -2 which is roughly 30 times that of the same MJs with a ratio of 1. In other words, the conductance decreases considerably when PF6 -is removed. A similar tendency is observed in Ru-NDI8nm MJs before and after polarization at -0.5V/SCE. The current density goes from 0.6 A.cm -2 to 0.0125

McCreery

  et al. reported that MJs based on e-C electrodes have distinct advantages, such as high current density, large area (200 to 500 µm in one dimension) and semitransparency in the visible range. 36 This last point is very important if one wishes to investigate the optical properties of MJs when current is flowing. Because of these advantages, the photoemission of MJs based on Ru(by3) and e-C electrodes was investigated by McCreery's team in University of Alberta in collaboration with our group. The MJ structure, Au30/eC10/Ru(by3)/eC10/Au20, is shown in Figure V-14a and was fabricated in high yield by deposition of a semi-transparent e-C/Au top contact, 36,45 and the Ru(by3) layer thickness was varied from 1.6 to 12.8 nm. The current density vs. bias (JV) curves for Ru(by3) MJs (Figure V-14b) are now symmetric (as opposed to the asymmetric 158 JV curves obtained when Au and Ti/Au contacts are used in Paris, (see Figure S V-11), show a strong dependence on thickness. Attenuation plots show a transition thickness around 4 nm (Figure S V-12) which is slightly greater than that observed when the same layers are incorporated between Au and Ti/Au contacts (see Figure V-4).

Figure V- 14

 14 Figure V-14. (a) Schematic illustration of Cr 4 /Au 30 /eC 10 /Ru(by3)/e-C 10 /Au 20 MJ. (b) JV curves for Cr 4 /Au 30 /eC 10 /Ru(by3)/eC 10 /Au 20 junctions (average of 8 MJs, in vacuum) with thickness (d, nm) indicated for each curve.

Figure V- 15 .

 15 Figure V-15. Light emission spectra from Ru(by3)12nm junctions for negative (a) and positive (b) bias, with Vapp = ±3.0, 3.2, 3.4, 3.6, 3.8, 4.0, 4.2 V. (c) Peak emission intensity normalized to junction current vs. Vapp with d = 3.2, 5.6, 8.3 and 12.8 nm (both +V and -V bias).

Figure S V- 3 .

 3 Figure S V-3. (a) 10 Cyclic voltammograms of Ru-NDI electrografting on Au electrode. (b) Electroactivity of Ru-NDI functionalized on Au surface (red).

  Ru(by3) deposited on Au substrate.

Figure S V- 4 .

 4 Figure S V-4. High-resolution XPS spectra of Ru(by3) films deposited on Au surface. (a) C1s-Ru3d (b) N1s (c) P2p and (d) P2s.

Figure S V- 5 .Figure S V- 6 .

 56 Figure S V-5. Characterization of Ru-C functionalized gold electrodes: (a) Survey XPS. (b) C1s-Ru3d high-resolution XPS spectra. (c) N1s high-resolution XPS spectra. (d) Atomic ratio of the layer determined by XPS. (e) AFM topography of the surface 5x5 μm2 (top) and cross-section (bottom) of the modified nanostructured electrode corresponding to the thickness measurement.(f) AFM statistical analysis of the thickness.

Figure S V- 7 .Figure S V- 8 .Figure S V- 9 .

 789 Figure S V-7. AFM measurements on Ru-NDI film functionalized on Au stripe by different electrochemical condition.

Figure S V- 10 .

 10 Figure S V-10. IV overlay of Co(tpyr2) MJs with different thickness.

Figure P- 1 .

 1 Figure P-1. a mixed layer molecular junction containing (a) the ionic channel and an active channel (BTB unit). (b) the n dopable channel and p dopable channel.

Figure P- 2

 2 Figure P-2 (a) nanoelectrode fabricated by e-beam lithography (b) plasmon induced Nanolocalized Reduction of Diazonium Salts. Figure adapted from reference 7 .

