Quyen Van Nguyen

Pascal Martin
email: pascal.martin@univ-paris-diderot.fr

Denis Frath

Maria Luisa

Della Rocca

Frederic Lafolet

Clément Barraud

Philippe Lafarge

Vineetha Mukundan

David James

Richard L Mccreery

Jean-Christophe Lacroix
email: lacroix@univ-paris-diderot.fr

Control of Rectification in Molecular Junctions: Contact Effects and Molecular Signature

Keywords: electronic materials, electron transport, molecular electronics, rectification, diazonium electroreduction

Thin layers of oligomers with thickness 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 chain. 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, i.e. tetrafluorobenzene, no rectification is observed while the direction of rectification is reversed if the molecular layer consists of naphtalene diimides having low LUMO energy level. 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 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 and the rectification mechanism is discussed in terms of Fermi level pinning and electronic coupling between molecules and contacts.

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 mechanism for transport 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 that is above the direct tunneling limit. 3 Rectification controlled by molecular structure in molecular junction 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 along the transport direction. 20 This can be done either by using contacts based on conductive materials of different work function, or by using bilayers with a donor (D) and an acceptor (A) part in which 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 hexadecyl-quinolinium tricyanoquino-dimethanide and yielded rectification ratios 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 on A--D molecules were widely used with preferred direction of the electron flow from A to D (Aviram-Ratner direction) or from D to A (reverse Aviram Ratner direction) 33,[START_REF] Metzger | Unimolecular and Supramolecular Electronics II: Chemistry and Physics Meet at Metal-Molecule Interfaces[END_REF] and rectification ratio rarely exceeded 100 [33][START_REF] Metzger | Unimolecular and Supramolecular Electronics II: Chemistry and Physics Meet at Metal-Molecule Interfaces[END_REF][START_REF] Lenfant | [END_REF][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 rectification ratio were also observed in alkyl-Ferrocene self-assembled monolayers, contacted by chemisorption to a bottom electrode and physisorption to a top electrode, and was 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 these devices suffer from the intrinsic stability limitation of the SAM based molecular layer and 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] were also reported but in each case rectification ratios remained 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 fields 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 thickness exhibit high rectification ratio depending both on asymmetric contact and molecular structure.

We have reported an Au/molecule/Ti-Au MJs structure to examine molecular layers made by diazonium electroreduction. Rectifiers were obtained when using BTB with thicknesses above 5 nm 62 and a 7 nm BTB film showed RR ratio 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 in for SAM based devices incorporating ferrocene units. 44 The same layers when contacted by carbon show symmetric I(V) curves for films thickness between 2 and 22 nm. 15 Moreover using bilayers with overall thicknesses above 5 nm but below 22 nm 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 junction stimulated consideration of molecular layers in the Au/molecule/Ti-Au configuration with thicknesses above 5 nm, in the hope it will be possible to develop reliable rectifiers based on a robust device design. Diazonium-based molecular junctions 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" on the electronic response of the devices, compared to that observed when direct tunneling between the contacts is the dominant transport mechanism.

The current report describes single component MJs made by electrochemical reduction of different diazonium reagents 19,64,66,67 yielding layers with 7 to 9 nm thicknesses. The contacts were Au and Ti/Au, so structural asymmetry from contacts with different work function was always present in all the studied devices. 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-naphtalenediimide (NDI) oligomers, with low energy LUMO and alkyl moieties and are shown in figure 1a. The contacts induce asymmetry in the junction and by varying only the composition of the layers, the effects of molecular structures on JV symmetry were observed. The rectification mechanism is discussed in terms of Fermi level pinning and electronic coupling between molecules and contacts. 68 Note that the frontier orbital energies in vacuum neglect the mutual interaction of the oligomers as well as their interaction with the electrode.

Experimental Methods.

Unless noted otherwise, the MJs were fabricated using a gold stripe substrate on SiO 2 /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/Gold 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 deposition systems 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 coat deposition was performed with identical conditions and all junctions were also completed using lithography and shadow mask for top electrode deposition. No significant differences were observed using either shadow mask or direct lithography on the molecular layer showing that the fabrication techniques used here are fully compatible with standard microfabrication techniques.

1-(2-bisthienyl)-4-aminobenzene and, N-(4-aminophenyl)-N'-hexylnaphtalene-1,8:4,5tetracarboxydiimide were synthesized using procedures adapted from literature, 66,68,72 this last molecule being synthesized for the first time its characteristics (NMR and High Resolution Mass Spectroscopy) are given in the Supplementary Information (SI files) while tetrafluoroaniline and para amino-nitrobenzene are commercial products and were purchased from Aldrich. Their corresponding diazonium salts were generated in situ using ter-butylnitrite (Aldrich) as a reagent. This procedure has been described in detail. 19,66,67,73,74 Organic layer thicknesses were determined by AFM measurements. The thickness of the gold stripe deposited on SiO 2 was first measured using AFM 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 as shown in Figure S1.

Standard deviations of thickness were in the range of 0.4 to 0.8 nm as indicated in Table S1.

Junction designations include subscripts indicating layer thicknesses in nanometers, with all devices using the same bottom and top contact electrodes. For example, Au 45 /BTB 9 /Ti 2 /Au 45 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 molecular junctions 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 applying a bias to the bottom electrode. The resulting current-density/voltage (JV) curves exhibited no hysteresis when acquired in air, or in vacuum (<10 -5 torr for several hours, Figure S2-a). All JV curves in figures were obtained in air, and each curve presented in figures is a representative curve of JV characterization obtained with the MJs fabricated on each sample.

Low temperature measurements were performed using a variable temperature insert at a base temperature of 5 K while keeping the sample in 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. 67 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, ability to generate robust, pinhole-free, electroactive layers with two redox states of different conductivity which can be considered switchable organic electrodes.

