Session: 08-02: Micro/Nanoscale Heat Conduction

Paper Number: 132133

132133 - Stacking Order, Thickness and Strain Dependent Thermal Conductivity of Res2 

Abstract: 

Since the discovery of single atomic layer graphene, 2D thin films have been realized in hundreds of different materials. Fascinating physical phenomena have been observed and new applications have been demonstrated. The single atomic layers of these materials are bonded together via weak van der Waals (vdW) force, hence are also called vdW solids. Thermal properties of vdW solids are essential in determining the performance, stability, and durability of electronic devices. Thermal conductivities of vdW solids have been simulated and measured in a wide range of materials, such as graphene (including ribbons, thins films, etc.), Boron Nitride, transition metal dichalcogenides (e.g., MoS₂, ReS₂, WSe₂, and black phosphorus  etc.). Majority of these studies focus on the high-value in-plane thermal conductivity (k_//). Only a few studies reported the thermal transport along the cross-plane direction (k_⊥).Based on the kinetic theory, where phonon mean free path (L) could be estimated through the measured k_⊥ via: k_⊥ ~ (1/3)Cn_g,⊥L_⊥ , where C is the heat capacity, n_g is the phonon group velocity. For MoS₂, the phonon mean free path is estimated to be 1.5~4 nm, equivalent to 2~6 layers. Kinetic theory suggests that the thermal conductivity of these materials should not show any thickness dependence trend beyond 10 layers. However, among the few studies about k_⊥, both experimental and theoretical studies have shown clear thickness dependence in graphene and MoS₂, up to several hundred nanometers. The discrepancy between the kinetic theory with the experimental/simulation results suggests that the cross-plane phonon mean free path in vdW solids could be much longer than previously thought, our paradigm about the heat transport across vdW layers may be inaccurate, and the nature of the phonon scattering and thermal transport in this regime is not well understood. 

In this study, we utilized the high pressure diamond anvil cell (DAC) to tune the interlayer vdW force in ReS₂ across a wide range and measured the evolution of thermal conductivity with picosecond transient thermo-reflectance technique. ReS₂ is chosen mainly due to two reasons: (i) It has the weakest interlayer vdW force among TMDs, hence can show the thermal transport change across a wide range of vdW force strength. (ii) It possesses a pure stacking order up to several microns, hence eliminates any complexity from effects of mixed stacking orders . ReS₂ possesses a distorted 1T triclinic crystal structure where the additional d valence electrons of Re atoms form zigzag Re chains parallel to the b axis, drastically reducing its symmetry. Recently, two distinct stacking orders of ReS₂ (AA and AB stacking) have been identified with the Raman spectroscopy.  For AA stacking, the adjacent layers have no relative shift; while for AB stacking, there is a one-unit cell shift between adjacent layers along a axis.

Firstly, we measured the thickness dependent thermal conductivity in both AA & AB stacking samples, and found that (a) both stackings show long range phonon mean path, up to about 1µm; and (b) AA stacking has higher thermal conductivity than AB stacking over the whole thickness range. Secondly, we applied the compressive strain on both AA and AB stacking samples in DAC and observed that: (a) thermal conductivity of AA stacking sample oscillates between 2 W/mK to 18 W/mK with pressure; and (b) AB stacking sample oscillates between 1.5 W/mK to 5 W/mK. Together with Raman spectroscopy that monitors the structural change, we hypothesize the observed thermal conductivity patterns to layer sliding and lattice distortion under compressive strain. 

 

 

Presenting Author: Yaguo Wang The University of Texas at Austin

Presenting Author Biography: Dr. Yaguo Wang is currently an Associate Professor with Temple Foundation Endowed Faculty Fellowship No. 1, as well as the Associate Chair for DEI at Walker Department of Mechanical Engineering at University of Texas at Austin. Yaguo Wang received her Bachelor's degree from the University of Science and Technology of China (USTC) in 2005 and her Ph.D. degree from the Department of Mechanical Engineering, Purdue University, Indiana, in 2011. After one-year's postdoctoral experience at Purdue University, she joined the faculty of UT Austin in 2013. Dr. Wang received the NSF Career Award in 2014. Dr. Wang is devoted to promoting an interdisciplinary experimental approach to address fundamental scientific problems in the areas of thermal sciences, ultrafast optics and material science. Dr. Wang’s research interests include developing and utilizing time-resolved laser spectroscopy to study interactions among photons, phonons and electrons, as well as to characterize macroscopic electrical and thermal properties in a wide-range of materials, including metal films, semiconductor nanostructures and 2D materials. Results from her research advance the understanding of fundamental physics of electron/phonon transport in these materials and facilitate the development of high-performance next-generation electronic devices with improved thermal stability. In recent years, Dr. Wang expanded her research endeavor to laser-based additive manufacturing and carbon capture. Dr. Wang has published more than 50 articles in prestigious journals and conference proceedings, including Physical Review Letters, Advanced Materials, ACS Nano, Additive Manufacturing Letters etc.

Authors:

Yaguo Wang The University of Texas at Austin
Zefang Ye The University of Texas at Austin