電極材料對單分子電性的影響：金、鈀及鉑電極與飽和烷硫醇和異硫氰基團之軌域偶合

Superior Contact for Single-Molecule Conductance: Electronic Coupling of Thiolate and Isothiocyanate on Pt, Pd, and Au

柯志宏(Chih-Hung Ko)

指導教授 ： 陳俊顯

國立臺灣大學/理學院/化學研究所/碩士(2009年)

https://doi.org/10.6342/NTU.2009.00623

摘要

在分子電子學中，提昇電子在金屬-分子-金屬接合點(Metal-Molecule-Metal, MMM)的傳遞效率是項重要議題。影響單分子電性之量測值的關鍵因素包括分子主體、末端官能基與電極材料，然而迄今仍無針對電極材料進行討論的實驗結果。有鑑於此，我們以金、鈀、鉑三種金屬做為電極，發現所測得的單分子導電值有數倍之差異，本論文將報導量測結果並探究其原因。我們使用的實驗儀器是掃描式穿隧顯微鏡(scanning tunneling microscope, STM)，以破裂接點法(break junction)測量直鏈飽和烷系列的雙硫醇和雙異硫氰分子之導電值，藉此得到官能基與電極間的接觸電阻(contact resistance)，用於比較各種分子-電極組合的差異。實驗結果顯示每種分子在三種金屬上皆呈現高、低兩組導電值(high-conductance, HC和low-conductance, LC)，相差約11倍。在接觸電阻方面，以鈀與鉑架接的雙異硫氰分子下降至金的20-60%，而對於雙硫醇分子，使用鉑電極減少成為金的50%。我們也檢視了三核鎳錯合物[Ni3(dpa)4(NCS)2] (dpa = 2,2'-dipyridylamide)，其導電值變化趨勢和雙異硫氰分子類似，證實鈀、鉑電極可提供較佳導電性之特性也適用於飽和烷以外的分子。分析連續取樣出現有效MMM訊號的機率，雙異硫氰分子在鈀和鉑上鍵結機率分別為37%與42%，較優於在金的27%；而比較金與鉑電極量測雙硫醇分子的機率分別為35%與29%。密度泛函理論(density functional theory, DFT)的模擬計算顯示實驗中的兩組導電值可能源自於頭基鍵結於基材表面原子的three-fold hollow或ontop位置，且分子在兩位向的吸附能差異與機率之相對大小吻合。此外，受軌域偶合影響，分子與表面鍵結的幾何角度將決定電子穿越此介面的難易。例如頭基的硫原子與金屬原子間以d軌域重疊為主，頭基與電極的夾角會因pi電子雲的參與而呈180度，直線通道將有助於提昇電子傳遞效能；反之，兩者間如以s軌域為主，角度為100度，導電值因而下降。所以我們能藉由更換電極材料來製造出通道含較多d軌域性質的接合面。

關鍵字

掃瞄式穿隧顯微鏡 ； 單分子電性 ； 破裂接點法 ； 分子電子學 ； 金屬奈米線

並列摘要

One of the critical issues for molecular electronics is the development of optimal molecule-electrode contacts which have long been expected to substantially influence the measured single-molecule conductance. However, other than gold, systematic studies of a homologous series of molecules to extract the headgroup-metal barrier have not been reported. This thesis work scrutinizes the effect of electrode materials on single-molecule conductance by examining alkanedithiols anchored onto Au and Pt electrodes as well as alkanediisothiocyanates on Au, Pd, and Pt. STM-BJ (scanning tunneling microscopy break junction) was employed to create thousands of molecular junctions and to obtain single-molecule conductance. The results show that all molecule-electrode combination exhibits high- and low-conductance datasets (HC and LC). Compared to the contact resistance measured using Au electrodes, alkanediisothiocyanates are about 20% ~ 60% less resistive on Pd and Pt and alkanedithiols are about 50% less resistive on Pt. The difference is ascribed to their Fermi energies and the pi characters of the atoms at the contact. The dependence of single-molecule conductance on the electrode materials is also true for a linear trimetal complex, [Ni3(dpa)4(NCS)2] (dpa = 2,2'-dipyridylamide), suggesting the generality of the findings for both saturated and highly conductive molecular wires. For alkanediisothiocyanates, the probability of acquiring staircase-like traces among all i-s traces increases from 27% on Au to 37-42% on Pd or Pt electrodes. Using density functional theory (DFT), adsorption energies on three-fold hollow site and ontop site are in agreement with probabilities of HC and LC. Calculations of transmission function and comformation of headgroup-metal interface are also carried out to facilitate the rationalization using HOMO-LUMO gap and electronic coupling at the contact.

並列關鍵字

molecular electronics ； STM, break junction ； quantum point contact ； single molecular conductance

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