Processing-Induced Electrically Active Defects in Black Silicon Nanowire Devices.

PubMed

Carapezzi, Stefania; Castaldini, Antonio; Mancarella, Fulvio; Poggi, Antonella; Cavallini, Anna

2016-04-27

Silicon nanowires (Si NWs) are widely investigated nowadays for implementation in advanced energy conversion and storage devices, as well as many other possible applications. Black silicon (BSi)-NWs are dry etched NWs that merge the advantages related to low-dimensionality with the special industrial appeal connected to deep reactive ion etching (RIE). In fact, RIE is a well established technique in microelectronics manufacturing. However, RIE processing could affect the electrical properties of BSi-NWs by introducing deep states into their forbidden gap. This work applies deep level transient spectroscopy (DLTS) to identify electrically active deep levels and the associated defects in dry etched Si NW arrays. Besides, the successful fitting of DLTS spectra of BSi-NWs-based Schottky barrier diodes is an experimental confirmation that the same theoretical framework of dynamic electronic behavior of deep levels applies in bulk as well as in low dimensional structures like NWs, when quantum confinement conditions do not occur. This has been validated for deep levels associated with simple pointlike defects as well as for deep levels associated with defects with richer structures, whose dynamic electronic behavior implies a more complex picture.

Defects in regular nanosystems and interference spectra at reemission of electromagnetic field attosecond pulses

NASA Astrophysics Data System (ADS)

Matveev, V. I.; Makarov, D. N.

2017-01-01

The effect of defects in nanostructured targets on interference spectra at the reemission of attosecond electromagnetic pulses has been considered. General expressions have been obtained for calculations of spectral distributions for one-, two-, and three-dimensional multiatomic nanosystems consisting of identical complex atoms with defects such as bends, vacancies, and breaks. Changes in interference spectra by a linear chain with several removed atoms (chain with breaks) and by a linear chain with a bend have been calculated as examples allowing a simple analytical representation. Generalization to two- and three-dimensional nanosystems has been developed.

Magnetic Resonance and Magnetoresistance for the Understanding of Defect Chemistry and Spin-Transport in Amorphous Semiconductors and Dielectrics

NASA Astrophysics Data System (ADS)

Mutch, Michael J.

