Highly luminescent (photoluminescence quantum yield (PLQY) as high as 96%) CdSe-based core/gradient alloy shell/shell (CGASS) quantum dots (QDs) have been synthesized in "one pot" using the reactivity difference between Cd and Zn precursors and Se and S precursors This procedure is highly reproducible and quite useful for large-scale synthesis. Upon photoexcitation these QDs show a multiexponential excited state decay behavior. Interestingly, with the growth of the shell the overall PL decay gets faster. All the decay traces have been fitted well with a three exponential decay function. Fitted decay traces reveal three different time constants, a faster one of 1-4 ns, moderate one of 13-16 ns, and slower one >25 ns. With the growth of the shell, the amplitude for the moderate time constant increases, and that of the slow time constant decreases consistently. The variation of PLQY could be correlated with the variation of amplitude of the moderate time constant. Slow and moderate time constants have been shown to be associated with two mutually interdependent excited state decay channels, and the competition between these two decay channels dictates the PLQY of these CGASS QDs. The moderate time constant is associated with an electron hole recombination process, and the slow time constant is associated with delayed emission from the band edge due to interaction with the manifold of shallow traps. The increase in magnitude of the amplitude of the moderate decay is reflected in higher PLQY. PL decay of blue-, green-, orange-, and red-emitting CGASS QDs follows a similar trend. This kind of uniform nature of PL decay of different color-emitting QDs is quite rare in the literature, and the fact that it has been observed in CGASS QDs perhaps hints toward the novelty of these systems. At the single-particle level these CGASS QDs are shown to be quite photostable without showing any blueing or bleaching for 1 h or even longer even under an air atmosphere. Thus, these CGASS QDs exhibit much improved optical behavior in comparison to CdSe/ZnS core/shell QDs. Quite interestingly all four differently emitting CGASS QDs optically behave in a similar way even at the single-particle level.
