以電化學方法調控單分子電性：五核金屬串分子與電極之能階匹配

單一分子的導電值會受到電極–分子–電極接合點中電子傳遞方式的影響，其中一個重要的因素是電極費米能階(Fermi level)與分子前緣分子軌域能階(frontier molecular orbitals)之間的能量差，即能階匹配(energy-level alignment)的程度。電化學是一種調控工作電極費米能階位置的方法，藉此可以輕易控制該能階接近或遠離分子軌域能階，即控制能階匹配的程度。本研究的量測對象為具有直線型金屬鏈的一系列五核金屬串分子(金屬核分別為鎳、鈷和鉻)，該分子們有明確的第一氧化態與相較於飽和烷碳鏈較小的 HOMO-LUMO 能量差，適合作為電化學系統探討的對象。ECSTM BJ (electrochemical scanning tunneling spectroscopy break junction)用於量測五核金屬串分子於中性態與第一氧化態時的單分子導電值，結果顯示導電值趨勢與中心金屬間的作用力有關。固定探針與表面間的偏壓進行的電化學電位掃描，可以在電化學電位連續變化的同時監控分子的導電值，結果發現當電化學電位掃描至分子氧化還原電位時具有較高的導電值，遠離該電位時導電值便跟著下降，顯示能階匹配程度上升時，電子較容易在接合點中傳遞。而在不同電化學電位下進行的偏壓掃描，經由轉換電壓能譜(transition voltage spectroscopy, TVS)可以獲得和能障高度(energy barrier height)呈正相關的轉換電壓，實驗觀察到當轉換電壓於電化學電位恰為氧化還原電位時具有最小值，即當能階匹配程度高的時候具有最小的能障高度，同時證明電化學電位掃描時觀察到的導電值變化是能階匹配效應。

關鍵字

電化學掃描式穿隧電子顯微術；單分子導電值；金屬串分子；能階匹配；轉換電壓能譜

並列摘要

The single-molecule conductance is affected by the electron transport through the electrode–molecule–electrode junctions. One of the most important factors is the energy-level difference between the electrode Fermi level and the frontier molecular orbitals. This energy difference can be controlled by electrochemical gating, which means pushing the potential of the working electrode toward the redox potential of the molecule. The compounds here are extended metal-atom chains (EMACs), which have well-defined one-electron oxidation reactions, to study the effect of energy-level alignment on the single-molecule conductance. For the scans of electrochemical potential, the single-molecule conductance is measured at a fixed bias and monitored as a function of electrochemical potential. On the other hand, single-molecule i–V curves are obtained at fixed electrochemical potentials. Transition voltages derived from the corresponding Fowler-Nordheim plots are well correlated with the energy barrier heights. Larger conductance and smaller energy barrier heights were found when electrochemical potential was just about the redox potential, indicating the effect of energy-level alignment.

並列關鍵字

electrochemical scanning tunneling spectroscopy ； single-molecule conductance ； extended metal-atom chains ； energy-level alignment ； transition voltage spectroscopy

參考文獻

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以電化學方法調控單分子電性：五核金屬串分子與電極之能階匹配

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