[论文解读] Non-Equilibrium Molecular Dynamics Study of Thermal Energy Transport in Au-SAM-Au junctions

Tengfei Luo, John R. Lloyd|arXiv (Cornell University)|May 13, 2009

Thermal properties of materials参考文献 40被引用 6

一句话总结

本研究采用非平衡态分子动力学（NEMD）模拟研究金-自组装单分子层-金结的热能传输，结果表明界面热阻主要由金-自组装单分子层界面主导，这是由于低频振动模式有限所致。主要发现包括：在250 K以下温度依赖的导热率，压力影响微弱，自组装单分子层覆盖度和键合强度影响显著，以及在界面处存在显著的非谐效应的弹道能量传输。

ABSTRACT

Non-equilibrium molecular dynamics (NEMD) simulations were performed on Au-SAM (self-assembly monolayer)-Au junctions to study the thermal energy transport across the junctions. Thermal conductance of the Au-SAM interfaces was calculated. Temperature effects, simulated external pressure effects, SAM molecule coverage effects and Au-SAM bond strength effects on the interfacial thermal conductance were studied. It was found that the interfacial thermal conductance increased with temperature increase at temperatures lower than 250K, but it did not have large changes at temperatures from 250K to 400K. Such a trend was found to be similar to experimental observations on similar junctions. The simulated external pressure did not affect the interfacial thermal conductance. SAM molecule coverage and Au-SAM bond strength were found to significantly affect on the thermal conductance. The vibration densities of state (VDOS) were calculated to explore the mechanism of thermal energy transport. Interfacial thermal resistance was found mainly due to the limited population of low-frequency vibration modes of the SAM molecule. Ballistic energy transport inside the SAM molecules was confirmed, and the anharmonicity played an important role in energy transport across the junctions. A heat pulse was imposed on the junction substrate, and heat dissipation inside the junction was studied. Analysis of the junction response to the heat pulse showed that the Au-SAM interfacial thermal resistance was much larger than the Au substrate and SAM resistances separately. This work showed that both the Au substrate and SAM molecules transported thermal energy efficiently, and it was the Au-SAM interfaces that dominated the thermal energy transport across the Au-SAM-Au junctions.

研究动机与目标

• 理解纳米尺度下金-自组装单分子层-金结的热能传输机制。
• 识别金-自组装单分子层体系中主导热阻的贡献因素。
• 研究温度、外加压力、自组装单分子层覆盖度及金-自组装单分子层键合强度对界面热导率的影响。
• 利用振动态密度（VDOS）分析振动模式与能量传输动力学。
• 评估弹道传输与扩散传输的差异，以及非谐性在热能传递中的作用。

提出的方法

• 采用非平衡态分子动力学（NEMD）模拟对金-自组装单分子层-金结的热能传输进行建模。
• 在基底上施加热脉冲，以研究瞬态热响应与能量耗散行为。
• 计算振动态密度（VDOS）以分析声子模式及其对热传导的贡献。
• 根据界面处的温度梯度与热流密度计算界面热导率。
• 系统性地改变温度、外加压力、自组装单分子层覆盖度及金-自组装单分子层键合强度等系统参数。
• 通过分析VDOS中偏离谐性振动行为的偏差，评估非谐性效应。

实验结果

研究问题

• RQ1 温度如何影响金-自组装单分子层-金结的界面热导率？
• RQ2 外加压力对金-自组装单分子层界面热导率有何影响？
• RQ3 自组装单分子层分子的覆盖度和金-自组装单分子层键合强度如何影响界面热阻？
• RQ4 低频振动模式在限制界面热传输中起什么作用？
• RQ5 在自组装单分子层内是否观察到弹道能量传输？非谐性如何影响界面能量传递？

主要发现

• 界面热导率在250 K以下随温度升高而增加，而在250 K至400 K之间保持相对稳定，与实验趋势一致。
• 外加压力对界面热导率影响可忽略，表明其对界面键合或声子耦合的影响微弱。
• 自组装单分子层覆盖度和金-自组装单分子层键合强度显著影响热导率，更强的键合与更高的覆盖度可改善界面耦合。
• 主要热阻源于金-自组装单分子层界面，这是由于自组装单分子层中低频振动模式数量有限所致。
• 在自组装单分子层内观察到弹道能量传输，且非谐性在界面能量耗散中起关键作用。
• 金-自组装单分子层界面的热阻远高于单独的金基底或自组装单分子层，是热传导的速率控制步骤。