Margaret A. Hines

PHOTOLUMINESCENCE WANDERING IN SINGLE CDSE NANOCRYSTALS

Low temperature two‐photon fluorescence excitation microscopy was used to obtain fluorescence spectra of single CdSe colloidal nanocrystals or quantum dots. The fluorescence spectra reveal large, 0.02 eV shifts of the peak emission wavelength on times of the order of 100 s were also observed in some instances. We attribute this behavior to excited state coupling to photoinduced changes at the nanocrystal surface.Low temperature two‐photon fluorescence excitation microscopy was used to obtain fluorescence spectra of single CdSe colloidal nanocrystals or quantum dots. The fluorescence spectra reveal large, 0.02 eV shifts of the peak emission wavelength on times of the order of 100 s were also observed in some instances. We attribute this behavior to excited state coupling to photoinduced changes at the nanocrystal surface.

Intraband transitions in semiconductor nanocrystals

Intraband spectra of CdSe nanocrystal colloids are observed. The photoexcited nanocrystals, of average diameters 31.5, 38, and 43 A, exhibit an isolated and strong infrared-induced absorption with peak energy between 0.5 and 0.3 eV. The energies and cross sections of the resonance are consistent with a one-electron transition between the delocalized IS e and IP e states of the strongly confined quantum dots.

POLAR CDSE NANOCRYSTALS : IMPLICATIONS FOR ELECTRONIC STRUCTURE

We report direct spectroscopic evidence for parity-forbidden electronic transitions in CdSe nanocrystals and relate it through perturbation theory to an internal electric field. This field is consistent with an estimated spontaneous polarization of Ps=−0.6 μC/cm2 for CdSe. The observed mixing of odd and even band edge hole states by this symmetry breaking internal electric field indicates that current theories of nanocrystal electronic structure, which assume inversion symmetry, require substantial revision.

Two-photon spectroscopy and microscopy of II–VI semiconductor nanocrystals

Abstract We review our recent nonlinear spectroscopies of nanocrystals and synthetic efforts to improve their luminescence properties. A two-photon spectroscopic study of CdSe nanocrystals as a function of size is presented and compared with predictions from the effective mass model with spherical confinement. We also detail our efforts at improving the luminescence properties of nanocrystals which have culminated in a 50% fluorescence quantum yield for inorganic capping of some sizes of CdSe nanocrystals. Finally, we present the application of two-photon microscopy to resolve fluorescence from single nanocrystals at room temperature and cryogenic temperatures.