Browsing by Author "Everitt, HO"
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Item Open Access Design and Signature Analysis of Remote Trace-Gas Identification Methodology Based on Infrared-Terahertz Double-Resonance Spectroscopy(PHYSICAL REVIEW APPLIED, 2014-11-26) Tanner, EA; Phillips, DJ; Persons, CM; De Lucia, FC; Everitt, HOItem Open Access Effect of pressure and Al doping on structural and optical properties of ZnO nanowires synthesized by chemical vapor deposition(JOURNAL OF LUMINESCENCE, 2014-02) Mohanta, A; Simmons, JG; Everitt, HO; Shen, G; Margaret Kim, S; Kung, PItem Open Access Effects of reabsorption and spatial trap distributions on the radiative quantum efficiencies of ZnO(Physical Review B - Condensed Matter and Materials Physics, 2010-03-15) Foreman, JV; Everitt, HO; Yang, J; McNicholas, T; Liu, JUltrafast time-resolved photoluminescence spectroscopy following one- and two-photon excitations of ZnO powder is used to gain unprecedented insight into the surprisingly high external quantum efficiency of its "green" defect emission band. The role of exciton diffusion, the effects of reabsorption, and the spatial distributions of radiative and nonradiative traps are comparatively elucidated for the ultraviolet excitonic and "green" defect emission bands in both unannealed nanometer-sized ZnO powders and annealed micrometer-sized ZnO:Zn powders. We find that the primary mechanism limiting quantum efficiency is surface recombination because of the high density of nonradiative surface traps in these powders. It is found that unannealed ZnO has a high density of bulk nonradiative traps as well, but the annealing process reduces the density of these bulk traps while simultaneously creating a high density of green-emitting defects near the particle surface. The data are discussed in the context of a simple rate equation model that accounts for the quantum efficiencies of both emission bands. The results indicate how defect engineering could improve the efficiency of ultraviolet-excited ZnO:Zn-based white light phosphors. © 2010 The American Physical Society.Item Open Access Global k-space analysis of electron-phonon interaction in graphene and application to M-point spectroscopy(PHYSICAL REVIEW B, 2016-02-08) Binder, R; Roberts, AT; Kwong, NH; Sandhu, A; Everitt, HOItem Open Access How Annealing and Charge Scavengers Affect Visible Emission from ZnO Nanocrystals(JOURNAL OF PHYSICAL CHEMISTRY C, 2016-03-10) Reish, ME; Zhang, Z; Ma, S; Harrison, I; Everitt, HOItem Open Access Optimal composition of europium gallium oxide thin films for device applications(2010) Wellenius, P; Smith, ER; Leboeuf, SM; Everitt, HO; Muth, JFEuropium gallium oxide (EuxGa1-x)(2)O-3 thin films were deposited on sapphire substrates by pulsed laser deposition with varying Eu content from x=2.4 to 20 mol %. The optical and physical effects of high europium concentration on these thin films were studied using photoluminescence (PL) spectroscopy, x-ray diffraction (XRD), and Rutherford backscattering spectrometry. PL spectra demonstrate that emission due to the D-5(0) to F-7(J) transitions in Eu3+ grows linearly with Eu content up to 10 mol %. Time-resolved PL indicates decay parameters remain similar for films with up to 10 mol % Eu. At 20 mol %, however, PL intensity decreases substantially and PL decay accelerates, indicative of parasitic energy transfer processes. XRD shows films to be polycrystalline and beta-phase for low Eu compositions. Increasing Eu content beyond 5 mol % does not continue to modify the film structure and thus, changes in PL spectra and decay cannot be attributed to structural changes in the host. These data indicate the optimal doping for optoelectronic devices based on (EuxGa1-x)(2)O-3 thin films is between 5 and 10 mol %. (C) 2010 American Institute of Physics. [doi:10.1063/1.3319670]Item Open Access Product selectivity in plasmonic photocatalysis for carbon dioxide hydrogenation(Nature Communications, 2017-02-23) Zhang, X; Li, X; Zhang, D; Su, NQ; Yang, W; Everitt, HO; Liu, JItem