13 results on '"Farida Selim"'
Search Results
2. Measurement and Simulation of Vacancy Formation in 2-MeV Self-irradiated Pure Fe
- Author
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Maciej Oskar Liedke, Andreas Wagner, Farida Selim, Peter Hosemann, R. Auguste, and Blas P. Uberuaga
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Materials science ,Ion beam ,0211 other engineering and technologies ,General Engineering ,02 engineering and technology ,021001 nanoscience & nanotechnology ,Molecular physics ,Positron annihilation spectroscopy ,Vacancy defect ,General Materials Science ,Neutron ,Irradiation ,0210 nano-technology ,021102 mining & metallurgy - Abstract
Positron annihilation spectroscopy is a powerful tool to quantify the amount of vacancies and vacancy clusters in materials. The technique has been utilized to study the induced defects in materials after ion beam and neutron irradiations. This paper makes the case for how the technique can and should be utilized to quantify the defects created by irradiation in situ during irradiation to foster a more thorough understanding of the surviving defects after initial collision cascades. This paper outlines a future experimental approach and its meaning for the nuclear materials community, being able to benchmark commonly used rate theory models of damage evolution.
- Published
- 2020
3. Localized UV emitters on the surface of β-Ga2O3
- Author
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Jesse Huso, Minhazul Islam, Matthew D. McCluskey, Yinchuan Yu, and Farida Selim
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010302 applied physics ,Multidisciplinary ,Materials science ,business.industry ,Band gap ,Doping ,02 engineering and technology ,021001 nanoscience & nanotechnology ,01 natural sciences ,Gallium oxide ,Semiconductor ,Excited state ,0103 physical sciences ,Optoelectronics ,Light emission ,0210 nano-technology ,business ,Luminescence ,Monoclinic crystal system - Abstract
Monoclinic gallium oxide (β-Ga2O3) is attracting intense focus as a material for power electronics, thanks to its ultra-wide bandgap (4.5–4.8 eV) and ability to be easily doped n-type. Because the holes self-trap, the band-edge luminescence is weak; hence, β-Ga2O3 has not been regarded as a promising material for light emission. In this work, optical and structural imaging methods revealed the presence of localized surface defects that emit in the near-UV (3.27 eV, 380 nm) when excited by sub-bandgap light. The PL emission of these centers is extremely bright—50 times brighter than that of single-crystal ZnO, a direct-gap semiconductor that has been touted as an active material for UV devices.
- Published
- 2020
4. Tuning the Phase and Microstructural Properties of TiO2 Films Through Pulsed Laser Deposition and Exploring Their Role as Buffer Layers for Conductive Films
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David Winarski, S. Agarwal, Le Zhang, Pooneh Saadatkia, E. Doyle, Micah Haseman, Vladimir Vasilyev, Tuoc Dang, Farida Selim, and Kevin D. Leedy
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010302 applied physics ,Indium gallium zinc oxide ,Anatase ,Materials science ,02 engineering and technology ,Substrate (electronics) ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,01 natural sciences ,Electronic, Optical and Magnetic Materials ,Pulsed laser deposition ,Chemical engineering ,Rutile ,0103 physical sciences ,Materials Chemistry ,Sapphire ,Electrical and Electronic Engineering ,Thin film ,0210 nano-technology ,Layer (electronics) - Abstract
Titanium oxide (TiO2) is a semiconducting oxide of increasing interest due to its chemical and thermal stability and broad applicability. In this study, thin films of TiO2 were deposited by pulsed laser deposition on sapphire and silicon substrates under various growth conditions, and characterized by x-ray diffraction (XRD), atomic force microscopy (AFM), optical absorption spectroscopy and Hall-effect measurements. XRD patterns revealed that a sapphire substrate is more suitable for the formation of the rutile phase in TiO2, while a silicon substrate yields a pure anatase phase, even at high-temperature growth. AFM images showed that the rutile TiO2 films grown at 805°C on a sapphire substrate have a smoother surface than anatase films grown at 620°C. Optical absorption spectra confirmed the band gap energy of 3.08 eV for the rutile phase and 3.29 eV for the anatase phase. All the deposited films exhibited the usual high resistivity of TiO2; however, when employed as a buffer layer, anatase TiO2 deposited on sapphire significantly improves the conductivity of indium gallium zinc oxide thin films. The study illustrates how to control the formation of TiO2 phases and reveals another interesting application for TiO2 as a buffer layer for transparent conducting oxides.
