Preparation And Optical Properties Of The Transition Metal Doped ZnO Nanostructures

ZnO nanostructure with a direct wide band gap and a large exciton binding energy isconsidered to be one of the most important semiconductor materials for applications inoptoelectronics. It is the ideal base to build photoelectronic devices. Intentional introductionof impurities into a semiconductor is an effective approach to control its energy levels andphysical properties. In this paper, transition metal doped ZnO nanostructures were synthesizedvia different routes, several characterization methods were adopted to study their structuraland optical properties. The major results are listed as follows:1. Zn_1-xCo_xO nanocrystals were synthesized through a simple solution route followed bya calcining process. The analysis of XRD and Raman spectra reveals that a small quantity ofCo ions were incorporated into ZnO lattice structure, whereas the secondary phase of Co₃O₄was segregated at high Co doping concentrations, the solid solubility of Co ions in ZnOnanocrystals could be lower than 0.05. Three additional absorption peaks are observed at 566nm, 609 nm and 654 nm for Co-doped ZnO nanocrystals, which are related to d–d transitionsof Co²⁺in a tetrahedral crystal field and can be attributed to the4A2（F）-2E（G）,4A2（F）-4T1（P）,and4A2（F）-4A1（G）, respectively. The emission peaks centered at 685 nm were observed in PLspectra, indicating that the incorporation of Co²⁺introduced impurity levels into ZnO.2. Zn1 xCuxO nanostructures were synthesized via solvothermal route. The effects ofdoping concentration and reaction temperature on the structural, morphological and opticalproperties of ZnO were investigated. Segregation of CuO phase was observed in Ramanspectra at high doping concentrations. The absorption speatra of Cu doped ZnO shows broadabsorption peak in visible region. The PL speatra of all samples display two emission peaks,which include a near band-edge emission and a defect emission. Blueshifts of these two peakswere observed after Cu doping, which may be related to changes in lattice stress and Cudoping, respectively. For samples with the same doping concentration, the higher reactiontemperature helped to improve the crystal quality. CuO related phase was segregated when thetemperature was higher than 120oC. The only major difference in the PL spectra of dopedsamples obtained in different temperature is the peak intensity. It may be related to segregationof Cu in ZnO.3. Zn_1-xCu_xO/ZnO core/shell nanocrystals were synthesized by water bath method. Thesubsequent additions of Zn²⁺and OH-precursors form ZnO shell outside ZnO:Cu core. Thesegregation of CuO phase is detected in Raman speatra of the doped sample without ZnOshell. It is suggested that some Cu atoms can be segregated from ZnO nanocrystals and theseparated Cu ions can be incorporated inside ZnO shell after the growth of ZnO shell. The PLanalysis indicates that core/shell structure helps to eliminate the surface-related emission.