Louis Caillard

Oliver Seitz

Philip M Campbell

Rachel P Doherty

Anne-Feĺicie Lamic-Humblot

Emmanuelle Lacaze

Yves J Chabal

Olivier Pluchery

Gold Nanoparticles on Oxide-Free Silicon-Molecule Interface for Single Electron Transport

Two different organic monolayers were prepared on silicon Si(111) and modified for attaching gold nanoparticles. The molecules are covalently bound to silicon and form very ordered monolayers sometimes improperly called self-assembled monolayers (SAM). They are designed to be electrically insulating and to have very few electrical interface states. By positioning the tip of an STM above a nanoparticle, a double barrier tunnel junction (DBTJ) is created, and Coulomb blockade is demonstrated at 40 K. This is the first time Coulomb blockade is observed with an organic monolayer on oxidefree silicon. This work focuses on the fabrication and initial electrical characterization of this double barrier tunnel junction. The organic layers were prepared by thermal hydrosilylation of two different alkene molecules with either a long carbon chain (C 11 ) or a shorter one (C 7 ), and both were modified to be amine-terminated. FTIR and XPS measurements confirm that the Si(111) substrate remains unoxidized during the whole chemical process. Colloidal gold nanoparticles were prepared using two methods: either with citrate molecules (Turkevich method) or with ascorbic acid as the surfactant. In both cases AFM and STM images show a wellcontrolled deposition on the grafted organic monolayer. I-V curves obtained by scanning tunneling spectroscopy (STS) are presented on 8 nm diameter nanoparticles and exhibit the well-known Coulomb staircases at low temperature. The curves are discussed as a function of the organic layer thickness and silicon substrate doping.

INTRODUCTION

Despite almost 40 years effort and the opening of great routes for future applications, molecular electronics is still in its infancy. Indeed, it offers an unprecedented way for controlling the morphology at the subnanometer level based on its ability to build functional organic layers on silicon substrates as detailed in several recent reviews. [START_REF] Vilan | Molecules on Si: electronics with chemistry[END_REF][START_REF] Vuillaume | Molecular nanoelectronics[END_REF] A functional organic layer should also be able to carry out electrical functions such as driving charges with a high mobility, storing electrons in a memory device, [START_REF] Vuillaume | The metal/organic monolayer interface in molecular electronic devices[END_REF] or controlling their flow in a transistor. [START_REF] Clement | Relaxation dynamics in covalently bonded organic monolayers on silicon[END_REF][START_REF] Dubey | Modulation of flat-band voltage on H-terminated silicon-on-insulator pseudo-metal-oxidesemiconductor field effect transistors by adsorption and reaction events[END_REF] However, this requires a high degree of morphologic organization close to what is achieved in a silicon crystal. If not, the memory will leak and exhibit a poor retention time, the electrons will be trapped, [START_REF] Aureau | In situ monitoring of the electronic properties and the pH stability of grafted Si(111)[END_REF] or the Fermi level will be pinned in a noncontrollable way. [START_REF] Lenfant | Electron transport through rectifying self-assembled monolayer diodes on silicon: Fermi-level pinning at the molecule-metal interface[END_REF] Therefore, it is crucial to succeed in assembling molecules on silicon with atomic precision, starting with the most straightforward layer, an insulating layer.

In this article we describe a chemical route for building two different organic monolayers grafted on silicon and acting as ultrathin insulating layers. In order to probe the electrical quality of the organic layer, gold nanoparticles (AuNP) from colloidal solutions are covalently attached on top of the monolayer. The whole process is achieved by wet chemistry. It preserves an oxide-free silicon substrate that remains stable under ambient conditions. This architecture serves as a first tunnel junction (silicon/organic layer/AuNP junction) and is completed by a second tunnel junction established with an STM tip (AuNP/vacuum/tip junction). Such a double barrier tunnel junction (DBTJ) exhibits an electric behavior characterized by Coulomb blockade which is detectable at 40 K in UHV. This work focuses on the fabrication and initial electrical characterization of this double barrier tunnel junction. Coulomb blockade is a typical single charge phenomenon where electrons can be controlled one by one by adjusting the potential applied to the DBTJ. It is based on the fact that the system is mostly capacitive and the value of the capacitance is roughly proportional to the nanoparticle diameter. The electrostatic energy needed to add one more electron to the nanoparticle (called the island) is expressed as e 2 /2C, where e is the unit charge and C the capacitance. For very small nanoparticles this energy becomes larger than kT and the electrostatic energy overcomes the thermal noise, so that the electron flux can be controlled by an external applied potential. At room temperature, this condition is fulfilled for nanoparticles smaller than 5 nm. In the early 2000s, a strong interest arose for single electron phenomena such as the Coulomb blockade, fueled by the hope of creating new architectures for single charge electronics. [START_REF] Asahi | Single-electron logic device based on the binary decision diagram[END_REF][START_REF] Davidovic | Coulomb blockade and discrete energy levels in Au nanoparticles[END_REF][START_REF] Likharev | Single-electron devices and their applications[END_REF][START_REF] Zabet-Khosousi | Charge transport in nanoparticle assemblies[END_REF] Some single electron transistors have been fabricated [START_REF] Yang | Room temperature single electron transistor with two-dimensional array of Au-SiO 2 core-shell nanoparticles[END_REF][START_REF] Ray | CMOS-compatible fabrication of room temperature single-electron devices[END_REF][START_REF] Khondaker | The fabrication of singleelectron transistors using dielectrophoretic trapping of individual gold nanoparticles[END_REF][START_REF] Tsai | Formation of single-electron-transistors using self-assembly of nanoparticle chains[END_REF] as well as nonvolatile memories. [START_REF] Novembre | Gold nanoparticle-pentacene memory transistors[END_REF][START_REF] Chan | Mesoscopic phenomena in Au nanocrystal floating gate memory structure[END_REF] However, the development of devices based on single electron transport faces serious challenges due to the poor reproducibility of these devices and the difficulty in precisely controlling Coulomb blockade phenomena. One reason for this lies in the insufficient control of the interface quality and the nanogap thickness. [START_REF] Homberger | On the application potential of gold nanoparticles in nanoelectronics and biomedicine[END_REF] In order to observe single electron phenomena, the targeted device is the DBTJ. Given that the tunnel current depends exponentially on the barrier thickness, an accurate control of the samples geometry is necessary. Moreover, the typical thickness of such junctions lies between 0.8 and 2 nm, which requires that the tunnel barrier thickness must be controlled at the atomic level. Such a control can best be achieved with self-assembled monolayers. Highly ordered monolayers on silicon have been explored for decades, particularly after the discovery of a process to form atomically flat hydrogen-terminated silicon surfaces in 1989. [START_REF] Higashi | Ideal hydrogen termination of the Si(111) surface[END_REF] Starting from this H-Si(111)-(1 × 1) surface, many chemical routes [START_REF] Tao | Electronic and structural factors in modification and functionalization of clean and passivated semiconductor surfaces with aromatic systems[END_REF] have been used for replacing the terminal hydrogen by other molecules: thermal activation [START_REF] Linford | Alkyl monolayers on silicon prepared from 1-alkenes and hydrogenterminated silicon[END_REF][START_REF] Hacker | Modifying electronic properties at the siliconmolecule interface using atomic tethers[END_REF][START_REF] Aureau | Controlled deposition of gold nanoparticles on welldefined organic monolayer grafted on silicon[END_REF][START_REF] Li | Hybrids of organic molecules and flat, oxide-free silicon: high-density monolayers, electronic properties, and functionalization[END_REF] UV activation, [START_REF] Li | Hybrids of organic molecules and flat, oxide-free silicon: high-density monolayers, electronic properties, and functionalization[END_REF][START_REF] Voicu | Formation, characterization, and chemistry of undecanoic acid-terminated silicon surfaces: patterning and immobilization of DNA[END_REF][START_REF] Miramond | 1-Octadecene monolayers on Si(111) hydrogen-terminated surfaces: Effect of substrate doping[END_REF][START_REF] Faucheux | Welldefined carboxyl-terminated alkyl monolayers grafted onto H-Si(111): Packing density from a combined AFM and quantitative IR study[END_REF] electrochemical activation, [START_REF] Fellah | Kinetics of electrochemical derivatization of the silicon surface by Grignards[END_REF][START_REF] Teyssot | Anodic grafting of organic groups on the silicon surface[END_REF] and chlorination. [START_REF] Li | Hybrids of organic molecules and flat, oxide-free silicon: high-density monolayers, electronic properties, and functionalization[END_REF][START_REF] Webb | Transmission infrared spectroscopy of methyl-and ethyl-terminated silicon(111) surfaces[END_REF][START_REF] Puniredd | Catalyst-free functionalization for versatile modification of nonoxidized silicon structures[END_REF] More recently, some of these groups started investigating the electrical quality of these organic layers in order to integrate them within future functional architectures. [START_REF] Vilan | Molecules on Si: electronics with chemistry[END_REF][START_REF] Salomon | How do electronic carriers cross Si-bound alkyl monolayers?[END_REF] The monolayer is probed either globally, with the mercury drop method, [START_REF] Seitz | Importance of monolayer quality for interpreting current transport through organic molecules: Alkyls on oxide-free Si[END_REF][START_REF] Thieblemont | Electronic current transport through molecular monolayers: Comparison between Hg/alkoxy and alkyl monolayer/ Si(100) junctions[END_REF] with electrochemistry, [START_REF] Faucheux | Welldefined carboxyl-terminated alkyl monolayers grafted onto H-Si(111): Packing density from a combined AFM and quantitative IR study[END_REF] by depositing an oxide layer [START_REF] Peng | Probing the intrinsic electrical properties of thin organic layers/ semiconductor interfaces using an atomic-layer-deposited Al 2 O 3 protective layer[END_REF] or a metallic electrode, or sometimes local probes such as KPFM or conductive AFM are used. [START_REF] Miramond | 1-Octadecene monolayers on Si(111) hydrogen-terminated surfaces: Effect of substrate doping[END_REF] From these studies, it appears that the Si-organics interface can be made with a notably low amount of surface defects, in line with expectations based on the H-Si(111) interface. [START_REF] Seitz | Importance of monolayer quality for interpreting current transport through organic molecules: Alkyls on oxide-free Si[END_REF][START_REF] Peng | Probing the intrinsic electrical properties of thin organic layers/ semiconductor interfaces using an atomic-layer-deposited Al 2 O 3 protective layer[END_REF][START_REF] Seitz | One-step selective chemistry for silicon-on-insulator sensor geometries[END_REF] Interestingly silicon surfaces functionalized with organics are usually much more stable against oxidation [START_REF] Aureau | In situ monitoring of the electronic properties and the pH stability of grafted Si(111)[END_REF] (e.g., up to 1 month) than H-Si(111) surfaces, which is encouraging for implementing a robust functional molecular passivation of silicon surfaces.

