HfOX monolayers (X = S, Se, Te): Atomically thin semiconductors with tailored band gaps and broadband optical response

Two-dimensional (2D) mixed-anion materials, particularly oxy-chalcogenides, offer a rich platform for tailoring electronic and optical properties through compositional engineering. In this work, we present a comprehensive first-principles investigation of hafnium-based oxy-chalcogenide monolayers (HfOX, X = S, Se, Te) using density functional theory (DFT) with generalized gradient approximation (GGA) and hybrid BLYP functionals. Structural optimizations reveal that all three monolayers adopt tetragonal lattices with a systematically increasing bond length and lattice parameters from S to Te, reflecting the chalcogen-dependent steric effects. Phonon dispersion calculations confirm their dynamic stability, while electronic structure analysis shows that HfOS and HfOSe are direct band gap semiconductors (1.08 eV and 0.97 eV, respectively), whereas HfOTe exhibits an indirect gap (0.65 eV). Charge density and electrostatic potential analyses highlight the polar covalent bonding nature and asymmetric charge distribution, which are crucial for piezoelectric and dielectric applications. Optical property calculations demonstrate strong broadband absorption across the ultraviolet to near-infrared spectrum (0.6–30 eV), with tunable peaks and high absorption coefficients ($>7.3\times 10^5$ cm⁻¹). Notably, HfOTe extends absorption into the infrared, while HfOS shows dominant UV activity. The refractive index and optical conductivity further reveal chalcogen-dependent trends, with static dielectric constants increasing from 1.82 (HfOS) to 2.24 (HfOTe). Our results establish HfOX monolayers as a promising class of 2D semiconductors with layer-dependent band gaps, anisotropic optical responses, and potential applications in flexible optoelectronics, photovoltaics, and nanoscale dielectric devices.