Figure A- 2 .

 2 Figure A-2. Metallic electrode nanofabrication process summary

  Figure A-3a.

Figure A- 3 .

 3 Figure A-3. (a) Picture of the Au stripe electrode (b) SEM Picture of MJs with 20×20 µm2

  /Molecules/e-C have been fabricated based on previous report at Mccreery's group at NINT, Alberta University. Shortly, the bottom electrode Cr3nm/Au30nm/eC10nm was ebeam fabricated by using a physical shadow mask (shown in FigureA-4a and b) where Cr, Au and Carbon were deposited respectively. The top contact electrode was comprised of electron beam deposition of 10 nm eC and 15 nm Au through a physical shadow mask with 250 µm openings oriented perpendicular to the bottom contact, which results in a crossbar junction of 0.125 mm 2 area. Deposition was done in an electron beam evaporator (Kurt J. Lesker PVD75) with typical pressure below 7×10-6 Torr in starting point of deposition. Deposition rate was 0.01 nm/second eC and 0.03nm/second Au. The schematics of the final fabricated chip is shown in FigureA-4c and as indicated in the FigureA-4c, one sample of eC contains up to 20 MJs.

FigureA- 4 .

 4 FigureA-4. Figures of (a) metallic mask for carbon e-beam evaporation (b) each layer of metallic mask (c) molecular junciton based on eC substrates.

Table I -1 . Common electron transport mechanisms in molecular electronics.

 I

  H NMR (400 MHz, CDCl 3 ) δ (ppm): 8.81 (4H, s, CH Arom), 4.21-4.18 (2H, m, CH 2 Hexyl), 1.76-1.70 (2H, m, CH 2 Hexyl), 1.45-1.41 (2H, m, CH 2 Hexyl), 1.37-1.32 (4H, m, CH 2 Hexyl), 0.91-0.87 (3H, m, CH 3 Hexyl); 13 C NMR (100 MHz, CDCl 3 ) δ (ppm): 162.4, 159.0, 133.3, 131.3, 129.0, 128.1, 127.0, 123.0, 41.4, 31.6, 28.1, 26.9, 22.7, 14.2; HRMS (ESI-TOF) m/z: [M + H + MeOH] + calcd for C 21 H 22 NO 6 384.1447; found 384.1435.

4-Aminophenyl)-N'-hexylnaphthalene-1,8:4,5-tetracarboxydiimide (3). 2

  H NMR (400 MHz, CDCl 3 ) δ (ppm): 8.79 (4H, s, CH Arom), 7.57 (2H, d, J = 8.7 Hz, CH Arom), 7.24-7.22 (2H, m, CH Arom), 6.64 (1H, s, NHBoc), 4.23-4.19 (2H, m, CH 2 Hexyl), 1.78-1.72 (2H, m, CH 2 Hexyl), 1.54 (9H, s, CH 3 Boc) 1.46-1.41 (2H, m, CH 2 Hexyl), 1.37-1.33 (4H, m, CH 2 Hexyl), 0.92-0.88 (3H, m, CH 3 Hexyl); 13 C NMR (100 MHz, CDCl 3 ) δ (ppm): 163.3, 162.9, 152.6, 139.3, 131.5, 131.1, 129.2, 127.2, 127.1, 127.0, 126.9, 119.2, 41.2, 31.6, 28.5, 28.2, 26.9, 22.7, 14.2; HRMS (ESI-TOF) m/z: [M + Na] + : calcd for C 31 H 31 N 3 O 6 Na 564.2111; found 564.2098. (160 mg, 0.3 mmol) was treated with trifluoroacetic acid (10 ml) and stirred at room temperature for 30 min. 20 ml of toluene was added and the mixture was washed with saturated NaHCO 3 solution to reach pH ≈ 8. Organic layers were collected, dried over MgSO 4 and the solvent evaporated. Yield, 82% (109 mg). 1 H NMR (400 MHz, DMSO-d 6 ) δ (ppm): 8.69-8.64 (4H, m, CH Arom), 7.57 (2H, d, J = 8.7 Hz, CH Arom), 7.01-6.99 (2H, m, CH Arom), 6.67-6.65 (2H, m, CH Arom), 5.29 (2H, s, NH 2 ), 4.08-4.05 (2H, m, CH 2 Hexyl), 1.69-1.65 (2H, m, CH 2 Hexyl), 1.39-1.30 (6H, m, CH 2 Hexyl), 0.90-0.86 (3H, m, CH 3 Hexyl); 13 C NMR (100 MHz, DMSO-d 6 ) δ (ppm): 163.2, 162.7, 130.5, 130.4, 129.1, 127.0, 126.2, 113.7, 30.9, 27.3, 26.2, 22.0, 13.9; ESI-HRMS (ESI-TOF) m/z: [M + H] + : calcd for C 26 H 24 N 3 O 4 442.1767; found 442.1765, m/z: [M + H + MeCN] + : calcd for C 28 H 27 N 4 O 4 483.2027; found 483.2018.

