Evidence for Charge Transfer at the Interface between Hybrid

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Evidence for Charge Transfer at the Interface between Hybrid Phosphomolybdate and Epitaxial Graphene. Loïc Huder, Corentin Rinfray, Denis Rouchon, Anass Benayad, Mira Baraket, Guillaume Izzet, Felipe Lipp-Bregolin, Gérard Lapertot, Lionel Dubois, Anna Proust, Louis Jansen, and Florence Duclairoir Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b00870 • Publication Date (Web): 27 Apr 2016 Downloaded from http://pubs.acs.org on April 30, 2016

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Scheme 1. Depiction of the grafting of KMoSn[N2+] to graphene with a C-C interface bond. In the polyhedral representation, the MoO6 octahedra are depicted, with oxygen atoms at the vertices and metal cations buried inside. Color code: MO6 octahedra, orange; PO4 tetrahedra, green. 155x133mm (150 x 150 DPI)

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Figure 1. Comparison of the AFM images of a bare graphene sample (a) and of a graphene sample after putting in contact with a solution of KMoSn[N2+] . 236x90mm (150 x 150 DPI)


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Figure 2. Comparison of the Raman spectra of bare graphene area (a) and an area after putting in contact with the POM solution (b) with the indicated signal attribution (see text). 257x98mm (150 x 150 DPI)


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Figure 3. Comparison of the XPS survey of a bare graphene area (a) and of an area in contact with the POM solution (b), with the indicated identification of the core-level peaks. 206x86mm (150 x 150 DPI)


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Figure 4. High resolution XPS spectra for the Mo 3d level outside (a) and inside the grafted region (b), for the O 1s level respectively (d) and (e), and for the C 1s level respectively (g) and (h). Black curves measured data, blue and red curves deconvoluted data. XPS-mapping for Mo (c), O (f) and C (i) performed at the position of the SXI image. 240x179mm (150 x 150 DPI)


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Figure 5. Cyclic voltammograms of the grafted surface. Annotation of the redox potentials E1 for the 1st redox wave POM/POM-1e- and E2 for the 2nd redox wave POM-1e-/POM-2e-. 96x86mm (150 x 150 DPI)


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Figure 6. Temperature dependence of the charge carrier mobility (a) and electron density (b) for the three successive surface states (bare graphene in blue, graphene dipped in solvent in green and graphene after functionalization in red). (c) UPS measurements with the extrapolated cut-off energy and (d) the associated schematic representation of the corresponding density of states (DOS) with respect to the vacuum level Ev in a rigid band model with the work function WF and Fermi level EF for the three successive surface states. 209x154mm (150 x 150 DPI)


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Figure 7. Relative positioning of the absolute redox potentials of the redox reactions of Aryl-N2+/ and KMoSn[I] (left part) and the work function of the graphene on SiC (right part). 204x146mm (150 x 150 DPI)


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TOC graphic 85x48mm (150 x 150 DPI)


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Evidence for Charge Transfer at the Interface between Hybrid Phosphomolybdate and Epitaxial Graphene.

Loïc Huder, †,‡ Corentin Rinfray, § Denis Rouchon, †,¦ Anass Benayad, †,ǁ Mira Baraket, †,± Guillaume Izzet, § Felipe Lipp-Bregolin, †,‡ Gérard Lapertot, †,‡ Lionel Dubois, †,± Anna Proust, § Louis Jansen, †,‡ Florence Duclairoir*,†,±

† Univ. Grenoble Alpes, F-38000 Grenoble, France ‡ CEA, INAC-PHELIQS, 17 rue des martyrs, F-38054 Grenoble, France § Sorbonne Universités, UPMC Univ. Paris 06, CNRS UMR 8232, Institut Parisien de Chimie Moléculaire, Université Pierre et Marie Curie, 4 Place Jussieu, Case 42, F-75252 Paris cedex 05, France ¦ CEA-LETI, MINATEC, 17 rue des martyrs, F-38 054 Grenoble, France ǁ CEA-LITEN, MINATEC, 17 rue des martyrs, F-38 054 Grenoble, France ± CEA, INAC-SyMMES, 17 rue des martyrs, F-38054 Grenoble, France