This work utilizes an electron paramagnetic resonance (EPR)-based approach, electrically detected magnetic resonance (EDMR), to study defect chemistry in amorphous semiconductors and dielectrics even when featureless spectra are present. EDMR is the electrically detected analog of EPR in which EPR induced changes in device current are detected. In this study, EDMR is detected via changes in amorphous semiconductor or dielectric tunneling current via spin-dependent trap assisted tunneling (SDTAT) events. Due to the nature of SDTAT, defects detected are directly linked to electronic transport; an additional benefit of EDMR relative to EPR. Unlike EPR, SDTAT/EDMR may also be detected at any field/frequency combination without loss of sensitivity. As will be explained, this field/frequency independence allows for a distinction between EDMR line width contributions from electronic g tensor components or electron-nuclear hyperfine interactions, thus providing insight into defect chemistry when featureless spectra are present. Additionally, performing EDMR measurements at multiple biases and comparing with MIS band diagrams allows for a rudimentary understanding of defect energy levels. Finally, we utilize EDMR to understand near-zero-field magnetoresistance (MR) phenomena. The EDMR techniques utilized in this study are relatively new, and have not been exploited to study a wide range of electronic materials. In Chapter 4, baseline EDMR measurements are provided in relatively simple amorphous systems including a-Si:H and a-C:H. We find that EDMR spectra in a-Si:H and a-C:H systems are due to silicon and carbon dangling bonds, respectively. Additionally, we utilize multiple frequency EDMR to provide additional information regarding contributions of line width due to the breadth of g tensor components in the featureless a-Si:H and a-C:H EDMR spectra. By providing a measurement of g tensor breadth, Deltag, we develop a baseline for distinguishing between silicon and carbon dangling bonds in more complex systems, such as low-dielectric constant (kappa) dielectrics a-SiOC:H and a-SiCN:H, in which silicon and/or carbon dangling bonds may be present. Low-kappa dielectric constant materials are critical for reducing parasitic capacitances due to the scaling of back-end of line interconnects. In Chapter 4, we first utilize conventional EPR measurements to study a variety of porous low-kappa dielectric powders. Via conventional EPR on these low-kappa powders, we are able to analyze the effects of UV radiation and remote hydrogen plasma upon the low-kappa systems. Our results indicate that UV treatments, which are utilized to eliminate sacrificial porogens to introduce pores, significantly increase defect density. Remote hydrogen plasma (RHP) treatments are found to decrease dangling bond concentration. However, due to the featureless EPR spectra, we are unable to provide insight into defect chemistry via conventional EPR. Thus, we utilize multiple field/frequency EDMR in these low-kappa systems, and compare Deltag measurements with previous baseline measurements, to provide insight into defect chemistry which was previously unavailable. We find a multitude of silicon and carbon dangling bonds in a-SiOCH and a-SiCN:H dielectrics. Defect chemistry seems to depend upon precursor chemistry. Additionally, EDMR measurements confirm that UV treatments in low-kappa systems introduce silicon dangling bonds, suggesting that these treatments may be damaging the Si-O-Si network in a-SiOC:H systems. Finally, we perform EDMR measurements at multiple biases to get a general understanding of defect energy levels in these systems. Band gaps are calculated via reflected electron energy loss spectroscopy (REELS), and band offsets are calculated via X-ray photoelectron spectroscopy (XPS). We find that carbon dangling bonds in a-SiOC:H systems have levels near the middle of the a-SiOC:H band gap, and silicon dangling bonds in a-SiCN:H systems have levels near the upper-middle part of the a-SiCN:H band gap. In Chapter 5, we analyze silicon nitride (a-SiN:H) thin films, which are widely utilized in the electronics industry as gate dielectrics for TFTs. However, defects and electronic transport in these systems are not fully understood. We utilize multiple frequency EDMR and variable bias EDMR to better understand defect chemistry and energy levels in a-SiN:H systems. It is found that K centers, which have been previously observed in a-SiN:H via EPR and electron nuclear double resonance (ENDOR), are primarily responsible for transport in these systems. Additionally, we find that K centers are about 3.1 eV above the a-SiN:H valence band edge, in agreement with previous theoretical calculations. In Chapter 6, we illustrate that near-zero field MR phenomena are ubiquitous in amorphous semiconductors and dielectrics. We link the MR and EDMR responses by measuring response amplitude for each technique versus bias. The observed EDMR and MR versus bias trends are nearly identical, suggesting that the defects responsible for each technique correspond to similar energy levels. Though circumstantial, our measurements provide strong evidence that the defects whose chemistry is plausibly identified via multiple frequency EDMR are primarily responsible for MR in the amorphous semiconductors and dielectrics in this study. (Abstract shortened by ProQuest.).

Self-compensation in arsenic doping of CdTe

DOE PAGES

Ablekim, Tursun; Swain, Santosh K.; Yin, Wan -Jian; ...

2017-07-04

Efficient p-type doping in CdTe has remained a critical challenge for decades, limiting the performance of CdTe-based semiconductor devices. Arsenic is a promising p-type dopant; however, reproducible doping with high concentration is difficult and carrier lifetime is low. We systematically studied defect structures in As-doped CdTe using high-purity single crystal wafers to investigate the mechanisms that limit p-type doping. Two As-doped CdTe with varying acceptor density and two undoped CdTe were grown in Cd-rich and Te-rich environments. The defect structures were investigated by thermoelectric-effect spectroscopy (TEES), and first-principles calculations were used for identifying and assigning the experimentally observed defects. Measurementsmore » revealed activation of As is very low in both As-doped samples with very short lifetimes indicating strong compensation and the presence of significant carrier trapping defects. Defect studies suggest two acceptors and one donor level were introduced by As doping with activation energies at ~88 meV, ~293 meV and ~377 meV. In particular, the peak shown at ~162 K in the TEES spectra is very prominent in both As-doped samples, indicating a signature of AX-center donors. In conclusion, the AX-centers are believed to be responsible for most of the compensation because of their low formation energy and very prominent peak intensity in TEES spectra.« less

Self-compensation in arsenic doping of CdTe

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ablekim, Tursun; Swain, Santosh K.; Yin, Wan -Jian

