Thermal transport in <b> <i>γ</i> </b> -InSe: Bulk single crystals and thin flakes

Abstract

We measure the temperature-dependent in-plane thermal conductivity, κ∥(T), of high-purity γ-InSe bulk single crystals and exfoliated thin flakes (30–50 nm) from 50 to 300 K. Our bulk results agree with prior bulk reports and provide a reproducible reference for phonon transport. In the literature, cross-plane thermal conductivity of supported InSe flakes shows relatively modest variation, whereas reported room-temperature in-plane values for supported flakes span a wide range, including outliers that exceed bulk despite much smaller thickness. In our measurements, the flake κ∥ at room temperature is lower than bulk, as expected, but exhibits substantial sample-to-sample variability; despite being thinner, intrinsic flakes show higher κ∥ than doped flakes, consistent with reduced impurity scattering and improved flake uniformity. We analyze our data using a Callaway-type phonon-scattering model in which substrate interactions (including plausible strain-related renormalization of acoustic parameters) contribute to the observed spread near room temperature. However, below ∼150 K, the measured trends cannot be captured without invoking changes in sound velocity far larger than realistic strain levels, indicating that additional mechanisms beyond simple strain renormalization are required at low temperature. These measurements provide low-temperature κ∥ benchmarks for γ-InSe flakes and constrain how much of the reported room-temperature spread can be explained by thickness, nonuniformity, and substrate effects alone.