電極材料對雙吡啶烷分子導電性之影響：分子-電極耦合及費米能階校準之研究

On the Mechanism of Charge Transport in Single-Molecule Junction: Energy Alignment and Electronic Coupling of Bipyridine and Alkylbipyridine on Au, Pd, and Pt

莊舒婷(Su-Ting Chuang)

指導教授 ： 陳俊顯

國立臺灣大學/理學院/化學研究所/碩士(2011年)

https://doi.org/10.6342/NTU.2011.03111

摘要

在電極-分子-電極架構下測量單分子導電性，電極材料的種類，會造成電極費米能階與分子能階之間能障高度的增減，以及分子-電極間耦合程度的改變，進而影響電子通過分子接合點的傳遞效率。本論文利用掃描式穿隧顯微術破裂接點法於金、鈀及鉑電極下，量測飽和烷雙吡啶和共軛聯吡啶(4,4'-bipyridine)分子導電值。相較於金，第10族的鈀和鉑金屬在費米能階附近有較高的d軌域特性和電子密度，且具有較大功函數，亦即費米能階能量較低。以吡啶為頭基的系列分子，費米能階較靠近於分子LUMO能階，在鉑電極下具有較大能障。實驗結果顯示，在共軛聯吡啶部分，電子傳遞通過分子LUMO能階，導電值的變化主要受到能障高度因素影響；在飽和烷雙吡啶部分，因具有極大HOMO-LUMO能隙，導電值趨勢隨吡啶頭基與電極耦合程度影響。藉由量測轉變電壓以及密度泛函理論的模擬計算亦證實，導電值在聯吡啶共軛性高的分子受能障高度主導，而飽和烷分子則以電極與末端官能基之間交互作用力為首要影響因素。

關鍵字

分子電性 ； 單分子導電值 ； 掃描式電子顯微術 ； 破裂接合點 ； 吡啶頭基

並列摘要

Understanding the factors that influence charge transport properties in MMM devices (metal-molecule-metal devices) is a critical issue. Here especially the influence of the metal work function and the metal-molecule interface are considered. We measure the single-molecule conductance of a series of pyridyl-terminated molecules, 4,4'-bipyridine (BPY) and Py–(CH2)n–Py (n = 2, 3, and 4; Py = 4-pyridyl), on Au, Pd, and Pt electrodes by the method of STM-BJ (scanning tunneling microscopy break junction). The experimental data show that the conductance of BPY decreases as the electrode work function increases. The results demonstrate electron transport takes place via LUMO of BPY. Moreover, the conductance of Py–(CH2)n–Py molecules on Au is inferior to those on Pd and Pt. Since the α,ω-alkane moiety has large HOMO－LUMO gap, the effect of different work function is not observed. The contact resistance of Py–(CH2)n–Py (n = 2, 3, and 4) is smallest on Pt. Finally, by analyzing the transport mechanism based on transition voltage spectroscopy and density functional theory calculation, we find out the major factor that influence charge transport properties through pyridyl-terminated molecules. For BPY, disparity in the conductance is attributed to the different value of barrier height. However, for Py–(CH2)n–Py, the major factor that influence charge transport is the degree of electronic coupling between the headgroup and the electrode.

並列關鍵字

molecular electronics ； single-molecule conductance ； STM ； break junction ； pyridine-based anchor

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