Konrad Wilczyński
supervisor: Mariusz Zdrojek
Although temperature-dependent Raman studies for 2D materials as a function of number of layers are available for the most popular compounds (e.g. MoS2), as well as individual studies of substrate impact on Raman bands (including strain and charge doping), a comprehensive ab-initio study of temperature-induced phonon evolution in supported multi-layered nanosheets is missing in the literature. However, studies of phonon propagation are important due it its significant impact on thermal and electrical conductivity, crucial for applications.
In this work, I study phonons in 1-5 layered WS2 thin films, including phonon-phonon anharmonic interactions, thermal expansion, and interaction with substrate. For representative Raman active phonon modes, I calculated temperature-dependent phonon shifts and lifetimes, using ab-initio DFT simulations. Moreover, I investigate phonon shifts caused by substrate-induced strain and charge doping. The simulation results are compared to temperature-dependent Raman spectra of exfoliated WS2 nanosheets.
The results indicate that the number of layers significantly affects the harmonic (temperature-independent) part of the phonon energy, and slightly affects the temperature-dependent shifts. The impact of temperature on phonon energies and lifetimes can be well described with models including phonon interactions and thermal expansion. Additionally, a significant impact of the substrate on the harmonic phonon frequencies was observed.