Strain induced modification in thermal properties of monolayer 1T-HfS2 and HfS2/HfSe2 heterojunction

We have calculated the phonon spectra of HfS2, HfSe2, and HfS2/HfSe2 heterojunction under biaxial tensile strain and analyzed the effect of strain on the stability of the structures using density functional perturbation theory (DFPT). The upper limit of the applied strain of three systems were investigated, and the results revealed that monolayer HfS2 and HfSe2 can withstand biaxial tensile strains of up to 8%. The HfS2/HfSe2 heterostructure can withstand biaxial tensile strains of up to 6%. In addition, the van der Waals forces between the layers of the HfS2/HfSe2 heterojunction have little effect in the x-y plane but can cause phonon splitting in the vertical direction. The biaxial strain can effectively change the entropy, enthalpy, and free energy of the three systems but has little effect on the heat capacity after high temperatures. When the strain reaches the critical strain, the thermal properties will decrease significantly due to structural instability.