Controllable Synthesis And Physical Properties Of Two-dimensional MoTe₂ Flakes

Earlier research on two-dimensional materials were mainly focused on graphene,and its excellent thermal conductivity and high carrier mobility endows its widely application in electrical,optical,electrochemical and biomedical fields.Besides graphene,there are various two-dimensional materials.Transition metal dichalcogenides?TMDs?are graphene-like materials.Among them,molybdenum ditelluride?MoTe₂?is one of the most unique transition metal dichalcogenides and can be stable in2H,Td and 1T'phases.2H-MoTe₂ has attracted lots of attentions due to smaller band gap?1.1eV?and high electron mobility(2500cm²·V^-1·S^-1).More interesting,1T'-MoTe₂ has a high electron mobility up to 4000 cm²·V^-1·S^-1 and its forbidden band width is 60 meV.Furthermore,the magnetic resistance of 1T'-MoTe₂ can reach 16000%under 14T magnetic field.In addition,theoretical calculations predict that single layer 1T'-MoTe₂ is a 2D topological insulator and a type II Weyl semimetal.MoTe₂ has a very broad application prospect due to these unique properties.However,there are still many problems in the controllable synthesis of MoTe₂ film.In this paper,we achieve1T'-MoTe₂ by chemical vapor deposition?CVD?.Not only the reaction mechanism and effects of various parameters on the morphology of MoTe₂ nanosheet but also the crystal quality and surface potential of MoTe₂ nanosheets were studied.The main contents of this paper are as follows:Firstly,MoTe₂ nanosheets were controllably prepared on SiO₂?300nm?/Si substrate by CVD method under atmospheric pressure.MoO₃ and Te are served as precursors during growth.The influence of the amount of Te precursor,the proportion of hydrogen in the carrier gas and the growth time on the morphology of MoTe₂ nanosheet were analyzed.It is found that the deuteration reaction is difficult to carry out thoroughly to obtain pure MoTe₂ film.Further,by adding NaCl as a kind of reaction reagent in the precursors,we found NaCl can reduce the evaporation temperature of MoO₃and greatly increase the concentration of Mo precursor.Two different morphologies of two dimensional?2D?1T'-MoTe₂ film:rectangular MoTe₂ and irregular MoTe₂ were successfully prepared.Secondly,we analyzed the morphology and surface friction of rectangular MoTe₂ and irregular MoTe₂ film by atomic force microscopy.Our results show that topography of rectangular MoTe₂nanosheets have more uniform thickness while irregular nanosheets have poorer crystallization quality with more grain boundaries.Moreover,the surface friction decreases with the increasing thickness of 2D MoTe₂,owing to the gradually decreased surface corrugation of nanosheets as the thickness increase.Finally,the MoTe₂ film was characterized by Kelvin probe force microscopy.The relationship between the thickness and surface potential of the two morphology 1T'-MoTe₂ films was analyzed.As the thickness of the 1T'-MoTe₂ film increases,electrons transfered from the tip in to MoTe₂ film,leading to a higher Fermi level,and the surface potential is also increased.At the same time,as the thickness increases,the electronic density of the film also increases,the transfer of the same amount of electrons causes a smaller shift in the Fermi level in the 1T'-MoTe₂ film.Thus,the greater the thickness,the smaller the magnitude of the surface potential change of the film.However,for the MoTe₂ film with irregular morphology,the thickness and the number of layers are not uniform.The change of surface potential and thickness of the irregular nanosheets do not show a regularity.