EXPERIMENTAL METHODS

Preparation of Highly Ordered Monolayers on Si(111). Samples of 5 × 12 mm 2 were cut from n-doped silicon (111) wafers with resistivity of either 0.03 Ω•cm (∼1 × 10 18 cm -3 ) or 7 Ω•cm (∼6 × 10 14 cm -3 ). Float zone (FZ) grown samples were chosen in order to minimize the amount of oxygen and allow an accurate monitoring of potential surface oxidation with FTIR. [START_REF] Aureau | Controlled deposition of gold nanoparticles on welldefined organic monolayer grafted on silicon[END_REF][START_REF] Webb | Transmission infrared spectroscopy of methyl-and ethyl-terminated silicon(111) surfaces[END_REF][START_REF] Ogawa | Initial stage of native oxide growth on hydrogen terminated silicon (111) surfaces[END_REF] It should be noted that the chemistry described here works equally well with Czochralskigrown wafers but depends on the level of doping. [START_REF] Miramond | 1-Octadecene monolayers on Si(111) hydrogen-terminated surfaces: Effect of substrate doping[END_REF] Functional organic monolayers were prepared within four steps (see Scheme 1). In step 1, samples were initially cleaned in a piranha solution at 80 °C (3:1 H 2 SO 4 98%: H 2 O 2 30%) and copiously rinsed with ultrapure water (18 Mohm•cm, Millipore). Atomically flat, hydrogen-terminated Si(111)-(1 × 1) surfaces were prepared by oxide removal in a concentrated HF solution for 30 s, followed by a 150 s immersion in 40% NH 4 F solution and thorough rinsing in deionized water. [START_REF] Higashi | Ideal hydrogen termination of the Si(111) surface[END_REF][START_REF] Miramond | 1-Octadecene monolayers on Si(111) hydrogen-terminated surfaces: Effect of substrate doping[END_REF][START_REF] Allongue | Molecular imaging and local density of states characterization at the Si(111)/NaOH interface[END_REF] Caution is recommended because the piranha solution reacts strongly with organics, and the HF solution is extremely harmf ul. Both solutions should be handled with due protection: goggles and suitable gloves. Starting from a set of two different hydrogen-terminated Si(111)-(1 × 1) surfaces, step 2 consists of covalently attaching the organic layer by direct thermal hydrosilylation with either a long alkyl chain of 11 carbon atoms (Si-C 11 samples) or a shorter chain with 7 carbon atoms (Si-C 7 samples).

Si-C 11 . 10-Carboxydecyl organic monolayers were grafted on the hydrogenated silicon via direct thermal hydrosilylation of ethyl undecylenate. The neat alkene was outgassed three times by freezing it using liquid nitrogen under low vacuum (10 -2 Torr) in a Schlenk tube. It was then heated up to 200 °C under continuous argon bubbling when inserting the freshly prepared H-terminated silicon sample. Grafting was performed for 4 h.

Si-C 7 . 6-Carboxydecyl organic monolayers were grafted on the hydrogenated silicon via direct thermal hydrosilylation of ethyl 6heptenoate diluted (1:1) in mesitylene following the same procedure described above. Notable differences are the use of a lower vacuum during the outgassing step (10 Torr) with liquid N 2, and thermal hydrosilylation is performed at 170 °C. Both modifications are done to avoid evaporation of the molecule and most of the solvent during the preparation and the reaction. At the end of this second step both organic monolayers are terminated with a protected carboxylic acid.

In step 3, the functionalized surfaces were sonicated in ethyl acetate and rinsed in boiling dichloromethane (40 °C). The carboxylic acids groups were then deprotected by using potassium tert-butoxide in DMSO (for 90 s and then rinsed thoroughly with DMSO) and by finally using an acid solution (HCl) to protonate the active group. Finally, step 4 is the transformation of the functional headgroup into succinimidyl ester-terminated. This was performed by reaction with an aqueous solution of N-ethyl-N′-(3-(dimethylamino)propyl)carbodiimide (EDC). After a few minutes in EDC solution, 1 mL of ethylenediamine is added to the solution for 1 h 30 min at room temperature. Finally, the surface was rinsed with ultrapure water. During the last step, one amine group of the ethylenediamine molecule (H 2 N-CH 2 -CH 2 -NH 2 ) reacts with the activated ester, leading to the attachment of the molecule via an amide bond (-(C O)-NH-CH 2 -CH 2 -NH 2 ). It is very unlikely that the ethylenediamine binds in a bridging configuration through its two amine ends because the molecule length (2.99 Å) is shorter than the typical distance between two COOH groups estimated from the Si-Si distance. Therefore, at the end of step 4, the resulting surface presents amine groups on top of the layer. [START_REF] Seitz | One-step selective chemistry for silicon-on-insulator sensor geometries[END_REF][START_REF] Seitz | Copper-metal deposition on self assembled monolayer for making top contacts in molecular electronic devices[END_REF][START_REF] Seitz | Optimizing non-radiative energy transfer in hybrid colloidal-nanocrystal/silicon structures by controlled nanopillar architectures for future photovoltaic cells[END_REF] FTIR Characterization of the Grafted Organic Monolayers. All FTIR spectra were taken using a Nicolet 6700 FTIR spectrometer from Thermo Scientific equipped with a DTGS detector in transmission mode at an angle of incidence of 64°with respect to the Si surface normal in a dry N 2 (g)-purged atmosphere and with an unpolarized IR beam. The spectra were obtained after the initial Scheme 1. The Four Steps for Preparing the Two Kinds of the Grafted Organic Monolayer Considered in This Study molecular attachment and deprotection of the carboxylic acid, i.e., after step 3. The reference spectra were recorded in both cases using the oxide-free, fully hydrogen-terminated Si(111) surfaces.

