Preparation And Photoluminescent Characteristics Of ZnS And Doped ZnS Quantum Dots

Semiconductor quantum dots (QDs) have recently drawn a great deal of attention due to their unique properties compared with corresponding bulk materials. As a wide-gap semiconductor, ZnS and ZnS doped with different ions are important luminescent materials. Because of quantum size effect, photoluminescent (PL) properties of ZnS QDs dramatically vary with not only decreasing the particle size but also impurity ions. In this paper, we have focused on the relationship between composition, structure and luminescent spectra of undoped and transition-metal ions doped ZnS QDs. Here, it mainly contains 4 parts as follows.(1) ZnS QDs (3⁴nm) were synthesized by chemical precipitation in aqueous solution, and characterized by XRD, TEM and PL spectra. PL spectra of ZnS QDs consist of three peaks centered at 416nm, 460nm, and 505nm excited by 330nm UV, which origins from surface state and S2- vacancy, respectively. When excited by 380nm, the peak at 416nm disappears and the hue of emitting light changes from blue to white. For those QDs with organic conjugante molecules, PL peaks from surface state disappeared due to surface modification.(2) By chemical precipitation, hydrothermal and liquid-solid-solution methods, ZnS:Mn²⁺ QDs with different sizes were prepared. The two excitation peaks of ZnS and Mn²⁺ apart from each other remarkably by increasing particle size, indicating there is no energy transfer between ZnS host and Mn²⁺ in ZnS:Mn²⁺ QDs. By increasing [S2-]/[Zn²⁺] ratio, we find the emission of ZnS vanishes, accompanying with enhancement of Mn²⁺ emission. These phenomena are attributed to the reduction of surface state and the increase of Mn²⁺ content in samples.(3) Cu-doped and Cu, M (M=F- or Ce3+) co-doped ZnS QDs with ⁵nm diameter have been obtained by chemical precipitation in aqueous solution. The effect of Cu+ ion concentration and codoping of F- or Ce3+ ion on fluorescent spectra are discussed. The emitting peaks centered at 460nm and 490nm are attributed to electron transition from shallow and deep donor level to Cu+ centers, respectively. As sensitizer, the luminescence intensity of ZnS:Cu QDs is decreased by doping F- ion but increased by doping Ce3+ ion.(4) Pb²⁺-doped and Pb²⁺-adsorbed ZnS QDs were synthesized by chemical precipitation in aqueous solution, respectively. The spectra consist of two bands peaked at 460nm and 515nm, which owes to 3P1â†'1S0 and 1P1â†'1S0 transitions of Pb²⁺, respectively. The optimized emission intensity of Pb²⁺-doped ZnS QDs is obtained at 0.3 at% of Pb²⁺ concentration. The peak position can be tuned to longer wavelength by increasing [S2-]/[Zn²⁺] ratio.