分子结中界面能级结构和低能电子共振隧穿机制研究

The minimum functional unit of a molecular electronic device is a metal-molecule-metal junction, that is so-called a molecular junction. It has not only molecular switch, rectifier, and field effect transistor, but also displays quantum interference from the development over decades of research. However, there still exists a giant challenge in characterization of microscopic structures of the molecular junctions. Meanwhile, there is an order of magnitude difference in molecular conductance between theoretical and experimental values. In the light of the challenges in the study of molecular devices, the project aims at studying the relationship of low energy electrons tunneling through the molecular junction from the following three aspects: a) the wave behavior and particle behavior of the low-energy electrons in confined space; b) the quantization feature resulted from small size of a molecular junction; and c) the molecular quantum conductance of a molecular wire embodied in a molecular junction. Accordingly, we propose to: 1) build the Hamilton of the molecular junction that can precisely describe the interfacial energy barrier and the energy level alignment between metal electrodes and organic molecules; 2) precisely simulate molecular current-voltage properties and predict molecular conductance is proposed to be developed in this project on basis of the non-equilibrium Green function method; 3) establish the relationship of transmission spectra and the resonant tunneling mechanism of low-energy electrons through a molecular junction; and 4) analyze the effect of perturbation under external fields on molecular current-voltage properties and molecular conductance. Finally, our goal of the project aims to elucidate the mechanism of low-energy electron tunneling through a molecular junction, to cognize the electronic functions and thermoelectricity functions, and to develop theoretical basis of designing molecular electronic functional device based on the unit of molecular junctions.

分子电子器件的最小功能结构单元是分子结，不仅具有开关、整流器和场效应的功能，经近十年来发展，也具有量子相干性。但是实验表征分子结的微观结构仍具有挑战，而理论预测分子电导值与实验值常有数量级的差距。针对以上分子器件研究存在的挑战，本项目拟将理论与实验结合，系统地开展低能电子分子结的共振隧穿机理、理论模型和计算研究，探索以下三者的关系，即低能电子在限域空间中的波动性和粒子性、由分子结小尺寸导致的量子化特征、以及低能电子隧穿分子结的量子电导。为此，首先建立能准确描述分子结界面能垒和能级排布的哈密顿量，基于非平衡格林函数方法，发展模拟分子伏安特性和预测分子电导的准确算法，探究低能电子隧穿分子结的透射谱与共振隧穿机理的关系，分析外场扰动对分子结伏安特性和分子电导的影响，阐明电子共振隧穿分子结的机理，认识分子结的电子学功能和热电功能，为发展以分子结作为分子电子器件基元的功能器件设计提供理论依据。

项目执行期间，紧密围绕分子电子器件中分子结的界面吸附结构、电学特性开关、整流器和场效应的功能，同时考虑量子相干性，将分子结的微观结构、理论预测分子电导值以及分子结的共振隧穿机理、理论模型和计算研究相关联，并与实验值比较，说明测量量子电导不是分子结的能量最低位置，而是外界拉伸力下分子结的稳定结构。将密度泛函理论方法与非平衡格林函数方法结合，选择在分子结研究中的典型分子，如含芳香硫酚、辛二硫醇、苯二甲硫酚、紫罗碱及聚苯炔为模型，从界面吸附结构、界面铆钉原子化学成键、外力拉伸、偏压效应以及量子干涉效应等方面，系统地研究了其对低能电子隧穿机制的影响。通过对系列含巯基硫、含氮铆钉原子在金、银表面吸附进行研究，发现其吸附与界面能级结构排布的关系；以苯二甲硫酚为连接分子，发现测量分子电导在Au-Au键断裂情况下，分子量子电导平台与实验值接近，而当断裂中若分子巯基硫与金字塔型金电极尖端金原子作用，可导致Au-S键断裂，同时分子端基硫原子上β自旋电子靠近费米能级，共振隧穿导致其量子电导显著升高；以辛二硫醇为分子线，系统地研究了界面拉伸中，界面金属原子结构畸变和弛豫，导致分子结电导平台的变化趋势；以取代紫罗碱为模型分子，考虑含氧化还原态分子作为分子线，分子电导随价态和烷基取代基链长的变化规律。在考虑紫罗碱吡啶环上取代时，发现相消量子干涉效应和界面作用导致自旋分裂效应，并在低偏压下产生显著的自旋滤波效应，而随偏压增加，这种自旋滤波效应消失，同时分子化学态转化。以上研究为阐明电子共振隧穿分子结的工作原理，为认识分子结的电子学功能和热电功能，为发展以分子结作为分子电子器件的设计提供了理论依据。项目执行期间，博士生毕业4人，硕士毕业3人，发表论文11篇。