Phonons in bulk and monolayer HfS2 and possibility of phonon-mediated superconductivity: A first-principles study

• Calculated phonon frequency and LO–TO splitting of bulk agree well with experimental data. The variation of phonon frequencies from bulk to monolayer are clarified.
• The ultimate strength under biaxial tensile strain is less than 6%. Phonon gap appears when goes to single layer from bulk and can be tuned by biaxial tension.
• Monolayer HfS2 cannot turn to a superconductor by electron–phonon interaction mechanism.

Using density functional perturbation theory (DFPT) with Born effective charges included, we calculate in detail the phonons of bulk and monolayer HfS2. Our calculated phonon frequencies and LO–TO splitting agree well with experimental data. The variation of phonon frequencies from bulk to monolayer can be accounted for in terms of the delicate balance of the long- and short-range force constants. The ultimate strength under biaxial tensile strain is explored for the first time. Monolayer HfS2 can suffer only no more than 6% tensile strain. Our important finding is that the phonon gap appears when goes to single layer from bulk and can be tuned by biaxial tension. By analysing the Eliashberg function we conclude that HfS2 cannot turn to superconductivity by electron–phonon interaction.