Unveiling vibrational and optoelectronic properties of HfX2 (X = Br, I) monolayers via DFT calculations

Abstract

The optoelectronic properties, phonon dispersion, stability, and quantum dynamics of HfX2-type transition-metal dihalides (TMDHs) were systematically investigated using first-principles calculations based on Density Functional Theory (DFT) with GGA-PBE and HSE06 functionals. Both HfBr2 and HfI2 monolayers exhibit structural stability, as indicated by phonon dispersion studies showing no negative frequencies, while cohesive energy and dynamic analysis confirm their energetic stability. Infrared (IR) and Raman spectra were calculated, assigned, and compared. The electronic properties of TMDHs reveal close direct and indirect bandgaps, with HfBr2 having a direct bandgap of 1.33 eV (HSE06) and an indirect bandgap of 1.19 eV (HSE06), and HfI2 displaying a direct bandgap of 1.04 eV (HSE06) and an indirect bandgap of 0.90 eV (HSE06), suggesting potential applications in field-effect transistors (FETs). These bandgap values are consistent with the experimental bandgap values of 2D and bulk HfSe2. The spectroscopic limited maximum efficiency method was used to estimate power conversion efficiency for both HfX2-type monolayers. Electronic excitations occur in the ultraviolet region of the electromagnetic spectrum, implying that HfX2 monolayers can serve as effective ultraviolet filters for polarized light. The calculations show that the HfBr2 and HfI2 monolayers are feasible and hold promise for various technological applications.