Electrochemical and Charge Transport Behavior of Molybdenum-Based Metallic Cluster Layers Immobilized on Modified n- and p-Type Si(111) Surfaces Bruno Fabre,* ,† Ste ´phane Cordier,* ,† Yann Molard, † Christiane Perrin, † Soraya Ababou-Girard, ‡ and Christian Godet* ,‡ Sciences Chimiques de Rennes, UMR 6226 CNRS/UniVersite ´ de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France, and Institut de Physique de Rennes, CNRS UMR 6251, Equipe de Physique des Surfaces et Interfaces, UniVersite ´ de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France ReceiVed: April 7, 2009; ReVised Manuscript ReceiVed: June 19, 2009 Octahedral molybdenum cluster cores [Mo 6 I 8 ] 4+ have been attached in apical positions to p- and n-type Si(111) surfaces through complexation with a pyridine-terminated organic monolayer (2 × 10 14 cm -2 ), which was previously covalently bound to hydrogen-terminated Si(111). This grafting procedure resulted in about a 4 nm thick Mo 6 -terminated layer. Similar XPS results were found for p- and n-type samples, suggesting that the grafting efficiency and composition of the resulting layers do not depend significantly on the doping type of the surface. The cluster footprint of 10 nm 2 indicates a fairly dense molecular packing on the Si(111) surface. The electrochemistry of such Mo 6 -modified surfaces in acetonitrile was characterized by a single irreversible oxidation peak at 0.92 V versus saturated calomel electrode (SCE) and flat band potential E fb values of -0.55 ( 0.05 V and 0.04 ( 0.05 V for the modified n- and p-type surfaces, respectively. The derivatization of silicon surfaces by Mo 6 introduces surface states that are probably due to some unavoidable oxidation of Si(111) and/or the possible presence of interfacial alkoxy species. From capacitance measurements, the total density of the surface states was estimated at 3.7 × 10 11 cm -2 and 2.7 × 10 11 cm -2 for the modified n-type and p-type Si(111), respectively. Electrical transport measurements through the Mo 6 -modified monolayer/ Si(111) devices were performed using mercury as a soft top contact. In contrast with the Hg/pyridine-alkyl/ Si(111) junctions, specific features appear in the current density-voltage characteristics of the Hg/Mo 6 / pyridine-alkyl/Si(111) junctions. The minima observed in the conductance-voltage plots for p-type and n-type Si(111) are attributed to the loss of one or two electrons from the highest occupied molecular orbital when a negative bias is applied to the substrate; the image charge in the mercury leads to a dipole layer at the Hg/Mo 6 interface, which creates an additional barrier as compared with those of the pyridine-modified surfaces. 1. Introduction The structures of Mo 6 cluster-based compounds are built up from [Mo 6 X i 8 X a 6 ] 2- building blocks (X ) halogen and/or chalcogen, i ) inner, a ) apical; Figure 1a) wherein the octahedral cluster is face capped by eight covalently bonded inner ligands (X i ) to form a [Mo 6 X i 8 ] 4+ cluster core that is ionically bonded to six additional terminal ligands (X a ). Their physical properties depend on the number of valence electrons involved in the Mo-Mo bonds (VEC) and on the strength of electronic interactions between the building blocks. 1-10 The metallic electrons are localized on a set of 12 metal-metal bonding orbitals, leading to a closed shell configuration with 24 electrons per cluster (Figure 1b). 6 A wide variety of interesting physicochemical characteristics have been reported: electrical, superconducting with high critical field, optical, catalytic, and thermoelectric. 11-14 For example, the host structure of the A x Mo 6 Q 8 series (A ) Cs + , n-C 4 H 9 N + ,Q ) chalcogen) with a fast reversible intercalation of cations should lead to applications as cathode material for Mg rechargeable batteries. 15,16 Interestingly, many routes afford soluble inorganic or hybrid organic/inorganic precursors containing Mo 6 X i 8 X a 6 cluster units. 17-22 Their intrinsic physicochemical and structural proper- * Corresponding authors. E-mails: bruno.fabre@univ-rennes1.fr (B.F.), stephane.cordier@univ-rennes1.fr (S.C.), christian.godet@univ-rennes1.fr (C.G.). Fax: + 33 (0) 223 236 799. † Sciences Chimiques de Rennes. ‡ Institut de Physique de Rennes. Figure 1. (a) Representation of the Mo 6 L 8 i L 6 a cluster unit. (b) Schematic representation of the molecular orbital diagram of a M 6 L 8 i L 6 a cluster unit in ideal O h symmetry. J. Phys. Chem. C 2009, 113, 17437–17446 17437 10.1021/jp903205a CCC: $40.75  2009 American Chemical Society Published on Web 09/15/2009