Effect of crystallinity and thickness on thermal transport in layered PtSe2

We present a comparative investigation of the influence of crystallinity and film thickness on the acoustic and thermal properties of layered PtSe2 films of varying thickness (1-40 layers) using frequency-domain thermo-reflectance, low-frequency Raman, and pump-probe coherent phonon spectroscopy. We find ballistic cross-plane heat transport up to similar to 30 layers PtSe2 and a 35% reduction in the cross-plane thermal conductivity of polycrystalline films with thickness larger than 20 layers compared to the crystalline films of the same thickness. First-principles calculations further reveal a high degree of thermal conductivity anisotropy and a remarkable large contribution of the optical phonons to the thermal conductivity in bulk (similar to 20%) and thin PtSe2 films (similar to 30%). Moreover, we show strong interlayer interactions in PtSe2, short acoustic phonon lifetimes in the range of picoseconds, an out-of-plane elastic constant of 31.8 GPa, and a layer-dependent group velocity ranging from 1340 ms(-1) in bilayer to 1873 ms(-1) in eight layers of PtSe2. The potential of tuning the lattice thermal conductivity of layered materials with the level of crystallinity and the real-time observation of coherent phonon dynamics open a new playground for research in 2D thermoelectric devices and provides guidelines for thermal management in 2D electronics.