Maxime Laurans

Kelly Trinh

Kevin Dalla

Guillaume Izzet

Sandra Alves

Etienne Derat

Vincent Humblot

Olivier Pluchery

Dominique Vuillaume

Stéphane Lenfant

Florence Volatron

Anna Proust

Covalent Grafting of Polyoxometalate Hybrids onto Flat Silicon/Silicon Oxide: Insights from POMs Layers on Oxides

Keywords: polyoxometalates, molecular oxides, surface functionalization, electron transport, silicon dioxide, covalent anchoring

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INTRODUCTION

Polyoxometalates (POMs) are nanometric molecular oxides made of early transition metals in their highest oxidation state. Their ability to be reduced with several electrons with minor structural changes makes them good electron reservoirs that could play important roles in various applications in molecular memories, energy storage and energy conversion, catalysis… [START_REF] Busche | Design and Fabrication of Memory Devices Based on Nanoscale Polyoxometalate Clusters[END_REF][START_REF] Chen | Keggin-Type Polyoxometalate Cluster as an Active Component for Redox-Based Nonvolatile Memory[END_REF][START_REF] Walsh | Hybrid Polyoxometalate Materials for Photo(Electro-) Chemical Applications[END_REF][START_REF] Miras | Engineering Polyoxometalates with Emergent Properties[END_REF][START_REF] Ji | Polyoxometalate-Functionalized Nanocarbon Materials for Energy Conversion, Energy Storage and Sensor Systems[END_REF][START_REF] Vangelder | Polyoxovanadate-Alkoxide Clusters as Multi-Electron Charge Carriers for Symmetric Non-Aqueous Redox Flow Batteries[END_REF][START_REF] Friedl | Asymmetric Polyoxometalate Electrolytes for Advanced Redox Flow Batteries[END_REF][START_REF] Chen | Polyoxometalates in Dye-Sensitized Solar Cells[END_REF][START_REF] Liu | Enhanced Proton and Electron Reservoir Abilities of Polyoxometalate Grafted on Graphene for High-Performance Hydrogen Evolution[END_REF] Some of these applications involve the use of oxide electrodes. Indeed, in capacitive molecular memories, the molecular floating gate is usually deposited onto the silicon channel protected by silicon dioxide. [START_REF] Busche | Design and Fabrication of Memory Devices Based on Nanoscale Polyoxometalate Clusters[END_REF][START_REF] Zhu | Redox-Active Molecular Nanowire Flash Memory for High-Endurance and High-Density Nonvolatile Memory Applications[END_REF][START_REF] Balliou | Tungsten Polyoxometalate Molecules as Active Nodes for Dynamic Carrier Exchange in Hybrid Molecular/Semiconductor Capacitors[END_REF] Thus, the molecules are not perturbed by the channel and keep their charge state and physical properties. Furthermore, in the field of solar energy conversion, e.g. photovoltaic and artificial photosynthesis (proton or CO 2 reduction, water oxidation...), nanostructured transparent metal oxide semiconductors like ITO, TiO 2 , NiO or ZnO, and more recently the promising Fe 2 O 3 or BiVO 4 semiconductors, often modified by an organic dye, are usually used to build photo-electrodes. [START_REF] Li | Enhanced Visible Photovoltaic Response of TiO 2 Thin Film with an All-Inorganic Donor-Acceptor Type Polyoxometalate[END_REF][START_REF] El Moll | Increasing P-Type Dye Sensitised Solar Cell Photovoltages Using Polyoxometalates[END_REF][START_REF] Xiang | Electron Transfer Dynamics in Semiconductor-Chromophore-Polyoxometalate Catalyst Photoanodes[END_REF][START_REF] Alaaeddine | Enhancement of Photovoltaic Efficiency by Insertion of a Polyoxometalate Layer at the Anode of an Organic Solar Cell[END_REF][START_REF] Bae | Catalytic Multilayers for Efficient Solar Water Oxidation through Catalyst Loading and Surface-State Passivation of BiVO 4 Photoanodes[END_REF] Exploiting the redox activity of POMs in such applications thus requires mastering their deposition on oxide surfaces.

Furthermore, the control of their organization on the surface is an essential and still challenging issue. Indeed, Busche et al., who recently reported the study of POM-based flash memories showed that the density of deposited POMs is crucial to control the write/erase voltage and switching time. [START_REF] Busche | Design and Fabrication of Memory Devices Based on Nanoscale Polyoxometalate Clusters[END_REF] In the energy field, it is well known that the activity of dye-sensitized photo-electrodes will be maximized if the contact between the active molecules and the electrode is optimized, which implies: (i) limiting the aggregation of the molecules, (ii) preventing the leaching of the molecules during operating conditions, (iii) ensuring a strong interaction between the molecules and the electrode to densify the molecules at the interface and speed up electron transfers. [START_REF] Walsh | Hybrid Polyoxometalate Materials for Photo(Electro-) Chemical Applications[END_REF][START_REF] Fielden | Water Splitting with Polyoxometalate-Treated Photoanodes: Enhancing Performance through Sensitizer Design[END_REF][START_REF] Lauinger | High Stability of Immobilized Polyoxometalates on TiO 2 Nanoparticles and Nanoporous Films for Robust, Light-Induced Water Oxidation[END_REF][START_REF] Lauinger | Stabilization of Polyoxometalate Water Oxidation Catalysts on Hematite by Atomic Layer Deposition[END_REF][START_REF] Zheng | A Strategy for Breaking Polyoxometalate-Based MOFs To Obtain High Loading Amounts of Nanosized Polyoxometalate Clusters to Improve the Performance of Dye-Sensitized Solar Cells[END_REF] An efficient answer to all these points is to covalently attach the redox active molecules on the electrode.

