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At least 1,513 records · Page 84
All sourcesNASA NTRSDOE OSTI
DOE OSTI2011

Magnetic and transport properties of transparent SrSn{sub 0.9}Sb{sub 0.05}Fe{sub 0.05}O{sub 3} semiconductor films

The effect of antimony doping on the structural, magnetic, and electrical properties of transparent SrSn{sub 0.9}Sb{sub 0.05}Fe{sub 0.05}O{sub 3} films synthesized by RF sputtering on oxidized Si and quartz substrates has been investigated. A reduction in electrical resistivity by two orders of magnitude compared to 5% Fe doped SrSnO{sub 3} film was observed. The electrical conductivity behavior has been analyzed using the Mott's Variable range hopping model. The nature of magnetic ordering were investigated by field cooled (FC) and zero field cooled (ZFC) magnetization measurements. The applicability of models based on oxygen vacancies to explain the magnetic ordering present in the sample has been discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2010

EPR and ab initio calculation study on the EI4 center in 4H- and 6H-SiC

We present results from electron paramagnetic resonance (EPR) studies of the EI4 EPR center in 4H- and 6H-SiC. The EPR signal of the EI4 center was found to be drastically enhanced in electron-irradiated high-purity semi-insulating materials after annealing at 700-750 deg. C. Strong EPR signals of the EI4 center with minimal interferences from other radiation-induced defects in irradiated high-purity semi-insulating materials allowed our more detailed study of the hyperfine (hf) structures. An additional large-splitting {sup 29}Si hf structure and {sup 13}C hf lines of the EI4 defect were observed. Comparing the data on the hf interactions and the annealing behavior obtained from EPR experiments and from ab initio supercell calculations of different carbon-vacancy-related complexes, we suggest a complex between a carbon vacancy-carbon antisite and a carbon vacancy at the third-neighbor site of the antisite in the neutral charge state, (V{sub C}-C{sub Si}V{sub C}){sup 0}, as a new defect model for the EI4 center.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2010

Excitons and photoluminescence in ZnO and Zn{sub 0.99}Mn{sub 0.01}O nanocrystals

The photoluminescence and photoluminescence excitation spectra for Zn{sub 1-x}Mn{sub x}O nanocrystals are presented. After annealing of powders in air, the intensity of the bands attributable to manganese decreases noticeably. This suggests that the oxygen vacancies affect the Zhang-Rice-like states appearing due to strong d-p-hybridization, which is confirmed by an increase in the band gap of Zn{sub 1-x}Mn{sub x}O for low x. The origin of the 2.9-eV peak and the shape of its excitation spectrum are discussed qualitatively. For Zn{sub 1-x}Mn{sub x}O nanocrystals, the shape of the excitation spectrum is as unusual as the intense absorption in the range (2.2-3.0) eV.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Local structure in solid solutions of stabilised zirconia with actinide dioxides (UO{sub 2}, NpO{sub 2})

The local structure of (Zr,Lu,U)O{sub 2-x} and (Zr,Y,Np)O{sub 2-x} solid solutions has been investigated by extended X-ray absorption fine structure (EXAFS). Samples were prepared by mixing reactive (Zr,Lu)O{sub 2-x} and (Zr,Y)O{sub 2-x} precursor materials with the actinide oxide powders, respectively. Sintering at 1600 {sup o}C in Ar/H{sub 2} yields a fluorite structure with U(IV) and Np(IV). As typical for stabilised zirconia the metal-oxygen and metal-metal distances are characteristic for the different metal ions. The bond lengths increase with actinide concentration, whereas highest adaptation to the bulk stabilised zirconia structure was observed for U---O and Np---O bonds. The Zr---O bond shows only a slight increase from 2.14 A at 6 mol% actinide to 2.18 A at infinite dilution in UO{sub 2} and NpO{sub 2}. The short interatomic distance between Zr and the surrounding oxygen and metal atoms indicate a low relaxation of Zr with respect to the bulk structure, i.e. a strong Pauling behaviour. -- Graphical abstract: Metal-oxygen bond distances in (Zr,Lu,U)O{sub 2-x} solid solutions with different oxygen vacancy concentrations (Lu/Zr=1 and Lu/Zr=0.5). Display Omitted Research Highlights: {yields} EXAFS indicates high U and Np adaption to the bulk structure of stabilised zirconia. {yields} Zr---O bond length is 2.18 A at infinite Zr dilution in UO{sub 2} and NpO{sub 2}. {yields} Low relaxation (strong Pauling behaviour) of Zr explains its low solubility in UO{sub 2}.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Major deep levels with the same microstructures observed in n-type 4H-SiC and 6H-SiC

