Session: Research Posters

Paper Number: 119922

119922 - Thickness and Strain Dependent Cross-Plane Thermal Conductivity in Res2 With Stacking Order 

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. The 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, thin films, etc.), Boron Nitride, transition metal dichalcogenides (e.g., MoS₂, ReS₂, WSe₂, etc.). The majority of these studies focus on high-value in-plane thermal conductivity (k_//). Only a few studies reported thermal transport along the cross-plane direction (k_⊥). Kinetic theory suggests that the thermal conductivity of these materials should not show any thickness dependence trend beyond 10 layers. Based on the kinetic theory, the phonon mean free path for MoS2 is estimated to be 1.5~4 nm, equivalent to 2~6 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 [1,2]. 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 the 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, thus eliminating any complexity from the effects of mixed stacking orders [3]. 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 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) the 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, which monitors the structural change, we hypothesize the observed thermal conductivity patterns to layer sliding and lattice distortion under compressive strain.

 

 

[1] Sood A, Xiong F, Chen S, Cheaito R, Lian F, Asheghi M, Cui Y, Donadio D, Goodson KE, Pop E, Quasi-Ballistic Thermal Transport Across MoS2 Thin Films. Nano Letters. 2019;19(4):2434-42.

[2] Fu Q, Yang J, Chen Y, Li D, Xu D. Experimental evidence of very long intrinsic phonon mean free path along the c-axis of graphite. Applied Physics Letters. 2015;106(3):031905.

[3] Zhou Y, Maity N, Rai A, Juneja R, Meng X, Roy A, Zhang Y, Xu X, Lin JF, Banerjee SK, Singh AK, Wang Y, Stacking-Order-Driven Optical Properties and Carrier Dynamics in ReS2. Advanced Materials. 2020;32(22):1908311.

Presenting Author: Zefang Ye The University of Texas at Austin

Presenting Author Biography: Zefang Ye is a Ph.D. candidate from Prof. Yaguo Wang's group in Mechanical Engineering Department at UT Austin. Her work focuses on thermal transport behavior at extreme conditions, such as low temperatures and high pressures.

Authors:

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