Efficient p-type doping in CdTe has remained a critical challenge for decades, limiting the performance of CdTe-based semiconductor devices. Arsenic is a promising p-type dopant; however, reproducible doping with high concentration is difficult and carrier lifetime is low. We systematically studied defect structures in As-doped CdTe using high-purity single crystal wafers to investigate the mechanisms that limit p-type doping. Two As-doped CdTe with varying acceptor density and two undoped CdTe were grown in Cd-rich and Te-rich environments. The defect structures were investigated by thermoelectric-effect spectroscopy (TEES), and first-principles calculations were used for identifying and assigning the experimentally observed defects. Measurementsmore » revealed activation of As is very low in both As-doped samples with very short lifetimes indicating strong compensation and the presence of significant carrier trapping defects. Defect studies suggest two acceptors and one donor level were introduced by As doping with activation energies at ~88 meV, ~293 meV and ~377 meV. In particular, the peak shown at ~162 K in the TEES spectra is very prominent in both As-doped samples, indicating a signature of AX-center donors. In conclusion, the AX-centers are believed to be responsible for most of the compensation because of their low formation energy and very prominent peak intensity in TEES spectra.« less

Impact of the silicon substrate resistivity and growth condition on the deep levels in Ni-Au/AlN/Si MIS Capacitors

NASA Astrophysics Data System (ADS)

Wang, Chong; Simoen, Eddy; Zhao, Ming; Li, Wei

2017-10-01

Deep levels formed under different growth conditions of a 200 nm AlN buffer layer on B-doped Czochralski Si(111) substrates with different resistivity were investigated by deep-level transient spectroscopy (DLTS) on metal-insulator-semiconductor capacitors. Growth-temperature-dependent Al diffusion in the Si substrate was derived from the free carrier density obtained by capacitance-voltage measurement on samples grown on p- substrates. The DLTS spectra revealed a high concentration of point and extended defects in the p- and p+ silicon substrates, respectively. This indicated a difference in the electrically active defects in the silicon substrate close to the AlN/Si interface, depending on the B doping concentration.

Simulation of electron energy loss spectra of nanomaterials with linear-scaling density functional theory

DOE PAGES

Tait, E. W.; Ratcliff, L. E.; Payne, M. C.; ...

2016-04-20

Experimental techniques for electron energy loss spectroscopy (EELS) combine high energy resolution with high spatial resolution. They are therefore powerful tools for investigating the local electronic structure of complex systems such as nanostructures, interfaces and even individual defects. Interpretation of experimental electron energy loss spectra is often challenging and can require theoretical modelling of candidate structures, which themselves may be large and complex, beyond the capabilities of traditional cubic-scaling density functional theory. In this work, we present functionality to compute electron energy loss spectra within the onetep linear-scaling density functional theory code. We first demonstrate that simulated spectra agree withmore » those computed using conventional plane wave pseudopotential methods to a high degree of precision. The ability of onetep to tackle large problems is then exploited to investigate convergence of spectra with respect to supercell size. As a result, we apply the novel functionality to a study of the electron energy loss spectra of defects on the (1 0 1) surface of an anatase slab and determine concentrations of defects which might be experimentally detectable.« less

Optical transitions of the silicon vacancy in 6H-SiC studied by positron annihilation spectroscopy

NASA Astrophysics Data System (ADS)

Arpiainen, S.; Saarinen, K.; Hautojärvi, P.; Henry, L.; Barthe, M.-F.; Corbel, C.

2002-08-01

Positron annihilation spectroscopy has been applied to identify Si and C vacancies as irradiation-induced defects in 6H-SiC. Si vacancies are shown to have ionization levels at EC-0.6 eV and EC-1.1 eV below the conduction-band edge EC by detecting changes of positron trapping under monochromatic illumination. These levels are attributed to (2-/1-) and (1-/0) ionizations of the isolated Si vacancy. In as-grown n-type 6H-SiC, a native defect complex involving VSi is shown to have an ionization level slightly closer to conduction band at roughly EC-0.3 eV. These results are used further to present microscopic interpretations to effects seen in optical-absorption spectra and to electrical levels observed previously by deep-level transient spectroscopy.

