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Browsing by Author "Van Orden, Alan K., advisor"

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    Characterizing the fluorescence intermittency of individual cadmium selenide/zinc sulfide quantum dot clusters with spatially correlated single molecule fluorescence spectroscopy and atomic force microscopy
    (Colorado State University. Libraries, 2008) Yu, Ming, author; Van Orden, Alan K., advisor
    In this thesis, I describe work done to study the optical behaviors of CdSe/ZnS quantum dots, especially the fluorescence blinking behavior of small quantum dot clusters. QDs have unique optical properties that impart several key advantages over molecular dyes. However, when examined at the single-molecule level, QDs emission exhibit novel fluorescence intermittency, or "blinking," behavior. This blinking is believed to be caused by trapping and de-trapping of the photoexcited carriers, causing the QDs to fluctuate between emissive and non-emissive states. A spatially correlated single molecule fluorescence spectroscopy and atomic force microscopy (AFM) apparatus was used to carry out these studies. Single molecule spectroscopy examines the blinking behavior of individual, isolated QDs and QD clusters, while the AFM images the nanometer scale topography of the particles. When multiple isolated QDs were probed simultaneously, the fluorescence behavior was consistent with independent blinking of the individual QDs. However, when close-packed QD clusters were probed, the fluorescence intermittency became much more rapid and intense than could be explained by the summation of multiple particles blinking independently. This suggests when the small QDs aggregate together, they become electronically coupled in some way that enhances the fluorescence blinking. Subsequently, we studied variations of the emission wavelengths of isolated small QD clusters possessing the enhanced blinking behavior. The emission wavelength of the coupled enhanced blinking is red shifted relative to that of normal blinking. We propose that red-shifting in emission is one of the characteristics of electronic coupling in the QD clusters and resulted from the quantum confinement Stark effect. In the following chapters, environment and substrate dependence were also studied. Compared with ambient air, dry nitrogen decreases the population, intensity and/or durations of "on" times. Both CTAB- and Mg 2+-mica substrates quench the fluorescence of single QDs and QD clusters, which is due to the dissociation of electron hole pairs of excited QDs by the electron attractive sites in CTAB molecules and Mg2+ ions.
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    Extending single molecule spectroscopic techniques to multi-particle systems of semiconductor nanocrystals
    (Colorado State University. Libraries, 2011) Shepherd, Douglas Parker, author; Gelfand, Martin Paul, advisor; Van Orden, Alan K., advisor; Roberts, Jacob Lyman, committee member; Prieto, Amy Lucia, committee member
    This dissertation describes the application of single molecule spectroscopic techniques to individual semiconductor nanocrystals (NCs), small clusters of NCs, and NCs used as the light harvesting layer in sensitized solar cells. We first examine how coupling between close-packed NCs may alter their photophysical properties by studying isolated NCs and small clusters of NCs via single molecule time-correlated single-photon counting, from which fluorescence intensity trajectories, autocorrelation functions, decay histograms, and lifetime-intensity distributions have been constructed. These measurements confirm that NC clusters exhibit unique photoluminescence behavior not observed in isolated NCs. In particular, the NC clusters exhibit a short-lifetime component in their photoluminescence decay that is correlated with low photoluminescence intensity of the cluster. A model based on radiative energy transfer to NCs within a cluster that have smaller energy gaps, combined with independent blinking for the NCs in a cluster, accounts for the main experimental features. This energy transfer may lead to energy sinks when an excitation is transferred to a NC that is in the off state. We then examine a model photovoltaic system where a sub-monolayer film of NCs is chemically coupled to a single crystal semiconductor (TiO2 or ZnO) substrate through a variety of capping ligands. Again, utilizing time-correlated single photon counting and internal photon conversion efficiency we have studied both the photoluminescence intensity, photoluminescence decay time, and sensitized photocurrents. We find that for all configurations of capping ligands and substrate the photoluminescence decay rate is quenched compared to the free NCs in solution; whereas, only the short chain capping ligands that promote electron coupling to the substrate produce photocurrents. The longer chain capping groups both inhibit the electron injection and promote NC clustering on the surface where interactions between the individual NCs or the NCs and substrate alter the radiative rate. This result confirms that the possibility of NC clusters leading to a loss of energy due to inter-NC coupling is present in devices and warrants further study.
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    Steady-state and time-resolved spectroscopy to probe the effects of confinement on Cy3 and the dynamics of AOT/iso-octane reverse micelles
    (Colorado State University. Libraries, 2010) McPhee, Jeffrey, author; Van Orden, Alan K., advisor; Levinger, Nancy E., committee member; Barisas, B. George, committee member; Prieto, Amy L., committee member; Luger, Karolin, committee member
    This dissertation describes the use of steady-state and time-resolved spectroscopy to probe the effects of localized confinement on the water soluble dye Cyanine-3 (Cy3) and the dynamics of intermicellar interactions using fluorescence correlation spectroscopy (FCS). The first set of experiments presents a wide range of steady-state and time-resolved spectroscopy data which indicate that the Cy3 molecules form H-aggregates at concentrations so dilute (nM) that this behavior is not observed in bulk aqueous solution. This unique behavior allowed for a series of FCS and dynamic light scattering measurements to be performed on the same system. These results indicate the formation of a transient reverse micelle dimer, whose lifetime has been identified to be on the order of 15 μs. Furthermore, preliminary experiments are presented on the same reverse micelle system containing the Rhodamine 6G and the results are consistent with those obtained for Cy3 in the reverse micelles. Lastly, fluorescence resonance energy transfer within the reverse micelles was investigated using Cy3 and Cy5. The preliminary results suggest that FRET may be occurring within this extremely confined environment. The work as a whole provides insight into the nature of confinement as well as the dynamics occurring within the world of reverse micelles.
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    The ongoing development of the technique of two-beam fluorescence cross-correlation spectroscopy coupled with continuous-flow capillary electrophoresis
    (Colorado State University. Libraries, 2008) Fogarty, Keir, author; Van Orden, Alan K., advisor
    This thesis describes work performed in the development and application of the technique of two-beam fluorescence cross-correlation spectroscopy coupled with continuous-flow capillary electrophoresis (2bFCCS-CFCE). In this approach, fluorescently-labeled molecules of interest are monitored as they electrophoretically migrate in continuous solution between two, spatially-separated laser foci. The work presented here illustrates a number of advantages that 2bFCCS-CFCE analysis has over more conventional capillary electrophoresis (CE) techniques. Three sets of experiments were performed that illustrate these advantages. In the first set of experiments, 2bFCCS-CFCE was used to detect and discriminate between three different analyte molecules electrophoretically migrating in opposite directions. The ability of 2bFCCS-CFCE to monitor molecules electrophoretically migrating in opposite directions without the need of sample injection steps or multiple buffers demonstrated a distinct advantage over conventional CE techniques, which require complex buffer conditions, sample injection, and can only analyze species migrating in uniform direction. The second set of experiments demonstrated 2bFCCS-CFCE on microchip platforms (2bFCCS-CFMCCE). Several design advantages of 2bFCCS-CFMCCE relative to separations-based CE microchip platforms, such as increased miniaturization and simpler fluid handling format, were demonstrated. The third set of experiments demonstrated the ability of 2bFCCS-CFCE to measure the effective charge of a single-stranded DNA (ssDNA) in the presence of magnesium ions. The measurement of effective charge requires analysis of both diffusion and electrophoretic migration simultaneously. Conventional CE experiments are not capable of making this measurement. 2bFCCS-CFCE was used to successfully measure the effective charge, revealing the interesting relationship between ssDNA charge and size.