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

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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.