Defect formation in MeV H+ implanted GaN and 4H-SiC investigated by cross-sectional Raman spectroscopy

NASA Astrophysics Data System (ADS)

Huang, Kai; Jia, Qi; You, Tiangui; Zhang, Shibin; Lin, Jiajie; Zhang, Runchun; Zhou, Min; Yu, Wenjie; Zhang, Bo; Ou, Xin; Wang, Xi

2017-09-01

Cross-sectional Raman spectroscopy is used to characterize the defect formation and the defect recovery in MeV H+ implanted bulk GaN and 4H-SiC in the high energy MeV ion-cut process. The Raman intensity decreases but the forbidden modes are activated at the damage region, and the intensity decrease is proportional to the damage level. The Raman spectrum is quite sensitive to detect the damage recovery after annealing. The main peak intensity increases and the forbidden mode disappears in both annealed GaN and 4H-SiC samples. The Raman spectra of GaN samples annealed at different temperatures suggest that higher annealing temperature is more efficient for damage recovery. While, the Raman spectra of SiC indicate that higher implantation temperature results in heavier lattice damage and other polytype clusters might be generated by high annealing temperature in the annealed SiC samples. The cross-sectional Raman spectroscopy is a straightforward method to characterize lattice damage and damage recovery in high energy ion-cut process. It can serve as a fast supplementary measurement technique to Rutherford backscattering spectrometry (RBS), nuclear reaction analysis (NRA) and transmission electron microscope (TEM) for the defect characterizations.

Study on the behavior and mechanism of polycarbonate with hot-water aging

NASA Astrophysics Data System (ADS)

Kong, L. P.; Zhao, Y. X.; Zhou, C. H.; Huang, Y. H.; Tang, M.; Gao, J. G.

2016-07-01

The present work was concerned with hot-water aging behavior and mechanism of Bisphenol A polycarbonate (PC) used as food and packaging materials. It indicated that with the aging time prolonged, PC sample had internal defects and the mechanical properties of PC materials changed not too much, molecular weight decreased, thermal stability declined. Phenolic hydroxyl absorption intensity enhanced in IR spectra and the maximum absorption wavelength red shift of benzene in UV-Vis spectra, the level of BPA increased. The color change of PC sample was not apparent.

Ab initio non-adiabatic study of the 4pσ B'' 1Σ+u state of H2

NASA Astrophysics Data System (ADS)

Glass-Maujean, M.; Schmoranzer, H.

2018-05-01

Fully ab initio non-adiabatic multichannel quantum defect calculations of the 4pσ B'' 1∑u+ energy levels, line intensities and widths, based on the latest quantum-chemical clamped-nuclei calculations of Wolniewicz and collaborators are presented for H2. The B″ state corresponds to the inner well of the ? state. The B'' v ≥ 1 levels are rapidly predissociated through vibrational coupling with the 3pσ B' 1Σ+u continuum so that coupled-equation calculations become unstable. Multichannel quantum defect theory, on the other hand, is demonstrated to be particularly suited to this situation. Experimental data as level energies, line intensities and dissociation widths were revisited and corrected. Reinvestigating previously published spectra, several new lines were assigned.

Eu2+ -induced enhancement of defect luminescence of ZnS.

PubMed

Xiao-Bo, Zhang; Fu-Xiang, Wei

2016-12-01

The Eu 2 + -induced enhancement of defect luminescence of ZnS was studied in this work. While photoluminescence (PL) spectra exhibited 460 nm and 520 nm emissions in both ZnS and ZnS:Eu nanophosphors, different excitation characteristics were shown in their photoluminescence excitation (PLE) spectra. In ZnS nanophosphors, there was no excitation signal in the PLE spectra at the excitation wavelength λ ex  > 337 nm (the bandgap energy 3.68 eV of ZnS); while in ZnS:Eu nanophosphors, two excitation bands appeared that were centered at 365 nm and 410 nm. Compared with ZnS nanophosphors, the 520 nm emission in the PL spectra was relatively enhanced in ZnS:Eu nanophosphors and, furthermore, in ZnS:Eu nanophosphors the 460 nm and 520 nm emissions increased more than 10 times in intensity. The reasons for these differences were analyzed. It is believed that the absorption of Eu 2 + intra-ion transition and subsequent energy transfer to sulfur vacancy, led to the relative enhancement of the 520 nm emission in ZnS:Eu nanophosphors. In addition, more importantly, Eu 2 + acceptor-bound excitons are formed in ZnS:Eu nanophosphors and their excited levels serve as the intermediate state of electronic relaxation, which decreases non-radiative electronic relaxation and thus increases the intensity of the 460 nm and 520 nm emission dramatically. In summary, the results in this work indicate a new mechanism for the enhancement of defect luminescence of ZnS in Eu 2 + -doped ZnS nanophosphors. Copyright © 2016 John Wiley & Sons, Ltd. Copyright © 2016 John Wiley & Sons, Ltd.

