Exploration of new direct gap semiconductor Na2X (X = S and Se) monolayers

Abstract

In this work, we report the theoretical prediction, based on density functional theory (DFT) calculations, of new Na2X (X = S and Se) monolayers considering their structural, electronic and magnetic properties. Formation enthalpy and phonon dispersion curves are calculated to examine the structural stability. Na2S and Na2Se single layers are Γ−Γ direct gap semiconductors with energy gaps of 2.17(2.99) and 2.06(2.84) eV, respectively, as determined by standard-PBE (hybrid-HSE06) functional. These two-dimensional (2D) materials are magnetized by creating single Na vacancy, where the magnetic properties are produced mainly by X atoms closest to the defect site. Meanwhile, single X vacancy causes a significant band gap reduction, preserving the paramagnetic nature. Similar effects are observed for oxygen doping, where the energy gap decreases linearly according to increase the doping concentration. In addition, aluminum doping is also efficient to tune this important parameter. It has been found an abrupt reduction once the monolayers are doped with Al atom, however further increasing Al composition will lead to a nearly linear increase. Results presented herein introduce new 2D direct gap semiconductors with band gaps values lying in the visible regime, prospective to be applied in optoelectronic devices, as well as suggest efficient approaches to modify their fundamental properties.