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- 2018
5. Optical and Electrical Properties of Sn-Doped Zinc Oxide Single Crystals
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J. T. Warfield, Armando Hernandez, Farida Selim, Pooneh Saadatkia, Micah Haseman, Gerald Earle Jellison Jr, Lynn A. Boatner, and J. Lawrence
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010302 applied physics ,Materials science ,Absorption spectroscopy ,Solid-state physics ,Dopant ,Doping ,Analytical chemistry ,02 engineering and technology ,Conductivity ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,01 natural sciences ,Thermoluminescence ,Electronic, Optical and Magnetic Materials ,Condensed Matter::Materials Science ,Condensed Matter::Superconductivity ,0103 physical sciences ,Materials Chemistry ,Condensed Matter::Strongly Correlated Electrons ,Electrical and Electronic Engineering ,Inductively coupled plasma ,0210 nano-technology ,Spectroscopy - Abstract
Sn dopant in ZnO may significantly improve the n-type conductivity of ZnO through a characteristic double effect. However, studies on bulk Sn-doped ZnO are rare, and the effect of Sn doping on the optoelectronic properties of bulk ZnO is not well understood. In this work, the effect of Sn doping on the optical and electrical properties of ZnO bulk single crystals was investigated through optical absorption spectroscopy, Hall-effect measurements, and thermoluminescence (TL) spectroscopy. Undoped and Sn-doped ZnO single crystals were grown by chemical vapor transport method and characterized by x-ray diffraction analysis. The Sn doping level in the crystals was evaluated by inductively coupled plasma mass spectroscopy measurements. Hall-effect measurements revealed an increase in conductivity and carrier concentration with increasing Sn doping, while TL measurements identified a few donor species in the crystals with donor ionization energy ranging from 35 meV to 118 meV. Increasing Sn doping was also associated with a color change of single crystals from colorless to dark blue.
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- 2017
6. Study of Trapping Phenomena in SrTiO3 by Thermally Stimulated Techniques
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David C. Look, Buguo Wang, Farida Selim, and Pooneh Saadatkia
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010302 applied physics ,Photocurrent ,Materials science ,Solid-state physics ,Thermally stimulated current spectroscopy ,Analytical chemistry ,02 engineering and technology ,Trapping ,Atmospheric temperature range ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,01 natural sciences ,Molecular physics ,Electronic, Optical and Magnetic Materials ,0103 physical sciences ,Materials Chemistry ,Electrical measurements ,Electrical and Electronic Engineering ,0210 nano-technology ,Luminescence ,Excitation - Abstract
Thermally stimulated current (TSC), thermally stimulated depolarization current (TSDC), and thermally stimulated luminescence (TSL) spectroscopies were combined to study trapping phenomena in undoped bulk SrTiO3 crystals. Electrical measurements were also performed and showed that the crystals are highly resistive in the dark but exhibited an unusually high photocurrent upon 400-nm illumination. Several traps were revealed in both TSC and TSDC spectra between 83 K and 450 K in such a broad temperature range and their activation energies were extrapolated from the trap positions (peaks). TSL spectra demonstrate similar characteristics comparable to TSC and TSDC spectra, though there are some differences because of different excitation and recombination mechanisms. This work reveals the presence of large number of traps in SrTiO3 single crystals, which are most likely the source of many of the interesting phenomena in SrTiO3 such as transient and persistent photoconductivity.