	N-(

Table S1 .

 S1 Grafting parameters on Ti 2 /Au 50 electrode: voltammogram scan range, number of CV scans, sweep rate, AFM thickness.

	Organic	CV scan range	No. of CV	Scan rate	AFM thickness
	compound	(V/SCE)	scans	(mV.s -1 )	(nm)
	BTB	0.4 -(-0.4)	7	100	9 ± 0.7
	NB	0.4 -(-0.6)	10	100	8 ± 0.8
	FB	0.4 -(-0.6)	15	100	8 ± 0.7
	NDI	0.4 -(-0.35)	10	100	7 ± 0.7

c, Determination of molecular layer thickness by AFM

Part 1 -Control of Rectification in Molecular Junctions: Contact Effects and Molecular Signature 53

Table S .1 Parameters for grafting onto Ti 2 /Au 45 electrode: scan range, number of CV scans, sweep rate, AFM thickness.

 S

	Layer	CV scan range (V	No. of CV scans	Scan rate (mV.s -1 )	AFM thickness
		vs. SCE)
	BTB@β-CD	0.4 :-0.5	4	100	2
	BTB@β-CD	0.4 :-0.5	8	100	3
	BTB@β-CD	0.4 :-0.6	10	100	4
	BTB@BTB	0.4 :-0.4	6	100	4
	BTB@BTB	0.4 :-0.4	10	100	7
	BTB@NB	0.4 :-0.4	8	100	4

http://www.research.ibm.com/cognitive-computing/neurosynaptic-chips.shtml#fbid=PjZ9Nl2HlG3

Cao, L.; Fang, G.; Wang, Y. Langmuir 2017, 33 (4), 980.

The energy barrier here in tunneling regime is different from that calculated in chapter III as the transport is intrachain hopping (above 5nm).

David D.James et al. Nanoscale Horiz., 2018,3, 45-52

Van-Quynh Nguyen et al. ACS Omega, 2017, 2 (5), pp 1947-1955 DOI: 10.1021/acsomega.7b00394

Acknowledgements

i ACKNOWLEDGEMENTS The Doctor thesis has been mainly done in Nano electrochemistry's group (NEC), at ITODYS laboratory, University Paris Diderot-Paris 7. In three years, probably the best moment in my life, I have worked with a great number of people coming from ITODYS laboratory (Interfaces Traitements Organisation et DYnamique des Systèmes), TELEM group (Transport Electronique à L'Echelle Moléculaire) at Laboratoire Matériaux et Phénomènes Quantiques (MPQ) laboratory, staffs in MPQ cleanroom and McCreery's group at University Acknowledgements iii Acknowledgements iv

