Probing the effects of thin-film thickness on transition metal dichalcogenides offer novel insights into their electronic properties and tunability, which leads to a new avenue of research and applications. A comprehensive first-principles study on thickness-dependent structural stabilities and electronic properties of ZrX2 (X = S, Se, or Te) thin films from 1 layer (L) to 6L and bulk was performed. The calculated formation energies show that ZrX2 adopts the 1T phase as the most stable structure. Furthermore, 1T-ZrS2 and ZrSe2 thin films and bulk are indirect semiconductors and their band gaps decrease as the number of layers is increased up to 6L, while 1T-ZrTe2 thin films and bulk are semimetallic. Interestingly, we demonstrate that the surface band structure of bulk and monolayer ZrTe2 under generalized gradient approximation + U and HSE06 methods is in excellent agreement with the angle-resolved photoelectron spectroscopy measurement. Finally, we discover the existence of van Hove singularities in strained 2L and unstrained 3L 1T-ZrS2 thin films, implying the existence of superconductivity in these thin films. These results showcase the tunable electronic properties of ZrX2 thin films because of thickness dependence and strain.

Recent studies have demonstrated the feasibility of synthesizing two-dimensional (2D) Janus materials which possess intrinsic structure asymmetry. Hence, we performed a systematic first-principles study of 2D Janus transition metal dichalcogenide...

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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 (HfX 2 , 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 HfTe 2 are semimetallic, while those of HfS 2 and HfSe 2 are insulating. Both HfS 2 and HfSe 2 thin films exhibit a decreasing band gap with increasing thickness, while HfTe 2 thin films remain semimetallic with increasing number of layers. Moreover, van Hove singularity (vHs), due to the contribution of the p z orbital from S atoms, is observed in 3L-HfS 2 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 HfTe 2 , 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 HfX 2 have sensitive and tunable electronic properties through film thickness control and strain for future potential applications.