Herein we demonstrate the robust and controlled grafting of POM hybrids onto Si/SiO 2 substrates. We chose this oxide for its direct applicability in CMOS (complementary metal-oxide semi-conductor)compatible devices, but also because very flat Si/SiO 2 substrates are available, which permits the use of numerous characterization techniques to probe the surface and interface. Whereas various strategies have been proposed to covalently anchor POMs on oxide nanoparticles surface [START_REF] Sang | A Novel Carboxyethyltin Functionalized Sandwich-Type Germanotungstate: Synthesis, Crystal Structure, Photosensitivity, and Application in Dye-Sensitized Solar Cells[END_REF][START_REF] Raula | Polyoxometalate Complexes of Anatase-Titanium Dioxide Cores in Water[END_REF][START_REF] Chakraborty | Design of an Inherently-Stable Water Oxidation Catalyst[END_REF] or mesoporous silica, [START_REF] Villanneau | Covalent Grafting of Organic-Inorganic Polyoxometalates Hybrids onto Mesoporous SBA-15: A Key Step for New Anchored Homogeneous Catalysts[END_REF] very few examples describe the grafting of POMs on flat oxide substrates. [START_REF] El Moll | Increasing P-Type Dye Sensitised Solar Cell Photovoltages Using Polyoxometalates[END_REF][START_REF] Tountas | A Silanol-Functionalized Polyoxometalate with Excellent Electron Transfer Mediating Behavior to ZnO and TiO 2 Cathode Interlayers for Highly Efficient and Extremely Stable Polymer Solar Cells[END_REF] In these last examples, few attention has been devoted to the control of the density and the characterization of the interface. In the present work, to ensure a strong bond between the POMs and the oxide, the POM hybrids with a pendant carboxylic acid group TBA (TBA = N(C 4 H 9 ) 4 + , tetrabutylammonium). Molecules containing carboxylic acid moieties were scarcely used to functionalize flat silicon oxide substrates [START_REF] García-López | Iron(II) Complex of 2-(1H-Pyrazol-1-Yl)Pyridine-4-Carboxylic Acid (PpCOOH) Suitable for Surface Deposition[END_REF][START_REF] Murata | Adsorption of a Carboxylic Acid-Functionalized Aminoxyl Radical onto SiO 2[END_REF] but the use of this anchoring function is recognized as a common way to modify non-planar silica surfaces and other oxides. [START_REF] Pujari | Covalent Surface Modification of Oxide Surfaces[END_REF][START_REF] Ichimura | Solid Phase Adsorption of Crystal Violet Lactone on Silica Nanoparticles to Probe Mechanochemical Surface Modification[END_REF][START_REF] Du | Carbon Dioxide Adsorbent Based on Rich Amines Loaded Nano-Silica[END_REF] Once synthesized, the POM hybrids were deposited on a silicon substrate surmounted by its native SiO 2 layer (hereafter named Si/SiO 2 substrate). Various surface characterization techniques were used to study the features of the monolayer and thorough attention was devoted to characterize the anchoring mode at the interface. To go further, the charge transport through the modified substrates was studied at the solid state.

EXPERIMENTAL SECTION

All chemicals and solvents were purchased form Aldrich or Acros and used as received, except for triethylamine and acetonitrile that were distilled from CaH 2 . The lacunar POM K 7 [PW 11 O 39 ] and the hybrid platform TBA 4 [PW 11 O 39 {SnC 6 H 4 I}] (K W Sn [I], TBA = N(C 4 H 9 ) 4 + , tetrabutylammonium) were synthesized following previously reported procedures. [START_REF] Matt | Elegant Approach to the Synthesis of a Unique Heteroleptic Cyclometalated Iridium(III)-Polyoxometalate Conjugate[END_REF][START_REF] Souchay | Polyanions and Polycations[END_REF] The silicon wafers (highly phosphorus-doped n-Si(100), resistivity <5.10 -3 .cm) for AFM, XPS and electrical characterizations were purchased from Siltronix and the silicon wafers for FTIR spectroscopy (Float-zone, low phosphorus-doped n-Si(100), resistivity 20-30 .cm) were purchased from Neyco.

NMR spectra were recorded on a Bruker AvanceIII Nanobay 400 MHz spectrometer equipped with a BBFO probehead. [START_REF] Busche | Design and Fabrication of Memory Devices Based on Nanoscale Polyoxometalate Clusters[END_REF] and k s = 0.018 were taken for the silicon wafer, n s = 1.5 and k s = 0 for the SiO 2 layer and n s = 1.48 and k s = 0 for the layer of POMs. [START_REF] Gam Derouich | Control of the Grafting of Hybrid Polyoxometalates on Metal and Carbon Surfaces: Toward Submonolayers[END_REF][START_REF] Zeghbroek | Principle of Conductor Devices[END_REF] The thickness of the SiO 2 layer was estimated by doing a measurement by ellipsometry after the bare silicon substrate rinsing steps, just before immersing the substrate in the POM solution. The thickness of the POM layer was deduced by the subtraction of the SiO 2 layer thickness to the final thickness. At each step, 5 measurements were performed on the substrate in different zones, to check the homogeneity of the layer. A mean value for the thickness was calculated when the standard deviation was lower than 0.2 nm.

Atomic Force Microscopy (AFM)

AFM images were recorded using a commercial AFM (NanoScope VIII MultiMode AFM, Bruker Nano Inc., Nano Surfaces Division, Santa Barbara, CA) equipped with a 150 × 150 × 5 μm scanner (J-scanner).

The substrates were fixed on a stainless steel sample puck using a small piece of adhesive tape. Images were recorded in peak force tapping mode in air at room temperature (22-24 °C) using oxidesharpened microfabricated Si 3 N 4 cantilevers (Bruker Nano Inc., Nano Surfaces Division, Santa Barbara, CA). The spring constants of the cantilevers were measured using the thermal noise method, yielding values ranging from 0. 4 to 0.5 N/m. The curvature radius of silicon nitride tips was about 10 nm (manufacturer specifications).

The raw data were processed using the imaging processing software NanoScope Analysis, mainly to correct the background slope between the tip and the surfaces.

X-ray Photoelectron Spectroscopy (XPS)

XPS analyses were performed using an Omicron Argus X-ray photoelectron spectrometer. The monochromated AlK α radiation source (hν = 1486.6 eV) had a 300 W electron beam power. The emission of photoelectrons from the sample was analyzed at a takeoff angle of 90° under ultra-high vacuum conditions (≤ 10 - 10 Torr). Spectra were carried out with a 100 eV pass energy for the survey scan and 20 eV pass energy for the C1s, O1s, N1s, Si 2p, P 2p, W 4f, Sn 3d, Pd 3d regions. Binding energies were calibrated against the Si2p binding energy at 99.4 eV and element peak intensities were corrected by Scofield factors. The spectra were fitted using Casa XPS v.2.3.15 software (Casa Software Ltd., U.K.) and applying a Gaussian/Lorentzian ratio G/L equal to 70/30.

Fourier Transform Infra Red (FTIR) spectroscopy on silicon

FTIR spectra were recorded with a Bruker Tensor 27 spectrometer, equipped with a DTGS detector, at a resolution of 4 cm -1 . Detection was performed in transmission at an incident angle of 70° (Brewster angle for the air-silicon interface) in order to minimize the interferometric patterns in the spectra. Sample compartment is purged from water vapor with a commercial air-dryer (relative humidity around 7%). The silicon substrates are chosen to be floatzone grown and lightly doped in order to have a very low amount of inserted oxygen and to be transparent for the mid-IR wavelengths used in this study.

I-V measurements

Current-voltage (I-V) curves were measured by contacting the POM monolayer by a Hg drop acting as the top electrode in a glove box filled under a nitrogen flow. The drop was gently brought into contact with the sample surface thanks to a camera. The voltage V was applied on the Hg drop and the highly doped silicon substrate is grounded through the ammeter to measure the current. I-V traces were acquired at different locations (around 10) on the POM monolayer surface, with ~ 5 traces per location, and they were not averaged. The surface contact area is estimated with the camera for each drop formed. Surfaces measured by this approach are comprised in the range 2 -5 x 10 -4 cm².

