費米能階附近的d能帶能態密度對電子傳輸機制的影響

Abstract

有機-金屬界面的電子傳輸研究對於許多材料應用有著重大的影響，例如，金屬表面電子結構以及表面的分子吸附作用等。為了研究此效應，典型的作法是量測單分子電性，其電性表現可以單能階模型描述。其中，能階匹配程度和耦合效率為主要的兩個參數，前者是前緣分子軌域和費米能階的能量差，後者則是象徵分子和金屬電極的作用，藉分析此兩參數可以了解表面能態密度的變化以及探討單分子結點的電子傳輸機制。 此研究包含飽和烷硫甲基和飽和烷氰基分子在金塊材、單原子層鉑和單原子層鈀修飾金電極上的電性量測，結果顯示單原子層鉑和單原子層鈀修飾金電極有著較高的導電值外，其導電值提升並非來自較好能階匹配程度而是更強的電極和頭基的耦合效率。藉由密度泛函理論計算和紫外光電子能譜，說明此較強的耦合效率來自於單原子層鉑和單原子層鈀修飾金電極的費米能階附近額外的能態密度，源於鉑、鈀原子的d電子能帶。因此，本研究提出，表面電子結構尤其是表面d電子能帶對於電子傳輸效應有著重大的影響。

Studies of charge transport at metal-organic interface have great impacts to numerous applications due to the complexity of metal surface electronic structure and a variety of surface adsorption behaviors. A typical method to describe the conductance is the single-level model. In this model, the major factors that affect junction conductance are the energy level alignment (ELA) and the coupling efficiency, where the former one is the difference between metal electrode’s Fermi level and the energy of adsorbate’s frontier molecular orbital (FMO), the latter one represents the interaction strength at metal-organic interface. These two parameters imply the consequence of shifted and broadened FMO that mixes with metal d electrons, aka d band in metal electronic structure macroscopically, upon adsorption. Accordingly, one can investigate the transport efficiency at metal-organic interface via tuning the surface electronic structure of electrodes. In this study, gold electrodes are modified by a platinum or palladium single atom adlayer via electrochemical methods. The conductance values of dimethyl sulfide, dinitrile-terminated molecules are found to be enhanced owing to the effect of the adlayer, which is proven to be not resulting in better ELA but stronger coupling efficiency. With the support of transmission function calculations and ultraviolet photoelectron spectroscopy which characterizes the surface states, the enhancement of coupling efficiency is ascribed to increased density of states (DOS) of metal d band around Fermi level comparing to bare gold. These results suggest that surface d band plays an essential role in transport across molecule-electrode interface.