Scalable production of CuInS2/ZnS quantum dots in a two-step droplet-based microfluidic platform

We report the scalable formation of CuInS2/ZnS nanocrystals using a two-stage microfluidic reactor integrated with a real-time optical detection system, which is able to monitor reaction parameters prior and subsequent to the addition of the shell material. By injecting a ZnS single source precursor in droplets containing CuInS2 cores and without the need of purification steps, we are able to obtain core-shell nanocrystal populations emitting between 580 and 760 nm with significant narrower size distributions (90-95 nm) than for the same material systems synthesized on the macroscale. In-line monitoring allowed for rapid assessment of optimum reaction parameters (Cu/In, S/(Cu + In), Zn/(Cu + In) molar ratios, temperatures and reaction time) and enabled the formation of CuInS2/ZnS nanocrystals with high photoluminescence quantum yields (similar to 55%) within a few seconds. We believe that this synthetic methodology will be of significant utility in controllable production of ternary and quaternary metal chalcogenides, complex core-shell and doped nanostructures.We would like to thank Dr Frank Krumeich for performing the HRTEM analysis as well as the elemental analysis of single particles and Dr Michael Worle for XRD measurements. This work was partially supported by a National Research Foundation (NRF) grant funded by the Ministry of Science, ICT and Future Planning (MSIP) of Korea through Global Research Laboratory (GRL) Program (Grant number 2009-00426)