RESULTS AND DISCUSSION

Synthesis of carboxylic acid terminated POMs

The POM hybrid TBA Fortunately, the coordinating carboxylic acid function of the reactant did not poison the catalyst and the POM hybrid was obtained as a yellow powder with a good yield (80% scale. [START_REF] Elgrishi | Molecular Polypyridine-Based Metal Complexes as Catalysts for the Reduction of CO 2[END_REF] However, on the second reduction wave, a slight inflexion is observed. To better understand its origin we recorded CVs at various scan rates and observed that: at low scan rate (50 mV.s -1 and below), the inflexion becomes a full wave at around -1.4 V vs SCE (E ' 2 , figure 1); at a scan rate higher than 100 mV.s -1 , the inflexion disappears (figure S5). We propose that after the first reduction process the formed "POM 5-" becomes more basic and undergoes intra or intermolecular protonation from a carboxylic acid group. The protonated "POM 5--H + " is thus more easily reduced and the wave E ' 2 corresponds to the reduction potential of a portion of POMs that are protonated (figure 1). [START_REF] Rinfray | A Covalent Polyoxomolybdate-Based Hybrid with Remarkable Electron Reservoir Properties[END_REF] The second reduction wave of this "POM 5--H + " species is probably hidden in the second or third reduction waves of the unprotonated "POM 4-". As the protonation reaction is slower than the electron transfer, [START_REF] Rinfray | Electron Transfer to a Phosphomolybdate Monolayer on Glassy Carbon: Ambivalent Effect of Protonation[END_REF] a portion of the "POM 5-" have time to be protonated then reduced at low scan rate whereas at a scan rate high enough, only the reduction of the unprotonated "POM 5-" occurs.

The POM hybrid TBA [START_REF] Duffort | Straightforward Synthesis of New Polyoxometalate-Based Hybrids Exemplified by the Covalent Bonding of a Polypyridyl Ligand[END_REF] The following waves at lower potentials are ill-defined probably due to the presence of the COOH functions (figure S10) but no study on their dependence upon the scan rate was carried out in this case.

As expected, the K W Si [COOH], less charged, is easier to reduce than the K W Sn [COOH].

UV-visible spectra and DFT calculations

The UV-visible spectra of K To gain more insights into their relative energies and the nature of the allowed electronic transitions, DFT and TD-DFT calculations have been undertaken, using the Turbomole package, version 6.4. 41 First of all, two model systems were selected: one corresponding (in blue boxes on Figure 3). Four relatively similar orbitals of this type can be found (HOMO, HOMO-1, HOMO-3, HOMO-5). Another block of occupied frontier orbitals can be characterized by oxygens' lone pairs located on the polyoxometalate part of the system (red boxes on Figure 3). The lowest unoccupied molecular orbitals (LUMOs) are characterized by tungsten d orbitals of the polyoxometalate (green boxes on Figure 3). Just above these orbitals can also be found antibonding  orbitals corresponding to the WO oxo bond (purple boxes on Figure 3). the same band can be found (band B on Figure 5) involving similar orbital contributions. But, on top of that, we can also attribute the band observed experimentally around 320 nm. From our calculations, it appears that this band can be attributed to a transition between the  system localized on the organic moiety of K W Sn [H] and d orbitals of POM's tungstens (band C on Figure 5).

Because it involves the  system, the position of this band is expected to be sensitive to the functionalization of the aryl unit, as observed experimentally (see Figure 2). The intensity of the calculated band is stronger than expected, which can be attributed to an overestimation of the oscillator strength involving extended conjugated system as previously noted by Tozer. [START_REF] Dwyer | Effect of Chemical Change on TDDFT Accuracy: Orbital Overlap Perspective of the Hydrogenation of Retinal[END_REF] In conclusion, the computational study gives some insights about the assignment of the main electronic transitions and confirms that the lowest unoccupied molecular orbitals are combination of tungsten d orbitals without any participation of the organic tether.

Grafting on silicon oxide

As mentioned in the introduction, whereas a huge literature exists about the deposition of POM films on oxide substrates by layer-by-layer, [START_REF] Bae | Catalytic Multilayers for Efficient Solar Water Oxidation through Catalyst Loading and Surface-State Passivation of BiVO 4 Photoanodes[END_REF][START_REF] Ahmed | A Molecular Photovoltaic System Based on Dawson Type Polyoxometalate and Porphyrin Formed by Layer-by-Layer Self Assembly[END_REF][START_REF] Yang | Enhanced Photovoltaic Response by Incorporating Polyoxometalate into a Phthalocyanine-Sensitized Electrode[END_REF] spin coating, [START_REF] Alaaeddine | Enhancement of Photovoltaic Efficiency by Insertion of a Polyoxometalate Layer at the Anode of an Organic Solar Cell[END_REF][START_REF] Siddarth | Photoelectrochemical Studies on Earth Abundant Pentanickel Polyoxometalates as Co-Catalysts for Solar Water Oxidation[END_REF] or dip coating on positively charged organic monolayers, [START_REF] Lauinger | Stabilization of Polyoxometalate Water Oxidation Catalysts on Hematite by Atomic Layer Deposition[END_REF][START_REF] Ye | Photoelectric Active Hybrid Film Based on RuII Terpyridyl Complex and EuIII Substituted Keggin Polyoxometalate of [Eu(BW11O39)2]15[END_REF] ] in a polymer solar cell at the metal oxide (TiO 2 or ZnO) / organic interface. [START_REF] Tountas | A Silanol-Functionalized Polyoxometalate with Excellent Electron Transfer Mediating Behavior to ZnO and TiO 2 Cathode Interlayers for Highly Efficient and Extremely Stable Polymer Solar Cells[END_REF] Interestingly, the authors proposed that strong POM-Si-O-metal bonds were formed at the oxide surface via the silanol groups inducing the formation of more homogeneous films and the passivation of the metal oxide surface stabilizing the system. This new type of interaction at the interface contributed to the improvement of the device. E. A.

Gibson and J. Fielden and coll. reported the study of pristine and organo-imido Lindqvist-type POM hybrids (with carboxylic acid and pyridine pendant group) as co-adsorbents in dye sensitized p-type NiO solar cells. [START_REF] El Moll | Increasing P-Type Dye Sensitised Solar Cell Photovoltages Using Polyoxometalates[END_REF] They showed that the presence of the POMs at best slightly improves the overall cell efficiency despite a significant increase of V oc . This was attributed to the combination of opposite effects such as retardation of both recombination to NiO and electron transfer to the electrolyte. Furthermore, no clear effect of the nature of the POM was observed. Specially, we could expect the POM hybrids with a pendant carboxylic acid function to induce a different behavior because of a strong anchoring to the NiO surface, albeit no experimental evidence was provided. One possible explanation is that the covalent grafting is not guaranteed in this study: the POMs layer is indeed added in a second step after prior coverage of the substrate by the dye sensitizer, also with a carboxylic acid anchor, which could hinder the subsequent interaction between the POMs and the oxide substrate.

