First-principles investigation of the thermodynamic properties of two-dimensional MoS2

Abstract

Density-functional perturbation theory has been applied to investigate the thermodynamic properties of two-dimensional MoS2 within the Perdew-Burke-Ernzerhof genealized gradient approximation (PBE-GGA). The Murnaghan’s isothermal equation of state has been derived for the monolayer, giving how the pressure and the total energy of the monolayer evolve versus the volume of the unit cell. The temperature-dependent behavior of some thermodynamic quantities, including the Helmholtz free energy, total energy, Debye temperature, mean square displacements, specific heat, entropy and number of microstates, have been determined as well as their functional forms within the quasi-harmonic approximation, by calculating the partial phonon density of states as a fundamental quantity. The value of 45.6 cm−1 has been found for the phonon band gap in the frequency range, separating the acoustic and the optical phonon bands in fair agreement with the literature. The results broadly support the view that the validity of the Debye T3-law for low-temperature specific heat of solids is violated for the case of two-dimensional MoS2 and therefore, a T2-law is proposed, accurately describing the behavior of the isochoric specific heat from 0 to 60 K.