Optical absorption and scattering spectra of pathological stomach tissues

NASA Astrophysics Data System (ADS)

Giraev, K. M.; Ashurbekov, N. A.; Lakhina, M. A.

2011-03-01

Diffuse reflection spectra of biotissues in vivo and transmission and reflection coefficients for biotissues in vitro are measured over 300-800 nm. These data are used to determine the spectral absorption and scattering indices and the scattering anisotropy factor for stomach mucous membranes under normal and various pathological conditions (chronic atrophic and ulcerous defects, malignant neoplasms). The most importan tphysiological (hemodynamic and oxygenation levels) and structural-morphological (scatterer size and density) parameters are also determined. The results of a morphofunctional study correlate well with the optical properties and are consistent with data from a histomorphological analysis of the corresponding tissues.

EPR of radiation defects in lithium-oxyfluoride glass ceramics

NASA Astrophysics Data System (ADS)

Fedotovs, A.; Rogulis, U.; Sarakovskis, A.; Dimitrocenko, L.

2010-11-01

We studied oxyfluoride composites based on lithium silicate glasses with yttrium fluorides and rare-earth dopants. The electron paramagnetic resonance (EPR) has been used to obtain information about radiation induced defects in these materials. Spectra have been measured before and after X-ray irradiation at room temperature and at liquid nitrogen temperature. Fluoride crystallites within samples were created by means of thermal treatment at specific temperatures. EPR spectra of radiation induced defects in oxyfluoride glass ceramics, in which crystallites have not been yet created, show no explicit hfs interaction of fluorine nuclei. However, in glass ceramics, which already contains fluoride crystallites, the hfs characteristic to fluorine nuclei appears in the EPR spectra. EPR hyperfine structure could be explained within a model of an F-type centre in YF3 crystalline phase.

A magnetically tunable non-Bragg defect mode in a corrugated waveguide filled with liquid crystals

NASA Astrophysics Data System (ADS)

Zhang, Lu; Fan, Ya-Xian; Liu, Huan; Han, Xu; Lu, Wen-Qiang; Tao, Zhi-Yong

2018-04-01

A magnetically tunable, non-Bragg defect mode (NBDM) was created in the terahertz frequency range by inserting a defect in the middle of a periodically corrugated waveguide filled with liquid crystals (LCs). In the periodic waveguide, non-Bragg gaps beyond the Bragg ones, which appear in the transmission spectra, are created by different transverse mode resonances. The transmission spectra of the waveguide containing a defect showed that a defect mode was present inside the non-Bragg gap. The NBDM has quite different features compared to the Bragg defect mode, which includes more complex, high-order guided wave modes. In our study, we filled the corrugated waveguide with LCs to realize the tunability of the NBDM. The simulated results showed that the NBDM in a corrugated waveguide filled with LCs can be used in filters, sensors, switches, and other terahertz integrated devices.

Vacancy-type defects induced by grinding of Si wafers studied by monoenergetic positron beams

DOE Office of Scientific and Technical Information (OSTI.GOV)

Uedono, Akira; Yoshihara, Nakaaki; Mizushima, Yoriko

2014-10-07

Vacancy-type defects introduced by the grinding of Czochralski-grown Si wafers were studied using monoenergetic positron beams. Measurements of Doppler broadening spectra of the annihilation radiation and the lifetime spectra of positrons showed that vacancy-type defects were introduced in the surface region (<98 nm), and the major defect species were identified as (i) relatively small vacancies incorporated in dislocations and (ii) large vacancy clusters. Annealing experiments showed that the defect concentration decreased with increasing annealing temperature in the range between 100 and 500°C. After 600–700°C annealing, the defect-rich region expanded up to about 170 nm, which was attributed to rearrangements ofmore » dislocation networks, and a resultant emission of point defects toward the inside of the sample. Above 800°C, the stability limit of those vacancies was reached and they started to disappear. After the vacancies were annealed out (900°C), oxygen-related defects were the major point defects and they were located at <25 nm.« less

Analysis of the defect clusters in congruent lithium tantalate

NASA Astrophysics Data System (ADS)

Vyalikh, Anastasia; Zschornak, Matthias; Köhler, Thomas; Nentwich, Melanie; Weigel, Tina; Hanzig, Juliane; Zaripov, Ruslan; Vavilova, Evgenia; Gemming, Sibylle; Brendler, Erica; Meyer, Dirk C.

