Large scale synthesis of stable tricolor Zn _{1 - X}Cd _xSe core/multishell nanocrystals via a facile phosphine-free colloidal method

Here we report a new "green" method to synthesize Zn _{1 - x}Cd _xSe (x = 0-1) and stable red-green-blue tricolor Zn _{1 - x}Cd _xSe core/shell nanocrystals using only low cost, phosphine-free and environmentally friendly reagents. The first excitonic absorption peak and photoluminescence (PL) position of the Zn _{1 - x}Cd _xSe nanocrystals (the value of x is in the range 0.005-0.2) can be fixed to any position in the range 456-540 nm. There is no red or blue shift in the entire reaction process. Three similar sizes of alloyed Zn _{1 - x}Cd _xSe nanocrystals with blue, green, and yellow emissions were successfully selected as cores to synthesize high quality blue, green, and red core/shell nanocrystal emitters. For the synthesis of core/shell nanocrystals with a high quantum yield (QY) and stability, the selection of shell materials has been proven to be very important. Therefore, alternative protocols have been used to optimize thick shell growth. ZnSe/ZnSe _xS _{1 - x} and CdS/Zn _{1 - x}Cd _xS have been found as an excellent middle multishell to overcoat between the alloyed Zn _{1 - x}Cd _xSe core and ZnS outshell. The QYs of the as-synthesized core/shell alloyed Zn _{1 - x}Cd _xSe nanocrystals can reach 40-75%. The Cd content is reduced to less than 0.1% for Zn _{1 - x}Cd _xSe core/shell nanocrystals with emissions in the range 456-540 nm. More than 15 g of high quality Zn _{1 - x}Cd _xSe core/shell nanocrystals were prepared successfully in a large scale, one-pot reaction. Importantly, the emissions of such thick multishell nanocrystals are not susceptible to ligand loss and stability in various physiological conditions.