Energy transfer interactions with single molecule phenomena in small clusters of quantum dots

Abstract

This dissertation describes the observed interactions between energy transfer in small clusters of nominally monodisperse semiconductor nanocrystals (quantum dots, QDs) and single molecule phenomena such as fluorescence intermittency (blinking) and antibunching. The relevant literature on energy transfer between QDs has typically invoked the Förster energy transfer mechanism to explain the observations in ensemble measurements. The size dispersion in QDs results in a dispersion in the electronic and optical properties of QDs due to size dependent confinement effects on photogenerated carriers. This size dispersion is thought to be the reason for energy transfer among nominally monodisperse QDs as in the single molecule work in this dissertation. The single molecule measurements in this dissertation were done using confocal microscopy and correlated atomic force microscopy (AFM). The experimental setup is described in detail. Confocal microscopy is used to excite a small region on a surface of sparsely deposited QDs or QD clusters. This allows for observation of individual QDs or individual clusters at a time. The fluorescence from these samples is collected through the microscope objective and spatially filtered using confocal techniques, i.e. spatially filtering the fluorescence with a pinhole. The excitation region can be correlated with a nanoscale topographical image using the light that is backscattered through the microscope objective by an atomic force microscope tip. This provides an additional method for distinguishing individual QDs from QD clusters. Methods for setup, alignment, maintenance of the instruments used will be described with sample preparation and practical measurement considerations. The interaction of energy transfer and QD blinking will be discussed in detail. The major findings are that the mechanism of energy transfer does not affect the individual blinking properties of QDs in a cluster, nor does the close proximity of other quantum dots. The findings will also show evidence that an individual QD governs the fluorescence state of the cluster through energy transfer. The clusters in this work were primarily identified and analyzed using fluorescence properties. The threshold in clusters is not as obvious as in individual QDs so an intensity threshold is set using a model of energy transfer that sets a threshold based on the lifetime. The findings impact future studies of QD clusters and applications that utilize QDs in close proximity to each other. The interaction of energy transfer and photon antibunching will also be discussed in detail. A simple model of energy transfer will be used to model the degree of antibunching in small clusters of QDs. The degree of antibunching observed in QD clusters is more characteristic of an individual emitter than multiple emitters which is a surprising find because it indicates that all QDs interact through energy transfer even in small nominally monodisperse aggregates. This work was done with correlated AFM to be sure that one QD or QD cluster is observed at a time. It is extremely important that only one emitter is in the excitation region because multiple independent emitters confound the analysis of antibunching and the observation of antibunching from multiple emitters heavily impacts single molecule study of QDs. Antibunching is thought to be the single definitive evidence that a single emitter is being probed but this is not so in the case of close proximity QDs even if the QDs are nominally the same size.