Theoretical prediction of alkali oxide M2O (M = Na and K) monolayers and formation of their Janus structure

Abstract

In this work, a first-principles study of Na₂O and K₂O monolayers and their Janus structure has been carried out using density functional theory calculations based on the projector-augmented plane wave method. Good structural stability is confirmed by phonon dispersion curves and formation enthalpy. The formation of the NaKO monolayer starting from either Na₂O or K₂O is analyzed via the surface formation energy method. The experimental feasibility with the exothermic synthesis process is demonstrated, mostly in Na-poor or K-poor conditions. Na₂O and NaKO monolayers are direct gap semiconductors with band gaps of 0.97 (1.97) and 0.51 (1.21) eV, respectively, as calculated using the PBE (HSE06) functional. Meanwhile, the K₂O monolayer exhibits an indirect gap semiconductor character with an energy gap of 0.52 (1.28) eV. The ionic nature is also confirmed by means of the projected density of states and charge density difference. In addition, the 2D materials studied herein exhibit good absorption in a wide energy range from the visible to ultraviolet region with large absorption coefficients of the order of 10⁵ (cm⁻¹), indicating their potential to be applied in optoelectronic devices.