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Quasi-Ballistic Thermal Transport Across MoS2 Thin Films

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journal contribution
posted on 27.02.2019, 00:00 by Aditya Sood, Feng Xiong, Shunda Chen, Ramez Cheaito, Feifei Lian, Mehdi Asheghi, Yi Cui, Davide Donadio, Kenneth E. Goodson, Eric Pop
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 (κz) are low, and therefore c-axis phonon mean free paths (MFPs) are small. Here, we measure κz across MoS2 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 κz of nanometer-thin films of MoS2 scales with their thickness and the volumetric thermal resistance asymptotes to a nonzero value, ∼10 m2 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 SiO2 substrate and top-side aluminum transducer. These findings are essential for understanding heat flow across nanometer-thin films of MoS2 for optoelectronic and thermoelectric applications.