Here, we chose to deposit the K 6a). Indeed, the length of the rigid organic arm is around 1.6 nm, the POMs size is 1 nm and the size of the flexible TBA counter cation can vary between 0.5 and 1 nm, as a function of its folding. If we consider molecules standing upright in the normal direction to the surface with extended TBA, we expect a maximal thickness of 3.6 nm. If tilted molecules with the classical tilt angle of 30° are considered, the minimal expected thickness is 2.7 nm. Based on our statistics (figure S11), we defined 2.3 nm as the lower acceptable limit. Over 17 samples, a majority (60%) gave satisfying thicknesses (between 2.3 and 3.9 nm, Figure S11 in the supporting information)) and were regarded as monolayers. The samples obtained with a thickness below 2.3 nm and above 4 nm were regarded as sub-monolayers and multilayers respectively and were sidelined. Surprisingly, a significant quantity of palladium at the +2 charge state is observed on the survey and the Pd high resolution spectra (figure S15). The only rational explanation is to attribute this impurity to the palladium-based catalyst used in the Sonogashira coupling. However, XPS measurements performed on the K W Sn [COOH] powder show the absence of the Pd element (on the survey spectrum as well as on the high resolution spectrum performed at the Pd level, figure S15). This means that palladium is present as traces only in the powder but probably accumulates on the substrate. was also characterized. The highest theoretical thickness was evaluated to be at 2.6 nm with a 0.6 nm height for the organic tether, 1 nm for the inorganic core of the POM and 1 nm for the extended TBA countercations. The lowest limit for the thickness is more delicate to determine theoretically as two more parameters have to be taken into account: the flexibility of the tether, and the fact that the POM can be attached by one or two anchoring groups (figure 7a). It is worth noting that the contribution of the carbon element linked to oxygen atoms is more important in the present case than in the case of the K W Sn [COOH] layer, in accordance with the two anchoring groups present in K W Si [COOH]. Moreover, as expected, no palladium was detected in this sample, confirming our previous explanations about the origin of the Pd (Figure S17).

Insights into the anchoring mode

The three surface characterization techniques (AFM, XPS and ellipsometry) have thus proved that we were able to build monolayers of POMs. To go further, we tried to probe more specifically the SiO 2 /POMs interface to elucidate the grafting mode of the POMs on the SiO 2 layer and particularly the exact nature of the bond between the carboxylic acid group and the SiO 2 layer. Indeed, various binding modes can occur between a carboxylic acid group and an oxide surface: [START_REF] Pujari | Covalent Surface Modification of Oxide Surfaces[END_REF] the bidentate carboxylate mode (chelating one silicon of the surface or bridging two adjacent silicon atoms of the surface), the monodentate carboxylate mode which results in the formation of an ester function, and the hydrogen bond mode, in which the carboxylic acid group forms H-bonds with the hydroxyl groups of the surface (see figure 8).

Figure 8. Possible binding modes of carboxylic acid or carboxylate groups to silicon oxide surfaces. [START_REF] Pujari | Covalent Surface Modification of Oxide Surfaces[END_REF] Note that the rinsing step supports a covalent grafting: indeed, despite a thorough rinsing, which consist in treating the POMs modified substrate in a TBAPF 6 solution by an ultrasonic bath, the POMs layer keeps attached, revealing a strong anchoring with the substrate. To confirm this observation, a blank sample was prepared by immersing a Si/SiO 2 substrate in an acetonitrile solution of To go further and try to determine the nature of the link between the POMs and the surface, FTIR measurements were performed on both carboxylic acid POMs layers, deposited on a special low-doped float-zone silicon substrate, transparent to IR radiation. To validate the technique, a drop-casting of the powders was performed on a freshly cleaned Si/SiO 2 substrate (Figure S18). In both cases, the FTIR spectrum is similar to the one registered on a KBr pellet, with the typical bands of the inorganic core of the POM below 1150 cm -1 and the vibration bands characteristic of the TBA countercations around 2900 cm -1 , and at 1382, 1467 and 1485 cm -1 . The band corresponding to water molecules of crystallization is also present in both samples, at 1675 cm -1 for the K W Sn [COOH] drop-casted powder and at 1656 cm -1 for the K W Si [COOH] drop-casted powder. More specifically on the K W Sn [COOH] spectrum, the C=O stretching band appears at 1704 cm -1 , and the aromatic C=C stretching band at 1606 cm -1 . The band corresponding to the asymmetric vibration of the carboxylate function can be guessed at 1548 cm -1 . On the K W Si [COOH] spectrum, the C=O stretching vibration band of the carboxylic acid group is far more intense at 1710 cm -1 . The band at 1602 cm -1 can be attributed to the asymmetric stretching vibration of the carboxylate group, visible in this case thanks to the slight shift of the band corresponding to the crystallization water molecules toward 1656 cm -1 compared to the spectrum of the powder registered in a KBr pellet.

K W Sn [H],
The spectra of the POMs monolayers show different features, particularly in the 1300-1700 cm -1 zone (Figure 9). On the spectrum of the K W Sn [COOH] monolayer, the C=O stretching band decreases drastically relatively to two bands at 1548 and 1390 cm -1 that can be attributed to the asymmetric and symmetric stretching mode of the carboxylate function; on the spectrum of the K W Si [COOH] monolayer, the C=O vibration band decreases also and two carboxylate bands appear at 1570 and 1420 cm -1 . As in this case the C=O was still quite intense, an additional rinsing of the surface was performed in an ultrasonic bath.

Interestingly, this new treatment induced a drastic decreasing of the C=O band on the FTIR spectrum and no modification of the carboxylate bands, showing that some additional physisorbed carboxylic acid POMs were removed by an additional rinsing, but that most of the POMs seem strongly attached to the surface by a carboxylate group. Moreover, as usually mentioned in carboxylate chemistry reports, [START_REF] Dobson | In Situ Infrared Spectroscopic Analysis of the Adsorption of Aromatic Carboxylic Acids to TiO2, ZrO2, Al2O3, and Ta2O5 from Aqueous Solutions[END_REF][START_REF] Zeleňák | Correlation of Infrared Spectra of Zinc(II) Carboxylates with Their Structures[END_REF] the separation between the carboxylate antisymmetric and symmetric stretching bands () can enlighten on the coordination mode of the carboxylate group: =350-500 cm -1 corresponds to a monodentate binding to the metal atom, =150-180 cm -1 corresponds to a bridging bidentate link to two adjacent metal atoms, and =60-100 cm -1 corresponds to a chelating bidentate binding mode. In the present case, the  separation is around 150 cm -1 for both the K W Sn [COOH] and the K W Si [COOH] layers, which leads to a bidentate binding of the carboxylate to the substrate.

