Covalent Anchoring of Re6Sei8 Cluster Cores Monolayers on Modified n- and p-Type Si(111) Surfaces: Effect of Coverage on Electronic Properties

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• 作者：Stphane Cordier ; Bruno Fabre ; Yann Molard ; Alain-Bruno Fadjie-Djomkam ; Nicolas Tournerie ; Alexandra Ledneva ; Nikola G. Naumov ; Alain Moreac ; Pascal Turban ; Sylvain Tricot ; Soraya Ababou-Girard ; Christi
• 刊名：Journal of Physical Chemistry C
• 出版年：2010
• 出版时间：November 4, 2010
• 年：2010
• 卷：114
• 期：43
• 页码：18622-18633
• 全文大小：576K
• 年卷期：v.114,no.43(November 4, 2010)
• ISSN：1932-7455

文摘

The electronic properties of redox-active transition metal clusters (Re₆Se₈) covalently immobilized on modified Si(111) surfaces through linear alkyl spacers have been studied as a function of the cluster coverage (1 × 10¹³−6 × 10¹³ cm⁻²). The latter is controlled by using Si(111)/H surfaces modified by dense mixed alkyl/acid-terminated monolayers with variable fraction of the acid grafting sites from 5 to 100% in solution. Quantitative X-ray photoemission analysis, spectroscopic ellipsometry, and scanning tunnelling microscopy indicate a covalent attachment of a submonolayer to densely packed monolayer of Re₆Se₈ clusters, while the vibrational Raman signature confirms the cluster integrity within the monolayer. Electrical band gaps as deduced from scanning tunnelling spectroscopy have been obtained for low Re₆Se₈ cluster coverage. Using ultraviolet photoemission spectroscopy, electronic properties such as ionization potential changes and energy level alignments at organic/inorganic interfaces are studied. We show that the lowest unoccupied molecular orbital of the Re₆Se₈ cluster is close to the bottom of the Si conduction band. At high cluster coverage, this affects the current−voltage characteristics measured using a weakly interacting top mercury contact onto the organic monolayer/silicon junctions. Indeed, on n-type silicon, the high level current at low bias and the shape of the conductance G(V) curve indicate a Schottky barrier height lowering. On the other hand, the current−voltage characteristics are the same for both acid-terminated and low coverage Re₆Se₈ cluster junctions at low bias; the high Schottky barrier height limits the current at low bias. When the forward bias increases, the current is tunnelling limited. As expected from the band alignment deduced from photoemission data, the opposite behavior is obtained on p-type silicon.