Current-density vs applied voltage (JV) curves for Au/BTB 9 /Ti 2 /Au MJ are shown in Figure 2a, for the case of ~ 9 nm thick films of BTB (overlay of several JV curves collected on various junctions are 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. Indeed large currents are observed when the bottom electrode is positively polarized at moderate bias whereas when negatively polarized the current flowing through the junction is much smaller. Figure S4 shows that the rectification ratio (RR) varies with applied potential and reaches a plateau at 2.7 V, and Figure 2b shows histograms of the rectification ratio at +2.7 V (RR 2.7V ) 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 using 9 nm BTB layers). Average RR 2.7V is 1200 with few devices showing RR 2.7V at 300 and some devices with RR 2.7V as high as 3000. Such rectification ratios are, to the best of our knowledge unprecedented for a single component molecular junction. They are reproducible and devices show long term stability (over a year). No 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 are not important for transport and rectification at low temperature and exclude redox events involving Titanium dioxide 69,76 as being at 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 ratios observed here to the use of two different metals with different work functions for contacting the BTB layer. 9 nm BTB (a)

Figure 2: a) Average Current-density vs applied voltage (JV) curves for Au/BTB 9 /Ti 2 /Au MJs inset Schematic illustration of a molecular junction 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 TiO 2 during the fabrication process, and has been already reported to be the origin of rectification in molecular junctions. 69,76 To rule out the involvement of TiO 2 in the rectification mechanism, we acquired XPS depth profiles during Ar + etching to yield the elemental composition of Si/SiO 2 /PPF/BTB 9 /Ti 2 /Au 20 made identically to the Au/BTB 9 /Ti 2 /Au molecular junctions 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 Au signal is observed, as expected, but after 360 s of etching the Au signal decreases and simultaneously Carbon and Titanium atomic percentage start to increase. Note that Titanium and Carbon signal increase before oxygen signal which start to be seen at 600 s etching time and the atomic percentage for O 1s remains lower than that of Titanium at all etching depths. Note also that the sulfur signal increases only 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 a moiety that 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 on gold by diazonium electroreduction fluorinated oligoparaphenylenes are generated. Polyparaphenylene is known to have a band gap of 3 eV 84 and the presence of four fluorine groups on each benzene rings in FB will increase the bandgap further by inducing a greater degree of 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.

Figure 4a and b compare the J(V) curves of Au/NB 8 /Ti 2 /Au (green curve) and of Au/FB 8 /Ti 2 /Au (blue curve) junctions to that of Au/BTB 9 /Ti 2 /Au (red curve). The same data are displayed in lnJ versus V in figure S7. NB and FB devices are consistently much less conductive than BTB devices as the measured current 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 and the current scale had to be zoomed in figure 4b for comparison with BTB junctions. Moreover, Figure 4a shows that NB devices also rectify with a rectification ratios (J(+2.7 V)/J(-2.7 V) between 50 and 100 that is clearly lower that those observed with BTB devices. On the contrary FB devices show weak rectification behavior (RR between 1 and 3) in the bias range between -2.7 and +2.7 V. These clear variations of the current densities and rectification ratios with molecular structure are a strong indication of a molecular signature relating structure to conductance and electronic behavior when using aromatic films of 8 to 9 nm thicknesses between Au and Ti/Au electrodes. Figure 4c compares the J(V) curve of Au/NDI 7 /Ti 2 /Au (gray curve) to that of Au/BTB 9 /Ti 2 /Au (red curve). NDI devices are again consistently less conductive than BTB devices over the entire -2.7 V, +2.7 V potential range investigated. Importantly, the direction of rectification in the NDI device is reversed compared to that observed in BTB devices with larger J 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 top contact.

Figure 4d shows the rectification ratio (J(+2.7 V)/J(-2. c) is observed when such differences were not 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 deviation 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- 88 However, above 5 nm, single-component layers of NDI, NB, FB and BTB current densities 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, all reported cases of single-component molecular layers between symmetric carbon contacts yielded nearly symmetric JV response, with RR < 2 8,15,89 while NDI 7 and BTB 9 layers between Au and Ti/Au contact 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 a strong dependence on molecular structure, of both rectification ratio and polarity.

As already stated, rectification must include an asymmetric feature along the transport 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 flowing through the device. The Fermi level of Au 90 is -5.1 to -5.2 eV vs vacuum while that of Ti and TiC is at -4.3 eV [START_REF] Santerre | [END_REF] . BTB HOMO is close to the Au Fermi level and is separated from that of Ti or TiC by only ~1 eV while the barrier between 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 using BTB layers where covalent bond formation has been experimentally evidenced both at bottom and top electrodes, we propose that the difference in barriers for hole injection associated with strong coupling at both electrodes is important to the origin of the large rectification (RR >1000) behavior observed for BTB based devices.

Figure 5 shows a plausible scheme explaining the rectification of 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. 67 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 and that it lies between 2 and 7 as the size of a hole in a 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 chemical bonds and the BTB HOMO being close to the Au FFermi level, and 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 holes 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. As a consequence Fermi level pinning is expected at both interfaces and the difference in work function between Au and TiC generates the energetic 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 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 participate in charge transport.

Moreover, a bias above 2.7 V will also be needed to allow a molecular state with small enough tunneling distance to reach resonance with the Fermi level of the titanium electrode. 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 explains also easily the temperature dependence of the current observed at positive or negative bias (see Figure S2). Indeed, for positive bias, the thermally activated current observed between 100 and 290 K with low activation energy of 150 meV can be attributed to the small barrier at the Au/BTB interface whereas, for negative bias no observable activation in the same temperature range are 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 linked to the initial barrier and to the Fermi level pinning at the BTB/TiC interface of the top contact.

Decreasing this barrier or the Fermi level pinning at this interface, by any means, will decrease the RR observed. 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 that case, it is easier to compensate for the initial ( 1 ) barrier located at the weakly coupled BTB/Ti interface by applying negative bias which decreases sharply the rectification ratio. 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 SIX) and a smaller RR observed for FB.

The lower energy HOMOs in the latter case creates significant barriers at both interfaces, thus decreasing 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 which 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 stronger for positive bias, i.e. in the same direction as that observed in BTB devices. This is clearly not observed here as electrons are experimentally more easily injected in the NDI layer at negative bias i.e. when the Au bottom electrode is negative and Ti/Au top electrode is positive. As a consequence, we 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 S10) 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 compared to BTB due to the long alkyl moiety which is known to induce a decoupling effect. 93 Assuming such situation, 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 junction. 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 profile 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 modelling that includes the entire system using a non-equilibrium Green's functions (NEGF) approach 96,97

Conclusions

Molecular junctions with thicknesses between 7 and 9 nm using two contacts of different work function exhibit significant changes in electronic behavior with variation of molecular structure of the layer. Using oligo(bisthienylbenzene) 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 and Ti/Au is negative.

