Spectroscopic Investigation Of Defects And Defect Engineering In Two-Dimensional Materials

Two-dimensional(2D)materials(e.g.graphene and transition metal chalcogenides)with unique optical and electronic properties,hold great potential for applications of future nanoelectronics and optoelectronic devices.However,different types of structural defects could present in 2D materials and have strong influence on their properties.Optical spectroscopic techniques,e.g.Raman and photoluminescence(PL)spectroscopy,have been widely used for defect characterization in 2D materials due to their time efficient and sensitive characteristics.In this thesis,Raman and PL spectroscopy are employed to investigate the defects in WSe2 introduced by electron beam and Ar+plasma irradiation and surface modification of graphene introduced by adsorbed chemical solvents.Based on characterization and analysis of PL at low temperature and Raman spectra,we realize the control of the quantification and type of defects,so as to further achieve the modulation of the electrical,optical and thermal performance of 2D materials.The main achievements are summarized as follows:1.The influence of electron beam irradiation on optical and electrical performance of WSe2 monolayer has been studied.(1)Defects are introduced by electron beam irradiation in WSe2 monolayer.Low temperature PL spectra of WSe2 presents a clear defect-induced PL emission due to excitons bound to defects.The PL intensity of bound exciton is vanished with increasing temperature,which is understandable since excitons are not tightly bound to defects.(2)We provide an optical spectroscopic characterization approach to correlate the number of structural defects and the electrical performance of WSe2 devices.It is found that the PL intensity of bound exciton increases monotonically with increasing electron dosage.Electrical performance test shows that mobility decays dramatically with the increase of electron irradiation density,which is attributed to carriers scattered by defects introduced by electron beam irradiation.(3)By adopting an e-beam-free transfer-electrode technique,we are able to prepare backgated WSe2 device with high mobility because of avoiding damage to 2D materials during electron beam lithography.2.The influence of Ar+ plasma treatment on optical performance of WSe2 monolayer has been studied.(1)Two defect-activated PL emission peaks are emerging in the low temperature PL spectra of WSe2 monolayer treated with Ar+ plasma.These emissions are attributed to the recombination of excitons bound to different types of structural defects.The shallow level emission originates from the recombination of excitons at chalcogen vacancies,while the deep level emission might arise from other types of defects,such as transition metal vacancies,cluster of vacancies,rotational defects,or antisite defects.(2)We compare PL spectra of sample treated by Ar+ plasma with sample treated by electron beam.The shallow level emission presents after short time electron irradiation,which corresponds to the formation of selenium vacancies.With the increase of electron irradiation dosage,deep level emission also can be detected.Through this work,the optical and electrical performance of WSe2 monolayer can be modulated by controlling the type of introduced defects.3.We study the influence of commonly used etching solvents during the transfer process,i.e.ammonium persulfate,ferric chloride,and ferric nitrate,on the properties of graphene grown on the copper suface by Raman spectroscopy.(1)The results of Raman spectroscopy show that residue iron compounds are presented on the surface of graphene during the transfer process,resulting in p-doping and reduction of phonon lifetime of graphene(2)The non-contact optothermal Raman technique is employed to measure the thermal conductivity.A great reduction of thermal conductivity of graphene is introduced due to reduction of phonon lifetime for graphene transferred by ferric chloride and ferric nitrate as compared to that transferred by ammonium persulfate,which illustrates that defects on the surface of graphene have great influence on the thermal conductivity.