Major deep levels observed in as-grown and irradiated n-type 4H-SiC and 6H-SiC epilayers have been investigated. After low-energy electron irradiation, by which only carbon atoms are displaced, five traps, EH1 (E{sub C}-0.36 eV), Z{sub 1}/Z{sub 2} (E{sub C}-0.65 eV), EH3 (E{sub C}-0.79 eV), EH5 (E{sub C}-1.0 eV), and EH6/7 (E{sub C}-1.48 eV), were detected in 4H-SiC and four traps, E{sub 1}/E{sub 2} (E{sub C}-0.45 eV), RD{sub 5} (E{sub C}-0.57 eV), ES (E{sub C}-0.80 eV), and R (E{sub C}-1.25 eV), were detected in 6H-SiC. The Z{sub 1}/Z{sub 2}, EH6/7 centers in 4H-SiC and the E{sub 1}/E{sub 2}, R centers in 6H-SiC exhibit common features as follows: their generation rates by the e{sup -}-irradiation were almost the same each other, their concentrations were not changed by heat treatments up to 1500 deg. C, and they showed very similar annealing behaviors at elevated temperatures. Furthermore, these defect centers were almost eliminated by thermal oxidation. Taking account of the observed results and the energy positions, the authors suggest that the Z{sub 1}/Z{sub 2} center in 4H-SiC corresponds to the E{sub 1}/E{sub 2} center in 6H-SiC, and the EH6/7 center in 4H-SiC to the R center in 6H-SiC, respectively. Since the concentrations of these four centers are almost the same for as-grown, electron-irradiated, annealed, and oxidized samples, these centers will contain a common intrinsic defect, most likely carbon vacancy. The authors also observed similar correspondence for other thermally unstable traps in 4H-SiC and 6H-SiC.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Analysis of Interstitial Elements in Niobium with Secondary Ion Mass Spectrometry (SIMS)

Superconducting Radio Frequency (SRF) cavities provide enhanced efficiency and reduced energy utilization in present day particle accelerators. Niobium (Nb) is the material of choice for these cavities due to its high critical temperature and critical magnetic field. In order to understand why certain treatments, especially a low temperature bake, improve performance, it is important to study Nb surface characteristics and identify elemental contamination that can affect the performance of the cavity. H, C, O, and N are of interest because they are interstitial impurities in Nb. In earlier work, SIMS analysis using a CAMECA IMS-6F with Cs{sup +} primary beam showed that C and N were probably not significant factors impacting performance but there was a very high level of H in the Nb. Ion implants of C, N, O, and D into Nb provided quantification of C, N, O and indicated that D is very mobile in the Nb. Further analyses showed that heat treated Nb has lower levels of surface H than non heat treated Nb and subsequent removal of surface oxide by etching causes intake of H in a heat treated Nb sample. This result helps confirm the role of surface oxide as a hydrogen barrier. To further understand the oxide, Nb samples were anodized to obtain a thicker surface oxide and H and D were implanted into this oxide to check for the appearance of implant peaks. SIMS depth profile analyses were carried out and confirmed the presence of the implant shape for these elements in the oxide. Relative Sensitivity Factor (RSFs) could then be calculated for quantification of H in the oxide. Since the Nb matrix signal showed little change from the oxide to the substrate, the same RSF was used to estimate the H concentration in the Nb at 2x10{sup 22} atoms/cm{sup 3}(approximately 40% mole fraction H).

