單分子接合點架接方式之研究： 以掃描穿隧顯微術探討金屬－頭基－金屬之結構

Abstract

本研究以掃描式穿隧顯微破裂接合法(Scanning Tunneling Microscopy break junction, STM-bj)量測一般作為頭基的飽和烷分子，探討分子與電極可能的架接方式。首先，量測1,6-hexanedithiol此典型分子的導電值，發現在1 G0到0.1 G0範圍有文獻未報導過的分子導電值平台訊號累積，推測可能有文獻未報導新的分子架接方式，而提出可能的架接方式為雙頭硫醇僅用一端頭基與電極橋接，即金-硫-金接合點，並探討金-硫-金接點導電值與硫醇分子導電值間的相關性，分析兩組接點構型間出現機率與延伸長度之消長。而為驗證1 G0到0.1 G0範圍的導電值平台為金-硫-金構型，量測一系列不同碳數的僅一端有硫醇官能基的飽和烷，觀察是否有相同導電值平台特徵，實驗結果顯示單頭硫醇在1 G0到0.1 G0範圍內有導電值平台訊號累積，且不同碳數的單頭硫醇導電值相近，支持此假設的架接方式。然而單頭硫醇量測的導電值較文獻理論計算金-硫-金小，推測由於硫醇分子與金電極有複雜接合點構型，造成實驗值與理論值的差異，目前較新的文獻以複雜接合點構型模擬硫醇分子的電性行為以期更符合實際實驗的構型。由於觀察到硫醇分子有金-硫-金的架接方式，因此檢驗其他作為電性量測的頭基官能基，是否也有金-元素-金接合點形成，實驗結果顯示僅含硫之頭基有金-元素-金的現象。最後，當有單頭硫醇和單頭異氰酸酯分子存在時，一條金原子串的導電值(1G0)發生波峰變寬的現象，推測是由於分子吸附於金電極，影響金原子導線的結構，進而影響金原子串電性之表現。

1,6-hexanedithiol (HDT) is measured by scanning tunneling microscopy break junction (STM-bj). In comparison to sets of conductance values previously reported, in this study another set of conductance value ranged from 1.0 G0 to 0.01 G0 is detected. This set of conductance values might arise from another contact geometry of molecule and electrode. This contact geometry is proposed here as a sulfur at the end of HDT being inserted into the gold contact, forming Au-S-Au junction. To examine the validity of the proposed mechanism, single molecule conductance of alkanethiols (CH3(CH2)n-1SH, n = 4, 6, 8, 10, 12) is measured by STM-bj. Results show that the conductance values of alkanethiols are almost the same. And the fact that the conductance values of alkanethiols are almost the same as the conductance values of this HDT might arise from another contact geometry, Au-S-Au. However, the conductance values of alkanethiols are smaller than the values calculated by theoretical models. It is speculated that these models are too simplified and can not simulate the experimental condition. Conductance histograms show that in solutions containing compounds with thiol or isothiocyanate as head group result in the broadening of peak at 1.0 G0. It is proposed that peak broadening arises from these compounds absorbing onto gold electrodes.