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We present a comparative investigation of the influence of crystallinity andfilm thickness on the acoustic and thermal properties of 2D layered PtSe2 thinfilms of varying thickness (0.6-24 nm) by combining a set of experimentaltechniques, namely, frequency domain thermo-reflectance, low-frequency Ramanand pump-probe coherent phonon spectroscopy. We find a 35% reduction in thecross-plane thermal conductivity of polycrystalline films with thickness largerthan 12 nm compared to the crystalline films of the same thickness due tophonon grain boundary scattering. Density functional theory calculations are ingood agreement with the experiments and further reveal the ballistic nature ofcross-plane heat transport in PtSe2 up to a certain thickness (~20 nm). Inaddition, our experiments revealed strong interlayer interactions in PtSe2,short acoustic phonon lifetimes in the range of picoseconds, out-of-planeelastic constant C33=31.8 GPa and layer-dependent group velocity ranging from1340 m/s in bilayer PtSe2 to 1873 m/s in 8 layers of PtSe2. The potential oftuning the lattice cross-plane thermal conductivity of layered 2D materialswith the level of crystallinity and the real-time observation of coherentphonon dynamics, which have direct implications on the cooling and transport ofelectrons, open a new playground for research in 2D thermoelectric devices andprovide guidelines for thermal management in 2D electronics.

Phonon dynamics and thermal conductivity of PtSe2 thin films: Impact of crystallinity and film thickness on heat dissipation