However, discrimination between a bridging or a chelating mode is more awkward with a  value at the lower limit of the bridging As in previous works on similar POMs, 56 the experimental J-V curves are analyzed using an electron tunneling model. We used the so-called "modified Simmons model", [START_REF] Holmlin | Electron Transport through Thin Organic Films in Metal-Insulator-Metal Junctions Based on Self-Assembled Monolayers[END_REF][START_REF] Cui | Making Electrical Contacts to Molecular Monolayers[END_REF][START_REF] Rampi | A Versatile Experimental Approach for Understanding Electron Transport through Organic Materials[END_REF][START_REF] Wang | Mechanism of Electron Conduction in Self-Assembled Alkanethiol Monolayer Devices[END_REF][START_REF] Vilan | Analyzing Molecular Current-Voltage Characteristics with the Simmons Tunneling Model: Scaling and Linearization[END_REF] which introduces in the usual Simmons model for tunneling through a rectangular energy barrier 62 a unitless adjustable non-ideality "shape factor" to take into account: (i) the deviation from the simple rectangular barrier in molecular junctions and/or a more complex barrier (here, a multilayer structure composed of the silicon oxide, the organic tether and the POM) (ii) a poorly defined effective mass in molecular junction. The case  = 1 corresponds to an ideal rectangular barrier and a bare electron mass (reduced effective electron mass=1).

𝐽 = ( 𝑒 4𝜋²ℏ𝑑² ) { ( Φ -𝑒𝑉 2 ) × 𝑒𝑥𝑝 [ -2(2𝑚) 1/2 ℏ 𝛼 ( Φ -𝑒𝑉 2 ) 1/2 𝑑 ] -( Φ + 𝑒𝑉 2 ) × 𝑒𝑥𝑝 [ -2(2𝑚) 1/2 ℏ 𝛼 ( Φ + 𝑒𝑉 2 ) 1/2 𝑑 ]} (1)
where J is the current density, e the electron charge, m is the bare electron mass, ℏ the reduced Planck constant, d is the barrier width corresponding to the thickness of the layers (native oxide, 1.4 nm, plus POM monolayer) measured by ellipsometry,  is an effective barrier height, V is the applied bias and  the shape factor described above.

All I-V curves (figure 10) were fitted by Eq. 1 with the two parameters  and  (see typical fits in the supporting information, For the reference sample, we have  = 1.91 eV (standard deviation 0.13 eV), which is consistent with previous values for native SiO 2 . [START_REF] Horiguchi | Evaluation of Interface Potential Barrier Heights between Ultrathin Silicon Oxides and Silicon[END_REF] We note that it seems counterintuitive to observe a higher current (figure 10) for the sample K W Sn [COOH] with a higher energy barrier (figure 11) and the thicker monolayer (see figures 2 and

3). This feature is understood if we consider the smaller values of the shape factor α (0. ). We suggest that this difference of the shape factor is likely related to the nature of the organic tether of the POM hybrid, π-conjugated vs.

σ-saturated. It is known that the exact potential distribution in a molecular junction is more complicated in a π-conjugated moiety, while it is more linear through alkyl chains (see a typical example in reference 62), [START_REF] Krzeminski | Theory of Electrical Rectification in a Molecular Monolayer[END_REF] which is consistent with a shape factor closer to ideality in this latter case. Note also that we cannot exclude an additional effect of the Pd 2+ cations present in the K W Sn [COOH] layers on the intensity of the observed current. In solution, solvation is also expected to stabilize the energy levels. The CV position is calculated from the first reduction waves (Figure S10), E LUMO =-(E 1/2 + E SCE/SHE )-4.4 (in eV with E SCE/SHE =0.24 eV). [START_REF] Cardona | Electrochemical Considerations for Determining Absolute Frontier Orbital Energy Levels of Conjugated Polymers for Solar Cell Applications[END_REF] The IV position is obtained from the ε POM value considering the n + doped silicon Fermi energy at -4.1 eV (Si electron affinity) and the junction energy diagram shown in the right (see details in the Supporting Information and in a previous publication [START_REF] Dalla Francesca | Charge Transport through Redox Active[END_REF] ).

Finally, we note that the LUMO position of the K

  H chemical shifts are quoted as parts per million (ppm) relative to tetramethylsilane using the solvent signals as secondary standard (s: singlet, d: doublet, t: triplet, sex: sextet, m: multiplet) and coupling constants (J) are quoted in Hertz (Hz). 31 P chemical shifts are quoted relative to 85% H 3 PO 4 . IR spectrum of the powder was recorded from a KBr pellet on a Jasco FT/IR 4100 spectrometer. Highresolution ESI mass spectra were recorded using an LTQ Orbitrap hybrid mass spectrometer (Thermofisher Scientific, Bremen, Germany) equipped with an external ESI source operated in the negative ion mode. Spray conditions included a spray voltage of 3.5 kV, a capillary temperature maintained at 270 °C, a capillary voltage of -40 V, and a tube lens offset of -100 V. Sample solutions in acetonitrile (10 pmol.μL -1 ) were infused into the ESI source by using a syringe pump at a flow rate of 180 μL.h -1 . Mass spectra were acquired in the Orbitrap analyzer with a theoretical mass resolving power (R p ) of 100 000 at m/z 400, after ion accumulation to a target value of 10 5 and a m/z range detection from m/z 300 to 2000. All data were acquired using external calibration with a mixture of caffeine, MRFA peptide and Ultramark 1600 dissolved in Milli-Q water/ HPLC grade acetonitrile (50/50, v/v). Elemental analyses were performed at the Institut de Chimie des Substances Naturelles, Gif sur Yvette, France. Electrochemical studies were performed on an Autolab PGSTAT 100 workstation (Metrohm) using a standard three-electrode set-up. Glassy carbon electrode, platinum wire and saturated calomel electrode (SCE) were used as the working, auxiliary and reference electrode, respectively. The cyclic voltammograms were recorded in 1 mM solutions of the POMs in acetonitrile with tetrabutylammonium hexafluorophosphate TBAPF 6 as electrolyte (0.1 M). UV-visible spectra were recorded on a Cary 5000 spectrophotometer.

Scheme 1 .

 1 Scheme 1. Synthetic route to the carboxylic acid terminated hybrids K W Sn [COOH] (top) and K W Si [COOH] (bottom).

  to K WSi[COOH] and one corresponding to K W Sn[COOH]. In the case of K W Sn [COOH], we chose to remove the carboxylic acid on the organic moiety for performing the calculations since the protonation state is unclear and we took K W Sn [H] as a model compound. Geometrical optimization for both systems was conducted at the B3LYP-D3/def2-SV(P) level, with solvation taken into account by adding a continuum solvation model (namely COSMO, with a dielectric constant of 48). A summary of the frontier orbitals for the two calculated systems can be found in Figures 3 and 4 for K W Si [COOH] and K W Sn [H] respectively. For K W Si [COOH], its orbital diagram is characterized by the following: the HOMO is localized on the sigma system of the two carboxylic acid interacting one with each other

Figure 3 .Figure 4 )

 34 Figure 3. Frontier molecular orbitals for K W Si [COOH], calculated at the B3LYP-D3/def2-SV(P). Color boxes indicates similar types of orbitals.Regarding K W Sn [H], the set of frontiers orbitals is slightly

Figure 4 .

