Theoretical study of F-type color center in rutile TiO2

Free-standing powder of doped ZnS nanoparticles has been synthesized by using a chemical co-precipitation of Zn2+, Co2+, Co3+ with sulfur ions in aqueous solution. Xray diffraction analysis shows that the diameter of the particles is ~2.6±0.2nm. Unique luminescent properties have been observed from ZnS nanoparticles doped with Co2+ and Co3+. The effect of Co3+ on emission spectra of doped samples is remarkably different from that of Co2+. Very strong and stable visible-light emission have been observed from ZnS nanoparticles doped with Co2+. However, the fluorescence intensity of Co3+-doped ZnS nanoparticles is much weaker than that of ZnS nanoparticles. Nanoparticles can be doped with cobalt during the synthesis without altering the X-ray diffraction pattern and emission wavelength. However, doping makes the fluorescence intensity vary. The fluorescence intensity of doped sample is about five times of that of pure ZnS nanocrystallines when the doped mole ratio of Co2+ is 0.5%. The fluorescence efficiencies of samples decreases as doped mole ratio of Co2+ and Co3+ increases. When Co3+ is doped in ZnS nanoparticles, the fluorescence efficiencies of doped ZnS nanoparticles almost died away. Therefore, Co3+ leads to fluorescence decay of ZnS nanocrystallites.