Elaboration, Characterizations, and Energetics of Robust Mo6 Cluster

Article pubs.acs.org/JPCC

Elaboration, Characterizations, and Energetics of Robust Mo6 Cluster-Terminated Silicon-Bound Molecular Junctions Stéphane Cordier,*,† Bruno Fabre,*,† Yann Molard,† Alain-Bruno Fadjie-Djomkam,‡ Pascal Turban,‡ Sylvain Tricot,‡ Soraya Ababou-Girard,*,‡ and Christian Godet‡ †

Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS-Université de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France ‡ Institut de Physique de Rennes, CNRS UMR 6251-Université de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France ABSTRACT: Reaction of a [Mo6Br8F6]2− octahedral cluster unit with functional carboxylic acid leads to the formation of the corresponding carboxylate along with the elimination of HF and the formation of a Mo−O bond between the cluster and the carboxylate. This offers an easy way to prepare Mo6 cluster functionalized surfaces by simple reaction of carboxylic acid-modified n-type Si(111) surfaces in a solution of [Mo6Br8F6]2− cluster units. The surface coverage of metallic clusters has been controlled in the range 0.3−5.8 × 1013 cm−2 by dilution of the carboxylic acid-terminated organic chains with inert n-dodecyl chains. The homogeneity of the grafted films on the oxide-free Si surface was followed by combined XPS and STM analyses whereas the electronic gap corresponding to the difference between the conduction band minimum (CBM) and the valence band maximum (VBM) of a single immobilized cluster was determined from tunneling spectroscopy data obtained from a surface with a low cluster coverage. The energy position of the VBM of the functional surface was determined by UPS for a surface with a high cluster coverage and charge transport characteristics through Hg//[Mo6Br8F6]terminated organic monolayer (OML)//n-Si junctions were measured using the mercury drop technique. These studies allow the determination of the electronic band diagram at the cluster−organic layer−Si(111) interfaces. Results are discussed and compared with other covalent assemblies of metallic clusters onto Si surfaces differing by the nature of cluster cores, [Re6Sei8]2+ or [Mo6Ii8]4+, and linker functionality, carboxylic acid or pyridine.

1. INTRODUCTION M6Qi8La6 cluster-based units (M = Mo, Re; Q = halogen and/ or chalcogen, L = donor ligand; i = inner and a = apical) are versatile building blocks exhibiting unique intrinsic structural and physicochemical properties (orthogonal disposition of metallic sites that can be selectively functionalized, dual photoluminescence, redox, and generation of singlet oxygen).1,2,3 The origin of their physical properties comes from the delocalization of valence electrons on all the metal atoms and depends to a lesser extent on the nature of ligands. They can be selectively functionalized to obtain structuration at the nanometric scale of hybrid organic−inorganic materials and supramolecular frameworks.4,5 After preparation by solid state synthesis at high temperature and further dissolution, processing the M6Qi8 cluster core via solution chemistry allows the elaboration of molecular assemblies or nanomaterials2,6 either for the patterning of materials at the nanometer scale and/or to provide specific properties.6−8 Interestingly, the functionalization of the inorganic M6Qi8 cluster core is obtained by grafting of inorganic or organic moieties in apical positions. The appropriate choice of the L ligand enables tailor-made cluster units to be prepared in order to control their arrangement in © 2016 American Chemical Society

nanoscale architectures, the so-called nanoarchitectonics. The concept of nanoarchitectonics developed by Ariga represents a total methodology for the construction of functional materials and systems from nanosized units, from which unusual properties or unexpected functionality might be induced based on uncertainty or local fluctuations.9,10 The self-assembly of metal atom clusters onto semiconducting surfaces, such as doped silicon, constitutes an elegant approach combining the advantages of semiconductor technology (doping, processing, and patterning) with the large flexibility in designing molecular structures; this strategy is very promising for the development of novel electrically addressable and switchable functional devices. These last years, we have synthesized and characterized several types of clusters and succeeded in the grafting on oxidefree hydrogen-terminated silicon surfaces (Si−H).11−13 Several immobilization strategies have been developed in order to obtain reproducible, stable and robust covalent assemblies. In earlier work, [Mo6I8]4+ cores were immobilized via an organic monomolecular layer (OML) containing terminal pyridine Received: December 21, 2015 Published: January 8, 2016 2324

DOI: 10.1021/acs.jpcc.5b12481 J. Phys. Chem. C 2016, 120, 2324−2334

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The Journal of Physical Chemistry C

SiMo5 prepared from SiAC100 (neat undecylenic acid), SiAC33 (33 molar % undecylenic acid) and SiAC5 (5 molar % undecylenic acid) acid-modified monolayers. Scanning tunnelling microscopy and spectroscopy (STM, STS) measurements were performed on SiMo5 to obtain the VBM/CBM electrical band gap of individual clusters. Spectroscopic ellipsometry, X-ray and UV photoelectron spectroscopies (XPS, UPS), electrochemical impedance spectroscopy (EIS), have been used to characterize extensively SiMo100, SiMo33, and SiMo5 surfaces and to derive the electronic band alignment for the molecular layer/Si assembly. The electrical properties of metal−insulator−semiconductor (MIS) diodes prepared from these functional films were also studied using a mercury drop as the soft electrical top contact. Surface coverage, silicon surface oxidation, transport properties, and electronic band alignments of the as-prepared junctions are discussed and compared with those reported for cluster-core-modified Si surfaces differing by the nature of cluster cores ((Re 6Se i8) 2+ and [Mo6Ii8]4+) and linkers (carboxylic acid or pyridine modified surface).

