聚噻吩单链量子热输运的第一性原理研究

聚噻吩块体通常被视为绝热材料,其热导率小于1W·m~(-1)·K~(-1).但近年发现对于室温下沿聚噻吩分子链方向排列的无定形聚噻吩纳米纤维,其热导率高于聚噻吩块体,可达4.4W·m~(-1)·K~(-1).为了相对准确地揭示纳米尺度聚噻吩单链热输运的微观特征,从量子力学出发,在密度泛函理论计算的基础上,应用中间插入延展方法结合非平衡格林函数方法,对长度为25.107nm、包含448个原子的聚噻吩单链的量子热输运及其同位素效应进行了研究,并与分子动力学方法模拟的结果进行了详细比较.结果表明:室温下32 nm长的纯聚噻吩单链热导率上限高达30.2 W·m~(-1)·K~(-1),与铅的热导率35 W·m~(-1).K~(-1)相近;相同掺杂比例(原子百分数)下C元素热导的同位素效应比S元素显著;室温下聚噻吩单链中~(12)C,~(13)C等比例随机掺杂时的同位素效应最为显著,此时聚噻吩单链的平均热导至少降低了30%;室温下纯聚噻吩单链的热导随C的相对原子质量增加近似呈反比例减小,随S的相对原子质量增加呈非线性单调增加.该研究对认识和调控聚噻吩这种新型功能材料的热输运特性具有积极的价值.

First-principle study on quantum thermal transport in a polythiophene chain

Bulk polythiophene material is usually regarded as thermal insulator because it has low thermal conductivity (less than 1 W·m^-1·K^-1). However, the report demonstrates that along the amorphous polythiophene nanofiber axis, the pure polythiophene nanofibers have high thermal conductivity (more than 4.4 W·m^-1·K^-1), which is obviously higher than that of the bulk polythiophene material. In order to throw light on this situation, molecular dynamics (MD) method is used to detect the high thermal conductivity of a polythiophene chain. However, the MD method is highly sensitive to the choice of empirical potential function or simulation method. Even if the same potential function (ReaxFF potential function) is adopted, the thermal conductivity of a polythiophene chain could also have obviously different results. To overcome the instability of MD method, we use the first-principles to calculate the force constant tensor. In such a case the properties of quantum mechanics in a polythiophene chain can be reflected. In our algorithm, several disadvantages of MD that different potential functions or different simulation methods probably lead to very different thermal conductivities for the same transport system are avoided. Based on the density functional theory (DFT), the central insertion scheme (CIS) method and nonequilibrium Green's function (NEGF) approach are used to evaluate the isotope effect on thermal transport in a polythiophene chain, which includes 448 atoms in a scattering region and has a length of 25.107 nm. It is found that the thermal conductivity of a 32-nm-long pure polythiophene chain reaches 30.2 W·m^-1·K^-1, which is close to the thermal conductivity of lead at room temperature. The reduction of average thermal conductance caused by C atom impurity is more remarkable than by S for a pure polythiophene chain when the mixing ratios of ¹³C to ¹²C and ³⁶S to ³²S are equal. The most outstanding isotope effect on quantum thermal transport appears when the mixing ratio of ¹³C to ¹²C is 1:1. It will cause the average thermal conductance to decrease by at least 30% in the polythiophene chain at room temperature. Moreover, we find that the thermal conductance of a pure polythiophene chain is inversely proportional to the atomic weight of carbon, and increases nonlinearly with the increasing atomic weight of sulfur. It is of significance to optimize the thermal conductance properties of polythiophene function material.