Study On Electronic Properties Of Two-dimensional Transition Metal Halides Based On First Principle

In this paper,the electronic properties of two classes crystal structures of monolayer transition metal dihalide MX2(M=Zr,Hf;X=C1,I)have been studied.Firstly,the phonon spectra of four monolayer crystals with 1H phase and 1T' phase are calculated respectively,which proves that these structures are dynamically stable.The structures of 1H phase transition metal halide is the same as that of transition metal dichalcogenides(TMDCs),which are three two-dimensional hexagonal lattices stacked in the form of A-B-A.In this paper,we use HfI2 as an example to describe the electronic properties of such monolayer compounds,and its band structure is calculated by the first principles.The results show that HfI2 is an indirect band gap semiconductor with a band gap of 0.387eV,and strong spin-orbit coupling opens up a large spin splitting ?v=0.203eV at the valence band,while the spin splitting at the conduction band minimum is Ac=82meV.The spin projection band structure indicates that the energy spectrum is almost completely spin polarized at the band edge.Based on the result of the projected band structures of the orbits near the Fermi surface,a three-band tight-binding model is constructed.dz2,dxy and dx2-y2 of metal atoms are chosen as the bases.The hopping parameters and coupling strength coefficients are obtained by fitting the DFT bands with least square method.Finally,biaxial stress is applied to adjust the energy spectrum.Compression strain can decrease the indirect band gap,while tension strain can increase the indirect band gap and decrease the direct band gap at K point.The results show that if the tension strain is large enough,the material can be transformed from indirect band gap semiconductor to direct band gap semiconductor.In addition,in-plane biaxial tensile strain can increase the spin splitting size at the same time.ZrI2 is chosen as an example of the distortion structure—1T'phase MX2.First principles calculations show that 1T'-ZrI2 is an indirect gap semiconductor with a band gap of 0.430eV.Because the spin-orbit coupling strength is very weak,the energy spectrum considering the SOC is still spin degenerate.According to the results of orbital projection band,a three-band tight-binding model based on dz2,dxy and dx2-y2 of Zr atom is also constructed,and the fitting parameters are given in this paper.In-plane biaxial compression strain makes the indirect band gap larger,while tensile strain decreases the indirect band gap and the direct band gap,the energy valley tends to be flat.When strain reaches 7.1%,1T'-ZrI2 changes from indirect band gap semiconductor to direct band gap semiconductor.In addition,the vertical electric field with 1V/A is calculated.It is found that the in-plane biaxial stress and the vertical electric field do not break the spin degeneracy around the band edge.