A DFT study on the substitutional effects on Hf3N2X2 (X = O, S) MXene monolayers

MXenes monolayers have shown potential applications ranging from electronic devices to water purification and antibacterial coatings since 2D MXene possesses properties significantly different from those of bulk materials. Here, density functional theory is used to investigate the structural, electronic, optical, and thermodynamic properties, as well as the energy stability and phonon dispersion of two-dimensional Hf₃N₂O₂ and Hf₃N₂S₂ MXene monolayers. The results show that, using approaches based on the generalized gradient (GGA) and the hybrid exchange–correlation functional HSE06 for the optimized structure with the minimum energy, an indirect bandgap of 0.387 eV and 0.577 eV was obtained within the GGA-PBE and HSE06 level of calculation, respectively, for the Hf₃N₂O₂ MXenes monolayers, while the Hf₃N₂S₂ structure is electronically a conductor. Replacing the sulfur (S) atom with oxygen (O) in the hafnium monolayer produced a clear electronic transition effect, turning it from a semiconductor to a conductor. The optical absorption was shown to be sensitive to the plane of polarization of the incident light, mainly in the UV–Vis regions. In addition, from the thermodynamic potential calculations within the PBE functional, the free energy (F) indicates that these MXene monolayers could be potentially synthesized spontaneously at low temperatures. All properties calculated in this study demonstrate that Hf₃N₂X₂ (X = O, S) MXenes monolayers have potential applications in optoelectronic and thermal devices at the nanoscopic scale.