groups grafted on Si(111)-H, using the reaction of [Mo6Ii8(CF3SO3)a6]2− cluster units with a pyridine functionalized surface.11,12 On the other hand, (Re6Sei8)2+ cores were immobilized by reaction of trans-Re6Sei8(TBP)a4(OH)a2 cluster units precursors (where TBP is tert-butylpyridine) with carboxylic acid groups end-capping an alkyl monolayer covalently bound to Si−H.13 In this work, we present a new strategy consisting in the selfassembly of (Mo6Bri8)4+ clusters core on a semiconducting surface by reaction of [Mo6Bri8Fa6]2− units with undecanoic acid modified Si−H. The properties and reactivity of the [Mo6Bri8Fa6]2− units have been previously described.14,15 In particular, the high ionicity of the Mo−F bonds leads to an increased affinity of fluorine for hydrogen. Indeed, insertion of [Mo 6 Bri 8F a 6] 2− units within the pores of mesoporous chromium carboxylate improved its hydrogen storage performances from 1.2 to 4.7 g cm−3 at room temperature.14 Moreover, this cluster exhibits an unusual reactivity with carboxylic acid derivatives, where F− reacts like a Brönstedt base with carboxylic acid leading to the formation of HF and carboxylate. During this process, the carboxylate group is grafted in situ on the cluster via the formation of a Mo−O bond. Reaction of [Mo6Bri8Fa6]2− units with functional aromatic molecules, containing a carboxylic acid group on one side and a cyanobiphenyl-terminated long chain on the opposite side, provided a new class of phosphorescent liquid crystals, the clustomesogens.15 This simple acido-basic reaction was used herein to immobilize the (Mo6Bri8)4+ cluster cores by reaction of [Mo6Br8F6]2− cluster unit with carboxylic acid groups endcapping an alkyl monolayer covalently bound to Si−H (Figure 1). After immobilization, the cluster core is bounded to fluorine

2. EXPERIMENTAL SECTION 2.1. Synthesis of the Molybdenum Cluster. Fluorinated clusters associated with tetrabutylammonium cation (n-Bu4N+) were initially reported by Preetz and co-workers.16 A slightly modified method has been developed in order to obtain (nBu4N)2[Mo6Bri8Fa6] units with higher yield and larger quantities.15 The [Mo6Bri8Fa6]2− unit is built up from a regular Mo6 octahedron with Mo−Mo distance of 2.621(1) Å. Each face of the octahedron is face-capped by eight covalently linked bromine atoms, the Mo−Br distance ranging from 2.615 to 2.631 Å. The resulting [Mo6Bri8]4+ cluster core is additionally bonded to six fluorine atoms (Mo−F = 1.994 to 2.011 Å) to form the [Mo 6 Br i 8 F a 6 ] 2− unit. Structural data of (nBu4N)2[Mo6Bri8Fa6] − 2 CH2Cl2 obtained by single crystal X-ray diffraction14 show that the [Mo6Bri8Fa6]2− unit is inscribed in a regular sphere with 9.06 Å diameter, whereas the (n-Bu4N)+ counterion is inscribed in an ellipsoid with a long axis length of 10.19 Å. This [Mo6Bri8Fa6]2− unit is chemically very stable, e.g., it can be stored in air without any risk of oxidation or exchange of apical ligand by water molecules from ambient atmosphere. Similar to the reaction of trans-Re6Sei8(TBP)a4(OH)a2 with carboxylic acid modified surfaces, the reaction of (n-Bu4N)2[Mo6Bri8Fa6] with such modified surfaces leads to stable cluster functionalized surfaces thanks to the formation of a covalent metal−oxygen bond. After immobilization, the cluster unit should be denoted as [Mo6Bri8Fax(OOC10H20-Sisurface)a6‑x] (Figure 2). For the sake of clarity the cluster/organic monolayer/Si assembly will be denoted Mo6Br8−OML−Si. As far as the robustness of the molecular assembly is concerned, (n-Bu4N)2[Mo6Bri8Fa6] is a much more convenient precursor than the air-sensitive (n-Bu4N)2[Mo6Ii8(CF3SO3)a6] used previously for the attachment of (Mo6I8)4+ clusters on pyridine-modified surfaces.11,12 Interestingly, this cluster immobilization strategy avoids the elimination of water molecules and the localized release of HF is believed to protect the underlying silicon surface against oxidation by residual water and oxygen. 2.2. Preparation of Molybdenum Cluster-Modified Si(111) Surfaces. The chemicals used for cleaning and etching silicon wafer pieces (30% H2O2, 96−97% H2SO4 and 40% NH4F solutions) were of VLSI semiconductor grade (Riedel-

Figure 1. Schematic representation of the (M6Qi8)n+ cluster core immobilization on functionalized Si surfaces: Mo6Bri8 (a) and Re6Sei8 (b) on carboxylic acid-modified silicon and Mo6Ii8 on pyridinemodified silicon (c). For the sake of clarity, inner ligands and counter ions are omitted. The cluster binding to the functionalized surfaces is representative of the highest cluster density (SiM100 with M = Mo or Re).

ligands that did not react with the surface and to the siliconbound carboxylate unit yielding the [Mo6Bri8Fax (OOC10H20Sisurface)a6‑x] unit. It is demonstrated that the surface coverage of the metallic cluster can be controlled using mixed alkyl/acidterminated monolayers with variable fractions prepared by using mixtures of 1-dodecene and undecylenic acid in the first step of the grafting process on Si(111)-H surfaces. We focus in particular on molybdenum cluster-modified surfaces with variable cluster coverage namely SiMo100, SiMo33, and 2325


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Figure 2. Schematics of the (Mo6Bri8)4+ cluster core immobilization on carboxylic acid-modified silicon forming a [Mo6Bri8Fax(OOC10H20Sisurface)a6‑x] cluster unit: (a) x = 5 from SiAC5 to SiMo5 and (b) x = 3, 4 from SiAC100 to SiMo100. For clarity, the inner ligands are not represented and the (n-Bu4N)+ cations are represented as dark blue spheres. Note that for SiMo5, the (n-Bu4N)+ cations are located between the cluster units within the cluster plane and that for SiMo100, the (n-Bu4N)+ cations form a monolayer above the cluster one.