36 MATERIALS SCIENCE↗
DOE OSTI2010

Annealing of induced absorption in quartz glasses by ArF laser radiation

Annealing of individual bands of electron-beam-induced absorption (IA) in the region of 150 - 400 nm in KS-4V, KU-1, and Corning 7980 (ArF Grade) quartz glasses by ArF laser radiation is studied. It is shown that the phototransformation of the IA spectra occurs mainly due to a significant decrease in the amplitudes of bands at {lambda} = 183.5, 213, and 260 nm. The role played by interstitial oxygen, hydrogen, and chlorine in the formation and relaxation of glass defects is considered. (effects of laser radiation on matter. laser plasma)

36 MATERIALS SCIENCE↗
DOE OSTI2010

K{alpha} satellite transitions in elements with 12{<=}Z{<=}30 produced by electron incidence

The emission of x-ray satellite lines in the K{alpha} region of Mg, Si, Sc, Ti, Cr, Fe, Ni, and Zn induced by electron incidence was studied by means of wavelength dispersive spectroscopy. The satellite lines studied were K{alpha}{sup '}, K{alpha}{sub 3}, K{alpha}{sub 4}, K{alpha}{sub 5}, K{alpha}{sub 6}, and two transitions denoted here as K{alpha}{sub 22} and K{alpha}{sub 12}. Energy shifts with respect to the main K{alpha}{sub 1} diagram line and transition probabilities relative to the whole K{alpha} group were determined for a number of lines through a careful spectral processing. The dependence of these parameters, as well as of the K{beta}:K{alpha} intensity ratio, on the atomic number was compared with previous experimental and theoretical determinations when available. A discussion about the different mechanisms responsible for vacancy creation involved in the production of double-ionization satellites was performed in the light of the results obtained. Finally, the behavior of the satellite intensities as a function of the incidence energy was discussed for silicon.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2010

Multifrequency spin resonance in diamond

Magnetic resonance techniques provide a powerful tool for controlling spin systems, with applications ranging from quantum information processing to medical imaging. Nevertheless, the behavior of a spin system under strong excitation remains a rich dynamical problem. In this paper, we examine spin resonance of the nitrogen-vacancy center in diamond under conditions outside the regime where the usual rotating-wave approximation applies, focusing on effects of multifrequency excitation and excitation with orientation parallel to the spin quantization axis. Strong-field phenomena such as multiphoton transitions and coherent destruction of tunneling are observed in the spectra and analyzed via numerical and analytic theory. In addition to illustrating the response of a spin system to strong multifrequency excitation, these observations may inform techniques for manipulating electron-nuclear spin quantum registers.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Quantum coherence and correlations of optical radiation by atomic ensembles interacting with a two-level atom in a microwave cavity

We examine quantum statistics of optical photons emitted from atomic ensembles which are classically driven and simultaneously coupled to a two-level atom via microwave photon exchange. Quantum statistics and correlations are analyzed by calculating second-order coherence degree, von Neumann entropy, spin squeezing for multiparticle entanglement, as well as genuine two- and three-mode entanglement parameters for steady-state and nonequilibrium situations. Coherent transfer of population between the radiation modes and quantum-state mapping between the two-level atom and the optical modes are discussed. A potential experimental realization of the theoretical results in a superconducting coplanar waveguide resonator containing diamond crystals with nitrogen-vacancy color centers and a superconducting artificial two-level atom is discussed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2010

