Photoluminescence And Raman Spectroscopy Of Defects In Two-dimensional WSe₂ And MoSe₂

The successfully discovery of graphene is drawing enthusiasm on two-dimensional materials.Among two-dimensional?2D?materials,transition metal dichalcogenides?TMDs?are a range of materials that include many kinds of materials attracting great attention of many researchers.Transition metal disulfides of group VI are semiconductors,representing as MoS2,when thinned to monolayer they transfer from indirect bandgap to direct bandgap semiconductors promoting the luminescence efficiency greatly.Moreover,field effect transistors?FETs?based on MoS2 monolayer have been reported to exhibit high mobility.These findings make 2D TMDs to be the very promising materials as the next generation of semiconductor.However,until now the characterization of their quality and defects is still challenging.In this thesis,based on WSe₂ and MoSe₂,we used chemical vapor deposition?CVD?method to prepare samples and studied the defects in them,and provided a convenient spectroscopic method to character their lattice quality and defects.Firstly,monolayer WSe₂ nanosheets were synthesized by CVD method.Photoluminescence?PL?and Raman spectra measurements indicated that there are defects at central area.A further atomic force microscopy?AFM?analysis demonstrated that the defect is arising from a solid-vapor reaction mechanism.During a certain growth stage,nuclei with the composition of WOx Sey do not fully react with the Se vapor,leading to the formation of oxygen defects.This type of defects permits radiative recombination of bound neutral excitons,which can make the emission from defect states as strong as the intrinsic excitation.The controllability of oxygen impurity concentration was realized by tailoring the heat-up process and growth time.This strong and controlable emission peak from defect states at room temperature in monolayer WSe₂ can be used in future application in optoelectronic devices.Secondly,Se deficient MoSe₂ monolayers were obtained through a simple CVD processing.The thermal annealing effect under the H2 environment is the key factor to introduce Se vacancies.The evolution of A1 g peak was explained by the phonon confinement theory and was well simulated by the RWL model.Density functional theory?DFT?calculation was utilized to confirm the type of Se vacancy and to assign this Se vacancy-activated Raman mode?D mode?.The atomic vibration images showed that D mode is an A1g-like localized mode bounded to single Se vacancy which has the same form of Raman tensor with A1 g mode.The results of polarized Raman scaterring experiments had a very good agreement with our DFT calculation results.These findings indicate that Raman spectra could be a powerful tool to define the quality of 2D TMDs.Finally,large area S doped and Te doped MoSe₂ nanosheets were grown by a modified Na Cl assisted CVD method,realizing the tunable bandgap.Combining Raman spectra and DFT calculation,we studied the influence of S doping and Te doping to the vibrational properties of monolayer MoSe₂.We found that S doping has the same effect to A1 g mode with Se vacancy,causing the splitting of A1 g mode,but Te doping dose not.The calculated Raman spectra obtained by DFT showed good agreement with experimental results.The atomic vibration images demonstrated the similar evolutions of the peak postion and shape of A1 g mode when doping S and Te.These findings give foundamental spectroscopic studies and play a significant role in research of TMDs alloys and their future applications espectially in near-infrared optoelectronic devices.