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Abstract

The interfacing of polyoxometalates and graphene can be considered as an innovative way to generate hybrid structures that take advantage of the properties of both components. Polyoxometalates are redox and photo sensitive compounds with high temperature stability (up to 400 °C for some) showing tunable properties depending on the metal incorporated inside the complex. Graphene has a unique electronic band structure combined with good material properties for electrical and optical applications. The spontaneous – rather than electrochemical – functionalization of epitaxial graphene on SiC with the Keggin phosphomolybdate derivative TBA3[PMo11O39{Sn(C6H4)C≡C(C6H4)N2}] (named KMoSn[N2+]) bearing a phenyl diazonium unit is investigated. Graphene decoration is evidenced by means of AFM, Raman, XPS and cyclic voltammetry indicating a successful immobilization of the polyoxomolybdate. The covalent bonding of the polyoxometalate to the graphene substrate can be deduced from the appearance of a D-band in the Raman spectra and from the loss of mobility in the electrical conduction. Highresolution XPS spectra reveal an electron transfer from the graphene to the Mo complex. The comparison of charge-carrier density measurements before and after grafting supports the p-type doping effect, which is further evidenced by work function UPS measurements.


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Introduction

Polyoxometalates (POMs) are negatively charged metal-oxides complexes that incorporate high valence metals such as Mo, W, V, Nb or Ta. The heteropolyanions can display different structures such as for example those of the Keggin [XM12O40]n- and the Dawson [X2M18O62]p− types, where M is a metal atom and X a heteroatom.1 POMs are thermally stable and they are also redox active with tunable redox potentials, which make them attractive key components for material development.2-12

POM-based materials have been obtained by coupling the inorganic core to different substrates using physisorption related methods (mixing, dispersing in a polymeric matrix, layer-by-layer deposition, …).13,14 In an alternative way, the POMs have been modified with an organic tail group for a more robust immobilization of the inorganic core to various substrates in order to avoid molecule dissolution and system instability.6,7,15-19 The POM tail groups contain terminal units such as a pyrene substituent to target π-π interactions with carbon nanotubes,20-22 a thiol to react with an Au electrode,23 or an aryl diazonium group to promote binding to various substrates (Au, C, Si).18,24-26 This latter organic group yields a covalent link between the redox POM core and the surface facilitating the charge-carrier transfer between the two.24

The combination of POMs and graphene has been employed to generate functional materials.27,28 For example, POM/graphene systems have been investigated for electrochemical storage,29-32 biosensing,33,34 adsorption,35 electrochemical detection,36 transistor applications,37 or catalysis.38,39 In most of the above cited examples the graphene used was derived from chemical-


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or sonication-driven graphite exfoliation. To the best of our knowledge no examples of POM immobilization on mechanically exfoliated, CVD or epitaxial graphene were reported in the literature. However such graphene samples of much better crystalline quality display remarkable electronic properties making them attractive for fundamental studies with potential applications in the new generation of electronic devices.40,41

Using various activation methods (electrochemistry,24,42 radical catalysis…), the diazonium salt function can react with most substrate types,43-46 including graphene.47-49 Diazonium salts are also known to react spontaneously with sufficiently reductive surfaces such as hydrogenated silicon,18,50,51 or graphene.52-56 So the scope of the study presented here is to investigate the spontaneous functionalization of epitaxial graphene on SiC by a POM hybrid bearing an aryl diazonium end group. The chosen molecule is the Keggin type POM-based hybrid TBA3[PMo11O39{Sn(C6H4)C≡C(C6H4)N2}] (acronym KMoSn[N2+]) (shown in Scheme 1),57 with the three positively-charged tetra-butyl ammonium (TBA) ions acting as counter ions.

The interest of this work is to provide a straightforward method to prepare a hybrid graphene/POM system based on the diazonium reaction (Scheme 1). Here the preparation method of this hybrid system is developed and the success of POM immobilization is highlighted by means of different complementary techniques such as AFM, Raman, XPS, and cyclic voltammetry. The nature of the chemical bond (non-covalent or covalent) between POM and graphene is discussed based on Raman and mobility measurements. The doping effect on the charge carrier density of graphene is studied by analyzing the electrical transport measurements completed by a UPS study of the work function before and after functionalization.


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Scheme 1. Depiction of the grafting of KMoSn[N2+] to graphene with a C-C interface bond. In the polyhedral representation, the MoO6 octahedra are depicted, with oxygen atoms at the vertices and metal cations buried inside. Color code: MO6 octahedra, orange; PO4 tetrahedra, green.

