Richard D. Robinson

Cerium oxide nanoparticles: Size-selective formation and structure analysis

Nanoparticles of cerium oxide with a narrow size distribution (±15%) are prepared by mixing cerium nitrate solution with an ammonium reagent. High-resolution transmission electron microscopy (TEM) indicates that over 99% of the synthesized particles are single crystals. TEM and photon absorption are used to monitor particle size. The lattice parameter increases up to 0.45% as the particle size decreases to 6 nm, as observed with x-ray diffraction. Raman spectra also suggest the particle-size effect and concomitant lattice expansion. The lattice expansion can be explained by increased concentrations of point defects with decreasing particle size.

Barium titanate nanocrystals and nanocrystal thin films: Synthesis, ferroelectricity, and dielectric properties

Advanced applications for high k dielectric and ferroelectric materials in the electronics industry continues to demand an understanding of the underlying physics in decreasing dimensions into the nanoscale. We report the synthesis, processing, and electrical characterization of thin (<100nm thick) nanostructured thin films of barium titanate (BaTiO3) built from uniform nanoparticles (<20nm in diameter). We introduce a form of processing as a step toward the ability to prepare textured films based on assembly of nanoparticles. Essential to this approach is an understanding of the nanoparticle as a building block, combined with an ability to integrate them into thin films that have uniform and characteristic electrical properties. Our method offers a versatile means of preparing BaTiO3 nanocrystals, which can be used as a basis for micropatterned or continuous BaTiO3 nanocrystal thin films. We observe the BaTiO3 nanocrystals crystallize with evidence of tetragonality. We investigated the preparation of wel...

Visible thermal emission from sub-band-gap laser excited cerium dioxide particles

Cerium dioxide particles excited in air with sub-band-gap radiation emit very broad radiation in the visible spectrum above a threshold intensity that decreases with increasing ambient temperature. Concomitant with this emission is the near disappearance of the Stokes and anti-Stokes Raman scattering peaks. Both phenomena are reversible in air up to just above threshold, and are seen for nanoparticles and several-micron-diameter particles with particle diameter comparable to or smaller than the laser focus. Temperature estimates using the Stokes/anti-Stokes scattering intensity ratio suggest there is laser heating due to small intragap absorption and possible nonlinear processes, given the very slow thermal conduction. The broad emission in this loose powder may well be due to thermal emission, on the basis of spectral fitting of the high-energy part of the spectrum to a blackbody radiator at ∼1200–1400u200a°C, although luminescence from a new phase is a possibility. The sudden decrease in Raman scattering and increase in emission in air are consistent with a transition to a new, possibly luminescent, phase, as is the continued disappearance of the Raman peaks in forming gas when the laser power is reduced below the upstroke threshold. Oxygen point defects and their complexes may play an important role in many of these processes.

Observation of Fano asymmetry in Raman spectra of SrTiO3 and CaxSr1−xTiO3 perovskite nanocubes

Bulk SrTiO3 is cubic and not expected to exhibit any first-order Raman scattering. However, nanocubes of SrTiO3 with an edge length of 80±10nm show strong first-order Raman scattering originating from the breaking of symmetry caused by frozen surface dipoles (local tetragonality) and the presence of nanoscopic polar domains (arising from incorporated impurities). Rapid polarization fluctuations within these nanoscopic ferroelectric regions interfere with a polar phonon, resulting in a Fano-like asymmetric line shape in these SrTiO3 nanocubes, as well as in Ca0.3Sr0.7TiO3 nanocubes.

Determination of size and composition of optically active CdZnSe/ZnBeSe quantum dots

The size and composition of optically active CdxZn1−xSe/Zn0.97Be0.03Se quantum dots (QDs) are determined using photoluminescence, photoluminescence excitation, and Raman scattering spectroscopies combined with a model of photoluminescence and LO phonon energies. The diameters of optically active QDs range from 5.1 to 8.0 nm with Cd composition in the range of 47%–54%, corresponding to the “small” QDs group. Additionally, surface phonons from QDs are observed in this system.

CdZnSe/Zn(Be)Se Quantum Dot Structures: Size, Chemical Composition and Phonons

The size and chemical composition of optically active CdZnSe/ZnSe and CdZnSe/Zn0.97Be0.03Se quantum dots (QDs) are determined using photoluminescence, photoluminescence excitation and polarized Raman scattering spectroscopies. We show that the addition of Be into the barrier enhances the Cd composition and the quantum size effect of optically active QDs. Additionally, surface phonons from QDs are observed in CdZnSe/ZnBeSe nanostructures.