Synthesis and characterization of new Cd-doped ZnO/ZnS core–shell quantum dots with tunable and highly visible photoluminescence
Novel Cd-doped ZnO/ZnS core–shell quantum dots (QDs) were successfully synthesized via a simple two-step chemical solution method. The morphologies and structures of the product and the precursor were verified using X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray analysis (EDX) and X-ray photoelectron spectroscopy (XPS). The exciton dynamics and fluorescence lifetime were studied by time-resolved photoluminescence spectroscopy (TRPL). The thickness of ZnS was controlled by modifying the concentration of thioacetamide (TAA). As the TAA content was increased, the UV-Vis absorption spectra of these novel QDs red shifted toward longer wavelengths; this shift is attributed to the leakage of excitons from the ZnO core to the ZnS shell. The PL emission spectra of the novel QDs blue shifted toward shorter wavelengths as the TAA content was increased, and when the molar ratio of the Cd-doped ZnO to TAA reached 5 : 1.5. For these novel QDs, the luminous intensity was observed to significantly increase with increasing Cd content at a Cd doping of 20% or less, and a small red shift in the peak was also observed with increasing Cd content. At appropriate Cd doping and ZnS coating, the absolute quantum yield can reach 21% for Cd-doped ZnO/ZnS QDs from 3% and 8% for undoped ZnO/ZnS and Cd-doped ZnO QDs, respectively. All results indicate that the described synthesis method is appropriate for the preparation of Cd-doped ZnO/ZnS core–shell quantum dots with tunable and highly visible photoluminescence.