Materials and methods

Solvents such as acetonitrile, anhydrous acetonitrile, acetone and isopropanol were purchased from Sigma Aldrich. For the electrochemistry acetonitrile distilled over CaH was used. Otherwise these solvents have been used as received.


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6H-SiC on-axis wafers with metallic n-type doping (resistivity 0.013 – 2.0 Ωcm) and with semi-insulating doping (resistivity ≥ 105 Ωcm) were purchased from NovaSiC SA. The nonconducting semi-insulating substrates were used for the electrical transport measurements. The wafers - received epitaxy-ready polished – were cut into squares (1 x 1 cm2 or 0.7 x 0.7 cm2).

Epitaxial growth of graphene on SiC was performed by placing the SiC at the bottom of a graphite enclosure inside a RF induction furnace. Prior to loading into the furnace, the SiC samples were cleaned by sonication in acetone and further in ethanol for 5 minutes to remove physically adsorbed contaminations. An automatic control system allows controlling the annealing and sublimation temperatures with the appropriate heating and cooling rates. The overall protocol of the sublimation process consists of a SiC preparation step involving an in situ hydrogen etching step (for 30 min at 1600 °C under 10% Ar/H2), and a graphene growth step (for 30 min at 1600 °C under 1-atm Ar).58 After the growth, the samples were cooled down to room temperature and then transferred to a desiccator for storage under vacuum until further ex situ characterizations.

POM synthesis has been done following a similar synthesis protocol as the one described for the synthesis of the polyoxotungstate analog KWGe[N2+].24 It involves the prior synthesis of the hybrid KMoSn[N3Et2] bearing a triazene function as a diazonium precursor. The diazonium terminated hybrid KMoSn[N2+] was formed by subsequent addition of trifluoroacetic acid to KMoSn[N3Et2] in acetonitrile.


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Functionalization of graphene was performed inside a cell made of polyoxomethylene of conical shape to be filled with a acetonitrile solution containing KMoSn[N2+] (see Figure S1). The graphene-SiC substrate was mounted at the bottom of this cell with an O-ring sealing in order to functionalize only a circular region of 4 mm diameter. A sealed cap of the cell with holes for access (electrochemistry experiments and Ar gas flow) is used to prevent the CH3CN evaporation and to keep the whole system under Ar. A 1 mM solution of KMoSn[N2+] prepared in degased anhydrous acetonitrile (2.5 mL) is syringed into the cell with the graphene mounted at the bottom. The spontaneous grafting lasts for 18 hours with the cell under Ar atmosphere. Finally, the reaction media is pipetted out and the graphene substrate is washed with acetonitrile, acetone and isopropanol before being dried under Ar flow and stored in a dessicator. For the transport measurements and UPS analyses it was necessary to functionalize the whole graphene surface using a different reaction cell.

Both the magneto resistance (Rxx) and Hall resistance (Rxy) were recorded using phasesensitive detection techniques, with typical excitation currents up to 1 µA at frequencies below 1 kHz, as a function of magnetic field up to 6 T with temperatures ranging from 4 to 300 K. To make the electrical contacts in a Van-der-Pauw geometry, a bilayer of 10nm-Ti/80nm-Au was evaporated at the corners of the 7 x 7 mm2 samples by means of a mechanical mask to serve as micro-bonding pads. Thus, not any lithography process was used in order to avoid the influence of solvents and polymer-deposition on the deduced values for charge-carrier density and mobility. The edges of the graphene sample were trimmed with a diamond tip to avoid transport through the graphitized borders. The charge-carrier density (n) and mobility (µ) were extracted


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from the resistance per square Rxx□(B=0) and the Hall resistance Rxy(B) using n = 1/[e(dRxy/dB)] and µ = Rxy/Rxx□B.

