On the origin of photons : understanding excitons and multiexcitons in colloidal semiconductor nanocrystals

Abstract

This thesis focuses on studies of exciton and multiexciton dynamics in colloidal semiconductor nanocrystals. I concentrated primarily on the development of correlation spectroscopies for the measurement of the statistics and dynamics of multiexciton emission. Under most conditions, the emission properties of nanocrystals are dominated by the photophysics of the single-exciton state. However, due to high density of states nanocrystals can readily undergo several successive excitations, leading to multiexcitonic states with distinct dynamics. As nanocrystals are more frequently used under conditions of high excitation ux for light-emitting applications, the often destructive nature of multiexcitons are increasingly relevant. Prior work on multiexcitons has largely focused on the biexciton, and here we develop tools to directly study triexciton emission. First, we develop the theoretical and analytical apparatus for studying multiexciton emission. The model and software are general to arbitrary numbers of excitons and provide the framework for further correlation-based methods. The software is general and may be used in any photon-timing application. InAs is a promising candidate material as an infrared emitter for down-shifting and biological applications. In one study, we characterize the excitonic dynamics of individual InAs nanocrystals. We nd that the nanocrystals qualitatively behave like CdSe nanocrystals, in that they exhibit blinking, monoexponential radiative dynamics, and (generally) low biexciton quantum yield. In a separate set of experiments, we study the temperature-dependent exciton emission dynamics of InAs quantum dots and nd that their emission is well-described by existing excitonic ne-structure models. CdSe is the classical colloidal quantum dot material, and is the ideal testing ground for methods development. We apply new high-order correlation methods to study the emission statistics and dynamics of the triexciton, biexciton, and monoexciton at the single-molecule level. We nd that multiexcitonic states have no memory: the relaxation of a biexciton yields a monoexciton which behaves identically to one formed by a single-photon excitation. We discuss a few examples of materials which may exhibit exotic biexcitonic states. Next, we explore the use of infrared-emitting nanocrystals for deep-tissue imaging in biological research. Earlier work on the subject involved the use of emitters with significantly lower quantum yield, and we demonstrated a variety of applications which make use of the high quantum yield and chemical functionality of colloidal nanocrystals. We used the unique combination of chemical functionality and high emission quantum yield to perform studies in mice: non-invasive measurement of vital signs in awake mice; real-time metabolic imaging of lipoproteins in an intact mouse; vascular imaging and blood velocimetry in a mouse with a cranial window model. Finally we discuss a few project ideas which arose during the work on this thesis, but were not successfully completed.