undecylenic acid), SiAC33, and SiAC5, respectively. Then the carboxylic acid-modified surface was soaked overnight under argon in gently stirred distilled THF solution (15 mL) containing ca. 7 × 10−4 M (n-Bu4N)2[Mo6Bri8Fa6]. The molybdenum cluster-modified surface was thoroughly rinsed with freshly distilled CH2Cl2, trichloroethylene (VLSI semiconductor grade) and acetone to obtain a contaminant-free perfectly clean surface for subsequent studies. The molybdenum cluster-modified surfaces prepared from SiAC100, SiAC33, and SiAC5 were denoted as SiMo100, SiMo33, and SiMo5, respectively. 2.3. Characterization Techniques. 2.3.1. Spectroscopic Ellipsometry. Spectroscopic ellipsometry (SE) experiments were performed in the range 1.6−4.6 eV, at an incidence angle of 70°, using a Horiba (UVISEL) ellipsometer, and analyzed with either two-layer (Si + alkyl) or three-layer (Si + alkyl + cluster layer) optical models. For the transparent (k = 0) alkyl molecular layers, an energy independent refractive index, nOML, was chosen because no improvement in the fitting result was found with a dispersion formula. To obtain the optical thickness dOML of mixed alkyl−acid layers, their refractive index was set at n = 1.48, while the cluster layer parameters (nCLUST, dCLUST) in the three-layer model were fitted to the SE spectra of Mo6Br8−OML−Si functional surfaces using fixed parameters (n = 1.48, dOML) obtained for the corresponding alkyl−Si surface. Some index contrast is found between the alkyl layer index (nOML = 1.48) and the index (nCLUST ≈ 1.7) of the densest Mo6Br8 cluster layer (SiMo100). A weak improvement in the fitting is obtained by using an absorbing cluster layer (k > 0, Cauchy-absorption model). 2.3.2. Scanning Tunneling Microscopy and Spectroscopy. STM and STS data were acquired using an OMICRON VT STM microscope in an ultrahigh vacuum chamber at a base pressure 99% from Fluka, then distilled over sodium under reduced pressure) with molar ratios (in %) of 33/67 and 5/95 respectively for SiAC33 and SiAC5 respectively. It has been demonstrated that this direct hydrosilylation route does not lead to appreciable reaction between the carboxyl groups and the surface provided that short UV irradiation times are used (typically, less than 4 h).25 The carboxylic acid-modified surface was rinsed copiously with acetone, then dipped in hot acetic acid at 65 °C for 2 × 20 min and dried under an argon stream.18 The carboxylic acidmodified surfaces were denoted as SiAC100 (for neat 2326


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and detection efficiencies of the analyzer, remains unchanged under identical experimental conditions. The total organic layer (acid + alkyl) coverage (∑OL) was estimated using the total carbon signal (denoted IOL hereafter) taking into account its attenuation by the organic layer itself using the following equation:

traces of water contamination and improve the tip−sample tunnel contact. All experiments took place at room temperature. STM images were acquired in constant current mode and were only postprocessed with a background plane correction unless stated otherwise. STS measurements were made by averaging 10 to 26 current−voltage (I−V) curves at specific locations either on clusters or on cluster-free parts of the substrate. The conductance dI/dV was numerically computed from I−V data. 2.3.3. X-ray Photoemission Spectroscopy Analysis (XPS) and Ultraviolet Photoemission Spectroscopy (UPS). Photoemission measurements were carried out with a base pressure in the 10−10 mbar range. The analysis chamber was equipped with an Omicron UVS 300 high current UV lamp, a VSW twin anode X-ray source and an Omicron HA100 electron energy analyzer. For the valence band measurements, He II (40.8 eV) radiation of the UV light was used. For the onset signal measurements, a fixed retardation ratio of 5 was chosen in order to avoid the saturation of the analyzer and a −9.0 V bias was applied to the sample to increase the kinetic energy of all photoelectrons. For the near valence band edge measurements, the electron pass energy of the analyzer was set at 10 eV in the fixed analyzer transmission mode, providing an overall energy resolution of 0.25 eV, as determined from the width of the Fermi step of a clean Au sample. This Fermi edge is the reference of the binding energies for UPS data. XPS spectra were measured using the Mg Kα (1253.6 eV) anode source with electron pass energy set either at 40 eV for survey spectra, or at 22 eV for resolved spectra. In the latter case, the overall resolution was 1.0 eV. For conductive samples, the binding energy was calibrated using the Au 4f7/2 peak set at 84.00 ± 0.05 eV, while for insulating samples (e.g., the (nBu4N)2[Mo6Bri8Fa6] reference powder) the binding energy was calibrated by setting the main C 1s peak at 285.0 eV. XPS spectra were fitted using mixed Gaussian−Lorentzian line shapes after a Shirley background subtraction. To derive the thicknesses from XPS analysis we assumed a homogeneous organic layer of thickness dOLon top of the silicon semi-infinite substrate. The Si 2p signal collected on grafted silicon (ISi,OL) was compared to that obtained on clean, uncovered Si(111)-H surface (ISi,clean). These values were obtained from the angular averaged signals to avoid the photoelectron diffraction effects observed on monocrystalline samples.26 The attenuation of the emitted photoelectrons was accounted for by the inelastic mean 27 free path parameter through the organic layer λOL Si = 3.6 nm. The thickness of the organic layer was then deduced according to ⎛ ISi,OL ⎞ ⎟⎟ dOL = −λSiOL ln⎜⎜ ⎝ ISi,clean ⎠

C1s IOL = Iplane

C1s 1 − exp( −dOL /λOL )

= K ΣOL

C1s 1 − exp( −a /λOL ) C1s 1 − exp( −dOL /λOL ) C1s 1 − exp( −a /λOL )

(3)

