First-principles Study Of Electronic Properties And Their Tuning In Heterostructures Based On Transition Metal Dichalcogenide Monolayers

Owing to their excellent physical properties,transition metal dichalcogenide(TMD)monolayers are considered to be ideal materials for the next generation electronic,optoelectronic devices.In particular,electron valleys in these systems can be used to store information,which are promising in valleytronics.However,the degeneracy of the electron valleys,i.e.,the K and K' valleys,are protected by the time-reversal symmetry.The key to make them to be useful in valleytronic devices is to break the energy degeneracy of the valleys,i.e.,valley polarization.Recently,several approaches have been proposed to manipulate the electron valleys in the TMD monolayers.One approach is to selectively pump the valley electrons by circularly polarized light.Another approach is to lift the energy degeneracy by external magnetic fields.However,the valley polarization achieved by both the approaches is volatile.Moreover,the effects of manipulating valley by external magnetic fields are rather small(?0.2 meV/T).Recently,theoretical studies show that large valley polarizations up to about 300 meV can be achieved in MoTe2/EuO using the magnetic proximity effect induced by the magnetic substrate.However,experiment found that the valley polarization in WS2/EuS is only about 19 meV.The reasons for such a large difference between theory and experiment remain to be explored.This thesis targets the above issue and further looking for new methods of manipulating the valley degree.To this end,we investigate the effects of surface reconstruction of EuO and EuS on their electronic structures and stability of the(111)surface.Then,we investigate the effect of surface reconstruction and orientation on the valley polarization in MoTe2 monolayer.Based on the understanding of the interface effects,we further propose a new kind of heterostructures,i.e.,MoTe2/CoCl2,and achieve a switchable valley polarization by electric fields only.To summarize,1.We investigate the stability and electronic structure of the reconstructed surface of EuO and EuS(111).We consider five types surface reconstructions,which are octopolar,P(1×2),((?))? and vacancy model,respectively.Our results show that the surface free energy of octopolar and P(1 × 2)remain unchanged with the chemical potential,suggesting that they have a higher stability than the other surfaces.Further electronic structure calculations reveal that the unreconstructed surface is metallic.While the octopolar and P(1 × 2)reconstructed surfaces are semiconducting,consistent with the bulk phase.2.After fully understanding the surface structures of EuO and EuS,we investigate the interface structure of MoTe2/EuO.We study the effects of the surface reconstruction and orientation on the valley polarization in MoTe2.Our results indicate that the value of valley splitting is one order of magnitude smaller than that for the unreconstructed surface.In the case of MoTe2 on top of EuO(001),the valley splitting is about 3.2 meV.We use a low-energy effective Hamiltonian to understand the valley splitting induced by magnetic proximity effect.The estimated effective Zeeman field induced by EuO(001)is about 27 T,which is comparable to the experimental value of WSe2/EuS.Our results are helpful in understanding the differences between theoretical calculations and experiment results.3.Finally,we propose a new way of manipulating the electron valleys in TMD monolayers.In the van der Waals heterostructures of MoTe2/CoCl2,we tune the interaction of the spin polarized states of CoCl2 and the valleys of MoTe2 by electric fields,which can achieve a valley polarization switching and a halfmetal to semiconductor transition.In addition,we construct CoCl2/MoTe2/CoCl2 heterostructures in which one can realize spin as well as valley valves.