Band engineering and van Hove singularity on HfX₂ thin films (X = S, Se, or Te)
Abstract
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.
