Wang Jonathan J, Gong Jie, McGaughey Alan J H, Segal Dvira
Chemical Physics Theory Group, Department of Chemistry, University of Toronto, 80 Saint George St., Toronto, Ontario M5S 3H6, Canada.
Department of Mechanical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA.
J Chem Phys. 2022 Nov 7;157(17):174105. doi: 10.1063/5.0125714.
With the objective of understanding microscopic principles governing thermal energy flow in nanojunctions, we study phononic heat transport through metal-molecule-metal junctions using classical molecular dynamics (MD) simulations. Considering a single-molecule gold-alkanedithiol-gold junction, we first focus on aspects of method development and compare two techniques for calculating thermal conductance: (i) The Reverse Nonequilibrium MD (RNEMD) method, where heat is inputted and extracted at a constant rate from opposite metals. In this case, the thermal conductance is calculated from the nonequilibrium temperature profile that is created at the junction. (ii) The Approach-to-Equilibrium MD (AEMD) method, with the thermal conductance of the junction obtained from the equilibration dynamics of the metals. In both methods, simulations of alkane chains of a growing size display an approximate length-independence of the thermal conductance, with calculated values matching computational and experimental studies. The RNEMD and AEMD methods offer different insights, and we discuss their benefits and shortcomings. Assessing the potential application of molecular junctions as thermal diodes, alkane junctions are made spatially asymmetric by modifying their contact regions with the bulk, either by using distinct endgroups or by replacing one of the Au contacts with Ag. Anharmonicity is built into the system within the molecular force-field. We find that, while the temperature profile strongly varies (compared with the gold-alkanedithiol-gold junctions) due to these structural modifications, the thermal diode effect is inconsequential in these systems-unless one goes to very large thermal biases. This finding suggests that one should seek molecules with considerable internal anharmonic effects for developing nonlinear thermal devices.
为了理解纳米结中热能流动的微观原理，我们使用经典分子动力学（MD）模拟研究了通过金属 - 分子 - 金属结的声子热传输。考虑一个单分子金 - 链烷二硫醇 - 金结，我们首先关注方法开发方面，并比较两种计算热导率的技术：（i）反向非平衡分子动力学（RNEMD）方法，其中热量以恒定速率从相对的金属输入和提取。在这种情况下，热导率是根据结处产生的非平衡温度分布计算的。（ii）平衡趋近分子动力学（AEMD）方法，结的热导率从金属的平衡动力学中获得。在这两种方法中，对不断增加尺寸的烷烃链的模拟都显示出热导率近似与长度无关，计算值与计算和实验研究结果相符。RNEMD和AEMD方法提供了不同的见解，我们讨论了它们的优缺点。评估分子结作为热二极管的潜在应用，通过用本体修饰其接触区域使烷烃结在空间上不对称，要么使用不同的端基，要么用银替换其中一个金接触。在分子力场中，系统内置了非谐性。我们发现，虽然由于这些结构修饰温度分布有很大变化（与金 - 链烷二硫醇 - 金结相比），但在这些系统中热二极管效应并不显著——除非施加非常大的热偏置。这一发现表明，为了开发非线性热器件，应该寻找具有相当大内部非谐效应的分子。
