丙二硫醇、丙二胺與三苯四乙炔化合物的電子傳輸能譜： 理論模擬與電化學電位控制之單分子i–Ewk 曲線之比較

Abstract

導電值是物質傳輸電子能力強弱的指標。在Metal–Molecule–Metal(MMM)系統中，單分子導電值可視為電子通過分子的穿透機率，分子能階離電極費米能階(Fermi level, EF)越近，即能階匹配程度(energy-level alignment)越高時，電子穿透機率越高，導電值越大。 本實驗室模擬了在小偏壓下，金電極與丙二硫醇及三苯二乙炔化合物(1,4-bis(2-(4-(triphenylphosphinylauryl)ethynylphenyl)ethynyl)benzene, OPE3)架接之穿透能譜(transmission spectra)，發現兩分子以HOMO 為主要導電之軌域，而頭基–電極耦合程度較佳的OPE3，HOMO 的波峰也較寬。由此可推論，因丙二胺的頭基–電極作用力較丙二硫醇差，HOMO 波峰是三分子中最窄的。為研究此三分子的導電值性質，我們利用掃描穿隧顯微術破裂點接合法(scanning tunneling microscopy break junction, STM bj)，搭配自製有機相參考電極，以電化學方法調控工作電極EF (= Ewk)位置，藉此調控能階匹配的程度。固定探針與表面間的偏壓(Ebias)對分子進行i-Ewk 掃描，可觀察靠近或遠離分子軌域時的導電值變化。結果顯示，隨Ewk往HOMO 掃描時，能階匹配程度提升，三個分子導電值皆隨之增加，但上升程度與頭基–電極耦合程度有關。然而，三個分子的i-Ewk 曲線都有兩種趨勢：導電值隨工作電位變化而上升，與導電值不隨工作電位改變。理論模擬顯示，電極形狀會影響分子的穿透能譜，當平面狀電極與分子鍵結時，分子穿透能譜會向左位移，HOMO 會往遠離電極EF 方向移動。故改變Ewk，能階匹配的程度低，導電值變化不明顯，與實驗結果一致。

Single-molecule conductance is an important index to show the electron transport efficiency through the Metal–Molecule–Metal junction. The closer the molecular frontier orbitals and Fermi levels are, namely higher degree of energy level alignment between Fermi levels and molecular frontier orbitals, the higher transmission probability would be, rendering higher conductance. Our group simulated the transmission spectra of 1,3-propanedithiol and 1,4-bis(2-(4-(triphenylphosphinylauryl)ethynylphenyl)ethynyl)benzene (OPE3) under small bias, finding the HOMOs of these molecules are the major conducting orbitals. OPE3 has better headgroup-electrode coupling, and also has wider HOMO peak on the spectra. Hence, we can infer that diamine will show the narrowest HOMO peak because of its weakest headgroup-electrode coupling. We use Scanning tunneling microscopy break junction combined with homemade organic phase reference electrode, using electrochemical system to control the potential of the working electrode, Ewk(=EF), for tuning the extent of energy level alignment. Operate i-Ewk scanning on the molecules with fixed bias between electrodes to observe the change of conductance while tuning Ewk. The results suggest that when Ewk scanning toward HOMO, the extent of energy level alignment increase, and the conductance of three molecules increase accordingly. However, the increase is due to the extent of the coupling. And we also found that the shape of the electrode would influence the transmission spectra, causing two trends of i-Ewk curves.