Results Graphene functionalization. The bare graphene samples have been functionalized with the polyoxomolybdate-based

hybrid

(TBA)3[PMo11O39{Sn(C6H4)C≡C(C6H4)N2}]

(KMoSn[N2+])

bearing an organic pendant function able to react via diazonium chemistry with carbonaceous surfaces (Scheme 1). The organic linker used consists of a C≡C triple bond bridging 2 phenyl groups with one being substituted in para position by a diazonium salt function. This linker is introduced onto a Sn atom inserted into a monovacant POM and adds rigidity to the system favoring the immobilization of the POM through a perpendicular orientation to the surface rather than a tilted one. Moreover, the choice of a fully conjugated linker should enhance the orbital overlap with the graphene surface enabling the charge transfer between the graphene substrate and the redox active POM core. After placing graphene samples in contact with a solution of KMoSn[N2+] for 18h at room temperature under Ar gas, a thorough rinsing was performed to remove as much as possible physisorbed molecules in order to mostly retain the covalently grafted ones.

Morphology. The scanning AFM images in Figure 1a reveal the surface morphology for a bare graphene sample grown on 6H-SiC. The picture reveals an atomically-flat finger-like domain structure with domain sizes up to 2-5 µm, corresponding to graphene layer thicknesses ranging from one to a few layers of carbon atoms. The AFM image after POM grafting (Figure 1b) shows a surface morphology really different from the bare graphene sample. The graphene is


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covered by a collection of spherical features. The average diameter of these features lays in the 20-100 nm range. The surface coverage is homogeneous and no underlying bare graphene regions are visible anymore. The graphene domain size is an order of magnitude larger (in the micrometer range) than the spherical features. The spherical units do not correspond to discrete POM but to POM assemblies. The formation of islets probably arises from ion pair association between the hybrids due to the presence of the cationic diazonium moieties and the negatively charged POMs. The mean film roughness is about 5 nm. To confirm the covalent grafting, further characterization was performed.

Figure 1. Comparison of the AFM images of a bare graphene sample (a) and of a graphene sample after putting in contact with a solution of KMoSn[N2+] .

Spectroscopic characterization. Figure 2a shows the Raman spectra of a bare graphene area. The observed G- and 2D- Raman peaks in the spectra indicate the presence of (few-layer) graphene on the grown surface. The G band at ~1580 cm-1 corresponds to the doubly degenerate E2g phonon mode at the Brillouin zone center; the 2D band at ~2700 cm-1 is the second order of the D-band for zone-boundary phonons.59 The G-band (1588 - 1605 cm-1) and 2D-band (2718 -


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2743 cm-1) positions fluctuate across the sample surface showing the non-uniformity of the number of graphene layers, which is often evidenced in graphene on SiC but could also come from variations in doping, strain, or stacking order. The weakness or absence of a D-peak reveals the crystalline quality of graphene with few defects, as this peak usually observed at ~1355 cm-1 arises from zone-boundary transverse optical (TO) phonon mode allowed in Raman via defect scattering. Additional Raman measurements on a graphene sample of the same batch normalized with the signal for HOPG allowed to determine the integrated intensity ratio of the G peak AGgraphene/AG-HOPG.

The obtained values for this ratio (0.030-0.044) indicate a small number of

graphene layers, since one expects a value of 0.03 for a single layer SiC graphene.58

The Raman spectra recorded on an area after putting in contact with the KMoSn[N2+] solution (Figure 2b) display the expected G and 2D bands for graphene. No obvious signals ascribable to the POM oxide core are observed, as expected for this wavenumber range. A peak located at 2210 cm-1 is visible on all the spectra recorded on the area after putting in contact with the solution, while it is never observed on any of the bare graphene samples. This peak clearly evidences a difference between both zones. This signal is attributed to the alkyne C≡C bond of the linker as the position expected for the Raman stretching mode of this bond is around 21902260 cm-1. Apart from the G- and 2D-peaks, as for the bare graphene, the first order D peak also appears in the grafted zone (Figure 2b). This peak arises from the defects related to the grafting and can be attributed to the conversion of the sp2 bonds of the graphene lattice into sp3 bonds acting as scattering centers.


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Figure 2. Comparison of the Raman spectra of bare graphene area (a) and an area after putting in contact with the POM solution (b) with the indicated signal attribution (see text).

A secondary electron X-ray image (SXI) was taken near the boundary between an area in contact and not in contact with the POM solution. It reveals a clear contrast between the different zones (Figure S2) originating from a difference in electron energy-loss mechanisms related to morphology and/or chemical environment. This contrast corroborates the AFM image that shows an increased roughness for the POM functionalized area. To identify if this contrast could also be related to a difference in chemical environment, a surface elemental analysis was performed by recording XPS survey spectra.

