The Synthesis And Optical Characterization Of 2-dimentional Transition Metal Dichalcogenides?TMDs?WS₂ Nanosheets

Inspired by the increasing development of science and novel technology,the sizes and weight of optical and optoelectronic equipment become smaller and smaller.This undoubtedly gives higher technical requirements:the devices to build up the equipment must be smaller and lighter without decreasing the necessary functionality.The achievement about the preparation and investigation of nanomaterials has been providing a prospect way to meet such a great requirement.Comparing with the conventional materials,nanomaterials have higher specific surface area and much smaller size,which can make it possible to integrate the complete functionality into a small unit.Besides the great effort on the studies of nanowires,the discovery of graphene gives another potential resolution for constructing electronic,optical and optoeletronics devices at nanoscale.Since its first preparation by the two scientists in the University of Manchester in 2004,the graphene has been attracting plenty of interest from the world due to their excellent optical,electrical,mechanical and electrochemical properties.However,the pristine graphene does not have a band gap,which strongly limits its application for optics and optoelectronics devices.The chemical modification can give a narrow band gap,but it will expense the other aspects of performance.Thus,new two-dimensional materials with desired properties are demanded.Different form the graphene,transition metal dichalcogenides(MoS2,WS2 and TiS2,etc.)have an appropriate bandgap and is believed to have great potential application for optical and optoelectronic fields.Therefore,the studies on transition metal dichalcogenides(TMDs)are important for scientific researches and future applications.In this thesis,I focus on the synthesis and optical characterization of WS2 nanosheets,which have the strongest photoluminescence(PL)among TMDs.I carefully studied the details of the synthesis,including the nucleation,the controlled growth and the affection from the surroundings.With the optimized parameters,large size and high quality WS2 nanosheets were obtained.Then,the optical characterization was performed on the as-grown nanosheets,including Raman spectra,PL spectra,transient spectra and nonlinear optical properties.The main results of this thesis are summarized as the following:(1)I obtained various 2D nanomaterials by chemical vapor deposition(CVD)method,such as monolayer,vertical multilayer and spiral WS2 nanoplates.On this basis,we further optimized the reaction parameters of CVD growth process and successfully realized the controlled growth of three kinds of structure:?The layer number of the vertical multilayer structures was increased from a few to majority;?The size of the monolayer was ranged from a few micrometers to tens of micrometers;?The morphology of spiral structure was changed from a single spiral to multiple spirals.The exploration of reaction parameter of three structures provided a convenience for the follow-up study.(2)We acquired a systematic understanding of Screw-Dislocation-Driven(SDD)growth which has a tight relationship with screw dislocations during the crystal growth process.The difference of the number of screw dislocations will results in different spiral structures.Due to the difficulty of sample preparation and the structural characterization,we have little understanding of this detailed mechanism up to my knowledge.We obtained a mass of spiral structures with the screw dislocations number range from a few to majority.After a series of atomic force microscope(AFM)analyses,we found that the number and included angle of screw dislocations have a huge influence to the morphology of spiral structures.With increasing screw dislocations,the structure of spiral nanoplates will became more complex.Besides,the included angle of the screw dislocation plays a critical role to the morphology of spiral structures.When the included angle is 0°or 120°,the spiral structure presents a triangular morphology,while the spiral structure presents a hexagonal morphology when the included angle is 60°.The discovery of these various spiral structures will greatly enrich and deepen the cognition and comprehension of screw-dislocation-driven(SDD)growth.(3)Due to the broken symmetry from the spiral structure,2D WS2 spirals will have strong nonlinear optical effect..As-grown spiral nanosheets were then transferred onto a quartz substrate to test their nonlinear optical properties with transmission mode.Strong second harmonic generation(SHG)and third harmonic generation(THG)were found in spiral structure.As layer numbers increase,SHG intensity increases rapidly.The slopes for the plots(output intensity vs incident power density)of SHG and THG are 2 and 3 respectively,which is well identical with the nonlinear optical principle.(4)Furthermore,polarization of SHG and THG output was tested on three kinds of spiral structure.The polarization direction for SHG output has a little shift and the azimuthal angle is about 5 degree compared to monolayer structure.The azimuthal angle is about 5 degree also be found in the TEM analysis between the top layer with bottom layer.Nevertheless,the shift of the azimuthal angle is attributed to the twist between bottom layer and top layer of spiral structure.(5)We have studied the excitation power and temperature dependence of recombination dynamics on chemical vapor deposition grown monolayer WS2 by the combination of Raman,PL and time-resolved PL spectra.We find a red-shift and a parabolic intensity increase of the PL emission of monolayer WS2 with elevating excitation power,and obtain the decay time constants corresponding to the recombination of trions and excitons from transient PL dynamics.We attribute the above nonlinear changes of PL peak positions and intensities to the combination of increasing carrier interaction and band structures renormalization,rather than thermal effect from a laser.In our experiments,no change in peak position of the E2g mode with the increase of excitation powers indicates that the in-plane lattice of the sample has no change during the radiation of the laser,which rules out the heating effect from the laser during our experiments.