- Published
- 2017
7. High dispersibility of α-Al2O3 powders from coprecipitation method by step-by-step horizontal ball-milling
- Author
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Hao Yang, Le Zhang, Farida Selim, Wei Shuai, Tianyuan Zhou, Hao Chen, Zheng Li, Yue Ben, and Ching-Ping Wong
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Materials science ,Coprecipitation ,Dispersity ,Mineralogy ,02 engineering and technology ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,01 natural sciences ,Atomic and Molecular Physics, and Optics ,0104 chemical sciences ,Electronic, Optical and Magnetic Materials ,law.invention ,Chemical engineering ,law ,visual_art ,Phase (matter) ,visual_art.visual_art_medium ,Calcination ,Ceramic ,Particle size ,Electrical and Electronic Engineering ,0210 nano-technology ,Dispersion (chemistry) ,Ball mill - Abstract
α-Al2O3 powder is widely used in numerous applications, especially in optical ceramics and crystals growth. However, achieving perfect dispersibility is essential in these applications and requires a complete understanding of the dispersion mechanism as well as high technology processing. In this paper, a step-by-step horizontal ball-milling method was adopted to improve the dispersibility of α-Al2O3 powders through coprecipitation. Effects of ball milling processes and Al3+ concentration on the particle size and the dispersity of alumina precursor were systematically investigated. The presence of absorbed sulfate ion on the surface of the particles could inhibit aggregation processes. Precursors were evolved into α-Al2O3 phase after subsequent calcination of 1180 °C in air. The work shows that step-by-step horizontal ball-milling achieves a well-dispersible powder with quasi-spherical morphology and narrow size distribution in the range of 150 ~ 200 nm.
- Published
- 2017
8. High sinterability nano-Y2O3 powders prepared via decomposition of hydroxyl-carbonate precursors for transparent ceramics
- Author
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Lixi Wang, Farida Selim, Hao Chen, Ching-Ping Wong, Rong Sun, Le Zhang, Zheng Li, Zhen Fangzheng, and Qitu Zhang
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010302 applied physics ,Materials science ,Transparent ceramics ,Coprecipitation ,Mechanical Engineering ,Sintering ,02 engineering and technology ,021001 nanoscience & nanotechnology ,01 natural sciences ,law.invention ,Chemical engineering ,Mechanics of Materials ,law ,visual_art ,0103 physical sciences ,Nano ,visual_art.visual_art_medium ,General Materials Science ,Calcination ,Ceramic ,Particle size ,Composite material ,0210 nano-technology ,Dispersion (chemistry) - Abstract
High sinterability nano-Y2O3 powders for transparent ceramics were successfully synthesized via the decomposition of hydroxyl-carbonate precursors from spray coprecipitation. The chemical composition of the precursor was determined as Y(CO3)(OH)·nH2O (n = 1–1.5), and it was evolved into Y2O3 particles with clear facets after calcination with the assistance of sulfate. Two dispersion mechanisms, “absorption” and “intercalation,” were proposed to work together to provide the dispersion effect. Microstructural and optical characterization of powders and as-fabricated transparent ceramics was employed to evaluate the sintering behavior of powders. The nanopowders calcined at 1250 °C had weakly agglomerated morphology with the mean particle size of ~140 nm and exhibited excellent sinterability. The in-line transmittance of Y2O3 ceramic of 1 mm thickness that was vacuum sintered at 1800 °C for 8 h without any sintering additives reached 78.7% at 1064 nm.