 4 Figure 4. Frontier molecular orbitals for K W Sn [H], calculated at the B3LYP-D3/def2-SV(P). Color boxes indicates similar types of orbitals.

Figure 5 .

 5 Figure 5. Theoretical UV-visible spectra for K W Sn [H] and K W Si [COOH], calculated at the B3LYP-D3/def2-SV(P) level. Inserts: main orbital contribution for selected electronic transitions.

Figure 6 .

 6 Figure 6. (a) Scheme of the extreme orientations of the K W Sn [COOH] layer on Si/SiO 2 (b) 1*1 μm AFM images and z-profile of the K W Sn [COOH] layer (top) compared to the freshly piranha treated Si/SiO 2 substrate (bottom) (c) W 4f, Sn 3d and C 1s high resolution XPS spectra of the K W Sn [COOH] layer on Si/SiO 2 .

Figure 7 .

 7 Figure 7. (a) Scheme of the possible anchoring modes of the K W Si [COOH] POMs on Si/SiO 2 (b) 1*1 μm AFM image and z-profile of the K W Si [COOH] layer (c) W 4f, C 1s and N 1s high resolution XPS spectra for the K W Si [COOH] layer on Si/SiO 2 .

Figure 9 .

 9 Figure 9. Top: FTIR spectra of the K W Sn [COOH] (left) and K W Si [COOH] (right) powders drop-casted on Si/SiO 2 . Bottom: FTIR absorption spectra of the carboxylic acid POMs monolayers: K W Sn [COOH] layer (blue ; K W Si [COOH] layer (green ; K W Si [COOH] layer (red) after an added treatment by ultrasonication. The spectra are referenced to the substrate with its cleaned thermal oxide.

Figure 10 .

 10 Figure10shows the J-V curves in a semi-log plot measured with

Figure

  Figure S19). The histograms of the effective energy barrier ()

Figure 11 .

 11 Figure 11. Effective barrier height  histograms for K W Sn [COOH], K W Si [COOH] monolayers and the reference bare Si/SiO 2 sample.

Figure 12 .

 12 Figure 12. LUMO energy positions with respect to the vacuum level. The DFT position is given by the first empty d orbital of the POMs (see "DFT section").

Synthesis of TBA 4.4 [PW 11 O 39 {Sn(C 6 H 4 )CC(C 6 H 4 )COOH 0.6 }] (K W Sn [COOH])

	K W 0.097 mmol), bis(triphenylphosphine) palladium (II) dichloride Sn [I] (200 mg, 0.050 mmol), 4-ethynylbenzoic acid (14.20 mg, (2.83 mg, 0.004 mmol) and CuI (1.56 mg, 0.008 mmol) were solubilised in 2 mL of anhydrous DMF preliminary degassed with argon. Then triethylamine (50 L, 0,378 mmol) was added. The mixture was degassed for 5 min more before being placed in a H), 7.75 (d+dd, 3 J H-H =7.74Hz, 3 J Sn-H = 93.4 Hz, 2H, Ar-H), 7.66 (d+dd, 3 J H-H =7.74Hz, 4 J Sn-H = 31 Hz, 2H, Ar-H), 7.60 (d, 3 J H-H =8.02Hz, 2H, Ar-H), 3.12 (m, 35H, N-CH 2 -CH 2 -CH 2 -CH 3 ), 1.63 (m, 35H, N-CH 2 -CH 2 -CH 2 -ACS Applied Materials & Interfaces -10.90 (s+d, 2 J Sn-P =12.11Hz); IR (KBr pellet, cm -1 ): =2961 (m), 2934 (m), 2873 (m), 1709(w), 1604(w), 1548 (w), 1483 (m), 1379 (w), 1070 (s), 962 (s), 886 (s), 798 (s); HRMS (ESI -): m/z: calcd for PW 11 O 41 SnC 15 H 9 : 754.30 [M] 4-; found 754.30 (100) ; calcd for PW 11 O 41 SnC 31 H 45 N : 1086.50 [M + TBA] 3-; found 1086.50 (50) ; calcd for PW 11 O 41 SnC 15 H 8 : 603.24 [M] 5-; found 603.24 (25) ; calcd for CH Page 8 of 57 PW 11 O 41 SnC 47 H 81 N 2 : 1750.89 [M + 2TBA] 2-; found 1750.89 (15); Anal.
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microwave oven for 1h at 80°C/80W. After the reaction, solid impurities were observed and removed by centrifugation. Diethyl ether was added to precipitate a powder that was solubilized again in a minimum of acetonitrile. The acetonitrile solution was stirred with TBA + enriched Amberlite ® ion exchange resin for 1hr. The addition of few drops of ethyl acetate leads to the apparition of a preliminary precipitate that was removed by centrifugation since it contains catalysts residues. The supernatant was then precipitated by further addition of ethyl acetate, K W Sn [COOH] was recovered by centrifugation and dried with a large excess of diethyl ether to give a yellow pale powder (70-80%). 1 H NMR (400 MHz, CD 3 CN): δ(ppm) 8.01 (d, 3 J H-H =8.02Hz, 2H, Ar-3 ), 1.39 (sex, 3 J H-H =7.74Hz, 35H, N-CH 2 -CH 2 -CH 2 -CH 3 ), 0.98 (t, 3 J H- H =7.74Hz, 53H, N-CH 2 -CH 2 -CH 2 -CH 3 ); 31 P NMR (121 MHz, CD 3 CN): δ(ppm) Calcd for TBA 4.4 [PW 11 O 39 {Sn(C 14 H 8 )COOH 0.6 }] (%): C 25.12, H 4.12, N 1.51; found : C 24.80, H 3.92, N 1.54; UV-vis spectroscopy (CH 3 CN):  (nm) (log  in mol -1 .L.cm -1 ): 263 (4.73), 283 (4.66), 300 (4.69), 319 (4.59) Synthesis

