The Study Of Defect Photoluminenscence And Its Quantum Yield In Two-Dimensition Tmdcs

Two-dimensional transition metal sulfides（TMDCs）are a class of newly developed layered semiconductor materials.They exhibit direct band gap,high exciton binding energy when their thickness is scaled down to a monolayer,providing an ideal platform for the study of light-emitting devices.Generally,defects are inevitable in atomically-thin TMDCs and the effects of these defects can be either detrimental or beneficial.Therefore,it is necessary to deeply understand the influence of defects for the PL modulation.In addition,the defect-related PL emission of TMDCs gives rise to some intriguing application of TMDC materials.In some traditional semiconductors,such as zinc oxide（ZnO）and gallium nitride（GaN）,the emission generated by defects can exist stably at room temperature and its quantum yield is equivalent to that of intrinsic PL emission.This property effectively promotes their applications in broadband light-emitting devices,single photon emitters and detectors.However,for monolayer TMDC materials,defect-related PL can only be detected at low temperature and usually accompanied by the overall PL decreasing and lower quantum yield,which highly limit their application.Thus,to deeply study the mechanism of defect induced PL emission and realize the modification of defect emission at room temperature,following works have been done:In Chapter 1,we first briefly reviewed the structure and common defects in TMDCs.Then the effects of defects on the PL emission are summarized from three aspects:defect-related radiative/nonradiative recombination mechanisms and the bandgap modification by defects.In Chapter 2,the design principle,optimization and application of micro-spectroscopy system are described in detail,which is mainly utilized for the measurement of PL spectra and quantum yield.After optimization,the PL collection efficiency of the system is equivalent to that of the commercial system.To meet the requirement for quantum yield measurement,an ideal diffuse reflector is used to compensate the PL collection loss caused by the receiving angle of the objective lens.Meanwhile,the response error of the spectrometer and the random error of diffuse reflector are calibrated to improve the test accuracy.In Chapter 3,the stable defect induced PL emission at room temperature was obtained in monolayer MoS₂ by mild argon plasma irradiation.This defect-related PL emission exhibits higher quantum yield than intrinsic PL emission at room temperature,which can be tuned by changing the irradiation time.This phenomenon may come from the sulfur vacancy defect and its interaction with gas molecules in the air.The origin of this strong defect emission at room temperature mainly includes two parts:firstly,the charge transfer caused by gas molecule inhibits some nonradiative recombination channels;Secondly,due to the high defect density of states,there are still a large number of bound excitons can radiatively recombine even if there is thermal dissociation during the recombination.