Electronic Structure And Properties Of Two-Dimensional WSe₂ And Its Heterostructure

The monolayer WSe₂ with excellent stability both in physics and chemistry is a novel two-dimensional material,which has a good performance in spintronics,photoelectronics and valleytronics due to its special electronic structure.At present,the problem of information storage capacity and the difficulty of material property expansion are increasingly prominent.The superiority of monolayer WSe₂ in spintronics and valleytronics will provide a significant solution to these problems.However,there're several issues of WSe₂ awaiting solution.On the one hand,people have contradictory views on the point defects appearing in the preparation of WSe₂ or cannot determine their magnetic source.On the other hand,the regulation of its spin splitting and valley splitting is not clear,few related research reports,and it is much difficult to regulate the valley splitting due to its small value.Furthermore,the band gap adjustment has not reached the ideal level.Aiming at these three aspects,this paper studies the electronic structure and physical properties of monolayer WSe₂ and its heterostructures by first-principles calculations based on density functional theory.First,by first-principles calculations(using Mede A software),the stability of the point defects(V_Se,V_Se2,V_W,V_W2,V_WSe3,V_WSe6 and Se _W)that may exist in the preparation of monolayer WSe₂ are systematically investigated,as well as the electronic structures and magnetism of the defect systems.The results show that in the presence of V_WSe6 defect,there're a large number of unpaired electrons in the system,and because of the metal bonding between the W atoms near the vacancy,these unpaired electrons are concentrated in the vicinity of the defect and spin-polarized,which induces the spin magnetic moment of 5.940?_B and exhibits semi-metallic characteristics.Then,the change of the defect system under strain is innovatively simulated,and it is found that V_Se,V_W and V_W2 also showed magnetism,which is caused by the change of the distance between atoms under stress,and the atomic bonding relationship changed accordingly,resulting in partial electron deloralization and spin magnetic moment.Then,the electronic structure and optical properties of g-C₃N₄/WSe₂ heterost-ructure are systematically investigated by first-principles calculations.The results show that by combining with g-C₃N₄,the g-C₃N₄/WSe₂ heterostructure has a wider absorption range and stronger UV absorption capacity,and the band gap of this heterostructure is adjustable.Under the electric field,its band gap can be adjusted from 1.101 e V to 0.527 e V,and a transition from type I to type II band alignment occurs,which effectively improves the separation ability of its photogenic carriers.Under the strain,the valence band spin splitting can be increased to 500.4 me V,and the conduction band spin splitting can reach 43.4 me V,with an indirect-direct band gap transition,showing obvious peak displacement in the absorption spectra.Finally,the electronic structures of novel heterostructures VI₃/WSe₂ and VI₃/WSe₂/VI₃ are investigated by first principles calculation.It is found that due to the coupling of W-V and W-W and the existence of a strong internal electric field in the system,spin splitting and valley splitting of WSe₂ are simultaneously induced at K and K'points.The values of them are independent of the stacking modes.When the V-atom spin magnetic moment is larger,there has a larger electric field and a greater valley splitting.The change of the interlayer spacing and the strain can effectively change the spin and valley splittings of the VI₃/WSe₂ heterostructures,which is achieved by changing the distance between atoms to enhance or weaken the W-V and W-W coupling,firmly confirming the controllability of the spin or valley polarization.For the VI₃/WSe₂/VI₃ sandwich heterostructures,due to the fact that the V atoms in the two VI₃ layers has spin parallel alignment,the coupling between W and V is stronger,so a larger spin and valley splitting can be induced.