Correlation between structure, phonon spectra, thermal expansion, and thermomechanics of single-layer MoS2

Using first-principles simulation, the correlation between structure, phonon spectra, thermal expansion, and thermomechanics of single-layer MoS2 is established. The laminar structure results in the low-dimension ZA mode with a parabolic dispersion and negative Gruneisen constants (gamma), while the nonorthogonal covalent Mo-S bonds (or intralayer thickness) result in the interatom and interdirection vibrational hybridizations, which tend to increase gamma. There is a negative-positive crossover in thermal expansion coefficient at 20 K, because of the competition between the modes with negative and positive gamma. Although the phononic activation at finite temperatures has a stiffening effect on the bulk modulus, the dominant effect from thermal expansion softens the lattice upon heating. The intralayer thickness results in the similarity between the thermal expansions of SL and bulk MoS2. Our numerical results explicitly support that the experimentally measured thermal shifts of the Raman modes are dominated by multiphonon scattering, but not thermal expansion.