Tuning Optical And Electrical Properties Of TMDs Homo/heterostructure Through Defects

Two-dimensional transition-metal dichalcogenides semiconductors(TMDs)becomes another star material after the discovery of graphene,which has attracted much attention from researchers in recent years.The band gap of the TMDs is controlled by the thickness,and it is direct transition in momentum space for the monolayer.The thickness of monolayer TMDs is only 0.7nm.The thickness induced effects of strong quantum confinement and weak dielectric shielding make the monolayer TMDs having huge exciton binding energy and higher fluorescence efficiency.Therefore,the TMDs offers an ideal platform for studying many-body effects.Due to the strong spin-orbit coupling interaction and central inversion symmetry breaking,the exciton spin and the valley pseudo-spin has been coupled each other in TMDs,leading to a special optical selection rule for the the valley pseudospine of monolayer TMDs,which can be initialized and controlled through circularly polarized light.In this context,two-dimensional TMDs materials provide a great application prospects for spintronic devices and spin-based quantum devices.However,owing to the strong optical dipole and electron-hole exchange interaction,the lifetimes of exciton and valley polarization of TMDs are on the order of picoseconds,which seriously restricts their practical application in the fields of optoelectronics and spintronics.Recently,it has been found that the interlayer free excitons in the heterojunction can have a long lifetime and a significant Zeeman splitting.The similar bevavior has also been observed for the defect-localized excitons,but the investigation is just in its infancy.In this dissertation,we will explore the defect effects on the optical and electrical properties of the WSez homojunction,the lifetime of valley polarization for the defect-localized exciton at weak magnetic field,and the polarization inversion of interlayer defect-localized excitons under near-resonance excitation in WSez-WSz heterostructure.The contents of the dissertation are summarized as follows:In chapter one,we first described briefly the lattice structure,energy band and unique valleytronics of the two-dimensional TMDs.Subsequently,we introduced the preparation methods of homojunction and heterojunction of two-dimensional TMDs and their research advances in optics,electricity and spintronics.Then the basic knowledge,advantage and progress of defects in two-dimensional TMDs were analyzed.Finally,we presented the significance and main results of our work.In chapter two,we successfully prepared a triangular inkslab-like morphology WSe2 homojunction by controlling the rapid rise and fall temperature of the physical vapor deposition(PVD)during the growth.The measurement of AFM reveals that the homojunction is monolayer in the inside and multilayers at the border.By using the characterizations of Raman and SAED,the monolayer WSe2 in the homojunction is found having a high crystal quality,while a great of defects exists in the frame areas.Moreover,it is confirmed by KPFM that the border has a charge doping effect on the internal monolayer,leading to the charge redistribution and various exciton species in the internal monolayer.The results demonstrate that the joint effects of the defects and the thickness on the energy band of WSe2 makes an effective built-in electric field in the connection region between the monolayer and multilayer.This built-in electric field in the inkslab-like homojunction can be fiurther confirmed by the scanning photocurrent mapping and the behavior of significant rectification.In chapter three,we first prepared the monolayer WSe2 and WS2 samples by chemical vapor deposition(CVD)method,and then stacked them successionally to form the WSe2-WS2 heterobilayer in AB configuration through the home-made transfer platform with wet transfer technique.By comparing the PL of the monolayer WSe2 and WS2 at low temperature of 10K,we found that the PL of monolayer WSe2 is dominantly originated from the defect-localized excitons.Simultaneously,it was found that both of the PL spectra of the monolayer WSe2 and the WSe2-WS2 heterojunction become saturation gradually as the increase of the excitation power,and have a similar transition temperature at?120K,indicating that the above characteristics come from the defect-localized excitons.The measurments of time-resolved PL for the monolayer WSe2 and WSe2-WS2 heterojunctions demonstrated that their dynamic features are consisted of two processes,the fast process and slow one.The former is derived from the free excitons,and the latyer is derived from the defect-localized excitons.More importantly,the lifetime of slow process can be as long as about l?s.Additionally,we also prefromed the investigations on the defect-localized excitons under the the external magnetic field.It was found that the valley polarization of the interlayer defect-localized excitons exhibits unusual magnetic sensitivity:it is independent of the directions of the magnetic field and shows a "?"shape tendency with the magnetic field.On the contrast,the valley polarization of the intralayer defect-localized excitons in the WSe2 depends on the direction of the magnetic field,and shows an "X" shape relationship with the magnetic field.The above phenomena were further investigated and confirmed by the time-resolved valley polarization spectrum and the exciton transition dynamics under the control of the out-of-plane magnetic field.In chapter four,the valley polarization of the interlayer defect-localized excitons in WSe2-WS2 heterojunctions for the AB and AA configurations were investigated under the near-resonance excitation.It is found that for the WSe2-WS2 heterojunction of AB alignment,when the excitation wavelength is less than 800 nm,the valley polarization of the interlayer defect-localized excitons is magnetically sensitive and exhibits a negative value,and shows a "?"shape change with the weak magnetic field.When the excitation wavelength is 800 nm,the magnetic sensitivity of the valley polarization for interlayer defect-localized excitons disappears,and the polarization changes with the stronger magnetic field as an,"X" shape.When the excitation wavelength is greater than 800 nm,the magnetic sensitivity of the valley polarization for interlayer defect-localized excitons recovers,however,the polarization shows a positive value and presents a "V" shape with the weak magnetic field.The polarization flip with the excitation wavelength for the interlayer defect-localized excitons in the AB heterojunction was also observed by time-resolved PL spectra.For the WSe2-WS2 heterojunction in the AA configuration,when the excitation wavelength is less than 800 nm,the interlayer defect-localized excitons exhibits a positive polarization and very weak magnetic sensitivity,which behaviors is opposite to that for the AB configuration heterojunction.When the excitation wavelength is higher than 800 nm,the polarization of interlayer defect-localized excitons shows "X" shape with the stronger magnetic field,which is also contrast to that for the AB configuration.In chapter five,we summarized the major issues and challenges,and presented the prospects in the defect engineering for two-dimensional TMDs materials,especially heterojunction.