Experimental evidence of strong intrinsic defect-induced phonon scattering in 2D non-layered β-In2S3

β-In2S3, as a typical two-dimensional (2D) non-layered material, has recently attracted growing attention for exploring novel physical properties beyond van der Waals layered materials. Here, we measured the cross-plane thermal conductivity of β-In2S3 thin films in the temperature range of 78–355 K and at various thicknesses using frequency-domain thermoreflectance. Remarkably, the thermal conductivity remains at around 1 W m−1 K−1 under all measured conditions, demonstrating no significant dependence on either temperature or thickness. This anomalously weak dependence originates from the abundant intrinsic indium vacancies in β-In2S3, which occupy one-third of the tetrahedral sites. These vacancies significantly shorten the phonon mean free path and enhance phonon scattering, resulting in a low and nearly constant thermal conductivity. This study fills the research gap regarding the thermal transport properties of β-In2S3 and provides new insights into the impact of intrinsic point defects on heat conduction in 2D non-layered materials.