Abnormal Thickness-Dependent Thermal Transport in Suspended 2D PdSe₂

Research on 2D materials originally focused on the highly symmetrical materials like graphene, h-BN. Recently, 2D materials with low-symmetry lattice such as PdSe₂ have drawn extensive attention, due to the interesting layer-dependent bandgap, promising mechanical properties and excellent thermoelectric performance, etc. In this work, the phonon thermal transport is studied in PdSe₂ with a pentagonal fold structure. The thermal conductivity of PdSe₂ flakes with different thicknesses ranging from few nanometers to several tens of nanometers is measured through the thermal bridge method, where the thermal conductivity increases from 5.04 W mk^-1 for 60 nm PdSe₂ to 34.51 W mk^-1 for the few-layer one. The atomistic modelings uncover that with the thickness thinning down, the lattice of PdSe₂ becomes contracted and the phonon group velocity is enhanced, leading to the abnormal increase in the thermal conductivity. And the upshift of the optical phonon modes contributes to the increase of the thermal conductivity as well by creating less acoustic phonon scattering as the thickness reduces. This study probes the interesting abnormal thickness-dependent thermal transport in 2D materials, which promotes the potential thermal management at nanoscale.