Facile synthesis of CuInS2/ZnS quantum dots with highly near-infrared photoluminescence via phosphor-free process

We present the synthesis of CuInS2 quantum dots (QDs) with a size range of 3-4 nm through a phosphor-free method. The photoluminescence (PL) properties of CuInS2 QDs synthesized using various Cu/In molar ratios were investigated. With the decrease of Cu/In molar ratio, the PL peak wavelength revealed a clear blue shift by ca. 100 nm and a noticeable increase in PL intensity. Being coated with a ZnS shell on CuInS2 QDs, the resulting core/shell QDs exhibited a dramatic increase of PL efficiency and stability, with the maximum PL efficiency up to 39 %. This is ascribed to the efficient reduction of non-radiative recombination after surface modification. In addition, the PL peak wavelengths were tuned from 670 to 800 nm. The transmission electron microscopy observation and X-ray diffraction analysis indicated that the CuInS2 and CuInS2/ZnS QDs revealed a dot shaped morphology and exhibited a wurtzite structure. Time-resolved fluorescence spectroscopy revealed that CuInS2/ZnS core/shell QDs apparently exhibited a slow decay compared with the CuInS2 cores (214 ns for CuInS2/ZnS QDs and 172 ns for CuInS2 cores). Because of the tunable near-infrared range emission, high stability and long lifetimes of the CuInS2/ZnS QDs, they will be very useful in applications such as solar cell and biological imaging.