When the molecular layer is based on molecule with 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 and incorporates alkyl moieties. Rectification persisted at very low temperature (7 K), and was activation-less between 7 and 100 K. The results show that rectification is induced by the asymmetric contacts but is also directly affected by 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 changing 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 such coupling depends on the molecular layer structure, since the same fabrication process has been used for all studied junctions. These observations clearly demonstrate a rectifying molecular junction can be "designed" by controlling the HOMO and LUMO energies as well as the electronic coupling at the interfaces.

ASSOCIATED CONTENT Supporting Information

Supplementary materials are available free of charge via the Internet at http://pubs.acs.org. Corresponding Authors *lacroix@univ-paris-diderot.fr *pascal.martin@univ-paris-diderot.fr

Figure 1 .

 1 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 HOMO level and absorption spectra. (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) in BTB, NB and FB device while NDI levels are for single NDI molecules since oligo(NDI) are weakly conjugated.68 Note that the frontier orbital energies in vacuum neglect the mutual interaction of the oligomers as well as their interaction with the electrode.

  and that the C/S ratio reaches the theoretical value of 7 expected for a BTB layer when etching time is between 800 and 960 s. Similar experiments were performed on a Si/SiO 2 /PPF/Ti 2 /Au 20 large-area sample and are shown in figureS6. In that case Au decreases and simultaneous Carbon, Titanium and Oxygen signals increase (but no sulfur) are also observed with Titanium atomic % being again always above that of oxygen. This excludes the formation of a 2 nm thick TiO 2 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 Ti 2 O 3 contain mobile conduction band electrons and are considered metallic.69,[77][78][79] There is no evidence that the TiO x layer contains significant TiO 2 , and it therefore behaves like a metal rather than a large band gap semiconductor.

Figure 3 :

 3 Figure 3: a) Atomic percentages (%) -versus-Etching time (s) for a Si/SiO 2 /PPF/BTB 9 / Ti 2 /Au 20 b) Titanium high resolution XPS signal, c) Carbon high resolution XPS signal.

Figure 3b and c

  Figure 3b and c show the high resolution XPS spectra of the Titanium and the 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

Figure 4 :

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

  Figure4dshows the rectification ratio (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 in lnJ versus V. The electronic behavior of the studied devices clearly depends on the molecular structure of the layers with a high level of confidence statistically. 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 molecular layer thicknesses of 9 nm, too thick to be explained by coherent tunneling between the contacts. Second, a large difference in conductance between NDI 7 , NB 8 , FB 8, and BTB 9 (Figure 4 a, b and c) is observed when such differences were not observed for different aromatic structures

  direction. Contacts based on conductive materials of different work function, 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 layer contacted by chemisorption to the bottom electrode and physisorption to a top electrode have been reported to be at the origin of rectification in previous work. Since the MJ structure in the present case differs from most of the previous studies, several factors might contribute to the rectification mechanism. However the current results clearly demonstrate that both contacts and molecular structure can affect rectification and we consider several of these factors below.

1 Figure 5 :

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

  Synthesis of NDI compound, electrochemical reduction conditions, determination of molecular layer thickness by AFM, temperature dependence measurement of Au/BTB 9nm /Ti 2nm /Au device, Overlaid JV curves of several MJs devices, Rectification ratio versus applied Voltage, Scan rate dependence on Au/BTB 9nm /Ti/Au MJs, Atomic percentages Versus Etching time (s) for a Si/SiO 2 /PPF/ Ti 2 /Au 20 , Ln J vs V curves of BTB, NDI, FB, NB molecule junctions, electroactivity of NDI modified carbon electrode, energetic diagram and plausible mechanism assuming electronic coupling is negligible at the top electrode, LUMO shape of NDI molecules

ACKNOWLEDGMENT

This work was supported the French Agence Nationale de la Recherche under Grant ANR-15-CE09 0001-01) and by the University of Alberta, the National Research Council of Canada, the National Science and Engineering Research Council and Alberta Innovates.


Verena ; Stockhausen

Van Quyen Nguyen

Pascal Martin

Jean Christophe Lacroix

Bottom-Up Electrochemical Fabrication of Conjugated Ultrathin Layers with Tailored switchable properties

Keywords: Bottom-up approach, nano-electrochemistry, ultrathin film, π-conjugated material, diazonium grafting, molecular electronics

In this work, we develop a Bottom-Up Electrochemical Fabrication process of Conjugated Ultrathin Layers with Tailored switchable properties. To do so, we use ultrathin layers of covalently grafted oligo(bisthienylbenzene) (oligo(BTB)) as a switchable organic electrodes and perform 3,4-ethylenedioxythiophene (EDOT) oxidation on such organic layer.

Adding only 3 nm of EDOT moieties (5 to 6 units) completely changes the switching properties of the layer without changing the surface concentration of the grafted electroactive species. As a consequence, a range of new materials, with tunable interfacial properties, is created through this bottom-up electrochemical process. They consist of oligo(BTB)-oligo(EDOT) di-block oligomers of various relative length grafted covalently on the underlying electrode. As an Page 1 of 24 ACS Paragon Plus Environment ACS Applied Materials & Interfaces 2

important result these ultrathin films retain reversible redox on/off switching and their switching potential can be finely tuned between 0.6 and -0.3 V/SCE despite an overall thickness that remains below 11 nm.

