Characterizing the fluorescence intermittency of individual cadmium selenide/zinc sulfide quantum dot clusters with spatially correlated single molecule fluorescence spectroscopy and atomic force microscopy.

摘要

In this thesis, I describe work done to study the optical behaviors of CdSe/ZnS quantum dots, especially the fluorescence blinking behavior of small quantum dot clusters. QDs have unique optical properties that impart several key advantages over molecular dyes. However, when examined at the single-molecule level, QDs emission exhibit novel fluorescence intermittency, or "blinking," behavior. This blinking is believed to be caused by trapping and de-trapping of the photoexcited carriers, causing the QDs to fluctuate between emissive and non-emissive states.In the following chapters, environment and substrate dependence were also studied. Compared with ambient air, dry nitrogen decreases the population, intensity and/or durations of "on" times. Both CTAB- and Mg 2+-mica substrates quench the fluorescence of single QDs and QD clusters, which is due to the dissociation of electron hole pairs of excited QDs by the electron attractive sites in CTAB molecules and Mg2+ ions.A spatially correlated single molecule fluorescence spectroscopy and atomic force microscopy (AFM) apparatus was used to carry out these studies. Single molecule spectroscopy examines the blinking behavior of individual, isolated QDs and QD clusters, while the AFM images the nanometer scale topography of the particles. When multiple isolated QDs were probed simultaneously, the fluorescence behavior was consistent with independent blinking of the individual QDs. However, when close-packed QD clusters were probed, the fluorescence intermittency became much more rapid and intense than could be explained by the summation of multiple particles blinking independently. This suggests when the small QDs aggregate together, they become electronically coupled in some way that enhances the fluorescence blinking. Subsequently, we studied variations of the emission wavelengths of isolated small QD clusters possessing the enhanced blinking behavior. The emission wavelength of the coupled enhanced blinking is red shifted relative to that of normal blinking. We propose that red-shifting in emission is one of the characteristics of electronic coupling in the QD clusters and resulted from the quantum confinement Stark effect.