The XPS survey spectrum of the bare graphene in Figure 3a displays mainly the expected signals for O, C, and Si. The XPS survey spectrum of the functionalized area (Figure 3b) reveals the signals of the various elements constituting the POM inorganic core. For example, at a binding energy (BE) of ~230 eV the Mo 3d core-level can be observed accompanied by the Mo 3p peak at ~400 eV. It is noteworthy that this last signal overlaps with the level of the N 1s coming from the TBA counter ion or from the diazo anchoring group. 18 Even though the POM


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core contains only one Sn atom, which is quite buried inside the molecular structure, the Sn 3d level was detected at 486.4 eV. Moreover on this survey spectrum (Figure 3b), the Si signals remain visible indicating a POM layer thickness lower than the XPS analyzing depth (5 nm).

Figure 3. Comparison of the XPS survey of a bare graphene area (a) and of an area in contact with the POM solution (b), with the indicated identification of the core-level peaks.

High Resolution XPS signals measured on both zones for the elements C, O and Mo are displayed in Figure 4. In the area that was in contact with the solution, the Mo 3d XPS spectrum (Figure 4b) reveals two doublets related to spin-orbit coupling around 230.0 and 236.0 eV. The doublet observed at the highest binding energies (3d3/2: 235.5 eV and 3d5/2: 232.3 eV) corresponds to the formal oxidation state Mo6+ while the one found with lower intensity at lower binding energies (3d3/2: 234.3 eV and 3d5/2: 230.9 eV) corresponds to Mo in a reduced state Mo5+.60,61 The deconvolution of the O 1s XPS spectrum (Figure 4e) reveals a peak at 530.1 eV characteristic of the O2- ions of the metal oxide core. Therefore, the Mo 3d and O 1s core level signals show the presence of the polyoxomolybdates on the surface. The deconvolution of the C 1s peak (Figure 4h) displays a component centered at 282.9 eV that can be considered to arise


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from the sp and sp2 bonds of the organic linker – and/or from the sp3 bond of the butyl chains of the counter ion (Figure 4e). For the area that was not in contact with the solution, the C 1s core peak assigned to the organic linker is barely visible. The Mo 3d signals are of extremely low intensities in this case (Figure 4a) and the O1s or C1s signals cannot be deconvoluted with contributions of the organic linker and the metal oxide core of the hybrid POM (Figure 4d and 4g).

For the HR XPS spectra an analysis in terms of atomic concentration was performed taking account for the mean-free-path corrections of the different elements. The Si signal intensity is less pronounced for the zone that was in contact with the solution than for the other zone (4.1 Si at.% and 13.7 Si at.%, respectively). This indicates that a layer of molecules screens the underlying substrate and decreases the percentage of the detected Si atoms. Similarly, the atomic concentrations of Mo and Sn are higher in the zone that was in contact with the solution (2 instead of 0.3 at.% for Mo and 0.3 instead of 0 at.% for Sn). Another clue for the presence of the POM lies in the atomic ratio of O2- to Mo. For the PMo11O39 core, it is equal to 39/11, i.e. 3.5. From our data the obtained ratio is 3.6, close to the expected value. The obtained information from the XPS data and its element analysis shows that the grafted graphene zone is covered with POM.

Considering the distinct HR XPS signals obtained for the Mo, O and C in the graphene zones in contact and not in contact with the KMoSn[N2+] solution, a XPS-mapping performed in unscanned mode was attempted for each of these elements (Figure 4c, 4f, and 4i respectively). The XPS mapping was recorded on an area of 1000 x700 µm2, using beam size of 10 µm2 (pixel


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size). After a linear least square spectral analysis, a RGB (Red-Green-Blue) image overlapping was performed, as shown in Figure 4c, 4f and 4i. On Figure 4c, the color code from red to blue corresponds to increasing Mo concentration. On Figure 4f, the green color code corresponds to the O 1s signal attributed to adsorbates on the bare graphene whereas the blue color code corresponds to the O 1s signal attributed to the O2- from the inorganic POM. On Figure 4i, the green color code corresponds to the C1s contribution of the sp2 C atoms of graphene and the red color code to the C atoms of the organic linker. These images clearly evidence the grafting in the circular region as defined by the conical cell used for localized functionalization. It corroborates that the contrast observed on the SXI image (Figure S2) not only originates from a difference in surface roughness but also from a difference in chemical composition. Interestingly we can detect on the XPS mapped area a surface of 100 µm2 where the grafting was not performed, evidencing a resolution of 10 µm of the chemical surface mapping. The origin of this non-grafted area may be explained by dust presence on the surface suppressing the direct contact between the KMoSn[N2+] solution and the graphene surface, or by a difference of graphene reactivity depending on the number of layers.