In the case of Si-C 11 , no oxide was detected. This surface remains robust against oxidation as evidenced by the absence of any detectable absorption at 1080 and 1240 cm -1 (TO and LO modes of SiO 2 oxide) in Figure 1. In the case of Si-C 7 a negligible amount of oxide shows up at 1080 cm -1 . This oxide uptake is due to the time spent by the sample in air, needed for recording the FTIR spectra, and this characterization step was discarded for the samples used for STM measures. The negative peak at 2093 cm -1 is due to the replacement of Si-H bonds with Si-C bonds in the hydrosilylation process. Knowing that a fully hydrogen-terminated Si surface exhibits a Si-H line with an area of 0.08 cm -1 , an area comparison between the Si-H peak before and after molecular grafting shows that a coverage of roughly 1/2 of the surface is obtained for the SiC 11 and slightly more for the SiC 7 . Half of the hydrogen atoms remains on the unreacted silicon atoms as depicted in Scheme 1. The peak at 1713 cm -1 is assigned to the terminal CO bond. The two peaks at 1280 and 1410 cm -1 correspond to the C-O bonds, and the 1461 cm -1 band is due to the bending mode of the methylene groups of the alkyl chains. The positive peaks in the range of 2880 and 2920 cm -1 are due to the CH 2 chains. They exhibit a higher area in the case of Si-C 11 , confirming the presence of a longer chain.

XPS Characterization of the Grafted Organic Monolayer. Surface analysis after molecular grafting using X-ray photoelectron spectroscopy after step 2 confirms that there is no oxide present (see Figure 2) and gave almost identical results for both monolayers: in the vicinity of Si 2p core level shift, there is no evidence of Si oxidation (i.e., at 103.2 eV). Moreover, the C 1s region shows the specific peaks corresponding to the ester (290.0 eV), the C-O bond (288.0 eV), the C-C bond (285.0 eV), and the Si-C bond (284.5 eV). This last peak is less intense than the C-O and the ester peaks because it corresponds to carbon located deeper in the sample; i.e., fewer photoelectrons can escape. An additional peak is also observed at 286 eV and is attributed to organic contamination because of exposure to air after surface chemistry during transport to the XPS chamber. Spectroscopic ellipsometry measurements were also performed on the samples before gold nanoparticle deposition using an HORIBA Jobin Yvon ellipsometer (iHR320). After organic modification, the thickness and the dielectric constant of each organic layer are determined using a simple two-layer model similarly to our previous study. [START_REF] Aureau | Controlled deposition of gold nanoparticles on welldefined organic monolayer grafted on silicon[END_REF] The thicknesses of the layers derived from the results are 1.6 nm for the Si-C 11 and 1.3 nm for the Si-C 7 . These results are in agreement with the expected values (1.67 and 1.28 nm, respectively) calculated from the geometry of the molecules.

After monolayer preparation and initial characterization, the samples were express shipped inside an argon-filled plastic container from Dallas to Paris. The samples were stored under vacuum at a pressure of 10 -8 Torr as soon as they were received. The surfaces were subsequently further functionalized by gold nanoparticles and then characterized with UHV STM. It is important to stress that the samples remain stable in air for several days, before notable oxidation of the Si-C 11 surface is observed. We checked that STM was able to image straightforwardly the surface as long as it is not kept in air for too long a time. Typically after 2 weeks in air, STM imaging is no longer possible because silicon oxide has regrown underneath the organic layers with a thicknesses greater than ca. 0.6 nm, an oxide thickness preventing proper STM under normal conditions. [START_REF] Xue | Local study of thickness-dependent electronic properties of ultrathin silicon oxide near SiO 2 /Si interface[END_REF] This stability against silicon oxidation of these grafted organic monolayers is one of their main interests.

Gold Nanoparticle Preparation and Deposition. Two different methods were used to prepare colloidal gold nanoparticles (AuNP):

1. Turkevich AuNP. The citrate reduction method for synthesizing gold nanoparticles in water was pioneered by Turkevich et al. in 1951 42 and modified by Frens in 1973. [START_REF] Frens | Controlled nucleation for regulation of particle-size in monodisperse gold suspensions[END_REF] It remains one of the most popular methods for producing spherical monodisperse gold nanoparticles. An aqueous solution of HAuCl 4 (20 mL, 0.25 mM) was boiled under refluxing conditions and vigorous stirring. Sodium citrate was quickly added (1 mL, 18 mM), and the solution color changed from faintly yellowish to clear gray, purple, and finally dark purple within 2-3 min. The resulting AuNP have a diameter of typically 15 nm as already established [START_REF] Diegoli | pH-dependent gold nanoparticle self-organization on functionalized Si/SiO 2 surfaces[END_REF][START_REF] Kimling | Turkevich method for gold nanoparticle synthesis revisited[END_REF][START_REF] Pluchery | Optical Characterization of Supported Gold Nanoparticles for Plasmonic Biosensors[END_REF] and demonstrated below by AFM. In this reaction, citrate first acts as a reducing agent and tends to adsorb onto the metallic gold clusters, slowing down their growth and resulting in size regulation. Moreover, citrate ions are negatively charged such that the nanoparticles repel each other. [START_REF] Kimling | Turkevich method for gold nanoparticle synthesis revisited[END_REF][START_REF] Roucoux | Reduced transition metal colloids: A novel family of reusable catalysts?[END_REF][START_REF] Ji | Size control of gold nanocrystals in citrate reduction: The third role of citrate[END_REF] 2. Ascorbic Acid Nanoparticles (asc-AuNP). The asc-AuNP were synthesized following a method described elsewhere. [START_REF] Stathis | Preparation of colloidal gold[END_REF] Typically, 200 μL of an aqueous solution of HAuCl 4 •3H 2 O (10 g L -1 of gold) was added to 25 mL of water at around 2 °C (ice-cooled) in a beaker. Then, 1.5 mL of a Na 2 CO 3 solution (21.2 g L -1 ) and 1 mL of an ascorbic acid solution (7 g L -1 ) were added under vigorous stirring. The solution became instantaneously dark red.

Deposition of AuNP on the organic monolayer was performed by dipping the functionalized silicon substrate into the colloidal solutions. The attachment occurs through the interaction between the -NH 2 moiety of the organic layer and gold surface. In the case of Turkevich AuNP the maximum nanoparticle coverage is reached after 1 h and occurs with an initial step of electrostatic attraction between the negatively charged citrate and the -NH 3 + end group which is protonated due to the acidity of the Turkevich solution (pH = 5). In the case of the asc-AuNP, the solution was first acidified to pH = 6 with a 0.1 M hydrochloric acid solution. The solution color remained unchanged. The functionalized silicon surface was dipped into the AuNP solutions for 12 min, allowing the particles to become grafted onto the amino-terminated surface while remaining well separated. In both cases, the substrate was cleaned after the dipping by several rinses in deionized water and dried with a nitrogen gas stream.

AFM Characterization of AuNP on the Monolayer. AFM was performed on these surfaces using the tapping mode with a Digital Instrument DI-3000 (Figure 3a) or a Multimode-8 instrument from Veeco (Figure 3b) and were processed with the software WSxM. [START_REF] Horcas | A software for scanning probe microscopy and a tool for nanotechnology[END_REF] In the case of the Turkevich AuNP, the deposition was carried out as a test experiment to make sure that attachment occurred on the oxide-free monolayer investigated here similarly to what had been previously demonstrated in other publications dealing with monolayer on oxidized silanized silicon surfaces. [START_REF] Diegoli | pH-dependent gold nanoparticle self-organization on functionalized Si/SiO 2 surfaces[END_REF][START_REF] Pluchery | Optical Characterization of Supported Gold Nanoparticles for Plasmonic Biosensors[END_REF][START_REF] Lee | Adsorption patterns of gold nanoparticles on methyl-terminated self-assembled monolayers[END_REF] The amino-terminated monolayer layer was confirmed as being very efficient at immobilizing the nanoparticles, [START_REF] Aureau | Controlled deposition of gold nanoparticles on welldefined organic monolayer grafted on silicon[END_REF] and the AFM images acquired in tapping mode showed no indication that the AuNP were moved by the tip. However, Turkevich nanoparticles are too large (diameter ∼15 nm) to exhibit Coulomb blockade at room temperature since this phenomenon requires NPs diameter around 5 nm. Therefore, we concentrate on the asc-AuNP. As shown in Figure 3b, the asc-AuNP are randomly distributed over the organic layer and there was no aggregation. Based on their profile height, the average size of the nanoparticles is 9.5 nm (standard deviation of 2.5 nm). Practically, this means that it is possible to find nanoparticles with a diameter between 4 and 15 nm on a typical AFM or STM image. The roughness of the underlying substrate without nanoparticles was measured to be 0.36 nm.

