Time resolved fluorescence of CdSe nanocrystals using single molecule spectroscopy

Abstract

A wide variety of spectroscopic studies of CdSe nanocrystals (NCs) are presented in this thesis, all studying some aspect of the temporal evolution of NC fluorescence tinder different conditions. In particular the methods of single molecule spectroscopy are used in many experiments allowing the behavior of individual NCs to be resolved from the blurring effect of averaging over the ensemble. Studies of the excited state lifetime of band edge fluorescence from single NCs reveal multiexponential relaxation dynamics that stem from fluctuations of non-radiative decay rates for the band edge exciton. Analysis of these fluctuations allows us to extract single exponential dynamics by sampling only "maximum-intensity" photons, and we find that this single exponential decay is remarkably uniform across a wide variety of NC samples and sizes. We also investigate luminescence from multiexciton (e.g. biexciton and triexciton) states of nanocrystals at both the ensemble and single NC level.

(cont.) Energy splittings, size and temperature dependencies, quantum yields and lifetimes of multiexciton states are measured and discussed. We show for the first time direct resolution of biexciton emission from single exciton emission using two different techniques, fluorescence line narrowing and single NC spectroscopy. We also study the non-classical light emission properties of single NCs and show how multiexciton emission leads to radiative quantum cascades of single photons in the emission of a single NC. Time resolved studies of fluorescence from NCs in solution environments conclude the thesis. The relationship between lifetime and quantum yield for non-homogeneous ensembles like NCs is studied in chapter 8. We show that a sub-population of non-luminescent nanocrystals can reduce the quantum yield of an ensemble of NCs even though the measured lifetime stays constant. A study of NCs in solution fluorescence correlation spectroscopy (FCS) is presented last. We find that FCS is a capable tool for distinguishing small differences in hydrodynamic radius of NCs in solution. We also find that blinking of NCs may have a significant impact on these FCS measurements.

(cont.) An appendix to this thesis presents a general summary the lifetime of various samples CdSe and CdTe NCs.