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。典型的原子间作用力包括弱的范德华力（例如，在液体中）和强的离子和共价力（例如，在固体中）。二维材料，如石墨，是特殊的，因为各个层通过范德华力耦合，而在层内，原子通过共价力结合。沿平面内方向的热传导速度可能比跨平面方向快数百倍。通过压缩材料，可以在很大程度上调整层间范德华力的强度。在石墨中，在极度压缩的情况下，来自不同层的原子甚至可以形成牢固的类似共价键的键。二维材料为研究热传导与原子间键的关系提供了一个很好的模型。高压金刚石砧单元将用于在二维材料中施加压缩应变。金刚石砧胞内产生的压力可高达100~ 200gpa，与地核内的压力相似，可使二维层之间的距离缩短约30%，从而大大提高了层间键的强度。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、结构和电学性能的平行应变调谐，从而实现具有改进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开展各种外联活动，改善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极短，导致导热系数低。这一范式受到了最近几项研究的挑战，这些研究表明声子的平均自由路径要长几个数量级，这表明人们对二维层间热传导的本质缺乏了解。假设跨二维层的长声子平均自由程需要较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。在本研究中，高压金刚石砧单元中产生的大压缩应变将在很大的强度范围内调节层间力。将利用光谱学（如x射线衍射、瞬态热反射）和先进的计算技术（如从头算、机器学习增强的分子动力学）对跨平面热输运进行全面研究。研究了三种材料体系：(1)环境条件下过渡金属二硫族中层间力最弱的原始ReS2；（ii）扭曲的ReS2，沿交叉平面方向具有破缺周期性；石墨在高压下通过sp2-sp3键转变发生层间屈曲。该奖项反映了美国国家科学基金会的法定使命，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.