INTRODUCTION

Bottom-up approach is one of the heart pieces of modern material science and provides an enormous creativity in its use as tool for the fabrication of an almost undetermined choice of smart materials [START_REF] Magasinski | Highperformance lithium-ion anodes using a hierarchical bottom-up approach[END_REF][START_REF] Godeau | Switchable surfaces from highly hydrophobic to highly hydrophilic using covalent imine bonds[END_REF][START_REF] Wang | A Bottom-Up Approach to Dual Shape-Memory Effects[END_REF][START_REF] Santos | Micro/Nano-Structured Polypyrrole Surfaces on Oxidizable Metals as Smart Electroswitchable Coatings[END_REF][START_REF] Lee | Multifunctional and Stable Monolayers on Carbon: A Simple and Reliable Method for Backfilling Sparse Layers Grafted from Protected Aryldiazonium Ions[END_REF][START_REF] Lee | Amine-Terminated Monolayers on Carbon: Preparation, Characterization, and Coupling Reactions[END_REF] ,interfaces [START_REF] Santos | Micro/Nano-Structured Polypyrrole Surfaces on Oxidizable Metals as Smart Electroswitchable Coatings[END_REF][START_REF] Lee | Amine-Terminated Monolayers on Carbon: Preparation, Characterization, and Coupling Reactions[END_REF][START_REF] Xu | Reversible Conversion of Conducting Polymer Films from Superhydrophobic to Superhydrophilic[END_REF][START_REF] Hermelin | Ultrafast Electrosynthesis of High Hydrophobic Polypyrrole Coatings on a Zinc Electrode: Applications to the Protection against Corrosion[END_REF][START_REF] Wang | Dynamic Control of Surface Energy and Topography of Microstructured Conducting Polymer Films[END_REF][START_REF] Xu | Electrically Tunable Polypyrrole Inverse Opals with Switchable Stopband, Conductivity, and Wettability[END_REF][START_REF] Santos | Electrografting polyaniline on carbon through the electroreduction of diazonium salts and the electrochemical polymerization of aniline[END_REF][START_REF] Nguyen | Highly Resolved Nanostructured PEDOT on Large Areas by Nanosphere Lithography and Electrodeposition[END_REF][START_REF] Santos | Formation of Mixed Organic Layers by Stepwise Electrochemical Reduction of Diazonium Compounds[END_REF][START_REF] Jacques | 1-Pyrrolyl-10decylammoniumphosphonate monolayer: a molecular nanolink between electropolymerized pyrrole films and nickel or titanium surfaces[END_REF] and nanoelectronic devices [START_REF] Lu | Nanoelectronics from the bottom up[END_REF][START_REF] Love | Self-Assembled Monolayers of Thiolates on Metals as a Form of Nanotechnology[END_REF][START_REF] Mccreery | Molecular Electronic Junctions[END_REF][START_REF] Sedghi | Longrange electron tunnelling in oligo-porphyrin molecular wires[END_REF][START_REF] Yan | Activationless charge transport across 4.5 to 22 nm in molecular electronic junctions[END_REF][START_REF] Yuan | A Molecular Diode with a Statistically Robust Rectification Ratio of Three Orders of Magnitude[END_REF][START_REF] Ho Choi | Electrical Resistance of Long Conjugated Molecular Wires[END_REF][START_REF] Tran | Experimental Approaches for Controlling Current Flowing through Metal-Molecule-Metal Junctions[END_REF][START_REF] Tuccitto | Highly conductive [sim]40-nm-long molecular wires assembled by stepwise incorporation of metal centres[END_REF][START_REF] Choi | Transition from Tunneling to Hopping Transport in Long, Conjugated Oligo-imine Wires Connected to Metals[END_REF] . Ultrathin films of conjugated oligomers grafted on surfaces from diazonium salt reduction are of particular interest for the fabrication of molecular electronic devices [START_REF] Mccreery | Molecular Electronic Junctions[END_REF][START_REF] Mccreery | A critical perspective on molecular electronic junctions: there is plenty of room in the middle[END_REF][START_REF] Martin | Organic Electrodes Based on Grafted Oligothiophene Units in Ultrathin, Large-Area Molecular Junctions[END_REF][START_REF] Sayed | Bilayer Molecular Electronics: All-Carbon Electronic Junctions Containing Molecular Bilayers Made with "Click" Chemistry[END_REF] . Indeed they allow the fabrication in high yield of robust molecular junctions using CMOS compatible processes [START_REF] Mccreery | A critical perspective on molecular electronic junctions: there is plenty of room in the middle[END_REF][START_REF] Martin | Organic Electrodes Based on Grafted Oligothiophene Units in Ultrathin, Large-Area Molecular Junctions[END_REF] . Among such layers, ultrathin films combining thiophene and EDOT units exhibit unique on/off switching of their transport properties in electrochemical environment [START_REF] Fave | Tunable Electrochemical Switches Based on Ultrathin Organic Films[END_REF][START_REF] Fave | Electrochemical Switches Based on Ultrathin Organic Films: From Diode-like Behavior to Charge Transfer Transparency[END_REF][START_REF] Stockhausen | Grafting π-Conjugated Oligomers Incorporating 3,4-Ethylenedioxythiophene (EDOT) and Thiophene Units on Surfaces by Diazonium Electroreduction[END_REF][START_REF] Stockhausen | Grafting Oligothiophenes on Surfaces by Diazonium Electroreduction: A Step toward Ultrathin Junction with Well-Defined Metal/Oligomer Interface[END_REF] . Insulating behavior is observed below an intrinsic film commutation potential whereas above this potential, they become conductive.

As a consequence, reversible redox probes with an E 0 below this intrinsic film commutation potential exhibit an anodic peak shifted towards higher potential and no cathodic peak, while reversible redox probes with an E 0 above the film commutation potential exhibit the same redox behaviour as on bare electrodes [START_REF] Fave | Tunable Electrochemical Switches Based on Ultrathin Organic Films[END_REF][START_REF] Fave | Electrochemical Switches Based on Ultrathin Organic Films: From Diode-like Behavior to Charge Transfer Transparency[END_REF] . The film commutation potential depends on the molecular structure of the grafted oligomers and incorporating EDOT in the molecular backbone decreases the layer switching potential [START_REF] Fave | Tunable Electrochemical Switches Based on Ultrathin Organic Films[END_REF][START_REF] Stockhausen | Grafting π-Conjugated Oligomers Incorporating 3,4-Ethylenedioxythiophene (EDOT) and Thiophene Units on Surfaces by Diazonium Electroreduction[END_REF] . Overall, such systems combine the advantages of conductive oligomers to those of diazonium based layers, i.e. fine control of the film thickness in the 2 to 20 nm range, ability to generate robust, pinhole free and electroactive layers, two redox states of different conductivity [START_REF] Martin | Organic Electrodes Based on Grafted Oligothiophene Units in Ultrathin, Large-Area Molecular Junctions[END_REF] . As a consequence they can be seen as switchable organic electrodes.

