Collaborative Research: Cross-plane Heat Conduction in 2D Materials under Large Compressive Strain

热传导在很大程度上取决于原子和分子之间的键合强度。典型的原子间力包括弱的货车范德华力（例如，在液体中）和强离子和共价力（例如，以固体计）。二维材料，如石墨，是特殊的，因为各个层通过货车德瓦尔斯力耦合，而在层内，原子通过共价力键合。沿着平面内方向的热传导沿着平面内方向的热传导可以比跨平面方向快数百倍。跨层的货车德瓦耳斯力的强度可以通过压缩材料在很大程度上调节。在石墨中，在极端的压缩下，来自不同层的原子甚至可以形成强大的共价键。二维材料提供了一个很好的模型来研究热传导如何与原子间键合相关联。将使用高压金刚石对顶砧单元在2D材料中施加压缩应变。金刚石对顶砧产生的压力可高达100~200 GPa，与地核内部的压力相当，可使二维层间距离缩短约30%，从而显著提高层间结合强度。对压缩下二维层间热传导的基本理解将有助于实现二维材料的热、结构和电学特性的并行应变调谐，以实现具有改进的热稳定性和性能的新功能。这项研究还将提供机会，通过对代表性不足的群体进行研究培训和各种推广活动，提高科学和工程领域的多样性。经典动力学理论预测，二维材料中的跨平面声子平均自由程非常短，导致热导率低。最近的几项研究对这种范式提出了挑战，这些研究表明声子平均自由程的数量级更长，这表明对2D层之间的热传导性质缺乏了解。的假设是，长的声子平均自由程在2D层需要弱的层间力和周期性，声子散射和热传导强烈依赖于层间力的强度。在这项拟议的研究中，在高压金刚石对顶砧单元中产生的大压缩应变将在很宽的强度范围内调整层间力。将用光学光谱法（例如，X射线衍射，瞬态热反射）和先进的计算技术（例如，从头算，机器学习增强的分子动力学）。本论文将研究三种材料体系：（i）原始ReS_2，其在室温条件下具有过渡金属二硫族化合物族中最弱的层间力;（ii）扭曲ReS_2，其沿交叉面方向沿着具有破坏的周期性;和（iii）石墨，其可以通过sp2-该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Heat conduction highly depends on the strength of bonding among atoms and molecules. Typical interatomic forces include the weak Van Der Waals forces (e.g., in liquids) and the strong ionic and covalent forces (e.g., in solids). 2D materials, such as graphite, are peculiar in that individual layers are coupled via Van Der Waals forces while within the layer, atoms are bonded via covalent forces. Heat conduction along the in-plane direction could be hundreds of times faster than the cross-plane direction. The strength of the Van Der Waals force across layers can be tuned to a great extent by compressing the material. In graphite, under extreme compression, atoms from different layers could even form strong, covalent-like bonds. 2D materials provide a good model to study how heat conduction is associated with interatomic bonding. A high-pressure diamond anvil cell will be used to apply compressive strain in 2D materials. Pressure produced in the diamond anvil cell can be as high as 100~200 GPa, similar to the pressure inside Earth core, and can reduce the distance between the 2D layers by about 30%, hence increasing the strength of interlayer bonds substantially. Fundamental understanding of heat conduction across 2D layers under compression will facilitate the realization of parallel strain tuning of thermal, structural, and electrical properties of 2D materials for novel functionalities with improved thermal stability and performance. This research will also provide opportunities to improve diversity in science and engineering through research training of underrepresented groups and various outreach activities.Classical kinetic theory predicts that cross-plane phonon mean free paths in 2D materials are extremely short, leading to low thermal conductivity. This paradigm was challenged by several recent studies that suggest phonon mean free paths orders of magnitude longer, which indicates the lack of understanding about the nature of heat conduction across 2D layers. The hypotheses are that long phonon mean free paths across 2D layers require both weak interlayer force and periodicity, and that phonon scattering and heat conduction depend strongly on the strength of interlayer force. In this proposed research, the large compressive strain generated in a high-pressure diamond anvil cell will tune the interlayer force over a wide range of strength. A comprehensive study of cross-plane thermal transport will be conducted with optical spectroscopies (e.g., x-ray diffraction, transient thermoreflectance) and advanced computational techniques (e.g., ab initio, machine learning-enhanced molecular dynamics). Three material systems will be studied: (i) pristine ReS2, which possesses the weakest interlayer force in the transition metal dichalcogenides family at the ambient condition; (ii) twisted ReS2, which has broken periodicity along the cross-plane direction; and (iii) graphite, which could experience interlayer buckling via sp2-sp3 bond transition under high pressure.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.