Quasi-Ballistic Thermal Transport Across MoS₂ Thin Films

Abstract

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 across MoS2 films of varying thickness (20 to 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 two 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 non-zero value, ~10 m 2 KGW -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.

Limits to Thermal-Piezoresistive Cooling in Silicon Micromechanical Resonators

“…Thermal-piezoresistive cooling is one application that would benefit from third-order and higher temperature coefficients of elasticity in doped silicon [52], [56]. The experiments reported here could be repeated in non-encapsulated devices, and the mechanical vibrations and microscale heat transport can be imaged at large currents using laser Doppler vibrometry and time-domain thermoreflectance, respectively [57], [58]. Large current experiments could be interesting in nanowire-based thermal-piezoresistive resonators [33], [34].…”

Thermal Conductivity Measurements in Atomically Thin Materials and Devices

“…Of course the discrepancy might arise from the differences in the crystal thicknesses. Cross-plane thermal conductivity is also strongly effected by the thickness of the crystals due to very long phonon mean free paths along the z-axis [13]. From 20 to 1000 nm crossplane thermal conductivity increases from 1 to 4 Wm −1 K −1 as shown in Fig.…”

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“…For example, there is a dire need to directly measure the spatial operating temperature of the device because micro-Raman is limited to point-by-point data measurements. Furthermore, new device concepts like ballistic devices [19] and thermal rectifiers [20] are to be investigated in detail. Also, Joule heating phenomenon in many emerging 2D materials like TMDCs, hBN, transition metal carbides, nitrides and carbonitrides need to be investigated.…”