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Figure 4. High resolution XPS spectra for the Mo 3d level outside (a) and inside the grafted region (b), for the O 1s level respectively (d) and (e), and for the C 1s level respectively (g) and (h) performed with pass-energy of 23.5 eV. Black curves measured data, blue and red curves deconvoluted data. XPS-mapping for Mo (c), O (f) and C (i) performed at the position of the SXI image (Figure S2) near the interface of grafted and non-grafted zones using pass-energy of 93 eV.

Control experiments have also been performed under similar conditions with the parent POMbased hybrid (TBA)4[PMo11O39{Sn(C6H4I)}] (KMoSn[I]) deprived of the diazonium functionality and thus not reactive toward a carbonaceous surface. The comparison of the XPS spectra for KMoSn[I] and KMoSn[N2+] is displayed in Figure S3b. On the survey spectra of the sample after


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putting in contact with KMoSn[I], the intensity of the signals coming from Mo and O are lower compared to that recorded for KMoSn[N2+]. Moreover, the signal coming from the Si atoms of the substrate is higher evidencing that the graphene surface is covered by a much lower quantity of POMs in the case of KMoSn[I]. Along the same line, the Raman spectra of a graphene sample after putting in contact with KMoSn[I] (Figure S3a) do not display the peak at 2200 cm-1 attributed to the C≡C bond nor the enhanced D peak. The control experiments carried out with KMoSn[I] confirm indirectly the covalent grafting between KMoSn[N2+] and the graphene surface, and the efficiency of the rinsing procedure in eliminating most of the adventitious physisorbed POMs. Indeed, POMs are known to spontaneously adsorb on surfaces or on pre-modified surfaces to form multi-layers.62,63,64

Electrochemical characterization. In order to probe the electro activity of the immobilized POMs, a cyclic voltammetry experiment was performed. For comparison, in solution using a glassy

carbon

working

electrode,

the

redox

waves

of

the

related

POM

(TBA)4[PMo11O39{Sn(C6H4I)}] (KMoSn[I]) were found at E1 = -0.50 V corresponding to the POM/POM-1e- redox couple and E2 = -0.91 V corresponding to the POM-1e-/POM-2e- redox couple versus a Saturated Calomel Electrode SCE (-0.26 and -0.67 V versus Standard Hydrogen Electrode SHE, respectively).5

The grafted graphene sample was connected to the working electrode of the electrochemical cell described before (Figure S1). The cyclic voltammogram performed in 0.1 M NBu4PF6 in CH3CN and recorded at 100 mV/s revealed two reversible reduction waves centered at E1 = -0.18 V and E2 = -0.33 V (Figure 5) versus a SCE (E10 = 0.06 and E20 = -0.09 V vs SHE, respectively).


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The presence of KMoSn on the graphene surface is confirmed by its preserved electroactivity. Since the redox potential positions of the PMo11O39Sn core are generally not impacted by the nature of the conjugated appended tether,5,65 the observed positive shift of the redox potential probably arises from the presence of adventitious protons. The electrochemical behavior of POMs is indeed very sensitive to protonation, especially that of polyoxomolybdates known to be more basic than polyoxotungstates.66 Here the presence of protons can be explained by the last step in the synthesis of KMoSn[N2+] consisting of the deprotection of a triazene terminal group in the presence of trifluoroacetic acid.

Figure 5. Cyclic voltammograms of the grafted surface. Annotation of the redox potentials E1 for the 1st redox wave POM/POM-1e- and E2 for the 2nd redox wave POM-1e-/POM-2e-.

