Theoretical Studies Of Magnetic And Electronic Properties Of 2D CoPX₃（X=S,Se） Semiconductors

Two-dimensional（2D）transition-metal phosphorous trichalcogenides CoPX₃（X=S,Se）,as a low dimensional magnetic semiconductor materials with the unique planar struc-ture,properties and potential application,indicate that 2D CoPX₃are the important part of spintronic devices in the future.Due to the magnetic fluctuation effect and the stability of electron spin increase with the decrease of dimension,the material has good stability in normal environment.In this paper,density functional theory（DFT）and Monte Carlo simulations（MCs）are used to study the ground state structure,magnetic properties and the effects of vacancy defects and molecular adsorption of 2D CoPS₃and CoPSe₃.First,the DFT calculated results show that the magnetic ground states of three-dimensional（3D）CoPX₃are AFM phases,in which CoPS₃show the C2/m space group（stacking sequence of“AAA”）,while CoPSe₃demonstrate the R（?） symmetry（stack-ing sequence of“ABC”）.In addition,the cleavage energies of CoPS₃and CoPSe₃are E_cl=0.19 J/m²and E_cl=0.28 J/m²,are smaller than that of graphite（0.36 J/m²）,indicat-ing the 2D CoPX₃can be obtained by exfoliation technology in experiments.Then,the magnetic ground states of 2D CoPS₃and CoPSe₃are n AFM phase and z AFM phase obtained by DFT and MCs,respectively.In order to obtain a reasonable N（?）el temperature（T_N）,the first,second and third nearest neighbor magnetic coupling are considered.Our MCs results show that the T_Nvalues of CoPS₃and CoPSe₃are 171 K and 98 K,which are larger than that of 3D crystals and confirm the existence of inter-layer magnetic coupling interaction.With the increase of defect formation energy,the band gap of 2D CoPX₃defect systems decreases gradually.Importantly,2D CoPX₃V_X2@1L defect system has a defect state near Fermi level,which is mainly contributed by S/Se（p）,P（p）and Co（3d）orbitals,and is related to the S/Se,P and Co atoms and their interaction.For the optical properties,2D CoPSe₃as a good candidate has stronger light absorption efficiency than that of CoPS₃,which is confirmed in the basic optical range.However,2D CoPS₃V_S2@1L and CoPSe₃V_Se2@1L defect systems with a defect peaks near 1.0 e V and 0.8 e V,which lead to the increase of optical absorption spectrum efficiency.Finally,our results indicate that the NO is easier to interact with defective CoPX₃by the hybridization of N（p）,Co（3d）and S/Se（p）,resulting in that the adsorption energy of NO is much higher than that of CO,that is,the adsorption of CO is physical adsorption,while NO is non physical adsorption.By comparison,the larger the adsorption energy of the defect systems,the better effect of electronic structure and magnetic properties,especially near the top of valence band and the bottom of conduction band.Based on the theoretical study of the 2D CoPX₃,which will enrich the application of 2D magnetic semiconductor materials in spintronics.