Effect of crystallinity and thickness on thermal transport in layered PtSe2
Alexandros El Sachat, Peng Xiao, Davide Donadio, Frédéric Bonell, Marianna Sledzinska, A. Marty, Céline Vergnaud, H. Boukari, Matthieu Jamet, Guillermo Arregui, Zekun Chen, F. Alzina, Clivia M. Sotomayor Torres, Emigdio Chávez‐Ángel
Abstract
Abstract We present a comparative investigation of the influence of crystallinity and film thickness on the acoustic and thermal properties of layered PtSe 2 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 ~30 layers PtSe 2 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 (~20%) and thin PtSe 2 films (~30%). Moreover, we show strong interlayer interactions in PtSe 2 , 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 PtSe 2 . 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.