在分子电子学领域中, 设计分子的结构可以实现特定的功能. 单分子二极管的整流行为是极具吸引力的器件功能之一. 研究了对称分子和非对称分子结的电子输运, 分别对应为四苯基和二嘧啶基二苯基单分子结, 二者均是共价结合到两金属电极. 与其同源对称嵌段相比, 非对称二嵌段分子表现出明显的整流行为, 且电子输运方向是从二苯基流向二嘧啶基. 利用密度泛函理论(density functional theory, DFT)和非平衡格林函数(non-equilibrium Green's function, NEGF)结合的第一性原理方法研究了单分子结的电子结构及其量子输运. 电流-电压 ($I$-$V$)曲线的非对称性可以用非对称分子二嵌段在偏压下由于电子态的局域性带来的非平衡效应进行解释. 本理论计算定性上符合其他小组的实验结果, 且尝试了不同的末端接触. 结果发现, 实验中的扫描隧道显微镜(scanning tunneling microscope, STM)针尖接触结构会一定程度地抵消非对称分子的整流效应, 而 STM 针尖接触结构的结果分析也符合之前的理论预测.

In the field of molecular electronics, structural design of a molecule is necessary to achieve different functionalities. The rectification behaviour of single-molecule diodes is one of the most attractive device features. The electron transport of symmetric and asymmetric single molecular junctions was studied, which correspond to tetraphenyl and dipyrimidiny-diphenyl monomolecular junctions, respectively. Both of them were covalently bonded to two gold electrodes. Asymmetric diblock molecules exhibited significant rectifying behaviour compared to their homologous symmetric blocks, and the electrons flow from diphenyl to dipyrimidinyl. The first-principles method was used, including density functional theory (DFT) and the non-equilibrium Green's function (NEGF) method, to study the electronic structure and quantum transport of single-molecule junctions. The asymmetry in the $I$-$V$ could be explained by the localisation of the electron density at one end of the asymmetric molecule, leading to a nonequilibrium effect under bias voltage. Theoretical results agreed qualitatively with the experimental works reported in the literature. Moreover, upon investigating different contacting ends, it was found that the scanning tunneling microscope (STM)-tip setup in the experiment would cancel the rectification effect to some extent; meanwhile, the STM-tip results also confirmed the previous theoretical prediction.