where a = 0.11 nm is the interplane distance of the organic layer, λC1s OL = 3.0 nm is the inelastic mean free path parameter through the organic layer for the electrons emitted from the C 1s level.27 2.3.4. Electrochemical Characterizations. Cyclic voltammetry measurements were performed with an Autolab electrochemical analyzer (PGSTAT 30 potentiostat/galvanostat from Eco Chemie B.V.) equipped with the GPES and FRA softwares in a homemade three-electrode Teflon cell. The working electrode, modified Si(111), was pressed against an opening in the cell bottom using a FETFE (Aldrich) O-ring seal. An ohmic contact was made on the previously polished rear side of the sample by applying a drop of In−Ga eutectic (Alfa-Aesar, 99.99%). The electrochemically active area of the Si(111) surface (namely 0.3 cm2) was estimated by measuring the charge under the voltammetric peak corresponding to the ferrocene oxidation on Si(111)-H and compared to that obtained with a 1 cm2-Pt electrode under the same conditions. The counter electrode was a platinum grid and the system 10−2 M Ag+|Ag in acetonitrile was used as the reference electrode (+0.29 V versus aqueous SCE). All reported potentials are referred to SCE (uncertainty ±5 mV). Tetra-n-butylammonium perchlorate Bu4NClO4 (Fluka, puriss, electrochemical grade) was used at 0.2 mol L−1 as supporting electrolyte in dichloromethane, previously distilled over P 2 O 5 . The (CH2Cl2 + 0.2 M Bu4NClO4) electrolytic medium was dried over activated, neutral alumina (Merck) for 30 min, under stirring and under argon. About 20 mL of this solution was transferred with a syringe into the electrochemical cell prior to experiments. All electrochemical measurements were carried out inside a homemade Faraday cage in the dark, at room temperature (20 ± 2 C) under constant argon gas flow. 2.3.5. Electrical Transport Measurements. DC and AC transport characteristics of Hg//[Mo6Br8]-terminated organic monolayer (OML) − n-Si junctions were measured in the dark as a function of bias, V, and temperature, T, using a homemade Teflon cell compatible with a Hg drop top electrode.28 The mercury (99.999% from Fluka) top contact area was 5.0 × 10−3 cm2 (0.8 mm diameter). The ohmic back contact was obtained by applying a silver paste electrode on the scratched Si backside. J(V) characteristics were measured at several locations of the fresh device, using a Keithley 6487 picoammeter. Voltage cycling in the range −4 V (reverse bias)/+2 V (forward bias) was applied without noticeable device evolution. The complex admittance was measured with a frequency response analyzer (Alpha-A, Novocontrol Technologies) at 1 MHz (VAC = 20 mV) and the Teflon cell capacitance (CPAR = 4.9 pF) was subtracted from the measured capacitance in order to derive the flat band voltage in Mott−Schottky plots C−2 vs V.

(1)

Carboxylic acid and cluster surface coverages, ΣCOOH and ΣMoBr respectively, were obtained by comparing the (C = O)OH and the molybdenum XPS intensities to the one-plane silicon intensity ISi,plane, calculated using the signal of the clean uncovered silicon through: a ISi,plane = ISi,clean SiSi = KNs λSi (2) where Ns is the number of Si atoms per unit surface (Ns = 7.8 × 1014 cm−2), λSiSi = 1.3 nm is the electron mean free path in silicon27 and aSi is the interplane distance perpendicular to the surface. K, which includes X-ray excitation flux, the transmission 2327


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The Journal of Physical Chemistry C As shown previously,28 the current at reverse and low forward bias can be modeled by a thermionic emission mechanism over the barrier at the semiconductor (SC)− metal interface; however, the measured effective barrier includes the transparency of the molecular tunnel barrier. At high forward bias, some current saturation results from the increasing role of tunnel barrier and series resistance. The admittance of a tunnel barrier reveals two major resonance frequencies, basically related to the semiconductor barrier and to the tunnel barrier, both being influenced by the density of states localized at the semiconductor/molecular insulator interface.29

cluster core based units. The measured thickness was more consistent with the greatest size of the Re6Sei8(TBP)a4(OH)a2 moiety which is included in an ellipsoid with short axis and long axis diameters roughly equal to 0.8 nm (O−O distance) and 1.8 nm (CH3−CH3 distance) respectively. 3.2. STM and STS Data. Figure 4a shows a large scale STM image of the carboxylic acid-modified Si(111) surface SiAC5.

3. RESULTS AND DISCUSSION 3.1. Ellipsometry Measurements. Similar optical properties (open symbols in Figure 3) are found for all single-

Figure 3. Pseudodielectric function (SE) data for mixed alkyl/acid monolayers grafted on Si(111)-H, before (open symbols) and after (filled symbols) immobilization of Mo6Bri8 cluster cores.

Figure 4. (a) 1100 × 1100 nm2 STM image of SiAC5 (Ugap = +3.929 V, It = 0.0316 nA). Atomic steps of the underlying Si(111) substrate are clearly visible. (b) 200 × 200 nm2 STM image of SiMo5 (Ugap = −3.207 V, It = 0.1037 nA). (Mo6Bri8)4+ cluster cores are sparsely distributed on the surface and the atomic steps of the substrate are still visible after grafting. (c) 10 × 16 nm2 STM image (zoom of b) of SiMo5 showing isolated clusters. (d) STS I−V curve (top) and differential conductance plot (bottom) obtained on (Mo6Bri8)4+ cluster cores grafted on SiMo5. A CBM/VBM energy gap of 2.5 eV is measured on single clusters.

component acid- and mixed alkyl/acid-terminated monolayers (SiAC100, SiAC33, SiAC5) grafted to n-Si(111), as shown in a previous report.13 The fitted optical thickness dOML, in the range from 1.0 nm (SiAC100, SiAC5) to 1.15 nm (SiAC33), is consistent with the geometrical length of C12 linear molecules, including a 40−50° tilt angle. No systematic dependence as a function of the acid dilution is found. Figure 3 shows that SE properties of [Mo6Br8]-OML-Si(111) surfaces depend on the initial acid dilution, with a clear difference observed between SiMo5 and SiMo100 spectra. This dependence reflects variations in the cluster coverage value. A large refractive index value (nCLUST = 1.70 ± 0.15) is found for the densest Mo6Br8 cluster layer (SiMo100). The fitted optical thickness dCLUST = 0.95 ± 0.15 nm (SiMo5, SiMo33) is consistent with the geometrical diameter of the cluster. We can then assume that for both these dilutions, clusters are grafted on the surface with the tetrabutylammonium counterions located within the same plane. On the other hand, the fitted optical thickness for SiMo100 is dCLUST = 2.1 ± 0.2 nm. This value is consistent with either two layers of cluster units (i.e., [Mo6Bri8Fax (OOC10H20-Sisurface)a6‑x] and [Mo6Bri8F6]2−) associated with their counter cations (n-Bu4N+) or a dense monolayer of [Mo6Bri8Fax (OOC10H20-Sisurface)a6‑x]2− cluster units with a monolayer of (n-Bu4N+) counterions laying on top. X-ray photoemission data presented hereafter do not support the first hypothesis (vide infra), the second hypothesis being probably more appropriate. The same behavior was observed in our previously published results on Re6Sei8(TBP)a4(OH)a2 building block grafted onto Si−H.13 The optical thickness, which was consistent with XPS data, appeared to be larger than the typical size derived from crystallographic data reported for the Re6Sei8