STM. STM images were recorded with a commercial apparatus (Omicron VT-STM) whose UHV chamber has a base pressure of 4 × 10 -11 Torr, in which the sample could be cooled down to 25 K with a coldfinger in contact with liquid helium. The samples were introduced into the preparation chamber with a load-lock and then annealed at 150 °C for 30 min before STM imaging. This annealing procedure removes any physisorbed water molecules and greatly improves the image quality by minimizing fuzzy structures during imaging. The organic layer is not affected by this thermal treatment because it was prepared at similar temperatures. Imaging a given sample before and after annealing (though not on the same area) allowed verification that the AuNP distribution did not change significantly before and after this annealing procedure. In the STM experiments, the indicated voltage is the bias of the surface relative to the tip.

RESULTS AND DISCUSSION

STM Characterization. STM images of the two kinds of organic-functionalized surfaces (see Scheme 1) were recorded in UHV at room temperature prior to AuNP deposition, as shown in Figures 4a and4b. The surfaces are remarkably flat with a rms roughness of 0.33 and 0.38 nm, respectively, in agreement with AFM experiments, which indicates that the monolayer is very well ordered. The surface shown in Figure 4a exhibits parallel lines that are attributed to the silicon terraces. [START_REF] Miramond | 1-Octadecene monolayers on Si(111) hydrogen-terminated surfaces: Effect of substrate doping[END_REF] The inset in Figure 4b shows that 3 nm large protrusions can be resolved, corresponding to bundles of ∼10 molecules. [START_REF] De Smet | Visiblelight attachment of SiC linked functionalized organic monolayers on silicon surfaces[END_REF] STM imaging found no notable differences between Si-C 7 and Si-C 11 surfaces.

After the asc-AuNP were grafted ex situ as described above, the samples were analyzed again by STM. As evidenced in Figures 4c and4d, AuNP are clearly visible, with a topographic distribution comparable to what was measured by AFM (Figure 3). The AuNP were strongly attached and could not been displaced by the STM tip as might occur on other systems such as self-assembled monolayer on Au(111). The diameters of two particles (marked with a cross in Figures 4c and4d) were evaluated, taking advantage of their height and found to be 7.5 nm for both particles.

STS Measurement. After acquiring an STM image, the current-voltage spectra I(V) were recorded on various AuNP selected on the basis of their size (closest to 5 nm) and having sufficient separation from neighboring particles. The tip was placed above each AuNP at a distance determined by the current set point used for imaging, typically between 20 and 50 pA. At ca. 40 K, the typical lateral drift was 0.4 nm/min after 1 h stabilization time. For each image and before I(V) acquisition, this drift was precisely calculated, and a correction was applied so that the tip did not move more than 0.01 nm over a nanoparticle during data acquisition. After having switched off the feedback loop, a series of 40 spectra were acquired and averaged. However, a few curves were usually dropped from the averages because of tip mechanical instability. As depicted in Scheme 2, positioning the STM tip over a gold nanoparticle creates a double barrier tunnel junction (DBTJ). The first barrier is composed of the organic layer and the second of the vacuum gap between the tip and the nanoparticle. This system can be electrically modeled by the circuit represented in Scheme 2 where each tunnel junction is represented by a capacitance and a resistance in parallel. If the two junctions are symmetric, the I = f(V) curve exhibits a Coulomb blockade region (-e/2C < V < e/2C) where no current flows through the DBTJ, and an Ohmic behavior is expected outside this region. On the other hand, if the DBTJ is asymmetric, the I = f(V) curve is staircase-like, and the number of extra electrons simultaneously present in the island is a discrete number directly controlled by V. [START_REF] Mullen | I-V characteristics of coupled ultrasmall-capacitance normal tunneljunctions[END_REF] About 100 AuNP were investigated under different conditions: three different temperature ranges (room temperature, 100 K with liquid nitrogen, and 25-40 K with liquid helium cooling), two different monolayer thicknesses (C 7 and C 11 ), two different substrate doping, and several AuNP diameters ranging from 4 to 10 nm. In this paper, we select a few examples to illustrate the performance of this organic monolayer-based system, and we restrict ourselves to nanoparticles of one size.

Coulomb Blockade. I(V) spectra on three AuNP of similar size (8 nm) are presented in Figure 5: two spectra were recorded with the same Si-C 11 monolayer, but with two different substrate doping levels, and the third spectrum was taken on the thinner Si-C 7 layer. They illustrate the understanding of the influence of the thickness of the tunnel barrier and the influence of the semiconductor doping level on electron transport. The spectra were recorded at low temperatures (25, 37, and 100 K, respectively). In all three samples, Coulomb staircases were observed (top graph of Figure 5) and are evidenced by differentiating the i(V) spectrum and calculating (di/dV)/(i/V). The quantity di/dV is proportional to the local density of states (LDOS) and allows detecting the electrostatic energy levels generated by Coulomb blockade in the metallic island. [START_REF] Zabet-Khosousi | Charge transport in nanoparticle assemblies[END_REF] Each Coulomb oscillation in the differentiated curve corresponds to a step of the Coulomb staircase, indicative of an additional electron stored in the nanoparticle. These results demonstrate that the quality of an organic monolayer on silicon can be such that they act as a local tunnel barrier between the substrate and a metallic nanoparticle and that it is possible to prevent an insulating oxide from being spontaneously formed. However, the exact Coulomb blockade behavior depends on many parameters such as the size and shape of nanoparticles, their crystallinity, [START_REF] Lu | Synthesis and characterization of crystalline and amorphous palladium nanoparticles[END_REF] the presence of an organic surfactant on the nanoparticle, and the actual bonding between the nanoparticle and the organic layer. These delicate issues have barely been investigated in the literature so far, and we do not address them in detail in the present article. A further analysis of our acquired data set will be reported in a forthcoming article and will focus on the transport measurements in a more quantitative approach. Nevertheless, we checked that on the three nanoparticles discussed herein Coulomb blockade was repeatedly observed during a measurement campaign and that for a given particle the positions of the di/dV peaks were reproduced within an interval of 0.08 V.

In the case of the low doped substrate (N d = 6 × 10 14 cm -3 ) and with the Si-C 11 layer (curve a in Figure 5), the current onset is measured at V 0 = 1.37 V, and the second step shows up at V 1 = 1.87 V (see Table 1 for an overview of the relevant experimental values). According to the theory developed for Coulomb phenomena on metals, [START_REF] Likharev | Single-electron devices and their applications[END_REF][START_REF] Mullen | I-V characteristics of coupled ultrasmall-capacitance normal tunneljunctions[END_REF] the step position is given by V n = (2n + 1)e 2 /2C: the current onset is expected at a potential that is the half of the step width. Obviously this is not the case in the present experiments since V 0 ≠ 1/2(V 1 -V 0 ). In order to understand the reason for this apparent discrepancy, the doping of the substrate was increased to N d = 7 × 10 17 cm -3 , keeping the same molecular insulating layer and the same nanoparticle diameter. In this case the current onset was lowered to V 0 = 0.85 V, and the first step is measured at V 1 = 1.22 V (curve b in Figure 5). The difference between the measured and theoretical onset is due to the strong band bending effect occurring in silicon substrates and particularly in those that are low doped. These effects are qualitatively shown Scheme 2. Configuration Used To Build a Double Barrier Tunnel Junction with the Tip of the STM Placed above a Nanoparticle Figure 5. Tunnel spectroscopy on 8 nm nanoparticles at low temperatures (25, 100, and 37 K, respectively) showing Coulomb staircases (upper graph) and its derivative (di/dV)/(i/V) (lower graph). Each peak of the derivative corresponds to a supplemental electron added to the nanoparticles. Spectra (a) and (c) were recorded on the AuNP marked with a cross in Figures 4c and4d, respectively. The silicon substrates were n-doped in all cases with a carrier density of 4 × 10 14 cm -3 , 7 × 10 17 cm -3 , and 2 × 10 18 cm -3 , respectively.