The structure and optical properties of nanocrytalline lead sulfide films

The crystal structure of lead sulfide (PbS) films produced by chemical deposition onto a glass substrate is studied by X-ray diffraction analysis. The thickness of the synthesized films is {approx}120 nm, the dimensions of the regions of coherent scattering are 70-80 nm, and the magnitude of microstrains is {approx}0.20%. It is established that the crystal structure of the synthesized PbS films and of the same films annealed at the temperature 293-423 K is cubic (space group Fm 3m) and corresponds to the D0{sub 3} type that differs from the B1 type typical of coarse-grained PbS. In the cubic structure of the PbS nanofilms, there is a latent nonstoichiometric distribution of S atoms and vacancies among octahedral sites 4(b) and tetrahedral positions 8(c). The optical transmittance of the nanocrystalline PbS films is measured in the wavelength range from 200 to 3270 nm. The most pronounced variations in the transmittance is observed in the wavelength range from 700-800 to 1600-2000 nm (corresponding to the photon energy range from {approx}1.8 to {approx}0.7 eV). It is established that the band gap E{sub g} is 0.83-0.85 eV, i.e., it is larger than the band gap of single-crystal PbS, 0.41 eV.

36 MATERIALS SCIENCE↗
DOE OSTI2010

ZnO as a buffer layer for growth of BiFeO{sub 3} thin films

Multiferroic BiFeO{sub 3} thin film was grown on the ZnO-buffered Pt/TiO{sub 2}/SiO{sub 2}/Si(100) substrate by off-axis radio frequency magnetron sputtering, where the ZnO buffer layer gave rise to a strong (110) texture for the BiFeO{sub 3} thin film. The resulting BiFeO{sub 3}/ZnO thin film exhibits diode-like and resistive hysteresis behavior, in which the resistive hysteresis and rectifying ratio are dependent on the applied voltage and temperature. The resistive switching behavior of the BiFeO{sub 3}/ZnO thin film is shown to relate to the trap-controlled space charge limited conduction and interface-limited Fowler-Nordheim tunneling, while the polarization reversal takes place in the BiFeO{sub 3} layer of the heterostructure. The BiFeO{sub 3}/ZnO thin film is also demonstrated with a higher remanent polarization (2P{sub r{approx}}153.6 {mu}C/cm{sup 2}), a much lower dielectric loss (tan {delta}{approx}0.012), and a better fatigue endurance as compared to those of the BiFeO{sub 3} thin film without a ZnO buffer layer, where the much reduced leakage is largely responsible for the enhanced ferroelectric behavior. The ZnO as a buffer layer for BiFeO{sub 3} significantly changes the dielectric relaxation and conduction mechanisms, when the dielectric relaxation and electrical conduction are governed by the thermal excitation of carriers from the second-ionization and short-range motion of oxygen vacancies, respectively, while the relaxation process remains the same over the entire temperature range of 20 to 200 deg. C investigated in the present study.

36 MATERIALS SCIENCE↗
DOE OSTI2010

Probing defects in chemically synthesized ZnO nanostrucures by positron annihilation and photoluminescence spectroscopy

The present article describes the size induced changes in the structural arrangement of intrinsic defects present in chemically synthesized ZnO nanoparticles of various sizes. Routine x-ray diffraction and transmission electron microscopy have been performed to determine the shapes and sizes of the nanocrystalline ZnO samples. Detailed studies using positron annihilation spectroscopy reveals the presence of zinc vacancy. Whereas analysis of photoluminescence results predict the signature of charged oxygen vacancies. The size induced changes in positron parameters as well as the photoluminescence properties, has shown contrasting or nonmonotonous trends as size varies from 4 to 85 nm. Small spherical particles below a critical size ({approx}23 nm) receive more positive surface charge due to the higher occupancy of the doubly charge oxygen vacancy as compared to the bigger nanostructures where singly charged oxygen vacancy predominates. This electronic alteration has been seen to trigger yet another interesting phenomenon, described as positron confinement inside nanoparticles. Finally, based on all the results, a model of the structural arrangement of the intrinsic defects in the present samples has been reconciled.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Hall mobility of cuprous oxide thin films deposited by reactive direct-current magnetron sputtering

