單層金屬原子修飾電極間的橋接分子之量子輸送：表面電子結構效應之研究

Abstract

過渡金屬的d原子軌域在表面吸附中扮演重要的角色。如何調控金屬表面的電子結構使分子形成穩定的金屬－分子－金屬系統並觀測其量子輸送的現象是一項重要的課題。通常由d軌域形成的d能帶越窄，表面吸附效應越強，分子軌域(molecular orbital, MO)越容易分裂為反鍵結軌域(anti-bonding MO)以及鍵結軌域(bonding MO)。由於反鍵結軌域相對靠近金屬的費米能階以致分子－電極間的能階匹配(energy-level alignment)程度提升，金屬－分子－金屬系統的電子輸送效率增加。為系統性探討表面電子結構對於量子傳輸效率之影響，本篇論文提出利用低電位沉積法(underpotential deposition)於金電極表面修飾銀（或銅）的單層原子膜使電極表面的d能帶變窄以致分子－電極間的能階匹配程度有所提升。吾人運用導電式原子力顯微術(conductive atomic force microscopy)測量飽和烷雙羧酸分子(HOOC(CH2)nCOOH, n = 4, 6, 8)的導電值，觀察到羧酸分子在修飾電極上的接觸電導(contact conductance)比在純金電極上高出一個數量級。常見的單能階模型(single-level model)無法充分解釋電極的表面電子結構和能階匹配程度間的關聯性。為驗證實驗結果吾人結合Nørskov提出之d能帶理論(d-band theory)，利用Newns-Anderson模型進行預測，並得到和實驗結果相符的趨勢。此研究揭示表面吸附效應對單分子電性的影響，並為尋找理想的分子－電極接觸位向(contact configuration)提供一種系統性、基於表面吸附概念的設計思路。

The surface d bands of transition metals play an essential role in the surface adsorption, which is associated with the formation of molecule-electrode contacts in a metal-molecule-metal junction (MMM junction). A relatively narrow surface d bands generally splits the adsorbate state into the anti-bonding and the bonding state. The anti-bonding state is generally closer to the Fermi-level than the adsorbate state, meaning a better degree of energy-level alignment. This effect leads to a better efficiency for electron transport through the MMM junctions. However, there is still lack of systematic studies on surveying molecule-electrode contacts considering effects of surface adsorption. In this thesis, the surface d bands of the Au substrates are narrowed by electrodepositing with a monolayer of Ag (or Cu). The model molecules are a series of α,ω-alkanoic acids (HOOC(CH2)nCOOH, n = 4, 6, 8). The electrical characterization was carried out by a non-destructive atomic force microscopy-based technique. The values of contact conductance for the MMM junctions of bimetallic electrodes were found higher than that of bare Au electrodes at an order of magnitude. The increased contact conductance of the MMM junctions is in agreement with that predicted by the Newns-Anderson incorporated with Nørskov’s d-band model. This research addressed the effects of surface adsorption on electron transport at the molecular level. A new strategy is proposed to rationally adjust surface d bands and to design superior interfacial contacts.