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

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• 作者：Bruno Fabre ; Stphane Cordier ; Yann Molard ; Christiane Perrin ; Soraya Ababou-Girard ; Christian Godet
• 刊名：Journal of Physical Chemistry C
• 出版年：2009
• 出版时间：October 8, 2009
• 年：2009
• 卷：113
• 期：40
• 页码：17437-17446
• 全文大小：458K
• 年卷期：v.113,no.40(October 8, 2009)
• ISSN：1932-7455

文摘

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 × 10¹⁴ cm⁻²), which was previously covalently bound to hydrogen-terminated Si(111). This grafting procedure resulted in about a 4 nm thick Mo₆-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 nm² indicates a fairly dense molecular packing on the Si(111) surface. The electrochemistry of such Mo₆-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 Mo₆ 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 × 10¹¹ cm⁻² and 2.7 × 10¹¹ cm⁻² for the modified n-type and p-type Si(111), respectively. Electrical transport measurements through the Mo₆-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/Mo₆/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/Mo₆ interface, which creates an additional barrier as compared with those of the pyridine-modified surfaces.