Cuprous oxide (Cu{sub 2}O) is a promising earth-abundant semiconductor for photovoltaic applications. We report Hall mobilities of polycrystalline Cu{sub 2}O thin films deposited by reactive dc magnetron sputtering. High substrate growth temperature enhances film grain structure and Hall mobility. Temperature-dependent Hall mobilities measured on these films are comparable to monocrystalline Cu{sub 2}O at temperatures above 250 K, reaching 62 cm{sup 2}/V s at room temperature. At lower temperatures, the Hall mobility appears limited by carrier scattering from ionized centers. These observations indicate that sputtered Cu{sub 2}O films at high substrate growth temperature may be suitable for thin-film photovoltaic applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Crystal structures and formation mechanisms of boron-rich tungsten borides

Boron-rich tungsten borides tend to adopt mechanically unfavorable layered structures but experimentally exhibit excellent mechanical properties rivalling traditional superhard solids. Unravelling the contraindicated structure-property relationship, however, has been impeded by their structural ambiguities because of the difficulty in probing boron and atomic deficiency of these borides. Here, we study crystal structures of boron-rich tungsten borides WB 3+x and WB 2+x by neutron diffraction based on high-quality samples prepared by a high-pressure method, leading to definitive structural resolutions for both borides with unique compositions of WB 5.14 and WB 2.34 . Combined with theoretical calculations, their structural stability is revealed to be closely related to atomic deficiency, which is governed by the valence-band filling with an optimal valence-electron concentration of ~10 per cell. The presence of interstitial boron trimers at the vacant W : 2b sites in WB 5.14 alters the crystal symmetry, making the Wyckoff 2d site more favorably occupied by W, rather than the 2c site, as previously misassigned. Here, the staggered planar boron layers and wrinkled boron bonding in WB 2.34 are identified to be crucial for stabilizing its structure. These findings unveil the longstanding structural mysteries of boron-rich tungsten borides and offer powerful insights for rational design of borides by defect chemistry.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Improving the structural consistency of C-S-H solid solution thermodynamic models

Simple aqueous-solid solution models of C-S-H (calcium silicate hydrate) are widely used in studies of cement hydration and waste-cement interactions. Even without a clear structural/mechanistic basis, such thermodynamic models yield a good description of solubility data in [Ca]-C/S space, while only satisfactory in [Si]-C/S, H{sub 2}O-C/S, [Ca]-[Si] spaces and in the 0.1 < C/S < 0.8 range. Here, using a multi-site (sublattice) concept, the ideal solid solution model of C-S-H is revised to make it consistent with the Richardson-Groves structural model of C-S-H and with the modern interpretation of spectroscopic ({sup 29}Si MAS NMR) and solubility data. Consideration of two site substitutions, (1) coupled H{sub 2}OCa{sup 2+} for SiO{sub 2}H{sub 2}{sup 2+} replacement in bridging tetrahedral and adjacent interlayer sites, and (2) substitution of interstitial Ca(OH){sub 2} for a vacancy, leads to a new CSHQ model of (A,B) (C,D)X type composed of two tobermorite-like and two jennite-like end members. Because this ideal sublattice SS model cannot fit solubility data well at 0.8 < C/S < 1.1, a simpler CSH3T model is constructed from a polymeric TobH (CaO){sub 2}(SiO{sub 2}){sub 3}(H{sub 2}O){sub 5}, a dimeric T2C (CaO){sub 3}(SiO{sub 2}){sub 2}(H{sub 2}O){sub 5}, and an ordered pentameric T5C (CaO){sub 2.5}(SiO{sub 2}){sub 2.5}(H{sub 2}O){sub 5} tobermorite-like end members. This solid solution model, limited to the range 0.67 < C/S < 1.5, has a correct built-in dependence of the mean silicate chain length on C/S, yields quite realistic fits to the solubility data, and provides a basis for extensions with foreign cations whose sites in the defect-tobermorite structure of C-S-H are known. To account for C-S-H compositions with C/S > 1.5, CSHQ end members were downscaled to one tetrahedral site and used within the simple mixing model. Despite some loss of structural consistency, the solubility and mean silicate chain length data can be reproduced well with this downscaled CSHQ model, capable of temperature corrections and dependencies of density and water content in fully-hydrated C-S-H on C/S ratio. Most literature solubility data sets can be modeled at the cost of moderate adjustments of CSHQ end-member solubility products within 0.2 to 0.6 pK units.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Room temperature ferromagnetism in undoped and Fe doped ZnO nanorods: Microwave-assisted synthesis