Large and flat terraces of 150−200 nm in length are clearly visible. These terraces are separated by a constant step height of 0.27 ± 0.03 nm according to the heights-histogram computed from this image. This value is very close to the inter-reticular distance of Si(111). This result indicates that the observed steps are monatomic steps of the Si substrate and that the covalent attachment of the mixed n-dodecyl/acid-terminated monolayer did not alter the underlying surface morphology. At smaller scales, STM images on a terrace reveal that the surface is perfectly atomically flat with a root-mean-square roughness of 0.17 nm. Such a small value is far below the molecular chains length (1.0 nm for SiAC5) deduced from ellipsometry measurements, allowing us to state that the molecular monolayer continuously covers the substrate without any pinholes. The STM images of the molybdenum cluster modified surface are shown in Figure 4b,c. Sparsely distributed clusters are clearly identifiable on the 200 × 200 nm2 area, and three steps of the underlying silicon substrate are still visible on this image (Figure 4b). As previously reported, the number of grafted clusters at the surface can efficiently be controlled by changing the molar ratios of the undecylenic acid/1-dodecene mixture. In the present case, dilution of carboxylic acid functions at a 5% level results in a low concentration of anchoring sites for the metal clusters in SiMo5 and gives the 2328


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The Journal of Physical Chemistry C opportunity to accurately measure the properties of individual clusters. The in-plane diameter of the observed clusters ranges from 3 to 5 nm; the lower value is believed to result from the experimental lateral resolution on individual clusters with their counterions lying in the same plane. Some aggregates with a larger radial extension (up to 8 nm) are also present with a rather homogeneous distribution on the surface. The height of a single cluster unit (1.0 ± 0.2 nm) is accurately measured from the difference between the surrounding substrate and a cluster. This value is consistent with the expected cluster core size with counterions in the same plane. It should also be noted that, while some clusters tend to aggregate laterally, the height of these aggregates is almost that of a single cluster. This indicates that vertical stacking is very limited on the observed samples. Scanning tunneling spectroscopy (STS) results are reported on Figure 4d. The measured tunnel current versus applied voltage shows a plateau centered at Vtip = 0 V. Assuming a uniform energy distribution of electronic states for the tip, the plot of the differential conductance (G = dI/dV) gives access to the VBM/CBM energy gap of isolated clusters grafted on the surface. A value of 2.5 ± 0.5 eV is found for immobilized Mo6Bri8 cluster core. This value is an average of 26 measurements made on isolated clusters of comparable radial size and height. The electrical band gap measured on the same sample in cluster-free regions is 1.9 eV. This value exactly matches the band gap measured on the SiAC5 sample (before grafting the (Mo6Br8)4+ cluster core). We point that higher band gap value (around 4 eV) has previously been obtained on isolated Re6Sei8 cluster cores grafted on silicon through mixed alkyl/acid monolayer. 3.3. X-ray Photoelectron Spectroscopy. Prior to the analysis of the grafted silicon surfaces, X-ray photoelectron measurements have been performed on (n-Bu4N)2[Mo6Bri8Fa6] cluster precursor powder to get reference signals for F, Mo, and Br elements. With the main carbon C 1s peak set at 285.0 eV, the characteristics peaks related to the cluster core, namely Mo 3d5/2 and Br 3d5/2, are observed at (229.40 ± 0.05) and (70.25 ± 0.05) eV, respectively. The apical F 1s fluorine is composed of a main peak at (683.20 ± 0.05) eV and a small shoulder (7% of the total fluorine) at 2.7 eV higher binding energy. XPS measurements have been performed on the mixed alkyl/ acid monolayers grafted on Si(111)-H using different acid molar ratios in the initial alkene mixture, before and after grafting of the (n-Bu4N)2[Mo6Bri8Fa6] cluster. Besides the Si substrate signal, only C 1s and O 1s signals appear after this first functionalization step. Typical survey scans after cluster grafting show the presence of characteristic peaks due to the Mo, Br, and N atoms of metallic moieties (Figure 5) which indicate the successful immobilization of the cluster. The thickness of the organic layers at each step has been calculated using the attenuation of Si 2p signal. The calculated values (dOL) are in good agreement with those deduced from spectroscopic ellipsometry data (dOML, dCLUST) within the experimental uncertainties. A good correlation is found between the Mo6Bri8 coverage and the COOH end group concentration initially present on the surface (Figure 5 and Table 1). The cluster coverage increases from 0.03 × 1014 to 0.58 × 1014 cm−2 when the acid concentration varies from 5% to 100% (Table 1). The higher coverage is half the density of (Mo6Ii8)4+ cluster cores grafted on silicon surfaces through pyridine linkers.11,12 Analysis of data reported in Table 1 shows that, for all the prepared surfaces, the amount of grafted clusters is much lower than the amount of available COOH groups.

Figure 5. XPS survey scans of SiMo5, SiMo33, and SiMo100 (top to bottom) grafted on the mixed alkyl/acid and single-component acidterminated monolayers.

Table 1. Acid and Metal Cluster Coverages per Surface Silicon Atom (θ) and Corresponding Surface Densities (Σ) Calculated from XPS Data of Modified Si(111) Surfaces as a Function of Undecylenic Acid Molar Concentration in the Initial Alkene Mixturea Acid coverage θCOOH ΣCOOH (1014 cm−2) (acid + alkyl) coverage θOL ΣOL (1014 cm−2) cluster coverage θMoBr ΣMoBr (1014 cm−2) ΣMoBr/ΣCOOH x 6−x

acid 5%

acid 33%

acid 100%

0.05 ± 0.02 0.39 ± 0.05 0.34 ± 0.02

0.10 ± 0.01 0.82 ± 0.05 0.39 ± 0.02

0.38 ± 0.02 2.8 ± 0.2 0.38 ± 0.02

2.6 ± 0.2 0.004 ± 0.001 0.03 ± 0.01

3.0 ± 0.2 0.020 ± 0.005 0.14 ± 0.02

2.8 ± 0.2 0.075 ± 0.010 0.58 ± 0.02

0.08 ± 0.03 4.8 1.2

0.17 ± 0.04 4.2 1.8

0.21 ± 0.02 3.6 2.4

a

The number of metal anchoring sites per cluster in the immobilized [Mo6Bri8Fax(OOC10H20-Sisurface)a6‑x] unit, i.e., (6 − x), is deduced from the number of remaining fluorine atoms per cluster (x) deduced from the F 1s peak area.