in Scheme 3 where the energy profile is drawn as a function of the distance away from the silicon surface. This scheme clearly shows the two barriers that an electron needs to cross in order to travel from the substrate to the STM tip. In the present case, both barriers are of the order of 1 nm and an electron can tunnel through them. The profile is derived from the work functions of all the materials of the DBTJ, knowing that the Fermi levels should be aligned through the entire structure when no bias is applied. [START_REF] Sze | Physics of Semiconductor Devices[END_REF][START_REF] Brennan | The Physics of Semiconductors: With Applications to Optoelectronic Devices[END_REF] For the first barrier tunnel junction (denoted BTJ #1), the work function of silicon W Si is given by the sum of the electron affinity of silicon χ Si and the energy difference ξ = E C -E F,Si such that W Si = χ Si + ξ ≈ 4.4 eV. χ Si is slightly attenuated by the electrostatic dipole generated by the molecular layer. The molecular dipole δ is independent of the applied bias and was evaluated at around -0.4 V in similar cases. [START_REF] Hacker | Modifying electronic properties at the siliconmolecule interface using atomic tethers[END_REF][START_REF] Hunger | Chemical and electronic characterization of methylterminated Si(111) surfaces by high-resolution synchrotron photoelectron spectroscopy[END_REF] In Scheme 3 this dipole shows up as shift of the vacuum level indicative of an electric field close to the organic surface. The Fermi levels of silicon and gold are aligned at equilibrium, and since the work function of gold is W Au = 5.0 eV, it results in the creation of an electric field between the two materials so that the bands of the semiconductor tend to bend upward even at zero applied bias. As a consequence of this band bending, an interface potential is generated, denoted Ψ S in Scheme 3. When a positive bias V Si is applied as depicted in case b of Scheme 3, the band bending is amplified and the electron needs to roll up this supplemental hill to reach the tip. As a result, a supplemental amount of energy needs to be provided, which is done by further increasing the applied voltage V Si . This effect is all the more important when the doping is low. This explains why the steps of the Coulomb staircases were detected at higher potential for the low doped samples. Therefore, band bending has two consequences: first, the actual interface potential applied to the silicon/organic layer/AuNP system is much weaker than the external potential applied by the STM tip, and the electrons need to climb this supplemental barrier. Second, the process depends on the thickness of this barrier, which can be very thin in the case of high doping allowing the charges to tunnel through it. This is not the case for low doped substrates due to band bending. The band bending potential at zero bias has been evaluated to be +0.60 V for low n-doped substrate (10 15 cm -3 ) and +0.50 V for highly n-doped silicon (10 19 cm -3 ) by Cahen's group. [START_REF] Yaffe | Hg/molecular monolayer-Si junctions: Electrical interplay between monolayer properties and semiconductor doping density[END_REF] Hacker found a value of +0.26 V with low n-doped substrate (4 × 10 14 cm -3 ). [START_REF] Hacker | Modifying electronic properties at the siliconmolecule interface using atomic tethers[END_REF] Although these two values differ, probably because of the different ways of estimating the molecular interface dipole, they confirm the order of magnitude of the voltage shift observed in our experiments.

Finally, with the shorter Si-C 7 monolayer, the Coulomb phenomena are even more pronounced, as shown in curve c of Figure 5: the steps are clearly visible, and the current is stronger due to a higher conductance of this thinner organic layer. The current onset is detected at 0.80 V, very similar to curve b, because the doping level and nanoparticle diameters are identical. The step widths are also comparable. It is not possible to compare the step height since the tunneling conditions were not the same for these three experiments (see last column of Table 1), and the actual tip-nanoparticle distances differ and are unknown in all three cases.

The inset in Figure 5 illustrates another consequence of working with a low doped substrate. With the high doped substrate, the i(V) is almost symmetric with stairs in positive and reverse bias. At positive and reverse biases, the current onset is measured at +0.80 and -0.89 V, respectively. However, for a low doped substrate, no current is detected in reverse bias, and the junction behaves like a Schottky diode. The first barrier tunnel junction is between the silicon and the selected AuNP, and the second BTJ is between this AuNP and the STM tip.

CONCLUSION

For the first time, a self-assembled monolayer made with organic molecules covalently grafted on oxide-free silicon has been used for measuring Coulomb blockade. This grafted organic monolayer consists of a highly ordered 10-carboxydecyl organic monolayer on silicon which is further modified with an amine termination designed for covalently attaching colloidal gold nanoparticles. By using the tip of an UHV-STM system, above a gold nanoparticle, a double barrier tunnel junction was formed and was found to exhibit clear Coulomb staircase, as evident in the I-V plots. The influence of the doping level of the silicon substrates on the electrical behavior of this system has been investigated. Low doped substrates induce a strong band bending near the Si/organic layer/Au junction, so such that the actual interface potential is much lower than the applied bias. The step width of the Coulomb staircases is therefore increased, compared to the case of highly doped silicon. Finally, the hydrosilylation method for preparing the organic layer was successfully applied for attaching shorter molecules, and we also investigated the influence of monolayer thickness on the step heights. Results suggest that the step heights are found to be higher for the thinner monolayer as expected. These results open a way for using such grafted organic monolayers on oxide-free silicon as active layers for molecular electronics.

■ AUTHOR INFORMATION

Corresponding Author *E-mail olivier.pluchery@insp.jussieu.fr.

Figure 1 .

 1 Figure 1. FTIR transmission spectra showing the organic monolayer fabrication when the surface is terminated with carboxylic acid at step 3 (see Scheme 1). The monolayer was obtained by thermal hydrosilylation with ethyl undecylenate (abbreviated Si-C 11 , spectrum a) and ethyl 6-heptenoate (Si-C 7 , spectrum b). The reference spectra were recorded in both cases using the oxide-free, fully hydrogenated Si(111)-(1 × 1) surface. These spectra demonstrate that the hydrosilylation process occurs without oxidizing the silicon substrate.

Figure 2 .

 2 Figure 2. XPS characterization of Si-C 11 and Si-C 7 molecules after thermal hydrosilylation. The C 1s peaks between 284.5 and 290.0 eV observed for both Si-C 7 (a) and Si-C 11 (b) confirm that the molecules are grafted on the surface. The Si 2p regions for Si-C 7 (c) and Si-C 11 (d) contain no evidence of oxidation.

Figure 3 .

 3 Figure 3. AFM images in tapping mode of AuNP deposited on highly ordered monolayers on Si(111)-(1 × 1) surfaces. The Turkevich nanoparticles (a, c) were deposited on the Si-C 11 organic layer and the asc-AuNP (b, d) on a Si-C 7 organic layer.

Figure 4 .

 4 Figure 4. STM images (100 × 100 nm 2 ) of different surfaces of organic layers grafted with a Si-C bond to the Si(111) substrate. Surfaces (a) and (c) were prepared with the long chain molecule (Si-C 11 ) and surfaces (b) and (d) with the shorter chain (Si-C 7 ). Inset in image (b) is a 20 × 20 nm 2 zoom of the surface with an enhanced contrast showing the molecular bundles on the surface. On both surfaces, asc-AuNP have been grafted through the -NH 2 terminal group of the organic monolayer (images c and d). The cross indicates the AuNP where STS was performed. Images (a) and (b) have been recorded at room temperature and images (c) and (d) at low temperatures (25 and 37 K, respectively). Scanning conditions are given on the images (gap voltage and current set point).

Table 1 .