of TBA 4 [PW 11 O 39 {Sn(C 6 H 4 )CC(C 6 H 5

	solid was solubilized in the minimum of acetonitrile, and stirred
	with TBA + enriched Amberlite ® ion exchange resin for 1hr. After
	filtration, the filtrate was precipitated with ether and the solid
	recovered by a last centrifugation. K W	Sn [H] was obtained with a 60%
	yield.
	1 H NMR (400 MHz, CD 3 CN): δ(ppm) 7.76 (d+dd, 3 J H-H =8.2Hz, 3 J Sn-H = 95
	Hz, 2H, Ar-H), 7.65 (d+dd, 3 J H-H =8.2Hz, 4 J Sn-H = 32 Hz, 2H, Ar-H), 7.6
	(m, 2H, Ar-H), 7.45 (m, 3H, Ar-H), 3.16 (m, 32H, N-CH 2 -CH 2 -CH 2 -
	CH 3 ), 1.66 (m, 32H, N-CH 2 -CH 2 -CH 2 -CH 3 ), 1.42 (sex, 3 J H-H =7.3Hz, 32H,
	N-CH 2 -CH 2 -CH 2 -CH 3 ), 1.01 (t, 3 J H-H =7.3Hz, 48H, N-CH 2 -CH 2 -CH 2 -CH 3 ); 31 P
	NMR (121 MHz, CD 3 CN): δ(ppm) -10.9 (s+d, 2 J Sn-P =12.15Hz); IR (KBr
	pellet, cm -1 ): =2962 (m), 2931 (m), 2869 (m), 1626(w), 1597(vw),
	1483 (m), 1380 (w), 1069 (s), 961 (s), 885 (s), 798 (s); HRMS (ESI -
	): m/z: calcd for PW 11 O 39 SnC 14 H 9 : 743.30 [M] 4-; found 743.30 (100)
	; calcd for PW 11 O 39 SnC 14 H 10 : 991.41 [M+H] 3-; found 991.41 (70) ;
	calcd for PW 11 O 39 SnC 30 H 46 N : 1608.25 [M + H + TBA] 2-; found 1608.26
	(30) ; calcd for PW 11 O 39 SnC 30 H 45 N : 1071.83 [M + TBA] 3-; found 1071.83
	(20); calcd for PW 11 O 39 SnC 46 H 81 N 2 : 1028.89 [M + 2TBA] 2-; found 1028.90
	(15) Anal. Calcd for PW 11 O 39 SnC 78 H 153 N 4 (%): C 23.75, H 3.88, N 1.42;
	more then left stirring under inert and ambient temperature during
	one night. TBABr (500 mg, 1.55 mmol) was added to the mixture then found: C 23.68, H 3.69, N 1.01; UV-vis spectroscopy (CH 3 CN): 
	an excess of absolute ethanol to precipitate the product. A solid (nm) (log  in mol -1 .L.cm -1 ): 271 (5.15), 278 (5.13), 285 (5.11),
	was recovered by centrifugation, washed with absolute ethanol and 304 (4.87).
	diethyl ether. To optimize the number of TBA counter-cations, the

)}] (K W Sn [H]) K W Sn [I] (400 mg, 0.1 mmol), ethynylbenzene (20 L, 0.18 mmol), bis(triphenylphosphine) palladium (II) dichloride (7 mg, 0.01 mmol) and CuI (3 mg, 0.015 mmol) were solubilised in 4 mL of anhydrous DMF preliminary degassed with argon. Then triethylamine (100 L, 0.756 mmol) was added. The mixture was degassed for 5 min

Synthesis of TBA 3.4 [PW 11 O 39 {O (SiC 2 H 4 COOH 0.8 ) 2 }] (K W Si [COOH])

	2TBA] 2-; found 1569.26 (8); Elemental Analysis calcd for Ellipsometry
	TBA 3.4 [PW 11 O 39 {O (SiC 2 H 4 COOH 0.8 ) 2 }] (%): C 19.50, H 3.55, N 1.28; Monowavelength ellipsometer SENTECH SE 400 equipped with a He-Ne
	K 7 [PW 11 O 39 ] found: C 19.51, H 3.50, N 1.24; UV-vis spectroscopy (CH 3 CN):  (0.64 g, 0.2 mmol) was dissolved in a laser at λ = 632.8 nm was used to perform ellipsometry
	water/acetonitrile mixture (30 mL, 1:2). A 1 M HCl aqueous solution (nm) (log  in mol -1 .L.cm -1 ): 265 (5.59) measurements. The incident angle was 70°. The values n s = 3.875
	was added drop by drop until an apparent pH equals to 3. The
	solution was cooled in an ice bath and the Si(OH) 3 (CH 2 ) 2 COONa (0.476 Surface grafting
	mL, 0.8 mmol) was inserted. The 1 M HCl solution was added drop by The silicon substrate (1 cm 2 approximately) was first rinsed with
	drop again to reach pH app =2. After an overnight reaction, TBABr a dichloromethane flow then dried under N 2 . It was then treated by
	(0.26 g, 0.8 mmol) was added and the solution concentrated with a sonication in a basic piranha bath NH 4 OH/H 2 O 2 /H 2 O (1/1/2 vol.)
	rotary evaporator to precipitate the product. The oily compound during 10 min. The treatment was repeated two more times, with
	obtained was dissolved in the minimum of acetonitrile then deionized water rinse between each bath. The substrate was then
	precipitated again with an excess of ether. A sticky solid was rinsed by immersion in two successive deionized water baths under
	recovered by centrifugation and washed thoroughly with ether to sonication during 5 min. After a thorough drying with N 2 flow, the
	obtain a white powder (0.6 g, 82%). substrate was immersed in an acetonitrile solution of the POM
	1 H NMR (400 MHz, CD 3 CN): δ(ppm) 3.14 (m, 27H), 2.53 (m, 4H), 1.65 hybrid (1 mmol.L -1 ) and heated at the reflux of the solvent during
	(m, 27H), 1.41 (sex, 3 J(H,H)=7.5 Hz, 27H), 1.05 (m, 4H), 1.01 (t, 22-24h. The modified substrate was rinsed following the following
	3 J(H,H)=7.5 Hz, 41H), 0.90 (m, 4H); 31 P NMR (121 MHz, CD 3 CN) δ (ppm) procedure: 5 min sonication in pure acetonitrile / rinsing with
	-12.28; IR (KBr pellet, cm -1 ):  =2963 (s), 2935 (m), 2874 (w), acetonitrile flow / 5 min sonication in a solution of 0.1 M of
	1710 (s), 1623 (w), 1483 (s), 1471 (s), 1420 (w), 1381 (m), 1112 TBAPF 6 in acetonitrile / rinsing with acetonitrile flow / 5 min
	(vs), 1064 (vs), 1052 (s), 1036 (s), 964 (vs), 870 (vs), 824 (vs); sonication in pure acetonitrile. The surface was finally rinsed
	HRMS (ESI-), m/z (%) : calcd for W 11 PSi 2 O 44 C 6 H 10 : 965.41 [M] 3-; with a last flow of acetonitrile before being dried with N 2 . This
	found : 965.42 (100) ; calcd for W 11 PSi 2 O 44 C 22 H 46 N : 1569.26 [M+TBA] 2-; rinsing sequence could be repeated once or twice depending on the
	found: 1569.27 (50) ; calcd for W 11 PSi 2 O 44 C 22 H 45 N: 1045.84 [M + TBA] 3-; ellipsometry results.
	found : 1045.84 (10) ; calcd for W 11 PSi 2 O 44 C 38 H 81 N 2 : 1569.27 [M +

  4.4 [PW 11 O 39 {Sn(C 6 H 4 )CC(C 6 H 4 )COOH 0.