From the highest cluster concentration (ΣMoBr = 0.58 × 1014 cm−2), we can deduce a specific area of 172 Å2 per cluster, which is much higher than the expected single cluster footprint for a closely packed monolayer, i.e. 78 Å2 assuming a 1 nm typical diameter. Since the highest experimental cluster density value is not consistent with the formation of a bilayer of clusters, the 2 nm thickness deduced from either ellipsometry or XPS measurements leads us to consider that counter cations are located above the (Mo6Bri8)4+ cluster cores in SiMo100 (Figure 2b). This conclusion is supported by the emission angle dependence of the C 1s peak shape. After grafting of the (Mo6Bri8)4+ cluster core, the intensity of the main COOH component at (290.10 ± 0.05) eV decreases and a new component appears as a shoulder on the high binding energy side of the main carbon peak (Figure 6). This new contribution and the main C 1s peak are both attributed mainly to the carbon atoms located in the counterions, since these last ones bring 32 carbon atoms per cluster. As the takeoff angle decreases from 90° to 45°, the slight increase in carbon signal is larger than that of the molybdenum signal on SiMo100, indicating that counterions are preferentially located closest to the external surface, i.e. on top of the clusters. Interestingly, the Mo and Br core levels are shifted to higher binding energy values with increasing grafted cluster concentration. The Mo 3d5/2 and Br 3d5/2 peaks shift by 0.3 and 0.4 2329


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semiconductor and indicate that the Fermi level is not pinned by surface states near midgap at the silicon surface. Moreover, the fact that the Si 2p3/2 peak maximum remains at the same value during the grafting process for all the analyzed samples, shifting by less than 0.07 eV, enables us to conclude that there is no charge transfer between the grafted molecules and the underlying semiconductor, which is in agreement with our previous results obtained with Re6Sei8-modified Si(111) surfaces.13 The grafting of organic layers on Si(111)-H allows the unpinning of the Fermi level, as reported by Mönch et al. in a recent paper.32 Careful examination of the high binding energy side of Si 2p (near 104 eV binding energy) does not show any oxidation of the silicon surface, neither after the first step nor after the second grafting step. In the present study, HF species released during the cluster immobilization reaction could likely have a protective role against silicon oxidation. In contrast, silicon oxidation was observed after grafting of Re6Sei8 cluster cores using an immobilization strategy based on a water molecule elimination reaction.13 3.4. UV Photoelectron Spectroscopy. Figure 7 shows He II-excited UPS spectra of Si(111) surfaces grafted with the single-component acid-terminated monolayer before (SiAc100) and after grafting of (Mo6Bri8)4+ cluster cores (SiMo100). For comparison, the UPS spectrum of the Si(111)H substrate recorded under the same experimental conditions was also included. All intensities can be directly compared since all data were collected under the same experimental conditions. We can notice that the SiAc100 organic layer presents a much larger UPS intensity than hydrogenated silicon due to the low cross section of Si as compared with carbon atoms. The left (Figure 7a) and right (Figure 7b) hand side of the energy distributions show the onset of the low kinetic energy secondary electron peak and the valence band structures, respectively. The valence band spectra for the acid-terminated n-alkyl chains are similar to those reported for linear alkyl chains on silicon.33 The slight differences in the relative heights between our measurements and literature data could be due to valence band states of COO and/or OH functional end groups. The signal between −4 eV and the Fermi level has been attributed either to electronic states arising from silicon and penetrating the organic layer,34 or to a slight degradation of the organic layer due to irradiation.35 After (Mo6Bri8)4+ cluster core grafting, the valence band spectra are significantly modified. A new component appearing at 3 eV below the Fermi level can be attributed to Mo2+ species.36 This component is indeed also observed in the UPS spectra of Mo-containing materials (e.g. (n-Bu4N)2[Mo6Bri8Fa6]

Figure 6. C 1s photoemission spectra recorded on SiAc100 (open symbols) and SiMo100 (filled symbols). The black arrow indicates the shift of the C = O-R component from 290 to 289 eV after grafting of the metal cluster.

eV to higher binding energy when going from SiMo5 to SiMo100, respectively (Table 2). Since an increasing number Table 2. Binding Energy Values (in eV) of Mo 3d5/2 and Br 3d5/2 Peaks Measured on the Grafted Surfaces, with the Corresponding Values Measured on the Reference Powder levels

SiMo5

SiMo33

SiMo100

reference powder

Mo 3d5/2 Br 3d5/2

229.7 70.5

229.9 70.7

230.0 70.9

229.5 70.3

of apical fluorine atoms per cluster are replaced by surface carboxylate bonds upon HF elimination (Table 1), resulting in the formation of covalent Mo−O units with larger binding energy, this stabilization of the (Mo6Bri8)4+ cluster core explains the increase in binding energy for these two elements. For the SiMo100 assembly, each (Mo6Br8)4+ cluster core is bound to the surface through multiple anchoring sites (6 − x = 2 or 3), indicating that acid-terminated alkyl chains are flexible enough to accommodate at least three bonds. For silicon substrate, the Si 2p3/2 peak has its maximum value at (99.60 ± 0.07) eV below the Fermi level. Subtracting this energy from the referenced value related to the valence band maximum,30 we can deduce that the Fermi level EF in silicon is 0.3 eV below the conduction band minimum Ec at the interface with the organic layer. This value is equal to that calculated N according to Ec − E F = kBT ln Nc , where kB is the Boltzmann d

constant, Nc the effective density of states in silicon, and Nd the donor concentration deduced from capacitance−voltage measurements (Nd = (6.0 ± 0.5) × 1014 donor cm−3, section 3.5).31 These results correspond to flat bands in the

Figure 7. (a) He II (hν = 40.8 eV) excited UPS spectra (onset of the photoemission spectra) of n-type Si(111)-H (black line), SiAC100 (red open circles), and SiMo100 (red filled circles) and (b) corresponding valence band photoemission spectra. The vertical arrows indicate the VBM positions (b), and the horizontal ones indicate the shift of the onset energy, Ecutoff, due to surface dipoles (a). 2330


