Electrochemical and Charge Transport Behavior of Molybdenum-Based Metallic Cluster Layers Immobilized on Modified n- and p-Type Si(111) Surfaces

Octahedral molybdenum cluster cores [Mo6I8](4+) have been attached in apical positions to p- and n-type Si(111) surfaces through complexation with a pyridine-terminated organic monolayer (2 x 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-tb 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, Front capacitance measurements. the total density of the surface states was estimated at 3.7 x 10(11) cm(-2) and 2.7 x 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 its 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 front 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.