One-dimensional (1D) undoped and Fe doped ZnO nanorods of average length {approx}1 {mu}m and diameter {approx}50 nm have been obtained using a microwave-assisted synthesis. The magnetization (M) and coercivity (H{sub c}) value obtained for undoped ZnO nanorods at room temperature is {approx}5x10{sup -3} emu/g and {approx}150 Oe, respectively. The Fe doped ZnO samples show significant changes in M -H loop with increasing doping concentration. Both undoped and Fe doped ZnO nanorods exhibit a Curie transition temperature (T{sub c}) above 390 K. Electron spin resonance and Moessbauer spectra indicate the presence of ferric ions. The origin of ferromagnetism in undoped ZnO nanorods is attributed to localized electron spin moments resulting from surface defects/vacancies, where as in Fe doped samples is explained by F center exchange mechanism. -- Graphical abstract: Room temperature ferromagnetism has been reported in undoped and Fe doped ZnO nanorods of average length {approx}1 {mu}m and diameter {approx}50 nm. Display Omitted Research Highlights: {yields} Microwave-assisted synthesis of undoped and Fe doped ZnO nanorods. {yields} Observation of room temperature ferromagnetism in undoped and Fe doped ZnO nanorods. {yields} Transition temperature (T{sub c}) obtained in undoped and doped samples is above 390 K. {yields} In undoped ZnO origin of ferromagnetism is explained in terms of defects/vacancies. {yields} Ferromagnetism in Fe doped ZnO is explained by F-center exchange mechanism.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2011

Energetics of intrinsic defects and their complexes in ZnO investigated by density functional calculations

Formation energies of various intrinsic defects and defect complexes in ZnO have been calculated using a density-functional-theory-based pseudopotential all-electron method. The various defects considered are oxygen vacancy (V{sub O}), zinc vacancy (V{sub Zn}), oxygen at an interstitial site (O{sub i}), Zn at an interstitial site (Zn{sub i}), Zn at V{sub O} (Zn{sub O}), O at V{sub Zn}(O{sub Zn}), and an antisite pair (combination of the preceding two defects). In addition, defect complexes like (V{sub O}+Zn{sub i}) and Zn-vacancy clusters are studied. The Schokkty pair (V{sub O}+V{sub Zn}) and Frenkel pairs [(V{sub O}+O{sub i}) and (V{sub Zn}+Zn{sub i})] are considered theoretically for the first time. Upon comparing the formation energies of these defects, we find that V{sub O} would be the dominant intrinsic defect under both Zn-rich and O-rich conditions and it is a deep double donor. Both Zn{sub O} and Zn{sub i} are found to be shallow donors. The low formation energy of donor-type intrinsic defects could lead to difficulty in achieving p-type conductivity in ZnO. Defect complexes have charge transitions deep inside the band gap. The red, yellow, and green photoluminescence peaks of undoped samples can be assigned to some of the defect complexes considered. It is believed that the red luminescence originates from an electronic transition in V{sub O}, but we find that it can originate from the antisite Zn{sub O} defect. Charge density and electron-localization function analyses have been used to understand the effect of these defects on the ZnO lattice. The electronic structure of ZnO with intrinsic defects has been studied using density-of-states and electronic band structure plots. The acceptor levels introduced by V{sub Zn} are relatively localized, making it difficult to achieve p-type conductivity with sufficient hole mobility.

36 MATERIALS SCIENCE↗