Molecular Foundry Seminar

"Structure-Specific Nanoscale Photodynamics"

Kenneth L. Knappenberger, Jr., Department of Chemistry and Biochemistry,
Florida State University
Wednesday, May 18th at 1:00 pm, Bldg. 67 - 3111

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Abstract:

Recent advances in two areas will be presented. 1) electronic energy relaxation in isolated and aggregated plasmonic nanosstructures. Femtosecond pump-surface plasmon probe spectroscopies have been developed to monitor electronic energy relaxation and interparticle coupling in tunable plasmonic nanospheres. 2) Magnetic field-mediated nanoparticle photophysics. External magnetic fields have been employed to optimize radiative relaxation processes in semiconducting nanoparticles.

The development of sensitive ultrashort laser-based spectroscopic probes to investigate dynamics of high surface-to-volume metal and alloy nanostructures will be discussed. Electronic relaxation and interparticle electromagnetic coupling processes in hollow gold nanospheres (HGNs) and HGN aggregates were studied using femtosecond pump- surface plasmon probe and second harmonic generation spectroscopies. In the case of HGNs, an unexpected, but systematic, blue shift of the spectral position of the surface plasmon resonance was observed upon nanoparticle aggregation. Femtosecond time-resolved measurements, high-resolution TEM, and Finite-Difference Time-Domain calculations demonstrate that this blue shift results from interparticle coupling, an effect not observed for solid nanospheres. The efficiency of this coupling was tailored by controlling HGN aspect ratio over a vast range of sizes (20 nm to 80 nm outer diameter).

Shape-dependent exciton spin polarization of semiconducting nanoparticles was investigated using time- and polarization-resolved magneto-photoluminescence spectroscopy. Experiments were performed at low temperature (<2 K) in magnetic fields up to 17.5 T to investigate the extent of spin polarization in CdSe quantum dots and nanorods. Even at low magnetic field strengths, induction of spin-polarization occurred in one-dimensional CdSe nanorods, as evidenced by intensity-integrated and wavelength-resolved photoluminescence and time-correlated single-photon counting measurements. This effect is not observed for 0-D quantum dots. The striking percentage of polarization was attributed to highly efficient mixing of “dark” and “bright” exciton fine-structure states in 1-D nanostructures, thus permitting formation of spin-polarized excitons. Field-mediated control over the relative populations of exciton fine-structures was also observed. These results demonstrate the potential use of particle shape and crystal structure, two key nanostructure design parameters, to control photoinduced dynamics.