三核金屬串的量子輸送現象之理論研究

Abstract

本篇論文主要是探討三核金屬串的量子輸送現象，內容主要包含兩部份，第一部分是利用非平衡格林函數法和拓展的休克爾模型(extended Huckel model)探討三核線型金屬串(M3(μ3-dpa)4(NCS)2 (dpa=2,2'-dipyridylamide), M = Co, Ni, 或 Cr) 的導電性質，指出了不同的金屬串中的主要導電通道；同時釐清了導電的主要通道在於中央的金屬原子，而配位基具有調控導電度大小的能力。最後透過一個假想實驗預測不同金屬的三核金屬串的導電度的趨勢，建立了導電度與鍵級(Bond Order)的連結。第二部份則是利用非平衡格林函數法和一階微擾理論探討電壓對單分子接點的穿透函數的影響，其中對稱的系統和不對稱的系統中的影響有明顯不同的結果。最後，我們對一個簡單模型系統和兩個真實分子系統進行計算，釐清這些系統中電壓對穿透函數的影響。

This master thesis is partitioned in three chapters: “Introduction”, “Charge transport through individual trinuclear EMACs” and “Bias dependence of intensities in the transmission spectra of single-molecule junctions”. Charge Transport Through Individual Trinuclear EMACs Charge Transport properties of single-molecule junction based on trinuclear extended metal atom chains (EMACs) are investigated using the Non-Equilibrium Green’s Function method and the extended Huckel theory. We identify the major conducting orbitals of the trichromium, tricobalt and trinickel complexes, which directly explain the trend in the single-molecule conductance. While the metal cores allow almost all of the current flow through, the conjugated ligands has the function to tune the conducting ability of the EMACs instead of provides another conducting pathways. Moreover, we have proposed a molecular orbital interpretation to the periodic trend in the single-molecule conductance of EMACs with the first-row transition metal cores, which illustrate the strong connections among the conductance, the orbital energies, the atomic number and the metal-metal bond order. Bias Dependence of Intensities in the Transmission Spectra of Single-molecule Junctions The modification of transport properties in single-molecule junctions in the region of finite bias is investigated by the Non-Equilibrium Green’s Function (NEGF) method and the first-order perturbation theory. Combining these two theories, we derive a semiquantitative formula in terms of molecular orbital coefficients to estimate the variation of transmission functions under the bias. The influence of bias for the symmetric and nonsymmetric junctions on the intensities of transmission functions is clarified. Finally, we applied this analysis to the following systems, a Huckel model system and two real single-molecule junctions, which consist of naphthalenedithiol and azulenedithiol, demonstrating the bias-induced change of the intensities of transmission functions.