New Insights into the Complexities of Shell Growth and the Strong Influence of Particle Volume in Nonblinking “Giant” Core/Shell Nanocrystal Quantum Dots

Abstract

The growth of ultra-thick inorganic CdS shells over CdSe nanocrystal quantum dot (NQD) cores gives rise to a distinct class of NQD called the "giant" NQD (g-NQD). g-NQDs are characterized by unique photophysical properties compared to their conventional core/shell NQD counterparts, including suppressed fluorescence intermittency (blinking), photobleaching, and nonradiative Auger recombination. Here, we report new insights into the numerous synthetic conditions that influence the complex process of thick-shell growth. We show the individual and collective effects of multiple reaction parameters (noncoordinating solvent and coordinating-ligand identities and concentrations, precursor/NQD ratios, precursor reaction times, etc.) on determining g-NQD shape and crystalline phase, and the relationship between these structural features and optical properties. We find that hexagonally faceted wurzite g-NQDs afford the highest ensemble quantum yields in emission and the most complete suppression of blinking. Significantly, we also reveal a clear correlation between g-NQD particle volume and blinking suppression, such that larger cores afford blinking-suppressed behavior at relatively thinner shells compared to smaller starting core sizes, which require application of thicker shells to realize the same level of blinking suppression. We show that there is a common, threshold g-NQD volume (~750 nm(3)) that is required to observe blinking suppression and that this particle volume corresponds to an NQD radiative lifetime of ~65 ns regardless of starting core size. Combining new understanding of key synthetic parameters with optimized core/shell particle volumes, we demonstrate effectively complete suppression of blinking even for long observation times of ~1 h.