In silico study of adsorption of oxide gases by MN4 (M = Be, Mg) monolayers

In response to the continuously increasing pollution and hazards due to gas leakage, density functional theory based simulation of MN4 (M = Be, Mg) monolayer (ML) was performed. The computed cohesive energy of -2.28 eV/atom for BeN4 ML as well as -2.03 eV/atom for MgN4 ML, and absence of imaginary frequencies in the phonon dispersion curves suggest dynamical stability of both the MLs. The stable structures of MN4 MLs attain monoclinic configuration. The electronic band structure and electron density of states show, the semi-metallic behaviour of both the MLs. With the adsorption energy of -1.64 eV and -2.68 eV, for BeN4 and MgN4 ML, respectively, NO2 is superiorly sensed by MN4 MLs. The charge transfer of -0.452e (BeN4 ML) and -0.646e (MgN4 ML), from gas molecule to the ML, provide assurance for strong adsorption. Also, both the MLs are good at sensing SO2, as the adsorption energies are -0.87 eV (BeN4 ML) and -1.64 eV (MgN4 ML). Both NO2 and SO2 gas molecules contribute at the Fermi level in the projected density of states (PDOS), referring to the transition of MN4 ML, from semi-metallic to metallic, after the adsorption. Besides, in the current-voltage (I-V) characteristics, the extremely sensitive behaviour of MN4 ML towards NO2 gas molecule can be observed, as the current for NO2 adsorbed MN4 ML shoots really high at voltage as low as 0.6 V, which makes MN4 MLs excellent pick as sensing arrays in NO2 gas sensors.