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- 2017
9. Fourier Transform Infrared Spectroscopy Measurements of Multi-phonon and Free-Carrier Absorption in ZnO
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G. Ariyawansa, Lynn A. Boatner, Farida Selim, David C. Look, Kevin D. Leedy, and Pooneh Saadatkia
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Absorption spectroscopy ,Hydrogen ,Analytical chemistry ,chemistry.chemical_element ,02 engineering and technology ,Atmospheric temperature range ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,01 natural sciences ,Electronic, Optical and Magnetic Materials ,Crystal ,chemistry ,0103 physical sciences ,Materials Chemistry ,Electrical and Electronic Engineering ,Free carrier absorption ,Fourier transform infrared spectroscopy ,Thin film ,010306 general physics ,0210 nano-technology ,Absorption (electromagnetic radiation) - Abstract
Fourier transform infrared (FTIR) measurements were carried out on thin films and bulk single crystals of ZnO over a wide temperature range to study the free-carrier and multi-phonon infrared absorptions and the effects of hydrogen incorporation on these properties. Aluminum-doped ZnO thin films were deposited on quartz substrates using atomic-layer deposition (ALD) and sol–gel methods. Hall-effect measurements showed that the ALD films have a resistivity of ρ = 1.11 × 10−3 Ω cm, three orders of magnitude lower than sol–gel films (ρ = 1.25 Ω cm). This result is consistent with the significant difference in their free-carrier absorption as revealed by FTIR spectra obtained at room temperature. By reducing the temperature to 80 K, the free carriers were frozen out, and their absorption spectrum was suppressed. From the FTIR measurements on ZnO single crystals that were grown by the chemical vapor transport method, we identified a shoulder around 3350 cm−1 and associated it with the presence of two or more hydrogen ions in a Zn vacancy. After reducing the hydrogen level in the crystal, the measurements revealed the multi-phonon absorption of ZnO in the range of 700–1200 cm−1. This study shows that the multi-phonon absorption bands can be completely masked by the presence of a large concentration of hydrogen in the crystals.
- Published
- 2016
10. Effects of Substrate and Post-Growth Treatments on the Microstructure and Properties of ZnO Thin Films Prepared by Atomic Layer Deposition
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Andreas Wagner, Pooneh Saadatkia, Kevin D. Leedy, Micah Haseman, W. Anwand, David Winarski, David C. Look, Farida Selim, and S. Tetlak
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010302 applied physics ,Electron mobility ,Materials science ,Annealing (metallurgy) ,Analytical chemistry ,02 engineering and technology ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,01 natural sciences ,Electronic, Optical and Magnetic Materials ,Positron annihilation spectroscopy ,Crystallinity ,Atomic layer deposition ,0103 physical sciences ,Materials Chemistry ,Electrical and Electronic Engineering ,Thin film ,0210 nano-technology ,Wurtzite crystal structure ,Transparent conducting film - Abstract
Aluminum-doped zinc oxide (ZnO:Al) thin films were synthesized by atomic layer deposition on silicon, quartz and sapphire substrates and characterized by x-ray diffraction (XRD), high-resolution scanning electron microscopy, optical spectroscopy, conductivity mapping, Hall effect measurements and positron annihilation spectroscopy. XRD showed that the as-grown films are of single-phase ZnO wurtzite structure and do not contain any secondary or impurity phases. The type of substrate was found to affect the orientation and degree of crystallinity of the films but had no effect on the defect structure or the transport properties of the films. High conductivity of 10−3 Ω cm, electron mobility of 20 cm2/Vs and carrier density of 1020 cm−3 were measured in most films. Thermal treatments in various atmospheres induced a large effect on the thickness, structure and electrical properties of the films. Annealing in a Zn and nitrogen environment at 400°C for 1 h led to a 16% increase in the thickness of the film; this indicates that Zn extracts oxygen atoms from the matrix and forms new layers of ZnO. On the other hand, annealing in a hydrogen atmosphere led to the emergence of an Al2O3 peak in the XRD pattern, which implies that hydrogen and Al atoms compete to occupy Zn sites in the ZnO lattice. Only ambient air annealing had an effect on film defect density and electrical properties, generating reductions in conductivity and electron mobility. Depth-resolved measurements of positron annihilation spectroscopy revealed short positron diffusion lengths and high concentrations of defects in all as-grown films. However, these defects did not diminish the electrical conductivity in the films.