In this work we investigate a bottom-up electrochemical process based on the electrochemical oxidation of irreversible redox systems on such a switchable organic electrodes, namely oligo(bisthienylbenzene) (BTB) layer. In doing so, we wish to create new ultrathin layers with tunable properties. For this purpose, we chose to study EDOT oxidation, a monomer whose oxidation is irreversible, gives unstable EDOT radical cations and yields on a bare electrode to PEDOT deposition. As EDOT oxidation potential is located beyond the switching potential of the grafted oligo(BTB), it will take place on a p-doped organic conductive electrode, that is a layer in which oligo(BTB) radical cations are already generated. As a consequence, two types of reactive radical cations will be created in the vicinity of the electrode and we will investigate the outcome of their chemical coupling and the resulting electrode modification. In doing so we wish to evaluate if it is possible to extend the ways to combine the advantages of conductive oligomers to those of diazonium based layers using a low cost bottom up nano-electrochemical process. Scheme 1 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 oligo(BTB) electrode and their relative reactivity towards oligo(BTB) radical cations.

Scheme 1:

Alternative scenarios that can be anticipated when performing EDOT oxidation on an oligo(BTB) layer. In scenario 1, a new material is generated by covalent attachement of EDOT on oligo(BTB). In scenario two, Oligo(BTB) layer is covered by PEDOT and an interface exists between the two materials First, at low EDOT radical cation concentration (i.e; low EDOT oxidation current density), EDOT radical cations can couple covalently to the grafted layer and thus extend the aromatic chain and create new block oligomers with new properties (Scenario 1). In this case, a new material is generated and its electrochemical and interfacial properties differ from those of the initial BTB layer.

Alternatively, at high current densities, EDOT radical cations may prefer to react with each other and form oligomers in solution or even a polymer. It can then precipitate onto the first 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 the two materials. As the BTB layer is not modified in this scenario, it keeps its electrochemical properties. As BTB potential switch is above that of PEDOT, BTB will command the electrochemical properties of the physisorbed PEDOT layer.

EXPERIMENTAL METHODS

Chemicals: 1-(2-bisthienyl)-4-aminobenzene were synthesized according to published procedures [START_REF] Fave | Tunable Electrochemical Switches Based on Ultrathin Organic Films[END_REF][START_REF] Trippé-Allard | Synthesis of nitro-and amino-functionalized πconjugated oligomers incorporating 3,4-ethylenedioxythiophene (EDOT) units[END_REF] . All chemicals were used as received. Ferrocene (Fc), decamethylferrocene (DmFc) and were purchased from Sigma Aldrich. Tert-butyl nitrite (tBuO-NO) was purchased from Aldrich. Lithium perchlorate (LiClO 4 ) and tetrabutylammonium tetrafluoroborate (Bu 4 NBF 4 ) from Aldrich were used as supporting electrolyte at 0.1 M concentration in acetonitrile (ACN). 3,4-ethylenedioxythiophene (EDOT) was from Bayer.

Electrochemical deposition of the oligo(BTB) layer:

The in situ formation of the diazonium cation from 1-(2-bisthienyl) precursor was performed without any acid in the solution, by adding tert-butyl nitrite (tBuO-NO) [START_REF] Fave | Tunable Electrochemical Switches Based on Ultrathin Organic Films[END_REF][START_REF] Doyle | Alkyl nitrite-metal halide deamination reactions. 6. Direct synthesis of arenediazonium tetrafluoroborate salts from aromatic amines, tert-butyl nitrite, and boron trifluoride etherate in anhydrous media[END_REF][START_REF] Delamar | Covalent modification of carbon surfaces by grafting of functionalized aryl radicals produced from electrochemical reduction of diazonium salts[END_REF][START_REF] Baranton | In situ generation of diazonium cations in organic electrolyte for electrochemical modification of electrode surface[END_REF] Briefly, an acetonitrile solution containing of 1-(2bisthienyl)-4-aminobenzene (0.5 mM) and Bu 4 NBF 4 (0.1 M) as supporting electrolyte was prepared and degassed by argon bubbling for at least 10 min. Following that, tBuO-NO was added in excess to the solution (30 equivalents). As the reaction between amine function and tertbutyl nitrite is relatively slow, grafting was only started after 5 minutes incubation time. BTB layer deposition was performed using multicyle voltammetry between 0.4 V/SCE and -0.4 V/SCE on glassy carbon electrodes as well as on gold stripes (20 µm wide and 5 mm long). The thickness of the deposited layer was measured using tapping mode AFM and was set to ~8 nm.

Each electrode was then characterized in acetonitrile containing 1 mM ferrocene and 0.1 M LiClO 4 .

Electrochemical oxidation of EDOT on the oligo(BTB) organic electrode: Electrodes previously functionalized by oligo(BTB) were sonicated in ACN for 10 minutes and plunged in a solution containing 2 x 10 -2 M EDOT. Cyclic voltammograms were performed while the potential window was adjusted in order to obtain oxidation charges turning 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 electrode was thoroughly rinsed with ACN and electrochemically characterized in electrolyte solution (0.1 M LiClO 4 in ACN), without any redox probe, at a scan rate of 10 V/s (with ohmic drop compensation). Then, voltammograms in presence of Fc and DmFc were recorded using the same modified electrode. This procedure (EDOT deposition on the organic electrode, rinsing characterisation in Fc, and DmFc, and in redox probe free solution) was repeated several times. AFM characterization: AFM measurements were performed on a Molecular Imaging system (picoscan 5500). AFM images are recorded in tapping mode with tapping tip from Budgetsensor (AFM tap 300G, resonant freq 300kHz, force constant 40nN/m). Blank studies were first realized on bare Au microelectrode in order to measure the thickness of Au stripes. The thickness of the grafted layer is then determined by subtracting the thickness of the Au bottom electrode [START_REF] Martin | Organic Electrodes Based on Grafted Oligothiophene Units in Ultrathin, Large-Area Molecular Junctions[END_REF][START_REF] Fluteau | Tuning the thickness of electrochemically grafted layers in large area molecular junctions[END_REF] .

