Investigation On Phase Transition Characteristics Of Two-Dimensional Material MoTe₂

Following graphene,two-dimensional graphene-like materials such as black phosphorus,two-dimensional transition metal chalcogenides（TMDCs）,and hexagonal BN have also attracted widespread attention in academic field all around the world.TMDCs are highly valued for their naturally controllable band gap,high carrier mobility and tunable phase transition.Among them,MoTe₂ has a huge application prospect in the field of phase engineering because the energy difference between the semiconductor phase（2H phase）and the semi-metal phase（1T’phase）of MoTe₂ is only 35 meV,which is much smaller than other similar two-dimensional materials.Previous studies have shown that high temperature annealing,laser burning,and regulating cooling temperature rate during CVD growth can achieve phase transition of MoTe₂,but these methods are relatively complicated,easy to damage the sample,and difficult to achieve uniform control over a large area.Therefore,it is extremely urgent to seek a phase transition method that is simple and non-destructible and can realize large-scale regulation.In view of this,this thesis systematically studies several phase transition methods and their corresponding mechanisms.Firstly,the phase transition mechanism based on laser burning method is analyzed by investigating the Raman spectroscopy of MoTe₂ with the variation of laser power,and the optimal power for the near-lossless phase transition of MoTe₂is determined.On this basis,the influence of substrate thermal conductivity on phase transition is further studied,and the relationship between thermal conductivity and phase transition power of different substrates is obtained.Finally,a simple and non-destructive mild hydrogen plasma technology is proposed to realize controllable and uniform phase transition of MoTe₂ with different layers.The optical and electrical properties of MoTe₂ before and after phase transition are systematically studied and the mechanism of phase transition is analyzed.The main research contents of the thesis are summarized as follows:1.The Raman spectroscopy of MoTe₂ with the variation of laser power is studied,and the phase transition mechanism based on laser burning method is discussed.It is found that in the laser irradiation region,the heat accumulated by the laser on the surface of the sample causes MoTe₂ to form a large number of defects that cause the change of its lattice structure,which is the primary cause of the phase transition.Then,the effects of four substrates（SiO₂/Si,Si,GaAs and PDMS）with different thermal conductivities on the optimal laser power of phase transition is systematically studied.The optimal laser power required for the phase transition is proportional to the substrate thermal conductivity.The PDMS substrate with the lowest thermal conductivity of 0.35 W/MK requires the lowest laser power of 2 mW to complete the phase transition.Then,based on low-power laser burning,the phase transition of MoTe₂ with a controlled micro-region is realized on PDMS substrate.Finally,it is proposed that the phase transition of MoTe₂ has a surface Raman enhancement effect on R6G,and its mechanism is explained.2.For the first time,the controllable conversion between 2H and 1T’phase of MoTe₂with different layers is achieved by using mild hydrogen plasma technology combined with high temperature annealing.The phase transition degree in different treatment stages is characterized by Raman spectroscopy.It is found that the half-widths of both E¹_2gand A_g Raman peaks become broaden when the phase transition takes place,and the peak intensities show a decreasing trend.In addition,the characterization results of both optical microscopy and atomic force microscopy show that the sample are not etched and destroyed during the plasma treatment.By comparing the oxygen and argon plasma treatment results,we attribute the lossless phase transition to the fact that the hydrogen plasma with a relatively smaller energy can ensure the slip of the upper Te atom layer in the MoTe₂ junction during the process without introducing additional defects.Moreover,the method can achieve reversible regulation of phase transition in combination with high temperature annealing at 300℃,and thus has incomparable advantages over other phase transformation methods.