6 }] (hereafter named K W Sn [COOH]) was synthesized (see experimental section) by post-functionalization of the hybrid platform TBA 4 [PW 11 O 39 {SnC 6 H 4 I}] (K W Sn [I]) via a Sonogashira coupling, following a procedure now

well mastered in our group (scheme 1).

[START_REF] Matt | Elegant Approach to the Synthesis of a Unique Heteroleptic Cyclometalated Iridium(III)-Polyoxometalate Conjugate[END_REF]

  3,4 [PW 11 O 39 {O(SiC 2 H 4 COOH 0,8 ) 2 }] (hereafter named 1M), the two first one electron reduction waves can be observed at E 1/2 =-0.38 and -0.88 V versus SCE in accordance with other Keggin type silicon derivatives reported in the literature.

	Page 19 of 57	ACS Applied Materials & Interfaces
	(0.
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		20

K W

Si

[COOH]

) was also synthesized (see experimental section), by adapting a published procedure.

[START_REF] Matt | Hybrid Polyoxometalates: Keggin and Dawson Silyl Derivatives as Versatile Platforms[END_REF]

This POM is quite different from the precedent since it displays aliphatic tethers, two anchoring groups and a total charge of -3 that should induce different electrochemical properties. It was prepared by a condensation of the carboxyethylsilanetriol on the lacunar POM K 7 [PW 11 O 39 ] (figure 1) in acidic acetronitrile-water. NMR, infrared spectroscopies and mass spectrometry confirmed the obtaining of the expected species, in particular by the presence on the 1 H NMR spectrum of both multiplets at 1.06 and 2.53 ppm integrating for 4 protons corresponding to the protons on the aliphatic chains of the POM hybrid (figure S6-S9). The excess of TBA cations measured by NMR spectroscopy and two contributions in the mass spectrometry spectrum again shows that deprotonated species are present, which is confirmed by elemental analysis that complies with the following general formula: TBA 3,4 [PW 11 O 39 {O(SiC 2 H 4 COOH 0,8 ) 2 }]. On the CV of the K W Si [COOH] recorded in a 1 mM solution in acetonitrile with TBAPF 6

Table 1 .

 1 Atomic percentages of W and Si elements in K W

	following exactly the same procedure as for the K W	Sn [COOH]
	layer. The thickness for the K W	Sn [H] layer measured by ellipsometry
	after thorough rinsing is only 0.5 nm, showing that only a few
	POMs are deposited on the surface. XPS measurements confirm this
	observation, as the atomic percentage ratio W/Si is around 25 times

2 . The carboxylic acid function is thus crucial to ensure a vertical and robust anchoring of the POMs on the substrate, which is an indirect proof that the POMs are attached via their terminal group.

  W Sn [COOH] POM is consistent with the position of similar molecule K W Sn [N 2 + ] grafted on hydrogen-terminated silicon (no oxide) using a diazonium group instead of a carboxylic acid one (ε POM = 1.8 eV in reference 53 vs. 1.7 eV here), 56 which indicates that the electronic structure of these molecular junctions is mainly controlled by the POM LUMO. As noted above the LUMOs of the POM hybrids are combination of tungsten d orbitals and do not involve the orbitals of the tether or the anchor group. The shifting down of about 0.6 eV on going from K W Sn [COOH] to K W Si [COOH] (molecule in solution, in a solvation continuum model, as well as for the "solid-state" self-assembled monolayers) is rather attributable to the change in the total charge (4-for K W Sn [COOH] vs 3-for K W Si [COOH]). 67 CONCLUSION Two POM hybrids TBA 4.4 [PW 11 O 39 {Sn(C 6 H 4 )CC(C 6 H 4 )COOH 0.6 }] (K W Sn [COOH]) and TBA 3.4 [PW 11 O 39 {O(SiC 2 H 4 COOH 0.8 ) 2 }] (K W Si [COOH]) have been prepared and thoroughly characterized in solution by NMR, UV-visible spectroscopies and cyclic-voltammetry. They display remote carboxylic acid/carboxylate groups that allow investigating their covalent grafting onto Si/SiO 2 flat substrates. Monolayers have been obtained and characterized by ellipsometry, AFM, and XPS. The K W Si [COOH] layer was found to be less uniform than the K W Sn [COOH] layer, presumably because of the presence of two, moreover more flexible, aliphatic arms. An FTIR study was carried out on a special low doped float-zone silicon substrate to probe the POM binding mode, from which a bidentate carboxylate was inferred. Finally, the electronic properties of the POM hybrids disclosed by cyclic-voltammetry, electronic absorption spectroscopy and calculated energy level diagrams have been confronted to the energetics of the Si/SiO 2 //POM layer//Hg junction as probed by electron transport measurements at the solid state. These three approaches are consistent, showing that the energy position of the LUMO of K W Si [COOH] is lower than the one of K W Sn [COOH] by a difference of 0.6 eV, in agreement with a higher total charge for the later. The LUMO energy of K W Sn [COOH] grafted on Si/SiO 2 and of K W Sn [N 2 + ] grafted on silicon (no oxide) are similar, showing that the electronic properties of these molecular junctions are driven by the POM LUMO (d-orbitals) and not by the π-orbitals of the tether or the anchor group. Such POM-based electrodes represent new functionalized silicon-based electrodes, which have important implications for improving and/or tuning the electronic properties of a variety of electronic devices: photovoltaics, sensors, memories and bioelectronics. 68 Some of us have recently shown that the controlled grafting of POMs on various oxides is also relevant to energy conversion. 69

	ASSOCIATED CONTENT
	Supporting Information file includes: 1 H and 31 P NMR spectra of
	K W	Sn [COOH] and K W	Si [COOH]; Infrared spectrum of K W	Sn [COOH] and
	K W	Si [COOH] in KBr pellets; ESI -spectra of 1 μmol.L -1 of K W	Sn [COOH]
	and K W	Si [COOH] in acetonitrile ; cyclic voltammograms of K W	Sn [COOH]
	in acetonitrile; thickness histograms inferred from ellipsometry;
	AFM images of the K W	Sn [COOH] and	K W	Si [COOH] layers; XPS
	characterization;	FTIR spectra of the K W	Sn [COOH] and K W	Si [COOH]
	powders drop-casted on Si/SiO 2 ; example of fits of IV curves using
	the modified Simmons model; description of the two layers staircase
	energy model; XYZ coordinates from the DFT calculation. The file
	is	available	free	of	charge	as	a	PDF	file.

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Funding Sources

This work was supported by Sorbonne Université and by the CNRS. K.D.F. thanks the program PER-SU of Sorbonne Universités for his post-doctoral grant.

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