RESULTS AND DISCUSSION

The voltammograms of EDOT oxidation onto the BTB layer are shown in Figure 1a, where the number of cycles used is indicated. EDOT oxidation onset starts at 0.8 V/SCE and is as expected irreversible. Successive oxidation cycles show that this onset shifts slightly to higher values as the BTB layer is modified by EDOT moieties deposition. In order to generate a small amount of EDOT radical-cations, the current density has been restricted to be below 0.3 mA/cm 2 . EDOT oxidation is here performed at the threshold of the EDOT oxidation signal which can reach current as high as 20 mA/cm 2 with such EDOT concentration. After each oxidation cycle, the electrode was thoroughly rinsed with ACN and electrochemically characterized in electrolyte solution (0.1 M LiClO 4 in ACN), with Fc then with DmFc redox probe. Voltammograms of Fc and DmFc were recorded using the same modified electrode. The electrochemical responses of these two redox probes are thus an indication of the modifications of the layer electrochemical properties, induced by each EDOT deposition cycle. This procedure (EDOT deposition on the organic electrode, rinsing characterisation in Fc, and DmFc,) was repeated several times in order to monitor the progressive evolution of the transport properties of the modified electrode. Ferrocene oxidation signal on the initial oligo(BTB) layer is curve labelled "0". As ferrocene redox potential is below the switching potential of the oligo(BTB) layer, it has a typical diode like signature, i.e. no current is observed at 0.3 V/SCE but ferrocene oxidation peak is shifted to positive values while no reduction peak associated to the ferricinium reduction, is observed during backward scan. Such behavior indicates that the oligo(BTB) film is pinhole free and switches from insulating to conductive at 0.65 V/SCE [START_REF] Fave | Electrochemical Switches Based on Ultrathin Organic Films: From Diode-like Behavior to Charge Transfer Transparency[END_REF][START_REF] Stockhausen | Grafting π-Conjugated Oligomers Incorporating 3,4-Ethylenedioxythiophene (EDOT) and Thiophene Units on Surfaces by Diazonium Electroreduction[END_REF] .

After the first oxidation cycle in presence of EDOT, the ferrocene signal on the new electrode is labelled "1" in Figure 1b. A typical diode like signature is still observed (Figure 1b curve 1), which indicates that the new electrode still switches from an insulating to a conductive state. However, the anodic peak potential is lowered compared to that observed on an unmodified BTB film. Upon further cycling, the anodic peak potential decreases more and after the third cycle, an emerging cathodic peak, corresponding to Fc + reduction, is observed. The intensity of this cathodic peak increases progressively while the layer is modified during 3rd to 7th cycle of EDOT oxidation. Simultaneously, the peak separation between anodic and cathodic peak, which is a measure of the reversible redox probe heterogeneous electron transfer kinetic constant, decreases. From the seventh cycle on, ferrocene shows reversible behaviour on the new organic electrode with ∆E p close to 60 mV and does not significantly changes upon further EDOT oxidation cycles.

DmFc redox probe was also used to report on the transport properties of the layer after each EDOT oxidation cycle. The voltammograms obtained on DmFc are depicted in Figure 1c.

They show a preserved diode-like behavior until the sixth cycle of EDOT oxidation, while after that, a growing cathodic signal appears, attributed to DmFc + reduction on the modified electrode.

Simultaneously ∆E p starts to get measurable at that point and decreases after each EDOT oxidation cycle and finally stabilises at 60 mV after nine successive EDOT cycles. After cycle 9, total reversibility is achieved also for DmFc. It is noticeable that at this moment, the film has become transparent towards Fc and DmFc.

