Studies On The Photoluminescence And Dynamic Process Of Monolayer MoS₂

Two-dimensional nanomaterials have attracted widespread attentions in the field of microelectronics because of its unique mechanical, optical and electromagnetism characteristics. Graphene, molybdenum disulfide(MoS2) and black phosphate are typical two-dimensional nanomaterials. The application of graphene in large scale integrated circuit is limited by its forbidden band problems. Single-layer MoS2 has an �direct bandgap� electronic structure which is different from the "zero band gap" electronic structure of graphene. This character gives it excellent optical and electrical properties and therefore has drawn wide attentions. The optical properties such like photoluminescence, fluorescence lifetime and the excitation dynamics of monolayer MoS2 are closely related to sample's layer thickness, substrate environment and preparation methods. We have done systematic and deep researches on the optical properties of the sample by means of photoluminescence, fluorescence lifetime and time-resolved measurements.The main work of this thesis includes two major parts. First, a monochromatic/ dichromatic pump-probe measurement system and a supercontinuum pump-probe measurement system were established to research the excitation dynamics of the sample. Second, the photoluminescence spectra and fluorescence lifetime of the monolayer MoS2 which was grown on silicon/silicon dioxide(Si/SiO2) substrates by Chemical vapor deposition(CVD) method as well as the photoluminescence and time-resolved transient absorption spectra of the monolayer MoS2 on sapphire(Al2O3) substrates have been measured and investigated by the photoluminescence and fluorescence lifetime measurement system and the established supercontinuum pump-probe measurement system. The main achievements are as follows:1. A monochromatic/dichromatic pump-probe measurement system has been successfully established. This system includes controlling section, optical circuit section and data collection section. The automation of the whole system is realized with the help of a self-composed Labview program. A lock-in amplifier and a chopper are used in the collection of signals in order to improve the signal-to-noise ratio of the systems. This system has been adopted to the research of transfer processes in dye-sensitized solar cells.2. A supercontinuum pump-probe measurement system has been set up. This system contains controlling section, optical circuit section and data collection section. The automation of the entire systems is implemented by a self-composed Labview program, and the time-resolved transient absorption spectra are collected by a spectrometer.3. The photoluminescent property and fluorescence lifetime of monolayer MoS2 on silicon/silicon dioxide substrate are measured and studied by the photoluminescence and fluorescence lifetime measurement system. The photoluminescence spectra of the sample show that there are two photoluminescent peaks corresponding to the composite transitions of A excition and B excition, respectively. As the temperature rises, the photoluminescent spectra of the sample change conforming to the evolution of general semiconductors. The fluorescence lifetime of the sample is about 20 ps and is independent on temperature. The results demonstrate that the monolayer MoS2 on silicon/silicon dioxide substrate prepared by CVD method has large film area, high quality, and stable optical properties. These characters help monolayer MoS2 materials find potential applications in optoelectronic devices.4. The photoluminescence and femtosecond time-resolved transient absorption properties of monolayer MoS2 sample on sapphire(Al2O3) substrate have been measured and studied respectively by the photoluminescence and fluorescence lifetime measurement system and the supercontinuum pump-probe measurement system, respectively. The results show that only one clear photoluminescent peak can been observed from the Al2O3 substrate, while there are two obvious photoluminescent peaks for the monolayer MoS2 sample. Time-resolved measurements indicate that, there is a strong coupling between the MoS2 layer and the Al2O3 substrate and an efficient energy transfer would occur between the MoS2 layer and the substrate.