Confocal Microscopy Studies of Fluorescence Blinking of Semiconductor Quantum Dots, Metal Nanoparticle Photogeneration, and Multiphoton Photoemission from Thin Metal Films and Metal Nanoparticles

<p>Since the advent of single molecule spectroscopy in 1989, advances in the field have revealed a wealth of information on dynamics and sample heterogeneity unobtainable by traditional ensemble studies. Microscopy experiments are a common technique to characterize and probe single molecule dynamics, due to the combination of the diffraction limited spatial resolution and the availability of sensitive single photon/electron detectors. Additionally, high excitation power densities can be achieved by the use of large numerical aperture objectives with moderately intense light sources. Fluorescence intermittency, or blinking, is a unique property found in the emission of single molecules. A series of experiments are undertaken to elucidate contributions to the blinking dynamics in nanocrystal semiconductors, or quantum dots (QDs). Investigations of the transitions from \textquotedbllefton\textquotedblright to \textquotedblleftoff\textquotedblright (and vice versa) in the absence of laser illumination allow for the determination of the roles of light versus non-light induced processes for single blinking QDs. Small molecules are found to influence QD blinking by altering the surface trap state distribution due to changes in the electrochemical potential of the solution. However, fluorescence detection is only one implementation to investigate single molecule systems by microscopy. Nanoscale metal materials possess many interesting electronic and optical properties that enable single molecule or particle detection. Silver and gold metal nanoparticles are of particular interest due to their surface plasmon resonances (SPRs), a\&nbsp;<span style="line-height: 1.6em;">collective electron oscillation excited in the near ultraviolet and visible range. As a result of the coherent electron oscillations on the surface of the particle, large electric fields are generated in the vicinity of the nanostructure. This local enhancement of the electric field enables molecular detection in the vicinity of particles by surface-enhanced Raman scattering (SERS). One difficulty with conventional systems used to study SERS is the large enhancement variability observed between nanoparticles on the same substrate, where typically only 1 in 100-1000 are found to have the necessary enhancement factors. Photogeneration of Ag nanoparticles within a thin silver percholorate/polystyrene polymer film form reproducible SERS active nanoparticles that can be monitored and characterized by Raman microscopy. Insight into the growth mechanism of the nanoparticles is provided by analysis of the time dependent data with an Avrami kinetic phase transformation model. The environment in which the nanoparticles are generated is found to influence both the photogeneration kinetics and the nanoparticles SERS activity. Information on the size and morphology of the nanoparticles provided by AFM and dark field scattering measurements allowing for correlation of photophysical properties with nanoparticle shape. Lastly, the electric field enhancements, exploited by SERS in the Ag nanoparticle system, are investigated for Au single nanoparticles by multi-photon photoemission (MPPE). The construction of a Scanning Photoionization Microscope (SPIM) combines the spatial resolution of an optical microscope with the sensitivity for electron detection arising from photoionization of metal nanostructures by femtosecond laser pulses. Investigations into thin metal films determine the viability of the technique to probe multi-photon photoemission. Examinations of various nanostructures provide insight into the role of the plasmon in the photoemission process. The magnitude of the enhancement field is intimately related to the </span><span style="line-height: 1.6em;">photoelectron yield, such that simulations of near-field effects are able to explain wavelength and polarization dependence for photoemission from metal nanostructures.\&nbsp;</span></p>
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University of Colorado Boulder
Boulder, CO
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