 1 Electric Characteristics of the Coulomb Staircases Measured with Our Silicon Substrates a The voltages are obtained with an accuracy of 0.05 V. Scheme 3. Energy Diagram of the DBTJ a

	Figure	doping/ cm -3	organic layer	temp (K)	NP diam (nm)	V 0 onset (V)	V 1 2nd step (V)	V 2 third step	V 3 2nd step (V)	ΔV Step width (mV)	Δi step height (pA)	tunnel conditions
	5a	low 6 × 10 14	Si-C 11	25	8.2	1.37	1.87	2.39		500	20	20 pA, +2.2 V
	5b	high 7 × 10 17	Si-C 11	100	8.0	0.85	1.22	1.58	1.92	370	2	5 pA, +2.0 V
	5c	high 2 × 10 18	Si-C 7	37	7.5	0.80	1.20	1.65	2.10	420	15	50 pA, -2.0 V

a

  Matisse program from UPMC. The work carried out at UTD (wet chemical modification and spectroscopic characterization) was supported by the National Science Foundation (Grant CHE-0911197) and partly by the Texas Higher Education Coordinating Board (NHAR Program). P.M.C. has taken part of the project through Eugene McDermott Scholars Program. Some aspects of the work described herein has benefited from the help and fruitful discussions with several colleagues to whom we want to express our sincere gratitude: Philippe Dollfus, Souhir Boujday, Irina Ionica, and Ahmed Naitabdi.

	Present Address

∥ O.S.: Rolith, Inc., 5880 W. Las Positas Blvd, Pleasanton, CA 94588.

Notes

The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS R.D. acknowledges a financial support from Labex-


A Nowacka

J Moughames

Z Adem

A.-A Quoineaud

M Rolland

F Guenneau
email: flavien.guenneau@upmc.fr

A Gédéon

In situ Magnetic Resonance Imaging study of the impregnation of γ-alumina pellets

Keywords: MRI, SPI, γ-Al 2 O 3, alumina impregnation, catalyst preparation, Ni 2+

SPI enables studying catalyst preparation in the presence of paramagnetic ions. The transport rate of ions in the pellet depends non-linearly on the concentration. A minimum Ni 2+ concentration is required to quickly reach uniform ion distribution.

Introduction

The removal of sulfur from diesel fractions is receiving increasing interest due to the environmental legislation enforcing the reduction of sulfur content in fuel. Since 2009, regulations required the sulfur content in diesel fractions to be lower than 10 ppm in Europe [1] and the "ultra low sulfur diesel" objective is now being considered. In order to face those demanding, environmental standards, the development of more active and selective hydrodesulfurization (HDS) catalysts is desired, to improve fuel quality without negative impact on the purification cost. Due to its high specific surface area, mechanical, chemical and thermal stability [START_REF] Misra | Industrial Alumina Catalysts[END_REF], the aluminum oxide (γ-Al 2 O 3 ), also called γ-alumina, is widely used as a solid support for catalysts in numerous industrial processes. Cobalt or nickel-promoted molybdenum sulfide catalysts are used as the active phase, called CoMoS or NiMoS, in hydrotreatment processes, such as HDS, due to their high activity and stability. The CoMoS and NiMoS phases are obtained by sulfidation of the oxide catalysts. However, when supported on -alumina, Ni or Co-promoted MoS 2 catalysts are composed of several other less active species such as unpromoted MoS 2 particles, NiS or Co 9 S 8 clusters, Ni 2+ or Co 2+ in Al 2 O 3 (nickel or cobalt aluminates, respectively), and Mo oxysulfides [START_REF] Topsøe | [END_REF][START_REF] Toulhoat | Catalysis by Transition Metal Sulphides from Molecular Theory to Industrial Application[END_REF]. The preparation method is crucial to improve the promotion of Mo and decrease the formation of the unpromoted species. Effort has been put into investigating the efficiency of the catalysis [START_REF] Broderick | [END_REF][6][7] and analyzing the factors influencing the catalyst preparation, such as pH [8,9] or the presence of complexing agents [10]. Highly active catalysts were obtained by modifying the sulfidation procedure [11], using carriers other than alumina [12][13][14], employing various precursors, such as thiometallates [15], carbonyl complexes [16], heteropolyanions [17] or even additives such as chelating agents [18,19]. Furthermore, the physicochemical properties of the γ-alumina support itself were studied [20], as well as those of the active species during the impregnation process [21][22][23].

Among the experimental techniques that can be used to characterize the impregnation process, Nuclear Magnetic Resonance (NMR) and Magnetic Resonance Imaging (MRI) provide a unique, nondestructive method of studying materials and processes in situ. The phenomenon of magnetic resonance (MR) is based on the presence of nuclear isotopes, such as 1 H, 13 C, 27 Al, which respond to a strong, external magnetic field [START_REF] Keeler | Understanding NMR Spectroscopy[END_REF]. The response is dependent on a number of parameters, allowing the observer to distinguish between the nuclei experiencing microenvironments different in terms of chemical composition or mobility, via the means of chemical shift or relaxation phenomena, respectively [START_REF] Keeler | Understanding NMR Spectroscopy[END_REF]. Thus, placing an (impregnated) alumina support in a magnetic field, it is possible to use the atomic nuclei present in the catalyst components themselves for the study. The nondestructive aspect of the method permits taking consecutive measurements to investigate dynamic processes, such as ion diffusion within an alumina extrudate. NMR spectroscopy, especially under the conditions of Magic Angle Spinning, has been extensively used to study catalysts and catalysis [START_REF] Ivanova | [END_REF][26][27], offering solutions for in situ observation in batch-like [28] and flow [29] conditions. Magnetic Resonance 1 H Imaging ( 1 H MRI) is becoming an increasingly popular technique, allowing for 2-and 3-dimensional imaging of the liquid components within the catalyst in order to not only identify the molecular species but also to map out their positions in the solid support. Thus the impregnation of the catalyst support with metal ions [30,31], as well as the flow of liquid/gas in a working trickle-bed catalyst [32], can be visualized in situ. Simultaneously, attempts have been made to analyze the structure of the alumina support itself, with the use of multinuclear solid-state MRI [33].

In the present study, we investigate the first step of the catalyst preparation: the impregnation of the alumina support with a metal salt solution. In order to ensure efficient HDS, the active phase should be homogeneously distributed throughout the pellet volume. Simultaneously, factors such as the time necessary for obtaining homogeneous ion distribution and the ion concentration that can be introduced into the pellet porosity come into play. In this article we propose an MRI-based method for estimating the first of those factors at a range of Ni 2+ concentrations. The aim is not to quantitatively describe the concentration profile during the impregnation, but to discuss the influence of the solution concentration on the time required for reaching a uniform ion distribution. Due to water 1 H interactions with the porous material surface, as well as the traces of paramagnetic elements present in the alumina, the transverse relaxation time (T 2 ) of pure water inside the alumina is reduced to values of the order of a couple of milliseconds. Such a short T 2 excludes the possibility of using the widely-spread spin echo based sequences [30,31,[34][35][36][37], which usually require a couple of milliseconds of preparation time and gradient encoding. A similar problem existing in the field of medical imaging, special spin echo sequences, using ultrashort echo time [38] or even zero echo time [39] have been designed, exploiting radial sampling schemes to reduce the necessary echo time. A considerable drawback, in case of studies of dynamic processes, relies in the an increased number of acquisitions [38], which leads to longer experimental times. There also exist pulse sequences designed to study paramagnetic samples with MRI, the most notable being continuous wave MRI [40,41] and Constant-Time Imaging, CTI [42,43], also known as Single Point Imaging (SPI). Both have been successfully used on bentonite [40,43], a clay where transverse relaxation times of the order of milliseconds can be expected, due to the presence of paramagnetic nuclei. Because continuous wave MRI requires considerable hardware adjustments, we propose SPI as a tool to follow the impregnation of γ-Al 2 O 3 pellets with Ni(NO 3 ) 2 solutions of varying concentration. Designed for studying materials with short T 2 values, SPI is based on simultaneous switching on of all three gradients prior to the excitation pulse, decreasing the required preparation time. It thus makes SPI an ideal tool for studying the impregnation of alumina pellets with traces of paramagnetic elements in situ.

