Fluorescence dynamics of semiconductor nanorod clusters studied by correlated atomic force, transmission electron, and fluorescence microscopy

Semiconductor nanocrystals have a wide range of applications as light emitters, especially in biomedical imaging. Elongated core - shell CdSe-based nanocrystals (nanorods) are particularly interesting as fluorescent markers because of their large absorption cross section, large surface area, and high brightness. While light intermittency ("blinking") in single nanorods has been studied previously, here we report the fluorescence properties of core - shell CdSe-based nanorod clusters. The time-dependent cluster fluorescence was correlated with the particle number by direct particle counting (from single nanoparticles to ∼10 000), cluster area, and the orientation and distribution of individual nanorods within these clusters. This was uniquely enabled by combined transmission electron and atomic force microscopy. In contrast to the "on/off" emission in single nanorods, we show that nanorod clusters containing as few as five nanorods exhibit a nonzero residual fluorescence in the "dark state"; that is, they can be "on" continuously, within the measurement time window of several tens of minutes. With increasing particle number, the cluster fluorescence increases in intensity and the relative fluorescence fluctuations, originating from single particle events, decrease in accordance with the central limit theorem. We report the effects of assembly patterns and nanorod orientation (on the silicon nitride substrate and relative to the laser polarization) on the fluorescence properties of the clusters. The fluorescence time-dependence of nanorod clusters at long time-scales, i.e. tens of hours, follows two characteristic trends depending on particle number and laser intensity. These data are compared to predictions made by a statistical model derived from single-particle dynamics. Finally, to investigate the possible role of charge traps on ensemble properties, we have also confirmed by electrical measurements across nanorod arrays that the electrical current exhibits statistical aging and memory effects. This complementary measurement provides a new way to relate the electrical and optical properties of nanoparticle ensembles and further suggests that filling of charge traps can possibly explain both the fluorescence and the anomalous transport dynamics of core-shell nanorod ensembles.