At that point, a first intermediate conclusion can be drawn. EDOT oxidation on oligo(BTB) layer chemically modifies the pristine oligo(BTB) layer and changes its transport properties. The shape of the electrochemical response of Fc and Dmfc clearly demonstrates that the new electrodes still switch from an insulating to a conductive state. The switching potential of EDOT modified BTB electrodes can be easily tuned from 0.65 V/SCE to below -0.3 V/SCE and at any intermediate values by careful control of the EDOT oxidation process. As a consequence, one can easily reach an intermediate situation in which EDOT modified oligo(BTB) electrode is transparent towards ferrocene, while a diode like behaviour is still observed with decamethylferrocene. This behaviour is expected from Scenario 1 (covalent grafting of oligo (EDOT) block on oligo(BTB)) as described in scheme 1 and is not compatible with scenario 2 in which the oligo(BTB) layer retain its switching potential. The differenciated ∆E p evolution of Fc and DmFc redox probes associated to the pinhole free compact oligo(BTB) layer (observed in electrochemistry and when metal evaporation is directly performed on the layer) exclude PEDOT polymer growth through pinholes of the oligo(BTB) layer [START_REF] Martin | Organic Electrodes Based on Grafted Oligothiophene Units in Ultrathin, Large-Area Molecular Junctions[END_REF][START_REF] Fluteau | Tuning the thickness of electrochemically grafted layers in large area molecular junctions[END_REF] . As an important result, these experiments demonstrate that it is possible to tune the switching potential of the layer from its initial value of 0.65 V/SCE to below -0.3 V/SCE by using the procedure described above. This distinctive evolution of the two redox probes signal is to our belief unique and has never been reported elsewhere. It is thus detailed in Figure 2 and Figure SI1-X where X designates the number of EDOT oxidation cycles used to modify the BTB layer. Each figure also contains the electrochemical response of Fc and DmFc on a bare GC electrode and on the pristine oligo(BTB) film. By comparing both probe responses in one graph, we nicely see how their respective electrochemical signal gradually changes. After 1 cycle of EDOT oxidation (Figure SI1-1), both redox probes show still irreversible behaviour while after cycle 2 (Figure 2a), Fc already starts to exhibit a cathodic wave. After cycle 4, Fc finally starts to behave like on bare electrode (Figure 2b) and does not shift any more from cycle 6 on (Figure 2c), indicating that now, Fc is quasireversible on this film. In the meantime, DmFc still preserves its diode-like behavior and only after cycle 7 (Figure SI1-7), a cathodic peak appears but the ∆E p is still superior to bare electrode, indicating that the electrode remains partially blocked [START_REF] Amatore | Charge transfer at partially blocked surfaces[END_REF] . After cycle 9 (Figure 2d), both redox probes show reversible behaviour onto the modified electrode at the same E 0 than on bare electrodes. Sonication in ACN during 15 minutes did not modify film responses, which is a further indication for covalent attachment of EDOT moities and thus strongly points to Scenario 1 (scheme 1). To characterize further the film modification, electroactivity of the EDOT-modified-BTB-electrode was recorded in ACN at 10 V/s with IR drop compensation(Figure 3a) after each EDOT oxidation cycle, and compared to that of an unmodified oligo(BTB) electrode. This electroactivity shows small anodic and cathodic peaks shifting progressively towards less positive values upon successive EDOT oxidation cycle. Initial anodic peak for BTB is above 0.75 V/SCE then shifts to 0.55 , 0.50, 0.35, 0.25 and 0.15 V/SCE after 1 3, 5, 7 and 8 EDOT oxidation cycles, respectively. Cathodic current peak follows the same tendency and the anodic current density onset also decreases to lower potential upon successive EDOT oxidation cycle. These measurements confirm that the switching potential of the EDOT modified oligo(BTB) electrode is progressively lowered. By integration of the CV, the amount of charge observed for doping the deposited layer (Q switch ) as a function of charge applied during EDOT anodic deposition (Q deposition ) was determined. Q switch was plotted against Q deposition (Figure 3b blue and magenta points) and compared to the results obtained when anodic polymerisation of EDOT is performed on a bare electrode (Figure 3b black points), without underlying oligo(BTB) layer. On a bare GC electrode, a linear increase of Q switch with a Q deposition is observed with a ratio close to 10 between Q deposition and Q switch 38-39 . Such behaviour has already been observed for many conducting polymers. When EDOT oxidation is performed on a oligo(BTB) layer, the evolution of Q switch is clearly different. It does not significantly increase upon the successive EDOT deposition cycle and remains around 0.2 mC/cm 2 (Γ= 2.10 -9 mol/cm 2 ) when Q deposition increases. However when Q deposition gets above 6.5 mC/cm 2 , the expected curve for PEDOT polymerisation is obtained as can be seen in figure SI3. This result is a very strong indication that covalent grafting of few EDOT units had taken place on the BTB oligomers yielding to a block oligo(BTB)-oligo(EDOT) that can be oxidized at a lower potential compared to oligo(BTB). (Note also that under application of this charge, a PEDOT film of around 30 nm would be formed if it was performed on a bare electrode.)

In order to measure the thickness variation of the oligo(BTB) film upon EDOT oxidation, similar experiments were performed on gold stripes, 42±0.5nm nm thick, e-beam deposited on Si/SiO 2 substrates. This allows to use AFM for thickness measurements at each step of the electrochemical bottom up process. As the stripes exhibit a very small size of 20 µm per 5 mm, the electrochemical response of ferrocene is close to that obtained on an ultramicroelectrode (UME) with stationary plateau currents observed when ferrocene is oxidized to ferricinium. [START_REF] Fave | Tunable Electrochemical Switches Based on Ultrathin Organic Films[END_REF][START_REF] Fave | Electrochemical Switches Based on Ultrathin Organic Films: From Diode-like Behavior to Charge Transfer Transparency[END_REF] Figure 4a shows the AFM profile of three different films generated on gold stripes while Figure 4b and4c show the behaviour of ferrocene and decamethylferrocene on these electrodes.

The first one is a oligo(BTB) electrode and Figure 4a reveals that the oligo(BTB) exhibits an average thickness of 7.5-8 nm with a root mean square roughness of 0.7 nm. As expected on the gold/BTB electrode, the electrochemical oxidation of Fc and DmcFe is shifted to 0.7 V/SCE whereas on a bare electrode, a plateau like current at 0.3 V/SCE and -0.2 V/SCE is observed.

Note also that the value of the diffusion limited current is the same on the bare and all modified electrodes.

The oligo(BTB) electrode was then modified by EDOT oxidation, generating a new modified electrode labelled BTB-EDOT1. The thickness of the organic layer is close to 9 nm and the roughness of this electrode appears almost the same as that of the initial oligo(BTB) electrode. However, the Fc and response on this electrode is significantly different with an onset for ferrocene oxidation at 0.3 V/SCE, a current rising on a larger potential range and a plateau current obtained only at 0.5 V/SCE. This result shows that despite a minimal amount of EDOT grafted on the BTB oligomers (around one nm thickness increase, i.e. 1 or 2 EDOT moieties), the switching potential of the new BTB-EDOT1 electrode decreases sharply from its initial value. This result is compatible with that observed when films are generated from new diazonium cations, derived from 2-(4-aminophenyl)-3,4-ethylenedioxy-thiophene and various aminofunctionalized π-conjugated oligomers incorporating 3,4-ethylenedioxythiophene (EDOT) and thiophene units [START_REF] Stockhausen | Grafting π-Conjugated Oligomers Incorporating 3,4-Ethylenedioxythiophene (EDOT) and Thiophene Units on Surfaces by Diazonium Electroreduction[END_REF] .

A second electrode labelled BTB-EDOT2 was then generated and studied. Its thickness is now 9-10 nm with a roughness of 0.9 nm. EDOT block of the electrode is thus only 2 to 3 nm which implies that the EDOT block grafted on the oligo(BTB) electrode consists of only 4 to 6 EDOT units. Fc signal on this electrode is at the same potential and has the same shape as on a bare gold stripe, showing that BTB-EDOT2 electrode has a switching potential below the standard redox potential of ferrocene. On the contrary, DmFc signal is still shifted to higher potential when compared to its position on a bare gold stripe. Onset of DmFc oxidation signal is now at 0.0 V/SCE and plateau current is reached at 0.2 V/SCE which is as expected below the standard redox potential of Fc.