Materials and Methods

Alumina pellet impregnation

Trilobe pellets of -alumina with 1.2-1.4 mm diameter and 5-6 mm length were prepared by kneading-extrusion of boehmite powder. Hg porosimetry (Micromeritics Autopore IV) and N 2 isotherm (Micromeritics ASAP 2420) were used to determine the specific surface area, porosity and mean pore size of the alumina to be 200-250 m² g -1 , 0.7-0.9 mL g -1 and 8-11 nm, respectively. The point of zero charge (pzc) was 8 -8.5. The alumina pellets were not submitted to any pre-treatment before impregnation. Ni(NO 3 ) 2 •6H 2 O of  98.5 % purity (Sigma-Aldrich, CAS number 13478-00-7) was used to prepare solutions at the desired concentrations of Ni 2+ ions: 0.05 M, 0.1 M, 0.2 M and 0.3 M. The pH of the prepared solutions varied from 6 to 5.1. Impregnation was conducted by rapidly submerging a semi-dry alumina pellet in a Ni(NO 3 ) 2 aqueous solution of chosen concentration for 30 s. The pellet was then taken out and the excess solution was removed by absorbing paper. The pellet was subsequently transferred into a 5 mm, thick-wall NMR tube having the internal diameter of 2.2 mm. Throughout the article, the samples will be referred to by the Ni 2+ concentration of the impregnating solution. After the MRI experiments, the samples were controlled with an electron microprobe (Jeol 8100 Electron Probe Micro Analyser with an EDS detector) to confirm the homogeneous ion distribution. MRI All experiments were performed at ambient temperature, on a Bruker Avance 300 MHz (7 T) magnet, equipped with a 5 mm micro imaging probe, having the maximum gradient capacity of 2 T m -1 in the x, y and z directions. The SPI pulse sequence was used with a field of view (FOV) of 2.5×2.5×8 mm and 32×32×8 acquisition points, resulting in the voxel size of 0.078×0.078×1 mm. The gradient encoding time, T P , was set to 100 µs with a maximum gradient of 1.4 T m -1 in x and y directions and 0.11 T m -1 in the z direction. An excitation pulse of 90° and 15 µs was used. With the recycle delay, T R , of 5 ms and one repetition per point the total experimental time of 41 s was achieved. As stipulated in the original CTI article [42], the use of long excitation pulse can lead to the blurring of sharp edges in the image and the loss of resolution. To preserve the image quality the pulse duration, T pul , must satisfy:

n T T p pul 2  , ( 1
)
where n is the number of points in the k-space. For the experimental parameters chosen in this study, T p = 100 µs and n = 32 in x and y directions, the maximum pulse duration is recommended to be 6.25 µs. However, the blurring effect was investigated experimentally and found to be unobservable on the images. In the z direction, where n = 8, the maximum recommended pulse duration is 25 µs. Inversion recovery and CPMG pulse sequences were used to measure the T 1 and T 2 relaxation times, respectively. The 180° pulse was set to 30 µs and the echo time was 400 µs. The measurements were made after homogenous ion distribution was obtained.

Image analysis

An automated procedure was implemented in the Halcon HDevelop image treatment environment to refine the image analysis. In each image (Figure 1a), the ion-rich, bright zone of the pellet was identified by choosing an appropriate threshold (Figure 1b). Subsequently, the unphysical dents and protrusions at the interface were removed (Figure 1c). The radius and the position of the smallest circle encompassing the extrudate were identified (Figure 1d). The value of pixel intensity at the pellet core was measured by taking the minimum value of the 4 pixels adjacent to the center of the circle (Figure 1e).

Results and discussion

As mentioned in the introduction, short transverse relaxation times exclude the use of traditional, spin-echo based imaging pulse sequences. Requirements of pulse length and gradient encoding time, as well as the delays necessary for correct functioning of the spectrometer, set the lower limit of the time required between the excitation pulse and the acquisition at values comparable to or higher than the T 2 of water 1 H in our alumina pellets (Table 1). In practice, the T 2 of the order of a couple of milliseconds leads to nearly complete signal decay before the end of the pulse sequence and gradient encoding, even in the absence of the paramagnetic, metal ions purposefully introduced into the pellet as the catalyst precursors. Single Point Imaging (SPI) is an MRI technique originally designed to study solid-state samples with short transverse relaxation times [42]. Rather than relying on echo techniques, SPI encodes the spatial coordinates by the use of three, simultaneous phase gradients (Figure 2). The gradients are switched on before the excitation pulse, decreasing the evolution period to the values of the order of hundreds of microseconds. Such an approach allows the acquisition of fast-relaxing signal components.

However, the acquisition of a single k-point per excitation results in the increase of the experimental time due to the recycle delay, necessary for complete longitudinal relaxation (related to T 1 ) after each acquisition. Two options can be used to shorten the recycle delay: using a small-angle excitation pulse or allowing T 1 contrast to be introduced into the image. In the current study, the excitation pulse flip angle and the recycle delay were set to 90° and 5 ms, respectively, in order to obtain the strongest T 1 contrast possible.

In the absence of nickel the water protons ( 1 H) experience T 1 of the order of hundreds of milliseconds (Table 1), due to the paramagnetic elements present in the solid support. The presence of nickel ions decreases the T 1 of water 1 H to 19 ms, at the lowest investigated concentration, 0.05 M. In a sense, the nickel ions act as a contrast agent, allowing the observation of the water 1 H signal in their proximity, while the water 1 H signal in other microenvironments is efficiently cancelled, as has been verified by recording images of an alumina pellet impregnated with pure water. The use of a long excitation pulse simultaneously to the incremented magnetic field gradients raises the question of insufficient edge definition because the magnetization corresponding to high k-space values might get attenuated [42]. Indeed, according to Equation 1, the used pulse length, T pul = 15 µs, is longer than recommended. To verify the correctness of the proposed set up, an experiment was conducted with T pul = 5 µs, well below the limit set by Equation 1. No difference between the images recorded with T pul = 15 µs and T pul = 5 µs was observed. It can be explained by the irregular shape of the alumina pellet and a relatively low resolution of the image, compared to the size of the pellet: a pixel on the image corresponds to 0.078x0.078 mm, while the pellet diameter is 1.2 -1.4 mm. Conversely, a significant amount of signal from the microenvironments not containing nickel ions was present in the images recorded with the T pul = 5 µs excitation pulse, decreasing the image contrast.

The current study deals with dynamic phenomena and a minimal total acquisition time was required, with the best possible contrast and SNR. Furthermore, the estimation of uniform distribution in the extrudate was made based on the signal in the centre of the pellet reaching equilibrium, which would be minimally affected by edge blurring. Consequently we have chosen to keep the 90° excitation pulse.

Figure 3 shows the results of the analysis conducted with Halcon HDevelop (Figure 1) for a series of experiments following the ion penetration of Ni 2+ at 0.1 M impregnating solution. Each point corresponds to the signal intensity of a pixel in the middle of the extrudate, averaged over 1 mm thickness in the z direction. Images one and eight were empty in this case, due to the length of the pellet.

For the first 50 minutes there is no change in signal intensity, after which a slope appears, leading to equilibrium at approximately 200 min. This trend is the same in all four inner images, plotted on Figure 3 in green and black. The edge images barely show a change of intensity, starting from a value nearly 30% higher than the inner images. While initially surprising, it can be explained by the impregnation method we used, which implies that the Ni 2+ ions enter into the pellet by the sides, the top and the bottom. The ions entering through the top and the bottom influence the signal in the middle of the edge images even at short times. Concurrently, we observe lower equilibrium intensity for the external slices, due to the pellet placement, illustrated on the legend of Figure 3. The space represented by the edge images does not contain a full 1 mm thickness of the alumina extrudate. The issues, of the top/bottom ion penetration and the pellet positioning in the magnet, become irrelevant if either of the two innermost images, namely four or five on Figure 3, are used for data analysis. Not only do they always contain a full 1 mm of pellet in the z direction, but also they are more than 1 mm away from the top and the bottom of the pellet. Considering homogenous ion penetration from all sides, as alumina structure has no preferential direction, the influence of top and bottom ion penetration is limited to distances smaller than half the pellet radius, 0.6 -0.7 mm, after which the ions should reach uniform distribution throughout the pellet. Consequently, the innermost images are considered henceforth, precluding the influence of the pellet length and exact position on the measurement results. Images, representing an average over 1 mm thickness in the middle of the extrudates are shown in Figure 4. The 1 H signal, which appears as the light grey areas, is only observed in the presence of Ni 2+ ions, providing a fast, visual estimation of the progress of ion penetration into the solid support.

While water molecules penetrate into the core of the pellet immediately, as verified by conventional spin echo imaging, the 1 H signal in the leftmost images in Figure 4, recorded within the first 3 min of ion diffusion, is only visible at the edges of the pellets. The paramagnetic ions are located close to the edge of the pellet, forming a band of a width increasing with increasing ion concentration of the impregnating solution.

