Research On Defects In Two-dimensional Transition Metal Dichalcogenides

In recent years,transition metal dichalcogenides(TMDCs)have been a hot spot in condensed matter physics research^[1-3].TMDCs are a kind of layered materials.Two-dimensional TMDCs have wonderful physical properties and can be widely used in energy conversion and collection.They have broad application prospects in future electronic and optoelectronic devices.Semiconductor TMDCs with a thickness of only a few atoms exhibit some novel physical phenomena due to quantum confinement effects.For example,atomic-thick molybdenum disulfide(MoS₂)has a significant electrocatalytic effect on reduction-oxidation reactions^[4].Spin polarization and valley polarization of electrons in monolayer MoS₂^[5].The energy gap in the molybdenum telluride(MoTe₂)band structure opens with the thickness of the layer decreases^[6],etc^[7].The defects in two-dimensional TMDCs have a very significant impact on the properties of the material itself.We can manipulate the properties of the material by studying and understanding the electronic structure and properties of defects.In this article,we take molybdenum diselenide(MoSe₂) and tungsten diselenide(WSe₂)as the research objects.MoSe₂ and WSe₂ films with many defects on the surface were successfully prepared by using molecular beam epitaxy(MBE).We characterize and measure our sample by using scanning tunneling microscopy(STM)and scanning tunneling spectroscopy(STS).The main results of our paper are as follow:As a two-dimensional TMDCs have a high surface area to volume ratio,making the two-dimensional TMDCs material very sensitive to the changes in the external environment,and the inertness of the van der Waals material surface will enhance defect states and edge states in the two-dimensional TMDCs materials.Therefore,the point defects and line defects in the ultrathin TMDCs film will have a very strong impact on the electronic properties,optical properties and catalytic properties of the material itself.In this paper,MoSe₂ are selected as the research objects.We grow MoSe₂ and WSe₂ thin films by the MBE method,and use STM to characterize the structure and electronic properties of surface defects.The results show that there are a series of equal spacing peaks in the STS spectrum of defects in the second layer of MoSe₂,but the energy separation between the first and second levels is twice that of other adjacent energy levels.The series of peaks with equal energy spacing is considered to be vibronic states generated by the non-radiative relaxation to the ground states of the excited local electrons in defect.The second energy level of the vibronic states has a missing phenomenon.We believe that the reason for the absence of the second level of the vibrational state is the spin-orbit coupling effect which causes the excited electron to transition from the excited singlet state to excited triplet state through the inter-system crossing resulting in the electron tunneling blocked through the second energy level.Through the spin reversal inelastic electron tunneling spectrum,we found that the ground states of the four defects are all single spin,which indicates that they are all magnetic.If we can prove that phosphorescence is the reason for the lack of the vibronic level,this can provide a theoretical basis for manipulating them and further promote their applications.The inherent two-dimensional magnetic materials are very rare in nature.Most of the two-dimensional magnetic materials have been discovered exhibit instability in the atmosphere,and these factors limit the in-depth research and application of two-dimensional magnetism.Therefore,it is particularly important to find two-dimensional materials that can generate magnetism in the atmosphere.According to theoretical calculations,Pt vacancies in PtSe₂ have a local magnetic moment.In the monolayer PtSe₂,the Pt vacancy is an antiferromagnetic defect with a magnetic moment about 4.66?_B.In the thick layer of PtSe₂,the Pt vacancy of the surface PtSe₂ also has a magnetic moment.In this paper,the MBE method was used to grow 1-5 layers of PtSe₂ thin films.We found the transition of PtSe₂ from an insulator to a metal as the layer thickness increased,And STM was used to find defects that may belong to Pt vacancies.In subsequent experiments,we can detect the local magnetic moment of the Pt vacancy through the inelastic electron tunneling spectrum of the extremely low temperature STM,and can also measure the Kondo effect caused by Pt vacancies on thick PtSe₂.If the local magnetic moment can be detected in the Pt vacancy,the growth of PtSe₂ using the MBE method will become a means to obtain magnetism from non-magnetic materials,which will open up a new way to control the magnetism in TMDCs materials.