Detection and spectroscopy of single CdSe nanocrystallite quantum dots

Abstract

Semiconductor nanocrystallites, with size dependent optical properties that have generated considerable interest over the past 10 years, are intrinsically difficult to study due to inhomogeneities in ensemble samples. In this thesis, I describe the motivation and development of an experimental program designed to detect and spectrally resolve the fluorescence from single CdSe nanocrystallites. Through these experiments, we uncover many new and unexpected physical phenomena such as ultra-narrow emission linewidths, fluorescence blinking on a timescale of seconds, and spectral shifting over a wide range of time and energy scales (from seconds to minutes and from less than 100[mu]eV to greater than 80me V). Ionization is found to play an important role in the optical characteristics of single nanocrystallites by quenching luminescence and by producing large local electric fields. Stark measurements of single nanocrystallites are able to directly measure local electric fields around individual nanocrystallites, and measure changes in the field that occur coincident with spectral shifts. Stark experiments also reveal a highly polarizable excited state (~105 A3) with a large induced excited state dipole (~80 De bye). Single nanocrystallite line shapes are found to primarily reflect fluctuations in the local field over time, which can be controlled by adjusting the excitation intensity, wavelength, sample temperature, and sample preparation. Measured linewidths can also be controlled by adjusting the integration time or by adding an applied electric field. Polarization spectroscopy is used to probe the nature of the transition dipole from the emitting state, uncovering a degenerate dipole that is oriented isotropically in the x-y plane of the nanocrystallite. The 2-dimensional nature of this dipole allows us to use polarization spectroscopy to directly measure the 3-dimensional orientation of each nanocrystallite within a sample. These experiments have provided a new perspective on the physics and dynamics of CdSe nanocrystallites that has been unavailable in ensemble experiments.