Thermal expansion coefficient and phonon dynamics in coexisting allotropes of monolayer WS₂ probed by Raman scattering

We report a comprehensive temperature dependent Raman measurement on three different phases of monolayer WS₂ from 4 K to 330 K in a wide spectral range. Our studies reveal the anomalous nature of the first, as well as the higher order combination modes reflected in the disappearance of the few modes and anomalous temperature evaluation of the phonon self-energy parameters attributed to the detuning of resonance condition and development of strain due to thermal expansion mismatch with the underlying substrate. Our detailed temperature dependence studies also decipher the ambiguity about the assignment of the two modes in literature near ∼297 cm⁻¹ and 325 cm⁻¹. The mode near 297 cm⁻¹ is assigned as first order Raman mode, which is forbidden in the backscattering geometry, and 325 cm⁻¹ is assigned to the combination of and mode. We also estimated thermal expansion coefficient by systematically disentangling the substrate effect in the temperature range of 4 K to 330 K and probed its temperature dependence in the 1H, 1T and 1T′ phases. (paper)