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 PtSe 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 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 films (~30%). Moreover, we show strong interlayer interactions in PtSe, 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 in bilayer to 1873 ms in eight layers of PtSe. 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.
This work has been supported by the Severo Ochoa program, the Spanish Research Agency (AEI, grant no. SEV-2017-0706), and the CERCA Program/Generalitat de Catalunya. The authors acknowledge support from the Spanish MICINN project SIP (PGC2018-101743-B-I00), and the EU project NANOPOLY (GA 289061). The LANEF framework (ANR-10-LABX-51-01) is acknowledged for its support of mutualized infrastructure. PX acknowledges support for the Ph.D. fellowship from the EU Marie Sklodowska-Curie COFUND PREBIST (Grant Agreement 754558). AES acknowledges support by the H2020-MSCA-IF project THERMIC-GA No. 101029727. The authors acknowledge Dr. John Cuffe for his critical comments.