Signal intensity of the middle pixel, normalized to the maximum of each series, is also shown in Figure 4. As time elapses, the ions penetrate further in, until the appearance of uniform ion distribution throughout the pellet, leading to the homogenous signal observed on the rightmost images in Figure 4. The ion penetration into the pellet is accompanied by signal intensity increase in the middle pixel, until an equilibrium value. While the signal intensity is related to the ion concentration, this relation is neither linear nor trivial. No concentration information is immediately accessible from the signal intensity.

The impregnation appears to be roughly symmetric, as can be expected from a material that is homogeneous on the scale of the voxel size. The shape of the bright band on the images changes with the ion diffusion time. At the early stage, the band mimics the trilobe shape of the extrudate, however, the penetration from two sides in the lobe leads to a simpler, triangular and circular shape of the inner edge of the band. Irregularities, such as in the beginning of the 0.05 M series (see the top lobe of the pellet in Figure 4), can be explained by the presence of preferential diffusion directions present in some extrudates. The case of the alumina extrudate impregnated with the 0.3 M solution is exceptional. Already at the first measurement, recorded at 1.6 min from the beginning of the immersion in the Ni(NO 3 ) 2 solution, a homogeneous ion distribution throughout the pellet is observed. Due to the sample preparation procedure and the acquisition parameters, it is impossible to reduce the time between the immersion and the first measurement. The time necessary for achieving homogenous ion distribution is estimated in Figure 4. Equilibrium intensity for each series was set to the average value of the last eight measurements, where the signal has reached a plateau (see horizontal lines in Figure 4). The slope of increasing signal intensity was drawn as a guide to the eye. The first image recorded with fully uniform signal (rightmost in Figure 4) was decided to be at or after the intersection of the equilibrium intensity line and the slope. The time of the appearance of homogeneous ion distribution, expressed in minutes from the beginning of the immersion, is plotted in Figure 5, with 0.3 M assumed at the time of the first measurement. A non-linear dependence on the concentration can be seen. Indeed a roughly ten-fold time decrease from concentration to concentration is observed (Table 2). The solutions' pH being lower than the alumina pzc, it renders the alumina surface positively charged, repulsing the ions and facilitating their passage into the pellet. Decreasing the pH with increasing ion concentration will increase the positive charge of the alumina, enhancing this effect. Additionally, higher ion concentration entails higher diffusion driving force. Both mechanisms will decrease the time required to achieve uniform distribution, when millimeter-size pellets are used. Whereas it is not trivial to quantify the influence of those two mechanisms, as well as van der Waals interactions or covalent binding of ions to the surface, the findings presented in this paper are of important consequence in the preparation of catalysts. While homogeneous impregnation can be achieved with salt solutions of as little concentration as 0.05 M, it is clearly convenient to increase the concentration to 0.2 M, the homogenous ion concentration to occur in a time of the order of minutes: the timescale of large-volume impregnation. Because the ion concentrations used in the HDS catalysts are higher than 0.3 M no additional waiting time for homogeneous impregnation is necessary in the catalyst preparation process.

To our knowledge no complete investigation following the ion penetration into the pellet, at different ion concentrations, has been conducted. Few studies of the duration of the impregnation process in general are found in the literature [30] and among them, the impregnation of a 3.85 mm diameter alumina pellet with Ni 2+ at the concentration of 0.5 M [31]. Authors report complete penetration of the pellet with the ions within 30 min and fully homogenous impregnation in an hour. However, already at 5 min of the impregnation time, a band comparable to our sample's diameter is homogeneously impregnated, insofar as can be deduced from the images. Our results remain in satisfactory agreement with the study of Espinosa-Alonso et al. Although we expect slightly faster ion penetration at the Ni 2+ concentration of 0.5 M, the use of different alumina support prevents a thorough comparison of both experiments. The influence of the impregnation method also remains an open question as immersing an extrudate in a salt solution leaves little control over the amount of ions introduced into the solid support.

A systematic study of pellets with different specific surface areas and with different immersion times could shed light on this issue. However, it is beyond the scope of the current study, which should be considered as a proof of concept. It must be added that the sample preparation with calculated solution content would be impractical in the case of pellets as small as 1.2 mm diameter and a couple of millimeters long. The solution volume necessary to fill the entirety of the pores in such a pellet is of the order of microliters, making a homogeneous impregnation difficult to achieve at timescales allowing in situ visualization of ion diffusion.

Conclusions

We have successfully used a solid state MRI pulse sequence, Single Point Imaging (SPI) for recording the signal of the water 1 H in the presence of paramagnetic elements. Although the experimental times are longer than for spin echo based sequences at equivalent image resolution, such an approach opens the possibilities for studying catalyst preparation in cases of fast relaxing NMR signal. It also shows that T 1 contrast can be used equivalently to T 2 contrast for differentiating between the environments with and without the impregnating, paramagnetic ions, such as Ni 2+ and Co 2+ . The obtained results show that already at a concentration as low as 0.05 M Ni 2+ a homogeneous ion distribution can be achieved in an alumina support with a high specific surface area and porous volume. However, in terms of HDS catalyst preparation, it is advantageous to reach homogeneous ion distribution times shorter than the duration of the impregnation process. Here, for the 1.2 mm diameter and 5-6 mm length pellets used, such short time was reached already at the concentration of 0.2 M Ni 2+ . Further increasing the ion concentration leads to nearly instantaneous, homogeneous ion distribution. Consequently, in catalyst preparation, where the concentrations higher than 0.3 M are used, no question of homogeneity needs to be posed.

Tables and figure captions

Table 1 The T

Figure 1

 1 Figure 1 Images illustrating the steps of the analysis performed in Halcon HDevelop: a) a raw image obtained from the spectrometer; b) choosing a threshold to discriminate the signal from noise; c) removing the unphysical dents on the pellet surface; d) finding the smallest circle encompassing the pellet; e) finding the 4-pixel middle of the circle, from which the minimum will be chosen to estimate the signal intensity.

Figure 2

 2 Figure 2 Single Point Imaging (SPI) pulse sequence. All three gradients (grad) are applied prior to the 90° radio frequency (RF) pulse. A single point is acquired after a constant gradient encoding time T P , marked by a circle. The sequence is repeated, with the repetition time T R , for the number of times equivalent to the number of points.

Figure 3

 3 Figure 3 Relative signal intensity in the center of the pellet, normalized to the maximum intensity value of the whole dataset, presented as a function of ion diffusion time, for an alumina pellet impregnated with 0.1 M Ni 2+ . The results from the edge images are plotted in red, the middle images in green and the innermost images in black (images 1 and 8 were empty in this case). A schematic drawing of the pellet positioning relative to the image definitions is included with the legend.

Figure 4

 4 Figure 4 Relative signal intensity, normalized for each series, in the center of the pellet as a function of ion diffusion time. The concentration on the left indicates the Ni2+ solution in which the pellet was impregnated. The images corresponding to 1 mm pellet thickness in the middle of the pellet, rescaled for the sake of presentation, show the progression of ions inside the pellet. The images were recorded at the times marked with squares on the relative intensity graphs. The

Figure 5

 5 Figure 5 Time necessary to achieve a homogeneous ion distribution, in minutes, as a function of ion concentration of the impregnating solution. The values for 0.05, 0.1 and 0.2 M were taken from the estimation presented in Figure 4, while the value for 0.3 M is the time of the first image: 1.6 min.

Figure

  Figure

1 and T 2 values for water 1 H nuclei, observed in the γ-alumina pellets at different Ni 2+ concentrations of the impregnating solution. The measurements were made on homogeneously impregnated pellets.

	Ni 2+ (M)	T 1 (ms)	T 2 (ms)
	0	344	2.6
	0.05	19	2
	0.1	10.3	1.45
	0.2	6.6	1
	0.3	3.9	0.73

Table 2 The time, in minutes, required for obtaining a homogeneous ion distribution throughout the pellet for the concentrations investigated. *time of the first measurement, showing homogeneous ion distribution

 2

	Ni 2+ (M)	0.05	0.1	0.2	0.3
	Time (min)	1972	186	15	1.6*

Acknowledgements

This work was supported by French state funds managed by the ANR within the Investissements d'Avenir programme under reference ANR-11-IDEX-0004-02, and more specifically within the framework of the Cluster of Excellence MATISSE led by Sorbonne Universités. Franck Launay is acknowledged for fruitful discussions.