原子/分子接触的热电转换效率及其输运性质的研究

The atomic and molecular contacts are the most important issues in various technological applications, such as the atomic-scale manipulation, nanoscale thermoelectric conversion and etc, and there has been an increasing interest in their unusual energy transport phenomena. The project tends to fabricate a microscale device, which can be used to measure the thermal properties of nanowires. A cryogenic system with the presence of a stable magnetic field will be established. At different magnetic density, the dependence of the thermal conductivity of metallic nanowire on its structures, such as length and diameter, is measured in the temperature range from 80 to 300 K. The self-assembly technique is applied to form a desired molecular layer on the target nanowires, and the atomic and molecular contact process is controlled accurately with the aid of the Lorentz force. By monitoring a large amount of the current-voltage curves, the quantized electrical contact conductance can be statistically obtained. Combined with the 3ω method and 2ω method, the corresponding Seebeck coefficient is obtained, and the thermal conductance is measured based on the crosswise structure formed by two nanowires. Finally, the figure of merit at the atomic and molecular contacts is exprimental obtained. The ab initio electronic structure calculations are based on the implementation of density functional theroy in commercial software, the transmission function is given by the nonequilibrium Green's function techniques, so the transport properties of the atomic and molecular contacts are calculated using the Landauer theory, and the effects of the molecular structure and dimensions on the figure of merit are analyzed. Based on the experimental and theoretical results, the energy conversion and transport mechanisms through the atomic and molecular contacts are confirmed, which in turns provides an important scientific support in devices synthesized by low dimensional materials.

原子/分子接触是原子操作、纳米热电转换利用等技术的关键问题，由于其特殊的能量输运过程而受到广泛关注。项目拟制备用于纳米线热物性测量的微器件装置，构建具有稳定磁场的低温真空实验系统，测量80-300K温度范围内纳米线热导率随长度、直径以及磁场强度的变化；采用自组装单分子膜技术对纳米线表面进行处理，通过磁场下的洛伦玆力实现原子/分子接触过程的准确控制，统计大量接触过程中的电流-电压曲线，测得接触电导的量子效应，结合3ω法和2ω法，得到对应的Seebeck系数，并在交叉纳米线结构的基础上，测得接触热导，进而实现原子/分子接触中热电转换优值系数的实验测量；利用商用软件计算电子能带结构，采用非平衡格林函数法得到透射系数，根据Landauer理论分析材料结构、尺寸等因素对优值系数的影响；结合实验测量和理论计算结果，揭示原子/分子接触过程中热电输运的本质规律，从而为低维材料器件化提供重要的科学依据。

金属接触是微器件开发、纳米热电转换、纳米材料热物性测量等技术的关键共性问题，甚至具有决定性作用，因此高空间分辨率（原子接触）和高时间分辨率（飞秒）下的能量输运过程近年来受到广泛关注。在高空间分辨率接触能量传递过程研究方面，项目搭建了具有稳定磁场的低温真空实验系统（80~300K温度范围），实现了Ag，Pt等金属单晶和多晶纳米线真实热学、电学性质的准确表征，定量给出了在自加热方法中接触电阻对金属纳米线热导率测量的影响规律，证明了热电比拟定律仍然适用于单晶金属纳米线材；测量得到的金属纳米线与电极间接触电阻近似与温度无关，证明接触处电子处于弹道输运过程；报道了Ag纳米线在低能电子辐射下电导率降低的实验现象，揭示了内在机理；证明了多晶纳米薄膜/纳米线中晶界反射系数与熔点关联式的适用性，建立了准确预测金属多晶纳米薄膜和纳米线热导率和电导率的理论模型；用洛伦玆力精确控制微米线的接触过程，测量了从弹道到扩散过程转变过程中金属接触电阻和接触热阻，证明了热电比拟定律对纯金属接触处于弹道/扩散输运过程都适用；开发了原子Green函数结合有限元的计算方法，根据Landauer理论计算得到声子弹道输运的透射系数和热导。掌握金属接触（包括晶粒界面接触）能量传递规律，准确预测纳米尺度材料热电性质，对微电子器件设计、热分析等都具有重要意义。在高时间分辨率能量传递过程研究方面，项目搭建了集成了脉冲整形器的共轴双色飞秒激光热反射实验系统，直接实验观测到了光学声子对超快能量传递过程的影响；建立了载流子吸收、能带填充和电子-空穴湮没等三过程对激光反射信号影响的理论模型，揭示了反射信号随脉冲强度变化的内在机理；成功利用具有不同时间间隔的两束泵浦脉冲精确调控了特定的晶格振动模式。光子与热电子、声子之间的超快能量传递过程研究，将为提高太阳能利用和光电晶体材料的转换效率提供理论依据，同时涉及非热致相变、激光精密加工等一系列前沿技术。