The cyclic voltammetry results were also used to extrapolate the POM density on the graphene surface. The analysis of the experimental voltammetry data is presented in the Supplementary Information. The surface density achieved is ~2.2 x 10-11 mol/cm2. It has to be specified that this


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value has been determined on a broad signal arising from both un-protonated and protonated POMs in unknown respective proportions, and that for the surface coverage calculations the number of electrons exchanged during the process was assumed to be 1 (one electron exchanged per redox wave). This coverage corresponds to the fourth of a closely-packed monolayer. On the XPS mapping (Figure 4), the non-complete coverage could be evidenced locally by the presence of a non-grafted zone.

Doping effect of the grafting. In the discussion of the high resolution XPS data in Figure 4b, the presence of the reduced Mo5+ state was deduced. This observation suggests that chargetransfer of electrons occurs from the n-type doped graphene to the POMs. The hypothesis of charge carrier transfer between the grafted POMs and graphene was further looked into by studying the magnetotransport data to get an estimation of the density n and the mobility µ of the charge carriers in graphene before and after grafting.

The magneto transport experiments were done after three successive process steps: the first one consists of the bare graphene after the growth process (referred as “bare” in the data). The second process step consists of dipping the graphene sample in the solvent with subsequent rinsing as done for the spontaneous grafting (referred as “dipped into solvent”). The third process step consists of the spontaneous POM grafting onto the graphene (referred as “functionalized”).

As shown in Figures 6a and 6b, the dipping in the solvent increases the density by 50% whereas the mobility stays nearly constant. From the sign of the Hall coefficient, it can be concluded that the charge carriers are electrons (n-type doping for graphene on SiC). It is known


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that adsorbed water vapor acts as a p-dopant for graphene. The rinsing process on an as-grown sample is thought to remove these p-doping species inducing an increase in electron density. The mobility of the charge carriers is not influenced by this cleaning procedure of only weakly bound species. For the functionalized sample, the electron density shows only a weak decrease with respect to the rinsed sample, but the mobility shows a significant effect with a 50% decrease from 1000 cm2/Vs to 500 cm2/Vs at 4.2 K. This decrease in mobility is compatible with the introduction of scattering centers originating from the covalent grafting (i.e. sp3 C between graphene and POMs). The change in carrier density from 9.2 to 8.9 x 1016 m-2 at 4.2 K would correspond to an estimated change of the kinetic Fermi energy EF from 352 to 347 meV assuming the two-dimensional density of states for single-layer graphene with

=ℏ

√

.

This quantitative evaluation has to be taken with care because of the inhomogeneous domain structure of graphene on SiC with multiple graphene layers.

To get more insight on the issue of charge carrier doping, UPS measurements (Figure 6c) were performed on a set of samples prepared in the same way than those prepared for the electrical measurements (bare, solvent and functionalized). In the inset of Figure 6c the UPS signal is presented as a function of the binding energy with the extrapolated estimation of the work function WF (photon excitation energy 21.2 eV (He I-UV source) minus cut-off electron energy). The bare-graphene sample displays a shoulder at the cut-off energy around 18 eV with several features in the valence band signal. The rinsed sample does not display these structures in the valence band, indicating that the rinsing in the solvent is efficient to yield a cleaner graphene surface. After dipping the sample into the solvent the WF does not vary significantly, it changes from 3.6 for bare graphene to 3.7 for graphene after dipping in solvent. The determined WF of


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the functionalized graphene reveals an increase to 4.1 eV. This increase in WF can be correlated to a decrease in Fermi level position assuming a rigid band model as schematically shown in Figure 6d. The POM grafting leads to p-type doping with a decrease of the electron density of the bare sample. These results corroborate the appearance of the Mo5+ signal in the XPS spectrum and the weak decrease of electron density observed in the electrical transport measurements.

Figure 6. Temperature dependence of the charge carrier mobility (a) and electron density (b) for the three successive surface states (bare graphene in blue, graphene dipped in solvent in green and graphene after functionalization in red). (c) UPS measurements with the extrapolated cut-off energy and (d) the associated schematic representation of the corresponding density of states


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(DOS) with respect to the vacuum level Ev in a rigid band model with the work function WF and Fermi level EF for the three successive surface states.

