單分子導電性研究：以偶氮基形成之苯–金共價鍵之分子–電極接觸結構

Abstract

電極–分子界面之接觸電阻(contact resistance)是影響分子的電性表現與測得導電值之重要因素，過大的接觸電阻將模糊或掩蓋刻意設計的分子主體之電性表現。以硫醇為頭基的硫–金電極界面是這個領域的典型接觸方式，雖優於其它頭基種類的接觸電阻，尋找比硫–金更小電阻之界面，仍是這個研究領域的重要課題之一。本研究目的為製造分子與電極之間的直接共價鍵結並測到單分子的導電性，希望如此之共價鍵結可以提供比物理吸附與化學吸附更小的接觸電阻。偶氮基(diazonium)是已知的良好離去基，並能夠在電化學還原或自發的條件下，與電極表面產生碳–金共價鍵結。為了與典型硫–金界面的性質比較，我們選擇金為電極材料，將四氟硼化1,4-二對偶氮基苯丁二炔以電化學還原到金表面上後，進行掃描穿隧顯微鏡斷裂接合法(scanning tunneling microscope break junction) 測量。為了比較此共價鍵結對分子導電性的影響，我們測量同樣主體為1,4-二苯基丁二炔而頭基分別為硝基、胺基與硫甲基三種分子之導電值。整體而言，本研究成功製作苯環上的碳與金之共價鍵結並運用於單分子導電性測量，也證實共價鍵結的導電值為其它頭基的10倍以上。由於分子主體皆相同，因此導電值的提高可歸因於共價鍵接觸提供較低的接觸電阻。另一方面，我們將電流對電壓圖做運算處理得到Fowler-Nordheim (F-N)圖，利用F-N圖間接得到分子能階與金電極能階之能障，探討頭基如何影響共軛有機分子的導電值。實驗結果指出，共軛有機分子之導電值與頭基金屬間吸附能之趨勢一致，而能障沒有顯著差異，因此在有機共軛分子系統中，頭基對分子導電質得主要影響還是來自於吸附能。

We introduce a novel approach to forming high conducting molecule-electrode covalent contacts for single molecular junction. In this study, aryl-gold contacts in molecular junction are derived from diazonium salt (R-N2+). Conductance of conjugated molecules covalently bonded to gold substrate is measured by scanning tunneling microscope break junction. To compare the effect of covalent contacts on conductance, we investigate in molecules with nitro-, amine-, and methylthio- anchoring groups. The conductance of junction with covalent contact is at least 10-fold higher than those of the same molecular framework yet with anchoring groups. By analyzing conductance to stretching distance traces, C-Au contacts are remained after break junction. On the other hand, we explore how anchoring groups affect the conductance of organic conjugated molecules. By Fowler-Nordheim plots, we conclude that anchoring groups affect their conductance by their adsorption energy with metal electrodes rather than energy alignment by adjusting the molecular orbitals.