Core–shell energy band engineering of cyan light-emitting ternary ZnGa2S4@ZnS quantum dots toward anti-counterfeiting and bioimaging applications

Abstract

The development of cadmium-free cyan-emitting quantum dots (QDs) is of paramount importance for application in anti-counterfeiting, display and bioimaging. II–VI alloyed ZnGa₂S₄ (ZGS) QDs have large Stokes shifts and wide bandgaps and are thus being rapidly developed as an attractive candidate for cadmium-free cyan-luminescent QDs. However, the synthesis method of monodisperse ZGS QDs and the construction of a core–shell energy band structure are not well established. In this paper, ternary ZGS core QDs were synthesized by a thermal decomposition method with a systematic reaction optimization. Furthermore, type-I core–shell structured ZnGa₂S₄@ZnS (ZGS@ZS) was constructed according to the core–shell energy band engineering principle. The obtained core–shell QDs had an average particle size of approximately 4 nm. A broad emission band appeared in the wavelength range from 400 to 600 nm, peaking at 480 nm with obvious cyan emission. The fluorescence quantum yield was enhanced from 24.6% to 37.3%, demonstrating superior fluorescent performance in ZGS@ZS core–shell QDs. Color-controlled ZGS@ZS showed a good application in anti-counterfeiting technology. Furthermore, after surface modification of polyethylenimine (PEI), water-soluble ZGS@ZS-PEI QDs showed good biocompatibility, thus exhibiting excellent fluorescent labelling application in bioimaging.