Sharony Inon, Chen Renai, Nitzan Abraham
School of Chemistry, Tel Aviv University, Tel Aviv 69978, Israel.
Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA.
J Chem Phys. 2020 Oct 14;153(14):144113. doi: 10.1063/5.0022423.
Understanding phononic heat transport processes in molecular junctions is a central issue in the developing field of nanoscale heat conduction. Here, we present a Langevin dynamics simulation framework to investigate heat transport processes in molecular junctions at and beyond the linear response regime and apply it to saturated and unsaturated linear hydrocarbon chains connecting two gold substrates. Thermal boundary conditions represented by Markovian noise and damping are filtered through several (up to four) gold layers to provide a realistic and controllable bath spectral density. Classical simulations using the full universal force field are compared with quantum calculations that use only the harmonic part of this field. The close agreement found at about room temperature between these very different calculations suggests that heat transport at such temperatures is dominated by lower frequency vibrations whose dynamics is described well by classical mechanics. The results obtained for alkanedithiol molecules connecting gold substrates agree with previous quantum calculations based on the Landauer formula and match recent experimental measurements [e.g., thermal conductance around 20 pW/K for alkanedithiols in single-molecule junctions (SMJs)]. Heat conductance simulations on polyynes of different lengths illuminate the effects of molecular conjugation on thermal transport. The difference between alkanes and polyynes is not large but correlates with the larger rigidity and stronger mode localization that characterize the polyyne structure. This computational approach has been recently used [R. Chen, I. Sharony, and A. Nitzan, J. Phys. Chem. Lett. 11, 4261-4268 (2020)] to unveil local atomic heat currents and phononic interference effect in aromatic-ring based SMJs.
理解分子结中的声子热输运过程是纳米尺度热传导这一新兴领域的核心问题。在此，我们提出一种朗之万动力学模拟框架，用于研究分子结在线性响应区域及以外的热输运过程，并将其应用于连接两个金基底的饱和与不饱和线性烃链。由马尔可夫噪声和阻尼表示的热边界条件通过几层（最多四层）金层进行过滤，以提供一个现实且可控的浴谱密度。使用完整通用力场的经典模拟与仅使用该场谐波部分的量子计算进行了比较。在大约室温下，这些截然不同的计算结果之间的紧密一致性表明，在此类温度下的热输运由低频振动主导，其动力学可用经典力学很好地描述。连接金基底的链烷二硫醇分子的计算结果与基于朗道尔公式的先前量子计算结果一致，并与最近的实验测量结果相符[例如，单分子结（SMJ）中链烷二硫醇的热导率约为20 pW/K]。对不同长度聚炔的热导率模拟揭示了分子共轭对热输运的影响。烷烃和聚炔之间的差异不大，但与聚炔结构所具有的更大刚性和更强模式局域化相关。这种计算方法最近已被用于[R. Chen, I. Sharony, and A. Nitzan, J. Phys. Chem. Lett. 11, 4261 - 4268 (2020)]揭示基于芳香环的单分子结中的局部原子热流和声子干涉效应。
