Geometric and Electronic Properties of Monolayer HfX2 (X = S, Se, or Te): A First-Principles Calculation

Abstract

The essential properties of monolayer HfX2 (X = S, Se, or Te) are fully explored by first-principles calculations. The optimal lattice symmetries, sublattice buckling, electronic energy spectra, and density of states are systematically investigated. Monolayer HfS2, HfSe2, and HfTe2, respectively, belong to middle-gap semiconductor, narrow-gap one and semimetal, with various energy dispersions. Moreover, the van Hove singularities (vHs) mainly arise from the band-edge states, and their special structures in the density of states strongly depend on their two or three-dimensional structures and the critical points in the energy-wave-vector space. The above-mentioned theoretical predictions are attributed to the multi-orbital hybridizations of [dx2−y2, dxy, dyz, dzx, dz2]–[s, px, py, pz] in the Hf-X chemical bonds. The diversified physical phenomena clearly indicate a high potential for applications, as observed in MoS2-related emergent materials ions.

Effect of biaxial strain on electronic and optical properties of vertically stacked HfS2/HfSe2 heterostructures

“…The structures were relaxed, the lattice parameters of HfX 2 (X = S, Se) for T phases (3.65 Å, 3.78 Å ) and H phases (3.54 Å,3.68 Å ) are obtained respectively and the calculated lattice parameters are consistent with the reported experimental results [26,31]. The bond distance of HfX 2 (X = S, Se) for the T phase is found to be (2.56 Å, 2.69 Å), and for the H phase is (2.58 Å, 2.71 Å) which is in agreement with reported data [31,34,35,54,55]. Although both the phases of HfX 2 (X = S, Se) i.e., (T-phase and H-phase) are energetically stable, the 1T phase has been found to be more energetic than the 2H phase as shown in table 1.…”

“…To verify the accuracy of the calculation parameters used in the calculations, a test calculation was performed on the 1T-HfS 2 monolayer. The test results of the lattice parameter 3.65 Å and electronic band gap of 2.07 eV are in good agreement with the reported experimental values of 3.63 Å and band gap of 1.96 eV [31,34].…”

“…The inclusion of the Hubbard U parameter in DFT calculation results in the lifting of partially filled d and f states and electronic band gap values improve to 2.07 eV, and 1.63 eV respectively. The electronic band gap with the PBE+U function is in good agreement with the reported value of 1.96 eV of HfS 2 (T) and the predicted value of 0.95 of HfSe 2 (H)results [31,34,35,55]. The band structure of monolayers is shown in figures 3(a) and (b).…”

“…2D HfX 2 is one of the recent additions to the ever-expanding family of 2D materials after their successful synthesis using the chemical vapor deposition technique [31,32]. HfX 2 (X = S, Se) monolayers have exhibited semiconducting behavior with an indirect band gap in the range (1.13-1.96)eV for T-phase [31,33,34] and (0.95 eV −1.15 eV) for H-phase [35]. In addition, HfS 2 and HfSe 2 have been predicted to exhibit high mobility of 1833 cm 2 V −1 s −1 and 3579 cm 2 V −1 s −1 [36].…”

From Two-, One-, to Zero-Dimensional Vacancies: A Densification Pattern for a Typcial Transition-Metal Dichalcogenide of TiSe2

“…Although another 8-fold Immm phase instead of the C 2/ m phase is proposed for the second phase, the vacancy’s transition pattern from 2D, 1D, to 0D agrees with that of TiSe 2 , realizing the transfomation from a layered structure at ambient pressure to a spatial covalent network structure at high-pressure. Considering those TMDs MX 2 (M = Ti, Zr, Hf; X = S, Se, Te) with the same P -3 m 1 phase at ambient pressure, − they should also follow this phase transition pattern at high-pressure. In particular, the C 2/ m phase may exist as a common transition phase during these MX 2 materials.…”