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The Journal of Physical Chemistry C reference powder and [Mo6I8]4+ cluster units). The position of the VBM is shown by the arrow at the knee. The ionization potential (IP) is obtained by subtracting the total width of the valence band spectra from the photon energy, IP = hν − (EVBM − Ecutoff) where Ecutoff and EVBM represent, respectively, the energy position of the secondary electron onset and the valence band maximum (VBM) of the substrate surface relative to EF. IPOL for the organic layer is obtained by considering the VBM edge of the organic layer in the previous equation. This gives IPOL = 7.8 ± 0.2 eV, for the acid-terminated n-alkyl chains, close to the values reported in the literature,37,38 and IPOL = 5.8 ± 0.2 eV on the cluster grafted surface. The shift observed in Ecutoff upon grafting of the organic layer can be interpreted as arising from interface dipoles (here positive dipoles pointing outward) while the energy barrier for hole transport is obtained by EVBM‑Si − EVBM‑SiAC100 and EVBM‑Si − EVBM‑SiMo100. As stressed above, the occurrence of charge transfer between clusters and silicon has been ruled out since no band bending has been observed after cluster immobilization. Consequently the dipole change after densification of the monolayer in SiMo100 compared to SiMo5 is due to charge distribution modification in the cluster unit itself driven by lateral interactions between clusters. Using these values, a diagram of the band alignment is proposed in Figure 8 for the (Mo6Bri8)-OML-Si covalent

acid-terminated organic chains in [Mo6Bri8Fax(OOC10H20Sisurface)a6‑x]- configuration. If the Mo−O bond is stronger than the initial MoF bond, this leads to the stabilization of the bonding energy levels of (Mo6Bri8)4+ cluster cores. This should give an opening of the HOMO−LUMO gap for individual [Mo6Bri8Fax(OOC10H20-Sisurface)a6‑x] cluster units due to the concomitant destabilization of antibonding levels of the cluster unit. This HOMO−LUMO gap is expected to be proportional to the (6 − x) number of anchoring sites. Additional secondary effects could also arise from lateral interactions between ligands. It is well-known in cluster chemistry that going from molecular cluster units with discrete levels of energy to condensed clusters in 1-D, 2-D, or 3-D extended solids leads to the formation of band structures.39,40 The dispersion of energy bands depends on the strength of electronic interactions between molecular units in cluster-based solids. Hence, in the case of immobilized clusters, since the coverage and lateral interaction between cluster units both increase from SiMo5 to SiMo100, an increase of electronic interactions between clusters units can be responsible in part for some energy dispersion of the valence and conduction bands. For SiMo5, the electrical band gap measured by STS is 2.5 eV. For higher surface coverage, the interaction of the cluster units with the surface is stronger as deduced from XPS results and it is rather speculative to place the Fermi level at the center between CBM and VBM. Finally, the location of tetrabutylammonium cations within (SiMo5 and SiMo33) or above (SiMo100) the clusters monolayer is explained by steric constraints reflecting cluster unit density and strengths of interactions between cluster units. High cluster coverage induces close contacts between units and enhanced mutual electronic interactions. 3.5. Electrochemical Characterizations of Molybdenum Cluster-Modified Si(111) Surfaces. Typical cyclic voltammograms of the (Mo6Bri8)-OML-Si surfaces in CH2Cl2 + 0.2 M Bu4NClO4 are shown in Figure 9. In the dark, no

Figure 8. Proposed energy band diagram deduced from XPS, UPS, and STS measurements for SiMo100 prepared in this work (left panel) and compared to that corresponding to the Re6Se8 clustermodified Si(111) surface (right panel) [ref 13].

assembly. The energy position ECBM is determined using the electrical gap of Si (1.12 eV). From this band structure diagram, the energy distance between the last filled electronic state of the grafted SiMo100 surface and the Fermi energy is found to be 1.6 eV. This value is slightly larger than the 1.25 eV measured by STS on an isolated cluster (SiMo5) (see Figure 4d). This demonstrates that lateral molecular interactions between clusters combined with the modification of the cluster structural arrangement for the dense grafted surface modify the energetic dispersion of the surface valence and conduction bands. Several hypotheses are proposed to explain the difference in the measured VBM for individual immobilized clusters in SiMo5 and close-packed cluster monolayer in SiMo100; the VBM values were −1.25 and −1.6 eV in SiMo5 (by STS) and SiMo100 (by UPS), respectively. As discussed in the XPS section, increasing grafted cluster concentration from SiMo5 to SiMo100 leads to an increase of the number, 6-x, of anchoring sites of a molybdenum cluster via

Figure 9. (a) Cyclic voltammograms at 0.1 V s−1 in CH2Cl2 + 0.2 M Bu4NClO4 of the three metal cluster-modified surfaces under illumination (colored traces) and in the dark (black trace). (b) Mott−Schottky Csc−2 − E plot at 50 kHz of SiMo100. The current fluctuations observed on the curves in (a) are thought to be caused by fluctuations in the intensity of the light source.

anodic current was measured for all the prepared surfaces, as expected for a semiconductor under depletion conditions, that is, when few majority charge carriers (i.e., electrons) are available for charge transfer.41 An excursion toward negative potentials shows only the beginning of a reduction wave without any distinct peak. This system can be reasonably ascribed to the reduction of the solvent and/or the electrolyte onto the modified surface. Upon white light illumination, a significant oxidation current is observed with no well-defined anodic peak. Furthermore, the intensity of the anodic current is found to increase with the amount of immobilized 2331


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The Journal of Physical Chemistry C molybdenum cluster. Such electrochemical characteristics of the (Mo6Bri8)-OML-Si assembly are in rather good agreement with those reported for Re6Se8-modified silicon surfaces, showing a poorly resolved irreversible anodic peak at ca. 0.60 V vs SCE, only for the higher cluster coverage.13 Such a result could be ascribed to the reduced electron transfer kinetics between the immobilized metallic cluster and the underlying silicon surface, due to both the insulating character and the length of the n-alkyl bridging molecular chain. To obtain further insights on the electrical properties of the modified Si(111) surfaces, differential capacitance measurements have been performed in the same electrolytic medium. In order to determine the energy levels of the semiconductor bands, it is essential to estimate the flat band potential Efb of the silicon surface, i.e., the electrode potential for which there is no space-charge region in the semiconductor. This parameter has been estimated from the commonly used Mott−Schottky plot (Csc−2 vs E, eq 4) that gives the space-charge capacitance Csc as a function of the electrode potential E under depletion conditions (i.e., depletion of conduction band electrons in the space charge region of the n-type surface).41 Csc−2 =

