Theoretical Study On The Electronic And Optical Properties Of M₂CO₂（M=Ti,Zr,Hf） Under Strain

MXenes have aroused research interest due to their unique electrical,magnetic,optical and electrochemical properties,which have a wide range of application prospects.As a representative material in MXenes,Ti₂C has low molar mass,thin layer thickness and large specific surface area per unit weight.Oxygen functional groups make MXenes semiconductor properties,greater mechanical strength,which has advantages in structural composites.In the functional Ti₂C structure,Ti₂CO₂ is a semiconductor with ultrahigh carrier mobility.It is of great value to study the structure of Ti₂CO₂ and its subgroup M2CO2（M=Zr,Hf）.In this paper,by density functional theory,the effect of strain on the electronic structure and optical properties of Ti₂CO₂ and the effect of interlayer coupling on the electronic structure of layered Ti₂CO₂ MXenes are firstly investigated.Secondly,the effects of strain on the electronic structure and optical properties of Zr₂CO₂and Hf₂CO₂ are compared.The specific research contents are as follows:1.By using hybrid functional calculation,it is found that Ti₂CO₂ monolayer is an indirect band gap semiconductor with a band gap of 1.044 e V.Due to the interlayer coupling,the band structures of Ti₂CO₂ nanosheet are split,generating a band gap of0.869 e V.Biaxial strain can be used to achieve the transitions of direct-indirect-negative band gaps semiconductor.At the same time,the interlayer interaction effectively inhibits the structural changes,resulting that the band gap of nanosheet be adjusted in a strain range of–6.4%to+6.0%.Furthermore,the change trend of imaginary partε₂ of the dielectric function with strain indicates that strain has an obvious regulatory effect on the optical transition.The different responses of monolayer and nanosheet under stress make them have their own advantages in optical devices.2.By using PBE functional calculation,it is found that Zr₂CO₂ monolayer is an indirect band-gap semiconductor with a band gap of 0.961 e V,while band structures of Zr₂CO₂ nanosheet is also split with a band gap of 0.808 e V.The band gap modulation of Zr₂CO₂ structure is realized by applying strain to the structures.Compared with Ti₂CO₂structure,Zr₂CO₂ has a larger band gap and can withstand larger strains while still maintaining semiconductor properties.In addition,by analyzing the imaginary partε₂ of the dielectric function of Zr₂CO₂ structures under strains,it is proved that the strain has an obvious regulating effect on the optical transition of Zr₂CO₂ structures.3.The PBE functional calculations show that Hf₂CO₂ monolayer is an indirect band gap semiconductor with a band gap of 1.049 e V,while the band gap of Hf₂CO₂ nanosheet is 0.876 e V,and its band gap is the largest in M2CO2（M=Ti,Zr,Hf）.By applying strain to the structure,the modulation of band gap and optical transition of Hf₂CO₂ structure is realized.It is found that Hf₂CO₂ can withstand greater stress than Ti₂CO₂ and Zr₂CO₂.The results show that the effects of strain on the electronic structures and optical properties of Ti₂CO₂,Zr₂CO₂ and Hf₂CO₂ are similar,which provides a theoretical basis for the potential application of M2CO2（M=Ti,Zr,Hf）in the stress-strain direction.