Study On Structure, Defects And Photoluminance Properties Of ZnO Quantum Dots

As a kind of II-VI compound semiconductor, Zinc oxide with wurtzite structure and 3.3 eV band-gap has excellent physical propertiy and easily achieves integration in photoelectric devices.In this paper, we summarize the characteristic and microstructure briefly, review preparation methods and progress of defect controlling of ZnO quantum dots. Subsequently, the controlled synthesis of ZnO quantum dots and its surface defect have been studied in detail. Our main work focuses on the factors of controlling the size of ZnO quantum dots in solution phase. Base on the photoluminescence (PL) spectrum, we study the influence of suface modification on the defect of ZnO quantum dots and doping of metal or lanthanon ion on ZnO quantum dots.The results of the main results are listed as follows:1. Discussion of the size controlled synthesis and growth kinetics of ZnO quantum dots: Size controlled synthesis of quantum dots is foundation of physics characteristic research, which has important influence on the application of optoelectronic devices. We discuss [0]the reaction temperature, pH value combined with the concentration of precursors on the size and fluorescence spectrum of ZnO quantum dots. Under optimized pH around 9 and the concentration of precursors at 0.02[0] M, the grain sized between 3 nm-6 nm of ZnO quantum dots are obtained by changing temperture and duration time. We mainly discuss the nucleation, growth and coarsening stages during the formation process of nanoparticle. The nucleation is nearly in identical duration at different temperatures. Growth process is the most sensitive to temperature upon three processes and the relatively gentle incline rate for coarsening process is well explained by Lifshitz-Slyozov-Wagner (LSW) model. By photoluminescence (PL) spectra of quantum dots at different temperatures and reactive time, we come to a conclusion that ultraviolet emission is mainly due to exciton recombination, and the origination of visible emission comes from the transition between the bottom of the conduction band and the defect band formed by oxygen.2. Surface defect controlling of the ZnO quantum dots: There has been a controversy on the origin of defects related luminescent for ZnO quantum dots. In this paper, we try to modify the surface of ZnO quantum dots to improve the surface defect and fluorescence intensity in ultraviolet range. CuO was first coated to form ZnO/CuO core-shell and enhance luminescent intensity of ultraviolet (UV) band with maximum at l: 2 of molar ratio of CuO: ZnO. We owe it to vanish of suspension bond by addition of hydrolytic CuO. By contraries, the intensity of green range is restrained. To achieve effective energy transfer, we prepare ZnO/ZnS core-shell by Na2S and TAA as precursors. The luminescent intensity of visible range decreases obviously because surface defects are clothed. In order to obtain water-soluble ZnO quantum dots, mercaptoacetic acid(MPA)is chosen to cover surface of ZnO quantum dots. The peak of UV band reaches maximum when the molar ratio of ZnO: MPA is l: 1 and then turns to degressive trend. This is because electrons transfer from conduction band to the LUMO of MPA through the nonradiative transition by excessive MPA added.[0] metal Au thin film is sputtered on ZnO quantum dots film. The fluorescence intensity of UV region reachs maximum after 10s deposition of Au and then droped as deposition duration prolonged, which is caused by density of Au particle deposit on sruface increase as deposition time prolong, so lattice tends to perfect and the luminous intensity reaches maximum.3.The luminescence spectrum of doped ZnO quantum dots[0]:Cu and lanthanide of rare earth Eu³⁺ doped ZnO quantum dots are prepared and related fluorescence characteristics are discussed. Under 320 nm excitation, the luminescent spectrum of Cu-doped ZnO quantum dots reaches maximal peaked at 490 nm for 0.2 % doping. Here is also discussion on Eu doping ZnO with energy band structure.