⎛ 2 kT ⎞ E − Efb − ⎟ 2⎜ q ⎠ qεε0NA ⎝

Figure 10. Temperature dependent current density−voltage J(V) characteristics of Hg//SiAc100 (black traces) and Hg//SiMo100 (red traces) junctions at 253 (dotted traces), 273 (dashed traces), and 293 K (solid traces).

show current density−voltage J(V) characteristics which are similar to their parent acid-terminated junctions. Hence, in the following, we will essentially discuss SiAC100 and SiMo100 devices. Using low doped n-type Si, the junction remains rectifying after the cluster layer immobilization, with a reverse bias current density increase by a factor of about 20. The reverse current density (JREV ≈ 2 × 10−5 A cm−2 at 293 K) depends strongly on temperature with an apparent activation energy EACT (JREV) ≈ 0.31 ± 0.03 eV for the SiMo100 junction, as compared with 0.49 eV for the SiAC100 junction. The exponential part of the forward J(V) (Figure 10) provides a Schottky barrier height, ΦEFF ≈ 0.70 ± 0.05 eV. This value is smaller than expected from the measured flat band voltage, VFB = 0.61 ± 0.03 V, derived from a Mott−Schottky plot (not shown) and from the bulk Fermi level position at 0.30 eV below the conduction band (293 K). This small apparent barrier height, derived from J(V) characteristics of the SiMo100 junction, reveals again some effect of lateral inhomogeneity of the potential barrier. The reverse current density increases monotonously in the series SiMo5, SiMo33, and SiMo100, up to 2 × 10−5 A cm−2, as the [Mo6Br8F6]2− cluster coverage increases in the range 0.3−6 × 1013 cm−2. The contrast with previous results using Hg//Re6Se8−OML−n-Si junctions is remarkable, since a very strong coverage dependence of the reverse current density, JREV, was found in the series SiRe5, SiRe33, and SiRe100, with a maximum value of 2 × 10−3 A cm−2, for the larger metal cluster coverage of 6 × 1013 cm−2.13 Different behavior in electrical transport through Hg//metal cluster−alkyl−Si (111) junctions may be related to the different covalent binding chemistry, to different energy position of HOMO−LUMO cluster orbitals or to variable short-term stability against oxidation of the OML−Si interface.

(4)

where ε is the relative permittivity of silicon (11.7), ε0 is the permittivity of free space, N is the dopant density of the semiconductor (expressed as Nd the donor density of silicon), A is the area of the electrode, k is the Boltzmann constant, T is temperature and q is the electronic charge. Linear Csc−2 − E plots are obtained for potentials above −0.5 V for the three (Mo6Bri8)-OML-Si surfaces, the slope and the intercept of which enable the flat band potential and the dopant density to be determined (Figure 9b). The calculated N value is consistent with the dopant density derived from the four-probe resistivity measurements of these silicon samples (Nd = (6.0 ± 0.5) × 1014 donor cm−3). The resulting value of Efb (−0.75 ± 0.05 V) does not significantly depend on the molybdenum cluster coverage and is not too far from that extracted from charge transport measurements using a Hg soft top contact (vide infra). Furthermore, this Efb value is quite similar to those measured for both acid-terminated monolayer and Re6Sei8-modified silicon surfaces.13 This demonstrates that the covalent attachment of metallic clusters to carboxylic acid terminated monolayers does not change the interfacial properties of the surface in CH2Cl2 + 0.2 M Bu4NClO4. 3.6. Charge Transport Characteristics of Hg// (Mo6Bri8)-OML-Si(111) Junctions. Current density−voltage J(V) characteristics of cluster-free Hg//OML−n-Si junctions show a strong rectification for all mixed alkyl/acid-terminated OML. Large values of the rectification factor at ±1 V, R (±1 V) > 1 × 106 (SiAC33 and SiAC5) are due to a very small reverse current density JREV (−1 V) < 10−7 A cm−2. In this set of devices, the reverse current density is larger for the SiAC100 junction (JREV ≈ 9 × 10−7 A cm−2) and some deviations from the expected linear ln(J) vs V are also observed at low forward bias (Figure 10); this excess current is attributed to some lateral inhomogeneity of the potential barrier.28 In the intermediate voltage domain, the current is limited by tunneling through the organic layer while at high forward bias some current saturation is due to a series resistance effect (RS ≈ 0.5 Ω cm−2). After cluster immobilization, at low metal cluster coverage (SiMo5, SiMo33), Hg//(Mo6Bri8)−OML−n-Si junctions

4. CONCLUSION In this study, a convenient and straightforward experimental procedure is reported to prepare Mo6 clusters functionalized silicon surfaces by simple reaction of carboxylic acid-modified n-type Si(111) with a solution of [Mo6Br8F6]2− cluster units. This novel surface grafting method involving acido-basic reaction leads to the release of HF molecules which is probably responsible for the efficient protection of the underlying silicon surface against oxidation as shown by the stabilized hydrogenated silicon surface after grafting of the metal cluster. In contrast, such favorable situation was not encountered with our previously reported experimental procedures used for the immobilization of Re6Sei8 on carboxylic acid-modified silicon13 2332


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The Journal of Physical Chemistry C or Mo6Ii8 on pyridine-modified silicon.11,12 A fine control of the surface coverage of the metal cluster is obtained by dilution of the reactive carboxylic acid-terminated organic chains with inert n-dodecyl chains in the initial alkene mixture. From XPS, UPS and STS measurements, an energy band diagram is deduced for the (Mo6Bri8)-terminated monolayer (OML)−n-Si interface. High cluster coverage induces close contacts between units and enhanced mutual electronic interactions. Further J−V measurements evidence a monotonous increase in the reverse current density as the cluster coverage increases. Knowing that [Mo6Bri8La6] cluster units exhibit luminescent, sensitization and catalytic properties,4,5,42,43 the fine control of both the surface coverage and the physical properties of the metal cluster-modified surface, as demonstrated in this work, are highly stimulating to prepare electrically addressable multifunctional systems.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.C.). Tel: +33 (0) 223 236 607. Fax: +33 (0)223 236 799. *E-mail: [email protected] (B.F.). *E-mail: [email protected] (S.A.-G.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank ANR CLUSTSURF for funding. REFERENCES

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