Electronic Structure,Electron Transport And Photocatalysis Of Vanadium Dichalcogenide Monolayers

Monolayered vanadium dichalcogenide as a two-dimensional material has promising applications in spintronics,thermoelectric materials,photocatalysts,and other fields.In order to investigate the properties of monolayered vanadium dichalcogenide in these areas,this study investigates the electronic structure,thermoelectric properties,electro-acoustic coupling,optical properties,and photocatalysis of monolayered vanadium dichalcogenide through first-principles calculations.Phonon and spintronic structures of Janus vanadium-dichalcogenide monolayers are calculated by the first-principles schemes of pseudopotential plane-wave based on spin-density functional theory,to study the dynamic structural stability and the electronic spin-splitting due by spin-orbit coupling（SOC）and spin-polarization.Geometry optimizations and phonon-dispersion spectra provide a consistent evidence that vanadium-dichalcogenide monolayers possess a high-enough cohesive energy for stationary stability,while only VSTe monolayer can represent a dynamically stable structure without any virtual frequency of atomic-vibration modes.Atomic population charges and electron density differences demonstrate that V-Te covalent bonds cause a high electrostatic potential gradient perpendicular to layer-plane internal VSTe and VSe Te monolayers.The spin polarization of vanadium 3d-orbital component causes a pronounced energetic spin-splitting of electronic-states near Fermi-level,leading to a semimetal band-structure and cause the increase of optoelectronic band-gap.Rashba spin-splitting around G point in Brillouin zone can be specifically introduced into Janus VSe Te monolayer by strong chalcogen SOC together with a high intrinsic electric field（potential gradient）perpendicular to layer-plane.The vertical splitting of band-edge at K point can be enhanced by a stronger SOC of the chalcogen element with larger atom number for constituting Janus V-dichalcogenide monolayers.The collinear spin-polarization causes the band-edge spin-splitting across Fermi-level and leads to a ferrimagnetic order in layer-plane between V and chalcogen cations with the higher up-spin and the lower down-spin densities respectively,which accounts for a large net spin as manifested more apparently in VSe Te monolayer.In a conclusion for Janus vanadium-dichalcogenide monolayers,the significant Rashba splitting with a enhanced K-point vertical splitting can be effectively introduced by strong SOC in VSe Te monolayer,which simultaneously represents the largest net spin of 1.64（?/2）per unit cell.The present study proposes a normative schemes for first-principles electronic structure calculations of low-dimensional spintronic materials,and suggests a prospective extension of applying two-dimensional compound materials to spintronics.Based on the deformation potential theory,the elastic constants,effective mass of charge carriers,deformation potential constants,carrier mobility of electrons and holes,relaxation time,Seebeck coefficient,thermoelectric figure of merit,and electromechanical coupling effect on the electronic band structure of vanadium-dichalcogenide monolayers VXY（X,Y=S,Se,and Te）are calculated.The carrier mobility of monolayer VXY（X and Y=S,Se,and Te）with Janus structure is lower than that of VX₂（X=S,Se,and Te）.Except for VSTe monolayer,the other five 2D materials show good thermoelectric properties,among which VS₂ has the best thermoelectric performance as a potential thermoelectric material.Under the effect of electromechanical coupling,the electron band-gap of these six 2D materials is monotonously reduced,while still maintaining a high thermal stability.The optical properties,adsorption energies and the Gibbs free energies in different reactions of single vanadium disulfide VXY（X and Y=S,Se and Te）have been calculated by first principles.The electron transition between the d orbitals V atoms and the p orbitals of chalcogenide atom of VXY（X and Y=S,Se and Te）is proved to exist.The photocatalytic properties of Vanadium-dichalcogenide monolayer have a high absorption coefficient in the near-ultraviolet range and be potential for photocatalysts.It was found that the catalytic activity of HER reaction and OER reaction on the surface of chalcogenide atoms decreased with the increase of atomic number and difference of chalcogenide atoms.That means the S atomic site of VSSe monolayer requires less excitation energy in the HER and OER reactions,indicating higher catalytic activity.Additionally,its larger optical band-gap is advantageous for increasing the catalytic rate.For CO₂RR reaction,VSe₂ has relatively higher catalytic activity.These computational results will provide theoretical guidance for the design of more excellent photocatalysts.