Band Engineering and Van Hove Singularity on HfX₂ Thin Films (X = S, Se, or Te)

Two-dimensional transition metal dichalcogenides (TMDs) have become well-known due to their versatile and tunable physical properties for potential applications, specifically on low-power and optical devices. Here, we explored the structural stability and electronic properties of bulk and thin-film (from 1 up to 6 layers) structures of hafnium dichalcogenides (HfX2, X = S, Se, or Te) using first-principles calculations. Our calculations reveal that the most stable phase is 1T for both thin films and bulk. The bulk and thin-film structures of HfTe2 are semimetallic, while those of HfS2 and HfSe2 are insulating. Both HfS2 and HfSe2 thin films exhibit a decreasing band gap with increasing thickness, while HfTe2 thin films remain semimetallic with increasing number of layers. Moreover, van Hove singularity (vHs), due to the contribution of the pz orbital from S atoms, is observed in 3L-HfS2 at the valence band maximum, which can be further enhanced by applying an in-plane biaxial strain, suggesting possible superconductivity. Finally, the bulk and monolayer band structures of HfTe2, under HSE06 and GGA + U with the effective Hubbard U parameter of 4.6 eV, are in good agreement with the experimental ARPES data. Our results indeed show that HfX2 have sensitive and tunable electronic properties through film thickness control and strain for future potential applications.