联苯基单分子结热电势的从头算研究
Ab-initio study of the thermopower of biphenyl-based single-molecule junctions
摘要：
采用从头算电子结构计算结合非平衡格林函数技术，我们研究了联苯基单分子结的热电势Q对构象的依赖关系。对于一系列实验上可用的联苯分子，烷基侧链使我们能够从0到90°逐渐调整两个苯环之间的扭转角θ，并以这种方式控制π电子共轭的程度。通过研究不同的锚定基团和结合位置，我们的理论预测，随着扭转角的增大，热电势的绝对值略有减小，并遵循a+b*cos^{2} π的依赖关系。锚定基团同时决定Q和a、b的符号。硫和胺基产生Q，a，b>0，而对于氰基Q，a，b<0。不同的结合位置可以导致热电势的实质性变化，主要是由于前线分子轨道能级和费米能的排列的变化。我们用π轨道紧束缚模型和最小两能级模型解释了我们的从头计算结果，该模型描述了两个苯环上的一对杂化前线轨道态。热电势随温度的变化似乎在实验分辨率之内。
英文摘要：
Employing ab-initio electronic structure calculations combined with the non-equilibrium Green's function technique, we study the dependence of the thermopower Q on the conformation in biphenyl-based single-molecule junctions. For the series of experimentally available biphenyl molecules, alkyl side chains allow us to gradually adjust the torsion angle ϕ between the two phenyl rings from 0 to 90° and to control in this way the degree of π-electron conjugation. Studying different anchoring groups and binding positions, our theory predicts that the absolute values of the thermopower decrease slightly towards larger torsion angles, following an a+b*cos^{2}ϕ dependence. The anchoring group determines the sign of Q and a,b, simultaneously. Sulfur and amine groups give rise to Q,a,b>0, while for cyano Q,a,b<0. The different binding positions can lead to substantial variations of the thermopower mostly due to changes in the alignment of the frontier molecular orbital levels and the Fermi energy. We explain our ab-initio results in terms of a π-orbital tight-binding model and a minimal two-level model, which describes the pair of hybridizing frontier orbital states on the two phenyl rings. The variations of the thermopower with ϕ seem to be within experimental resolution.
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