Prediction Of New Two-dimensional Metal Nitrides And Their Electronic And Optical Properties

Due to the characteristics of appropriate band gap and excellent electronic and optical properties,two-dimensional(2D)metal nitrides have potential applications in the field of electronic devices and optoelectronic devices,and have attracted extensive attention in recent years.Based on density functional theory(DFT)and particle swarm optimization algorithm,we predicted several two-dimensional metal nitrides,and systematically explored their crystal structure,stability,electronic,optical,and mechanical properties.The main results are summarized as follows:(1)Study for MoSi₂N₄and MoGe₂N₄monolayers:At first,both monolayers are hexagonal structures.The dynamic stabilities of MoSi₂N₄and MoGe₂N₄monolayers are verified by calculating the phonon dispersions.Band structure calculations show that they are indirect band gap semiconductors with moderate band gaps(2.48eV and 1.46eV,respectively).By calculating the optical absorption spectrum of MoSi₂N₄monolayer,it is found that there is an obvious absorption peak in the visible light range(1.61eV-3.10eV),indicating its strong ability for light absorption.By calculating the band alignments relative to the vacuum energy level,it is found that the band edge position of MoSi₂N₄monolayer can perfectly straddle the redox potential of water,while for MoGe₂N₄monolayer,the appropriate band edge positions can also be realized by applying a moderate strain.This indicates that both of the structures have potential applications in photocatalytic water splitting.(2)Study on the properties of 2D ternary compound TiNX(X=F,Cl,Br,I)monolayer:At first,the calculation of phonon spectrum confirms their dynamic stabilities.The calculation of electron band structure shows that TiNX(X=F,Cl,Br)monolayers have moderate direct band gap,which are 1.57eV,1.42eV and 1.40eV respectively.The investigation of optical properties found its excellent optical absorption.We also found that surface passivation and stress engineering can effectively adjust the band gap of TiNI monolayer.(3)Structure prediction and property investigation of Be₃N₂monolayer:Be₃N₂monolayer is a hexagonal and indirect band gap semiconductor with the gap value of 3.52eV.At first,through phonon spectrum calculation and molecular dynamics simulation show that the structure has dynamic and thermodynamic stability.Through the analysis of electron localization function and charge density,it is found that electrons are mainly localized around N atoms and form Be₃N₂monolayer structure in the form of ionic bond.In addition,we studied the mechanical properties of single-layer Be₃N₂.The calculated the young's modulus along the x direction and y direction are 440.86GPa·nm and 375.26GPa·nm,respectively.The mechanical stability of Be₃N₂monolayer is verified by calculating the elastic parameters and referring to the mechanical stability standard.By applying strain,we found that the band gap of the material gradually decreases with the increase of strain.When the strain reaches up to 50%,the structure is still not damaged,indicating that it is a flexible material.Finally through the calculation of optical absorption spectrum,it is found that it has strong ability to absorb ultraviolet light.Based on our systematic study of the above structures,we believe that it can provide a valuable theoretical basis for the experimental exploration of two-dimensional metal nitrides in the future.