In Figure 7, we have summarized the data of the measured redox potentials and the WF in an energy level scheme that could help to deduce the possible electron transfers involved in the grafting reaction (N2+ reduction) and in the POM-graphene interaction. The axes of the redox potential (difference with respect to the SHE) and the WF have been rescaled in such a way that the redox potential at 0 V coincides with the WF at 4.44 eV.67 The plotted Aryl-N2+/Aryl standard redox potential (-0.25 V vs SHE) has been taken from that obtained for a comparable polyoxotungstate POM with the same end group.24 The graphene samples studied here (bare and after dipping in CH3CN) display WF around 3.6/3.7 eV. This allows us to consider the relative positioning of the Fermi level of the bare graphene given by the work-function (on the right part of Figure 7) and the LUMO levels of the starting materials given by the Aryl-N2+/Aryl

24

and

POM/POM-1e- 5 standard redox potentials (on the left part of the Figure 7).

From the relative positioning of these energy levels in Figure 7 (EF (graphene) >LUMO (ArylN2+/Aryl )) one can conclude that a spontaneous charge transfer can occur from the bare graphene to the diazonium functionality initiating the grafting reaction. It is noteworthy that such chemistry was addressed in experimental studies of the spontaneous grafting of diazonium derivatives on epitaxial graphene.52-56,68 Another charge transfer leading to the POM reduction (EF (graphene) >LUMO (POM/POM-1e-)) may take place by electron hoping or tunneling through the conjugated linker from the substrate to the POM moiety. These transfer mechanisms inferred from Figure 7 explain the increase of work-function (i.e. hole doping of the graphene)


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upon graphene functionalization deduced from UPS measurements and the partial reduction of Mo6+ observed in XPS.

Figure 7. Relative positioning of the absolute redox potentials of the redox reactions of Aryl-N2+/ and KMoSn[I] (left part) and the work function of the graphene on SiC (right part).

Conclusion

The interfacing of graphene and POMs is an interesting step towards the development of functional materials. The diazonium chemistry offers a path for covalent functionalization, which has the advantage to yield a robust surface modification. The presented use of hybrid POMs bearing an organic conjugated linker with a terminal diazonium end group KMoSn[N2+] has thus enabled us to implement the spontaneous grafting on graphene.

The immobilization of the POMs on epitaxial graphene on SiC is evidenced with several experimental techniques. AFM surface imaging reveals a drastic change in surface topography


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after functionalization. The XPS spectra reveal core-level signals of POM atoms, especially of Mo, Sn, and N. The Raman spectra show a signal of the triple C≡C bond from the conjugated tail group. The redox activity of the Keggin POM species is preserved for the grafted samples.

Information on the nature of the bonding has been obtained from the Raman data. The enhancement of the D-peak, which is only allowed for defect-mediated phonon scattering, suggests a covalent bonding to the graphene with the conversion from sp2 C to sp3 C upon grafting. The measured decrease of mobility of the charge carriers can also be related to this appearance of point-like scattering centers at the grafting sites.

The high-resolution XPS measurements give strong indications of electron-charge transfer between the graphene and the POMs with the appearance of signals from the Mo5+ core levels. The observed increase of the Work Function for the functionalized sample also illustrates this ptype doping of graphene upon covalent bonding of the POMs. Additionally, the magnetotransport experiments show a slight decrease in electron charge carrier density. The spontaneous grafting of the polyoxomolybdates to graphene mediated by diazonium chemistry is thus coupled with electron transfer from the graphene to the POMs. A simple comparative representation of the values for the POM redox potentials in solution and for the bare graphene WF confirms this scenario.

It would be very interesting to extend the grafting of POMs to gated graphene samples in order to test the behavior of gated field-effect transistors upon functionalization. In further studies, the


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interplay between the tuning of the Dirac point of the graphene and that of the redox potential of the POMs could be addressed.

Supporting Information Contains details about the cell used for functionalization, the SXI image of the area characterized by XPS-mapping and a comparison of Raman/XPS analyses of a reference compound not bearing a diazonium salt end group. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author *E-mail : [email protected]

Acknowledgements For their financial supports, the authors would like to acknowledge the French P2N ANR program (Grafonics ANR project - 2010-NANO-004-01) as well as the Chimtronique CEA research program. The authors are obliged to the PFNC Nanocharacterization Minatec plateform and LANEF framework that provide access to mutualized equipment and infrastructures. The authors are grateful to G. Bidan and P. Maldivi for fruitful discussions.

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Table of content graphic (8.5 cm x 4. 75 cm)


Evidence for Charge Transfer at the Interface between Hybrid

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