Daniel Mayo

Anti-reflective polymer-nanocomposite coatings fabricated by RIR-MAPLE

There is increasing demand for functional polymeric optical coatings for plastic substrates. In the case of anti-reflective (AR) coatings, this is challenging because polymers exhibit a relatively narrow range of refractive indices. We synthesized a four-layer AR stack using hybrid polymer:nanoparticle materials deposited by resonant infrared matrixassisted pulsed laser evaporation (RIR-MAPLE). An Er:YAG laser ablated frozen solutions of a high-index composite containing TiO2 nanoparticles and PMMA, alternating with a low-index solution of PMMA. The optimized AR coatings, with thicknesses calculated using commercial software, yielded a coating for polycarbonate with relative transmission over 94%, scattering less than 5% and a reflection coefficient below 0.8% across the visible range.

Probing plasmons in three dimensions by combining complementary spectroscopies in a scanning transmission electron microscope

The nanoscale optical response of surface plasmons in three-dimensional metallic nanostructures plays an important role in many nanotechnology applications, where precise spatial and spectral characteristics of plasmonic elements control device performance. Electron energy loss spectroscopy (EELS) and cathodoluminescence (CL) within a scanning transmission electron microscope have proven to be valuable tools for studying plasmonics at the nanoscale. Each technique has been used separately, producing three-dimensional reconstructions through tomography, often aided by simulations for complete characterization. Here we demonstrate that the complementary nature of the two techniques, namely that EELS probes beam-induced electronic excitations while CL probes radiative decay, allows us to directly obtain a spatially- and spectrally-resolved picture of the plasmonic characteristics of nanostructures in three dimensions. The approach enables nanoparticle-by-nanoparticle plasmonic analysis in three dimensions to aid in the design of diverse nanoplasmonic applications.

Zinc oxide nanowire gamma ray detector with high spatiotemporal resolution

Conventional scintillation detectors are typically single crystals of heavy-metal oxides or halides doped with rare-earth ions that record the recombination of electron-hole pairs by photon emission in the visible to ultraviolet. However, the light yields are typically low enough to require photomultiplier detection with the attendant instrumental complications. Here we report initial studies of gamma ray detection by zinc oxide (ZnO) nanowires, grown by vapor-solid deposition. The nanowires grow along the c-axis in a wurtzite structure; they are typically 80 nm in diameter and have lengths of 1- 2 μm. The nanowires are single crystals of high quality, with a photoluminescence (PL) yield from band-edge exciton emission in the ultraviolet that is typically one hundred times larger than the PL yield from defect centers in the visible. Nanowire ensembles were irradiated by 662 keV gamma rays from a Cs-137 source for periods of up to ten hours; gamma rays in this energy range interact by Compton scattering, which in ZnO creates F+ centers that relax to form singly-charged positive oxygen vacancies. Following irradiation, we fit the PL spectra of the visible emission with a sum of Gaussians at the energies of the known defects. We find highly efficient PL from the irradiated area, with a figure of merit approaching 106 photons/s/MeV of deposited energy. Over a period of days, the singly charged O+ vacancies relax to the more stable doubly charged O++ vacancies. However, the overall defect PL returns to pre-irradiation values after about a week, as the vacancies diffuse to the surface of these very thin nanowires, indicating that a self-healing process restores the nanowires to their original state.

Direct Observation of Plasmonic Enhancement of Emission in Ag-nanoparticle-decorated ZnO nanostructures

1. Vanderbilt University, Department of Physics and Astronomy, Nashville, TN USA 2. Oak Ridge National Laboratory, Materials Science and Technology Division, Oak Ridge, TN USA 3. Vanderbilt University, Interdisciplinary Materials Science Program, Nashville TN, USA 4. Fisk University, Department of Physics and Astronomy, Nashville, TN USA 5. National University of Singapore, Department of Materials Science and Engineering, Singapore 6. Vanderbilt University, Department of Electrical Engineering and Computer Science, Nashville, TN USA

Probing Plasmons in Three Dimensions within Random Morphology Nanostructures

1. Vanderbilt University, Department of Physics and Astronomy, Nashville, TN USA 2. Oak Ridge National Laboratory, Materials Science and Technology Division, Oak Ridge, TN USA 3. Vanderbilt University, Interdisciplinary Materials Science Program, Nashville TN, USA 4. Fisk University, Department of Physics and Astronomy, Nashville, TN USA 5. National University of Singapore, Department of Materials Science and Engineering, Singapore 6. Vanderbilt University, Department of Electrical Engineering and Computer Science, Nashville, TN USA

Polymer-nanocomposite anti-reflective coating fabricated by resonant IR matrix-assisted pulsed laser evaporation

We demonstrate a multilayer anti-reflective conformal coating for polycarbonate substrates, using a a polymer nanocomposite, fabricated by resonant infrared pulsed laser evaporation. The coating has 97% transmission, and less than 0.6% reflectivity over the visible spectrum.