Non-heavy-metal ZnS quantum dots with bright blue photoluminescence by a one-stepaqueous synthesis

Abstract

We have examined the aqueous synthesis of non-heavy-metal ZnS quantum dots(QDs) using 3-mercaptopropionic acid (MPA) as the capping molecule atvarious pH and MPA:Zn:S ratios. Transmission electron microscopy (TEM)and x-ray diffraction (XRD) indicated that the aqueous ZnS QDs were3–5 nm in size with a zinc blende structure. We showed that, at pH 12 with aMPA:Zn:S = 8:4:1 ratio, the ZnS QDs with optimal blue emission could be obtained in a one-step,room-temperature aqueous process that exhibited a quantum yield of 31%, higher than that ofthe commercial CdSe/ZnS core–shell QDs. The present ZnS QDs could pass through a50 kD filter. This indicated that they were smaller than 5 nm in size, consistent with thoseestimated from the UV–vis absorption edge and the TEM image. At a lower pH (e.g.pH = 8), the room-temperature synthesized ZnS QDs exhibited nophotoluminescence. Although further hydrothermal annealing at100 °C could improve the photoluminescence of the ZnS QDs, the resultant emission was not asbright as that obtained at pH 12 at room temperature. The blue emission of aqueous ZnSQDs was likely the result of trap-state emissions involving the defect states of the QDs.The present ZnS QDs were bright, small and contained non-heavy-metal elements, thusoffering the potential for in vivo bioimaging.