Quasi-Ballistic Thermal Transport Across MoS 2 Thin Films

作者：Aditya Sood, Feng Xiong, Shunda Chen, Ramez Cheaito, Feifei Lian, Mehdi Asheghi, Yi Cui, Davide Donadio, Kenneth E. Goodson, Eric Pop · 发表于：Nano Letters · 年份：2019 · DOI：10.1021/acs.nanolett.8b05174 · 被引用次数：103 · 研究领域：Thermal properties of materials、Advanced Thermoelectric Materials and Devices、Thermal Radiation and Cooling Technologies

Layered two-dimensional (2D) materials have highly anisotropic thermal properties between the in-plane and cross-plane directions. Conventionally, it is thought that cross-plane thermal conductivities (κ z ) are low, and therefore c -axis phonon mean free paths (MFPs) are small. Here, we measure κ z across MoS 2 films of varying thickness (20–240 nm) and uncover evidence of very long c -axis phonon MFPs at room temperature in these layered semiconductors. Experimental data obtained using time-domain thermoreflectance (TDTR) are in good agreement with first-principles density functional theory (DFT). These calculations suggest that ∼50% of the heat is carried by phonons with MFP > 200 nm, exceeding kinetic theory estimates by nearly 2 orders of magnitude. Because of quasi-ballistic effects, the κ z of nanometer-thin films of MoS 2 scales with their thickness and the volumetric thermal resistance asymptotes to a nonzero value, ∼10 m 2 K GW –1 . This contributes as much as 30% to the total thermal resistance of a 20 nm thick film, the rest being limited by thermal interface resistance with the SiO 2 substrate and top-side aluminum transducer. These findings are essential for understanding heat flow across nanometer-thin films of MoS 2 for optoelectronic and thermoelectric applications.