Thin layers of oligomers with thicknesses between 7 and 9 nm were deposited on flat gold electrode surfaces by electrochemical reduction of diazonium reagents, then a Ti (2 nm)/Au top contact was applied to complete a solid-state molecular junction. The molecular layers investigated included donor molecules with relatively high energy HOMO, molecules with high HOMO-LUMO gaps and acceptor molecules with low energy LUMO and terminal alkyl chains. Using an oligo(bisthienylbenzene)-based layer, a molecule whose HOMO energy level in vacuum is close to the Fermi level of the gold bottom electrode, the devices exhibit robust and highly reproducible rectification ratios above 1000 at low voltage (2.7 V). Higher current is observed when the bottom gold electrode is biased positively. When the molecular layer is based on a molecule with a high HOMO-LUMO gap, e.g. tetrafluorobenzene, no rectification is observed, while the direction of rectification is reversed if the molecular layer consists of naphthalene diimides having low LUMO energy levels. Rectification persisted at low temperature (7 K), and was activationless between 7 and 100 K. The results show that rectification is induced by the asymmetric contact but is also directly affected by the orbital energies of the molecular layer. A "molecular signature" on transport through layers with thicknesses above those used when direct tunneling dominates is thus clearly observed. The rectification mechanism is discussed in terms of Fermi level pinning and electronic coupling between molecules and contacts.

f, Rectification ratio versus applied voltage

Scheme II-1. Chemical structures of the VIO-C1 and VIO-C6 used in this section.

There are some differences between these viologen-based compounds and NDI. Firstly, the reductive peaks of NDI molecules in solution appear at -0.6 and -1.0 V/SCE which is -0.2 V lower than that of the viologens. Secondly, the viologens are ionic in their oxidized state, positively charged and associated with one or two PF6 -counter-ions, and it is only below -0.8 V that this molecule is neutral. This is fundamentally different from a neutral NDI molecule which becomes negatively charged when it is reduced. Thirdly, according to DFT calculation (B3LYP/6-31G*), the dihedral angle between the phenyl group and the viologen motif in both compounds is around 46° whereas it is 90° (between phenyl and the NDI core) for NDI. Consequently, the LUMO of both VIO-C1 and VIO-C6 is delocalized into the phenyl group (see Figure S II-1) whereas in the case of NDI, the LUMO level is localized on the core. Hence, electronic coupling between the LUMO of both viologens and the Au bottom electrode is expected to be stronger than for NDI. In this chapter, we will develop MJs using mixed molecular layers, i.e. layers in which two different molecules, with different electronic properties, are used and positioned in parallel and may create separate or independent parallel transport channels, as depicted in Scheme IV-1.

Scheme IV-1 -a) MJs based on bilayer (left) or (b) on mixed layers (right) using two different molecules.

The two molecules can be of various types, one being -conjugated while the other is saturated or both being -conjugated. We anticipate that new electronic functions, not observed in single-component and bilayer MJs, may be obtained when molecular junctions are based on such mixed layers. Moreover, comparison of the transport properties of MJs using single-component, bilayers and mixed layers may provide new insights into molecular electronics. Among them, we will focus in this chapter on how the current density scales with the number of active molecules involved in the junction.

To illustrate this question, let us recall that the simplified Simmon equation, J = Joe - d , provides an approximate relationship between the rate of charge transport and the energy barrier, and also predicts exponential decay in current density with increasing width of the tunneling barrier, d. 1,2 . J is the total current density through the MJ while Jo is the preexponential factor or the current density when the thickness of the molecular layer, d = 0. The attenuation factor or the tunneling decay coefficient, β, defines how fast the current changes Low temperature measurements were performed using a variable temperature insert at a base temperature of 5 K while keeping the sample in a He atmosphere. Firstly, the sample were bonded into a chip using wired setup as presented in FigureA-5b. Then the chip was inserted into the He atmosphere and then the temperature of the device can be adjusted from 7 K to room temperature (FigureA-5c). Temperature dependence measurements were done by heating from 7 K to room temperature and reversely cooling down from room temperature into 7K.