External Control Of The Physical Properties Of Two-dimensional Transition Metal Chalcogenides

Since the discovery of graphene,two-dimensional layered materials have regained widespread attention.Among the many layered materials,transition metal chalcogenides have become a research hotspot due to their various electronic properties such as insulation,metallicity,and superconductivity.And because of its band gap characteristics,it makes up for the deficiencies of graphene,thus becoming an excellent candidate material for the optoelectronic devices.Under the background of the era when portable electronic devices have become mainstream,we need to integrate more highperformance electronic devices in a smaller area,so people are no longer satisfied with the single nature of materials.Although there are many diverse materials in the transition metal chalcogenide family,the band gap structure of each specific material is certain.Therefore,if we want to achieve a variety of electronic properties on the same material,we need to introduce various control methods.In this thesis,we combine electric double-layer control methods,electrochemical control methods,and highpressure technology to control two-dimensional layered transition metal chalcogenides,in order to achieve different electronic properties on the same material.At the same time,we combined Raman spectroscopy,photoluminescence spectroscopy,electrical transport measurement to study the properties of two-dimensional layered transition metal chalcogenides under control.The specific research content is as follows:1.Firstly,we studied the evolution of interlayer coupling on Mo S2 with gate voltage.We conducted a systematic study on the evolution of both low-frequency Raman mode and high-frequency Raman mode with thickness of 2H-Mo S2 and 3R-Mo S2.We found that the high-frequency Raman mode of the two stacking sequence of Mo S2 show the same evolution trend with the increasing thickness,while the evolution of the low-frequency Raman mode with thickness show a dependence on the stacking sequence.Through the discussion of the low-frequency Raman mode,we also draw the conclusion that the interlayer coupling of 3R-Mo S2 is smaller than that of 2H-Mo S2.In addition,we also experimentally combined the electric doublelayer technology to realize the control of the low-frequency Raman vibration mode of Mo S2 for the first time.Through electric double-layer technology,we found that the increase of electronic doping level will lead to the enhancement of the interlayer coupling in the double-layer Mo S2.We also explained the experimental results theoretically through first-principles calculations.In terms of carrier concentration,the gate voltage control can increase the electron doping level.The results of firstprinciples calculations show that the electron cloud of carriers doped in Mo S2 begins to diffuse and distribute in the Van der Waals gap.As the level of electron doping increases,the electron cloud of one layer even tends to overlap with the electron cloud of the adjacent layer,which means that the interaction between two adjacent layers is stronger and the stack is tighter.From the perspective of electric field,the interlayer lattice parameters gradually decrease with the increasing electric field,which in turn reduces the interlayer coupling.2.Secondly,we studied the spectra of monolayer transition metal chalcogenides and their homojunctions and heterojunctions under hydrostatic pressure.In the study of the high-pressure photoluminescence spectra of monolayer Mo S2,WS2,MoSe₂,and WSe2,we found that the energy and intensity of the photoluminescence peaks of these monolayer materials do not show a linear trend with increasing pressure,and this phenomenon can be passed through K-? crossover transition in the conduction band to explain.We also studied the high-pressure Raman spectrum of the Mo S2 homojunction,and the results show that the 22)2)1 and 12)2)modes of the Mo S2 homojunction change with pressure faster than the monolayer and bilayer Mo S2.We also studied the high-pressure photoluminescence spectra of heterojunctions of transition metal chalcogenides.We took the MoSe₂/WSe2 heterojunction as the research object and found that the layer coupling of the MoSe₂/WSe2 heterojunction increases as the pressure increasing.In addition,we found that it is difficult to distinguish and identify photoluminescence peaks in a low-temperature and highpressure environment.Therefore,we pioneered the combination of low-temperature and high-pressure technology with electrical control to achieve electrical control of samples in a high-pressure environment.We change the carrier concentration in the sample by gate voltage to control the relative intensity between the exciton and trion,thereby realizing the distinction between the exciton and the trion.3.Thirdly,by using the electric double-layer gating technology,we explore the superconductivity of the noncentrosymmetric 3R-Mo S2 and the twisted bilayer Mo S2.Since we cannot control the fragmentation of the ionic liquid in the lowtemperature,we turn the control method into electrochemical doping.In our experiment,we did not observe the superconductivity of 3R-Mo S2.In addition,we also performed electrochemical doping on the bilayer Mo S2 with 56.5° twist angle,and observed a superconductivity trend at a carrier concentration of the order of 1014.However,we still have not observed the zero resistance in the twisted bilayer Mo S2.We believe that this situation may be due to the lowest temperature we can reach,which is not enough to make the twisted bilayer Mo S2 achieve zero resistance;or it may be caused by the uneven doping of lithium ions during the electrochemical doping process.