Spectroscopy and external control of optical dynamics in single semiconductor nanocrystals

Abstract

Single molecule spectroscopy has progressed substantially in the past ten years and the accompanying progress in the optical study of single semiconductor nanocrystals has opened a new dimension in our understanding of the photophysical properties of these quantum-confined materials. In this thesis, I describe experimental advances that investigate and manipulate optical dynamics--blinking and spectral diffusion--in single CdSe and CdTe nanocrystals caused by complex interactions of electric fields and charge migration between a nanocrystal and its nearby environment. With the aid of a simple yet powerful fluorescence far-field microscope, we examine the blinking phenomenon using statistical analysis to determine a temperature-independent physical mechanism (charge tunneling) that is universal to every nanocrystal. Furthermore, we uncover a strong correlation between the blinking and spectral diffusion processes. This correlation proves to be critical in connecting the power-law statistics observed from blinking in isolated nanocrystals with the binary spectral shifts in surface plasmon coupled nanocrystals. Moreover, we identify charged nanocrystal emission signatures and create charged nanocrystals on command. This level of control, whether to turn the nanocrystals off or to eliminate the blinking behavior, also improves the prospects for nanocrystal device applications. Finally, we explore the impact of external magnetic fields on single nanocrystal optical properties and reveal behavior that is dependent on nanocrystal orientations relative to the applied field. In addition, we observe evidence of zero-field splitting in a subset of nanocrystals. This corroborates theoretical models that propose paramagnetic interactions as an explanation for the fundamental physics of CdSe nanocrystal emission.