Open Access Room temperature photoluminescence from InxAl(1-x)N films deposited by plasma-assisted molecular beam epitaxy(Applied Physics Letters, 2014-09-29) Kong, W; Mohanta, A; Roberts, AT; Jiao, WY; Fournelle, J; Kim, TH; Losurdo, M; Everitt, HO; Brown, AS© 2014 AIP Publishing LLC.InAlN films deposited by plasma-assisted molecular beam epitaxy exhibited a lateral composition modulation characterized by 10-12 nm diameter, honeycomb-shaped, columnar domains with Al-rich cores and In-rich boundaries. To ascertain the effect of this microstructure on its optical properties, room temperature absorption and photoluminescence characteristics of InxAl(1-x)N were comparatively investigated for indium compositions ranging from x = 0.092 to 0.235, including x = 0.166 lattice matched to GaN. The Stokes shift of the emission was significantly greater than reported for films grown by metalorganic chemical vapor deposition, possibly due to the phase separation in these nanocolumnar domains. The room temperature photoluminescence also provided evidence of carrier transfer from the InAlN film to the GaN template.Item Open Access Spectroscopic investigation of coupling among asymmetric InGaN/GaN multiple quantum wells grown on non-polar a-plane GaN substrates(APPLIED PHYSICS LETTERS, 2013-10-28) Roberts, AT; Mohanta, A; Everitt, HO; Leach, JH; Van Den Broeck, D; Hosalli, AM; Paskova, T; Bedair, SMItem Open Access Terahertz Digital Holographic Imaging of Voids Within Visibly Opaque Dielectrics(IEEE Transactions on Terahertz Science and Technology, 2014-11-11) Heimbeck, MS; Ng, WR; Golish, DR; Gehm, ME; Everitt, HOTerahertz digital off-axis holography (THzDH) has been demonstrated as a non-destructive tool for imaging voids within visually opaque dielectrics. Using a raster scanning heterodyne detector, the imager captures lensless transmission holograms formed by the interaction of a highly coherent, monochromatic beam with 3-D printed structures. Digital hologram reconstructions from two structures were used to measure the imager's modulation transfer function and to show that terahertz digital holography can provide sub-millimeter resolution images of voids within visually opaque printed structures. As a demonstration we imaged embedded air- and lossy dielectric filled-voids whose refractive indices differ from the host material.Item Open Access Terahertz photovoltaic detection of cyclotron resonance in the regime of radiation-induced magnetoresistance oscillations(PHYSICAL REVIEW B, 2013-06-17) Mani, RG; Ramanayaka, AN; Ye, T; Heimbeck, MS; Everitt, HO; Wegscheider, WItem Open Access Ultraviolet-Visible Plasmonic Properties of Gallium Nanoparticles Investigated by Variable-Angle Spectroscopic and Mueller Matrix Ellipsometry(ACS Photonics, 2014-07-16) Yang, Y; Akozbek, N; Kim, TH; Sanz, JM; Moreno, F; Losurdo, M; Brown, AS; Everitt, HO© 2014 American Chemical Society.Self-assembled, irregular ensembles of hemispherical Ga nanoparticles (NPs) were deposited on sapphire by molecular beam epitaxy. These samples, whose constituent unimodal or bimodal distribution of NP sizes was controlled by deposition time, exhibited localized surface plasmon resonances tunable from the ultraviolet to the visible (UV/vis) spectral range. The optical response of each sample was characterized using a variable-angle spectroscopic ellipsometer, and the dielectric response of the ensemble of NPs on each sample was parametrized using Lorentz oscillators. From this, a relationship was found between NP size and the deduced Lorentzian parameters (resonant frequency, damping, oscillator strength) for most unimodal and bimodal samples at most frequencies and angles of incidence. However, for samples with a bimodal size distribution, Mueller matrix ellipsometry revealed nonspecular scattering at particular frequencies and angles, suggesting a resonant interparticle coupling effect consistent with recently observed strong local field enhancements in the ultraviolet. (Graph presented).