Thermal expansion and thermodynamic properties of Janus WSSe monolayer: A first-principles study

Materials with Janus structure play an important role in the emerging family of 2D materials. In this paper, based on a quasi-harmonic approach, the elastic, thermal expansion, and related thermodynamic properties of Janus WSSe monolayer are investigated by using first-principles calculations. For comparison, WS2 and WSe2 monolayers are studied as well. The linear thermal expansion coefficients of the three monolayers exhibit isotropy as their isotropic structures. All three monolayers show negative thermal expansion behavior at low temperatures (T < 30 K), with very small magnitudes. The value of the largest magnitude of negative thermal expansion is −0.376 × 10−6 K−1, contributed by the Janus WSSe monolayer. The linear thermal expansion coefficients also change from negative to positive as the temperature increases. This is consistent with macroscopic Grüneisen parameters. Remarkably negative values of the macroscopic Grüneisen parameter directly lead to negative thermal expansion at low temperatures. The linear thermal expansion coefficient of Janus WSSe is between those of WS2 and WSe2. The 2D bulk modulus has an important effect on the thermal expansion of three monolayers.