Origins of heat transport anisotropy in MoTe2 and other bulk van der Waals materials

Journal Article · Wed Aug 09 04:00:00 EDT 2023 · Materials Today Physics

DOI:https://doi.org/10.1016/j.mtphys.2023.101196· OSTI ID:1999048

Li, Hongkun ^[1]; Pandey, Tribhuwan ^[2]; Jiang, Yi ^[3]; Gu, Xiaokun ^[4]; Lindsay, Lucas ^[5]; Koh, Yee Kan ^[3]

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1. National Univ. of Singapore (Singapore); Southwest Jiaotong University, Chengdu (China)
2. Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States); Univ. of Antwerp (Belgium)
3. National Univ. of Singapore (Singapore)
4. Shanghai Jiao Tong Univ. (China)
5. Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States)

Knowledge of how heat flows anisotropically in van der Waals (vdW) materials is crucial for thermal management of emerging 2D materials devices and design of novel anisotropic thermoelectric materials. Despite the importance, anisotropic heat transport in vdW materials is yet to be systematically studied and is often presumably attributed to anisotropic speeds of sound in vdW materials due to soft interlayer bonding relative to covalent in-plane networks of atoms. In this work, we investigate the origins of the anisotropic heat transport in vdW materials, through time-domain thermoreflectance (TDTR) measurements and first-principles calculations of anisotropic thermal conductivity of three different phases of MoTe₂. MoTe₂ is ideal for the study due to its weak anisotropy in the speeds of sound. We find that even when the speeds of sound are roughly isotropic, the measured thermal conductivity of MoTe₂ along the c-axis is 5–8 times lower than that along the in-plane axes. We derive meaningful characteristic heat capacity, phonon group velocity, and relaxation times from our first principles calculations for selected vdW materials (MoTe₂, BP, h-BN, and MoS₂), to assess the contributions of these factors to the anisotropic heat transport. Interestingly, we find that the main contributor to the heat transport anisotropy in vdW materials is anisotropy in heat capacity of the dominant heat-carrying phonon modes in different directions, which originates from anisotropic optical phonon dispersion and disparity in the frequency of heat-carrying phonons in different directions. Further, the discrepancy in frequency of the heat-carrying phonons also leads to ~2 times larger average relaxation times in the cross-plane direction, and partially explains the apparent dependence of the anisotropic heat transport on the anisotropic speeds of sound. This work provides insight into understanding of the anisotropic heat transport in vdW materials.

• Research Organization: Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States); Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States). National Energy Research Scientific Computing Center (NERSC)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES). Materials Sciences & Engineering Division (MSE); Singapore Ministry of Education
• Grant/Contract Number: AC05-00OR22725; AC02-05CH11231
• OSTI ID: 1999048
• Alternate ID(s): OSTI ID: 1997311
• Journal Information: Materials Today Physics, Journal Name: Materials Today Physics Vol. 37; ISSN 2542-5293
• Publisher: Elsevier
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
anisotropic effective relaxation time
anisotropic speeds of sound
anisotropic thermal conductivity
anisotropy in effective heat capacity
van der Waals materials