Finally, a third electrode labelled BTB-EDOT3 was generated. The thickness of the organic layer is 10-11 nm with an average roughness of 1.1 nm and this electrode is transparent for ferrocene but not yet fully transparent for DmFc as DmFc oxidation onset is seen at -0.1 V/SCE with plateau current reached at 0V/SCE. Thickness measurements indicate that the oligo(EDOT) block grafted on the initial oligo(BTB) units is only 3-4 nm thick that is close to 6 to 8 EDOT units. Overall, Scheme 2 summarizes the finding of this work. Starting from and 8 nm oligo(BTB) layer acting as a switchable electrode, it is possible to create a large set of new materials with tunable electrochemical properties. Oligo(EDOT) block of various length are grafted on the oligo(BTB) moieties leading to chain extension. As a consequence the new oligomers have higher energy HOMOs as seen in scheme 2c and the layer can switch from a conductive to insulating state at lower potential than the initial oligo(BTB) electrode. As long as the overall thickness remains below 11 nm, the charge involved in the film electroactivity remains that of the initial oligo(BTB) layer with polaron, spreading over most of the oligomer [START_REF] Lacroix | Intrachain Electron Transfer in Conducting Oligomers and Polymers: The Mixed Valence Approach[END_REF] , likely localized on the growing EDOT blocl as depicted in Scheme 2b. Ultimately further growth will generate PEDOT chains on the electrodes as observed in figure SI3. This set of new material with tunable switching properties through a bottom-up electrochemical process leading to ultrathin layers with tailored switchable properties. Such layers switch from an insulating, to a conductive state and acts above their switching potential as organic electrodes This switch is strongly dependent on the molecular structure of the diazonium salt precursor. Here, EDOT oxidation was performed on ultrathin oligo(BTB) layers. We show first that EDOT can be easily oxidized on such organic electrode and that by using small current densities, EDOT oxidation does not generate PEDOT film on the oligo(BTB) layer but oligo(EDOT) oligomers grafted covalently on the oligo(BTB) units. As a consequence, new elongated oligomers with a block structure are generated on the surface. Moreover, the degree of elongation can be easily controlled as we observe successive switching potential decrease as a function of the EDOT overgrafting cycles, evidenced in voltammograms recorded in presence of several redox probes with different redox potentials. Depending on the length of the oligo(EDOT) block, the switching potential of the new layers can be tuned at will from 0.6 V/SCE to -0.3 V/SCE. As an exciting result, these large switching potential variations are obtained by grafting less than 3-4 nm of oligo(EDOT) on the oligo(BTB) layer.

Overall, a bottom-up electrochemical process for oligomer elongation is indeed possible and leads to thin films with very low switching potential being covalently grafted to the electrode surface. Furthermore, film switching potential can be fine-tuned to the desired potential, facilitating device fabrication. Finally, overgrafting electron withdrawing groups by a similar procedure, is likely to raise the switching potential of the layer instead of decreasing it, a feature which could enlarge even more the possibilities for tailoring film switching potentials to any desired potential. Such experiments are under investigation.

FigureFigure 1 :

 1 Figure 1b and 1c illustrate the evolution of Fc and DmFc voltamograms on such BTB

Figure 2 :

 2 Figure 2: Comparison of Fc (pink line) and DmFc (blue line) (both at 1mM concentration in ACN with 0.1 M LiClO 4 as supporting electrolyte) response on oligo(BTB) modified electrodes (thin solid line) and after cycling in EDOT solution (2 x 10-2 M) (dashed line) with redox probe response on bare glassy carbon electrode (thick solid line). (a) After 2 EDOT overgrating cycles, (b) after 4 EDOT overgrafting cycles (c) after 6 EDOT overgrafting cycles, (d) after 9 overgrafting cycles. Scan rate: 0.1 V/s.

Figure 3 :

 3 Figure 3: a) Electroactivity after 1, 3, 5, 7 and 8 EDOT oxidation cycle respectively on BTBmodified GC electrode. CVs are recorded in ACN with LiClO 4 (0.1 M) as supporting electrolyte. Grey line is bare electrode, scan rate: 10 V/s (ohmic drop correction). (b) Evolution of Q switch as a

Figure 4 :

 4 Figure 4: a) AFM profiles of a gold stripe covered with various organic layers. Bare gold, Gold-BTB and Gold-BTB-EDOT1, Gold-BTB-EDOT2 and Gold-BTB-EDOT3 electrodes obtained by successive EDOT oxidation cycles at low current densities on the BTB-gold electrode. Note that the results reported here are obtained on a unique electrode that is modified in several successive steps. b) 3D AFM image of a Gold-BTB-EDOT3 electrode c) Response of ferrocene on the BTB-EDOT modified electrodes (1 mM) with LiClO4 (0.1 M) as supporting electrolyte in ACN, (d) Response of decamethylferrocene on the same BTB-EDOT modified electrodes (1 mM) with LiClO4 (0.1 M) as supporting electrolyte in ACN.

Scheme 2 :

 2 Scheme 2: Overall evolutions of the organic electrode properties while grafting 3 nm of oligo(EDOT) block on an 8 nm thick oligo(BTB) organic electrode. The yellow and green envelops represents the plausible extension a polaron on each oligomer.

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ACKNOWLEDGMENT

This work was supported by the CNRS, the University Paris Diderot (Paris, France) and by a Ph.D. grant from the University of Science and Technology of Hanoi.

ASSOCIATED CONTENT

Supporting Information. Comparison of Fc and DmFc response on oligo(BTB) modified electrodes and after cycling in EDOT solution after 1 and 7 EDOT overgrafting cycle, Evolution of Q switch as a function of Q deposition when. Q deposition is above 7 mC/cm 2 . This material is available free of charge via the Internet at http://pubs.acs.org."

AUTHOR INFORMATION

Corresponding Author

* lacroix@univ-paris-diderot.fr

Notes

The authors declare no competing financial interest