- Published
- 2016
11. Scintillation of Un-doped ZnO Single Crystals
- Author
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P. S. Stepanov, Jianfeng Ji, Farida Selim, Lynn A. Boatner, and A. M. Colosimo
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Photomultiplier ,Materials science ,Physics::Instrumentation and Detectors ,Mineralogy ,Crystal growth ,02 engineering and technology ,01 natural sciences ,Particle detector ,Condensed Matter::Materials Science ,0103 physical sciences ,Physics::Atomic and Molecular Clusters ,General Materials Science ,010306 general physics ,Spectroscopy ,Scintillation ,Condensed Matter::Other ,business.industry ,Mechanical Engineering ,Doping ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,Anode ,Mechanics of Materials ,Optoelectronics ,0210 nano-technology ,Luminescence ,business - Abstract
Scintillation properties are often studied by photo-luminescence (PL) and scintillation measurements. In this work, we combine X-ray-induced luminescence (XRIL) spectroscopy [Review of Scientific Instruments 83, 103112 (2012)] with PL and standard scintillation measurements to give insight into the scintillation properties of un-doped ZnO single crystals. XRIL revealed that ZnO luminescence proportionally increases with X-ray power and exhibits excellent linearity - indicating the possibility of developing radiation detectors with good energy resolution. By coupling ZnO crystals to fast photomultiplier tubes and monitoring the anode signal, rise times as fast as 0.9 ns were measured.
- Published
- 2016
12. Diffusion of 111Cd probes in Ga7Pt3 studied via nuclear quadrupole relaxation
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Gary S. Collins, Farida Selim, and John P. Bevington
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Arrhenius equation ,Nuclear and High Energy Physics ,Chemistry ,Terahertz radiation ,Intermetallic ,Gamma ray ,Perturbation (astronomy) ,Condensed Matter Physics ,Atomic and Molecular Physics, and Optics ,symbols.namesake ,Impurity ,Quadrupole ,symbols ,Physical and Theoretical Chemistry ,Atomic physics ,Electric field gradient - Abstract
Diffusion of 111In/Cd impurity atoms was studied in Ga7Pt3 at the atomic level using the method of perturbed angular correlation of gamma rays (PAC). As in previous measurements on Ga7Pd3, probes were observed to occupy two inequivalent Ga-sublattices. Quadrupole interaction perturbation functions for each site exhibited damping at elevated temperatures that is attributed to nuclear relaxation caused by stochastic jumps of the probe atoms accompanied by reorientation of axes of the electric field gradient tensor. Fitted relaxation frequencies, proportional to mean jump frequencies of the probe, were thermally activated. Arrhenius plots revealed jump-frequency activation enthalpies of 0.94 (8) and 0.67 (4) eV for the two sites and frequency prefactors close to 1 THz. Results were similar to those found previously for Ga7Pd3, although jump frequencies are about 100 times lower in Ga7Pt3.
- Published
- 2007
13. Spatial sampling of crystal electrons by in-flight annihilation of fast positrons
- Author
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Thomas E. Cowan, Richard H. Howell, Jene Andrew Golovchenko, R. Haakenaasen, Kelvin G. Lynn, Farida Selim, David Cassidy, and Alan W. Hunt
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Physics ,Multidisciplinary ,Annihilation ,Positron ,Recoil ,Scattering ,Annihilation radiation ,Electron ,Atomic physics ,Channelling ,Charged particle - Abstract
Energetic, positively charged particles travelling along a low-index crystal direction undergo many highly correlated, small-angle scattering events; the effect of these interactions is to guide or ‘channel’ (refs 1,2,3,4,5,6,7,8) the particles through the lattice. Channelling effectively focuses positive particles into the interstitial regions of the crystal: nuclear collisional processes such as Rutherford backscattering are suppressed, while the number of interactions with valence electrons increases. The interaction of channelled positrons with electrons produces annihilation radiation that can in principle9,10,11,12 serve as a quantitative, spatially selective probe of electronic charge and spin densities within the crystal: in the interstitial regions, two-photon annihilation is enhanced relative to single-photon annihilation, because the latter process requires a nuclear recoil to conserve momentum. Here we report observations of single- and two-photon annihilation from channelled positrons, using a monoenergetic beam flux of 105 particles per second. Comparison of these two annihilation modes demonstrates the ability of channelled positrons to selectively sample valence electrons in a crystal. Useful practical implementation of the technique will require the development of more intense positron beams with fluxes approaching 107 particles per second.
- Published
- 1999
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