Synthesis of highly luminescent and photo-stable, graded shell CdSe/Cd xZn1-xS nanoparticles by in situ alloying

We report a facile and robust synthesis of Cd_xZn_1-xS graded shells on CdSe nanoparticles that are prepared by interface alloying between CdS and ZnS shells at elevated temperatures. Alloying provides systematic control over the electronic structure and enables switching between Type-I and quasi-Type-II configurations. Good control of particle shape, shell thickness, and composition is achieved by slowly adding zinc oleate and octane thiol via syringe pump to readily prepared CdSe/CdS particles. The resultant quantum dots exhibit PL quantum yields of up to 97% and superior robustness toward environmental influences and quenching agents. Alloying promotes a blue-shift of both the absorption and PL spectra compared to pure CdSe/CdS particles and an increased Stokes shift, opening a new synthetic pathway to stable, green-emitting core/shell/shell quantum dots. High PL quantum yields are correlated to a narrow distribution of single-particle lifetimes and suppressed fluorescence intermittency. We introduce a new method to characterize the PL intermittency of single quantum dots based on the autocorrelation function of their PL time trajectories.