Study On Preparation And Photoluminescence Properties Of ZnS And ZnO Nanomaterials

As wide band-gap semiconductor materials, zinc sulfide (ZnS) and Zinc oxide (ZnO) nanoparticles have attracted considerable interest because of their quantum confinement effects, size effects, and superior luminescence characteristics. In the last two decades, ZnS and ZnO nanoparticles have been found broad applications in many different technological areas, including photo-catalysis, sensors, transparent electrodes, fluorescent probes, light-emitting diodes, solar cells, and lasers. Moreover, the electrical, optical, and magnetic properties of the wide band-gap nano-semiconductors can be conveniently adjusted by impurity doping for different applications. One of the important properties of ZnS and ZnO nanoparticles is the luminescence property. The investigation on the luminescent properties of ZnX (X=S and O) and ZnX:M (M is the doping ion) is significative for both scientific research and practical applications. So, the preparation and photoluminescence properties of ZnS and ZnO nano-luminescence materials were studied in this thesis.In this thesis, the following work has been finished:(1) ZnS nanoparticles were successfully prepared using a facile chemical solution method and solid state reaction method at low temperature, respectively. The size, the crystal structure and luminescent properties of ZnS nanoparticles were characterized by X-ray diffraction (XRD), transmission electronic microscope (TEM) and scanning electron microscopy (SEM). The result showed that the product had a cubic crystal structure and the average crystallite size was6.40and9.48nm, respectively. As compared with chemical solution method, ZnS nanoparticles, which were obtained by solid state reaction method, had better crystallinity and higher emission intensity.(2) Zn1-xMgxS nanoparticles were synthesized using a chemical solution method. The result showed that the energy band gap of the Zn1-xMgxS nanoparticles increased from3.70eV at x=0to3.98eV at x=0.55, which indicated that Mg2+-doped can adjust the energy band gap of the Zn1-xMgxS nanoparticles. However, the PL intensity of the Zn1-xMgxS nanoparticles was found to decrease with x due to an increase in the number of non-radiative deep traps.(3) ZnS:O2-nanoparticles were synthesized using a solid state reaction method at low temperature. The effect of O2-contents, reaction condition and anneal condition on the PL properties of samples was studied in detail. The result showed that O2-doping remarkably increased the luminescence intensity of ZnS nanoparticles, and a maximum emission was reached when Zn/O=10:5.3in the starting materials. The emission intensity of the ZnS:O2-(Zn/O=10:5.3) nanoparticles was about9times as high as that of the undoped ZnS sample. The average diameter of ZnS:O2-(Zn/O=10:5.3) nanoparticles increased from7.93to19.10nm with the increase of reaction time (from1to5h) and temperature (from120to160℃). A maximum emission of the ZnS:O2-(Zn/O=10:5.3) nanoparticles was observed when the reaction time and temperature were3h and130℃, respectively. The result also indicated that after annealing all samples showed larger radius and stronger visible emission for ZnS:O2-(Zn/O=10:5.3) nanoparticles. In this work, the optimal anneal temperature and anneal time were found to be100℃and0.5h for air annealed, and150℃and3h for vacuum annealed, respectively.(4) ZnS:X’(X=F, Cl, Br and I) nanoparticles was prepared by solid state reaction method and researched their PL properties. The result showed that after doping with halogen ion, all samples showed stronger visible emission for ZnS:X-(X=F, Cl, Br and I) nanoparticles, and the optimal molar ratio of X/Zn was found to be0.3,0.35,0.4and0.45for F, Cl, Br and I, respectively. It was also observed that PVA can decrease the number of the surface dangling bonds and then resulted in an increase in PL intensity of ZnS:X-(X=F, Cl, Br and I) nanoparticles.(5) ZnO nanoparticles were successfully prepared using sol-gel method and solid state reaction method at low temperature, respectively. The result showed that the product had a hexagonal wurtzite crystal structure and the average crystallite size was17.42and20.28nm, respectively. As compared with sol-gel method, ZnO nanoparticles, which were obtained by solid state reaction method, had better crystallinity and higher emission intensity. The effects of the reaction time and temperature on the microstructures and photoluminescence properties of ZnO nanoparticles were studied in detail. The result indicated that with the reaction time increasing (from6to10h), the diameter of ZnO nanoparticles was increased from16.0to30.2nm and the maximum intensity of emission was obtained while the reaction time was7h, under the same reaction temperature60℃. And under the same reaction time7h, with the reaction temperature increasing (from50to90℃), the diameter of ZnO nanoparticles was augmented from17.3to32.2nm and the maximum emission was attained when the reaction temperature was60℃. The effects of annealing conditions on the crystallinity, diameter, and PL properties of ZnO nanoparticles prepared by sol-gel method were investigated. All annealed ZnO nanoparticles showed stronger UV-visible emission and crystallinity than the as-grown ZnO nanoparticles. In this work, the optimal anneal temperature and anneal time were found to be100C and0.5h, respectively. Under the optimal anneal conditions, the vacuum annealed ZnO nanoparticles showed stronger emission intensity than the air annealed ZnO nanoparticles.(6) ZnO:S2-nanoparticles and ZnS/ZnO:S2-nanoheterostructure were obtained by solid state reaction method. The result showed that when S2-content was less than the solid solubility of ZnS and ZnO, S2-replaced O2-and formed ZnO1-xSx nanoparticles, and when S2-content was higher than the solid solubility of ZnS and ZnO, some parts of S2-had replaced O2-and form ZnO1-y-Sy (y is the maximum of the solid solubility between ZnS and ZnO) nanoparticles, others could not incorporate into ZnO1-+ySy nanoparticles and would react with Zn2+formed ZnS, which resulted in forming ZnS/ZnO1-ySy, nanoheterostructure. It can be seen that the emission intensity of samples follows the order ZnS/ZnO:S2->ZnO:S2->ZnO. And the photoluminescence intensity of ZnS/ZnO:S2-nanoheterostructure is much stronger than ZnO nanoparticles by25times. This may suggest that S2-dopants increase the number of radiative recombination sites in ZnO:S2-nanoparticles, but when ZnO:S2-nanoparticles were caped by ZnS nanoparticles, hackly surface was modified to some extent, which can decrease the number of the surface dangling bonds and then resulted in an increase in PL intensity.(7) Mg2+--doped ZnO nanoparticles were prepared by solid state reaction method. The result showed that doped with Mg2+can induce defect states in the ZnO nanocrystals such as VZn (zinc vacancy) and Mgi (interstitial magnesium), which resulted in enhance radiative recombination and PL intensity.