2018-01-01

A wide range of technological applications of lithium tantalate (LT) is closely related to the defect chemistry. In literature, several intrinsic defect models have been proposed. Here, using a combinational approach based on DFT and solid-state NMR, we demonstrate that distribution of electric field gradients (EFGs) can be employed as a fingerprint of a specific defect configuration. Analyzing the distribution of 7Li EFGs, the FT-IR and electron spin resonance (ESR) spectra, and the 7Li spin-lattice relaxation behavior, we have found that the congruent LT samples provided by two manufacturers show rather different defect concentrations and distributions although both were grown by the Czochralski method. After thermal treatment hydrogen out-diffusion and homogeneous distribution of other defects have been observed by ESR, NMR, and FT-IR. The defect structure in one of two congruent LT crystals after annealing has been identified and proved by defect formation energy considerations, whereas the more complex defect configuration, including the presence of extrinsic defects, has been suggested for the other LT sample. The approach of searching the EFG fingerprints from DFT calculations in NMR spectra can be applied for identifying the defect clusters in other complex oxides.

Chirality of Single-Handed Twisted Titania Tubular Nanoribbons Prepared Through Sol-gel Transcription.

PubMed

Wang, Sibing; Zhang, Chuanyong; Li, Yi; Li, Baozong; Yang, Yonggang

2015-08-01

Single-handed twisted titania tubular nanoribbons were prepared through sol-gel transcription using a pair of enantiomers. Handedness was controlled by that of the template. The obtained samples were characterized using field-emission electron microscopy, transmission electron microscopy, diffuse reflectance circular dichroism (DRCD), and X-ray diffraction. The DRCD spectra indicated that the titania nanotubes exhibit optical activity. Although the tubular structure was destroyed after being calcined at 700 °C for 2.0 h, DRCD signals were still identified. However, the DRCD signals disappeared after being calcined at 1000 °C for 2.0 h. The optical activity of titania was proposed to be due to chiral defects. Previous results showed that straight titania tubes could be used as asymmetric autocatalysts, indicating that titania exhibit chirality at the angstrom level. Herein, it was found that they also exhibit DRCD signals, indicating that there are no obvious relationships between morphology at the nano level and chirality at the angstrom level. The nanotube chirality should originate from the chiral defects on the nanotube inner surface. The Fourier transform infrared spectra indicated that the chirality of the titania was transferred from the gelators through the hydrogen bonding between N-H and Ti-OH. © 2015 Wiley Periodicals, Inc.

Characterization of deep level defects and thermally stimulated depolarization phenomena in La-doped TlInS{sub 2} layered semiconductor

DOE Office of Scientific and Technical Information (OSTI.GOV)

Seyidov, MirHasan Yu., E-mail: smirhasan@gyte.edu.tr; Suleymanov, Rauf A.; Mikailzade, Faik A.

2015-06-14

Lanthanum-doped high quality TlInS{sub 2} (TlInS{sub 2}:La) ferroelectric-semiconductor was characterized by photo-induced current transient spectroscopy (PICTS). Different impurity centers are resolved and identified. Analyses of the experimental data were performed in order to determine the characteristic parameters of the extrinsic and intrinsic defects. The energies and capturing cross section of deep traps were obtained by using the heating rate method. The observed changes in the Thermally Stimulated Depolarization Currents (TSDC) near the phase transition points in TlInS{sub 2}:La ferroelectric-semiconductor are interpreted as a result of self-polarization of the crystal due to the internal electric field caused by charged defects. Themore » TSDC spectra show the depolarization peaks, which are attributed to defects of dipolar origin. These peaks provide important information on the defect structure and localized energy states in TlInS{sub 2}:La. Thermal treatments of TlInS{sub 2}:La under an external electric field, which was applied at different temperatures, allowed us to identify a peak in TSDC which was originated from La-dopant. It was established that deep energy level trap BTE43, which are active at low temperature (T ≤ 156 K) and have activation energy 0.29 eV and the capture cross section 2.2 × 10{sup −14} cm{sup 2}, corresponds to the La dopant. According to the PICTS results, the deep level trap center B5 is activated in the temperature region of incommensurate (IC) phases of TlInS{sub 2}:La, having the giant static dielectric constant due to the structural disorders. From the PICTS simulation results for B5, native deep level trap having an activation energy of 0.3 eV and the capture cross section of 1.8 × 10{sup −16} cm{sup 2} were established. A substantial amount of residual space charges is trapped by the deep level localized energy states of B5 in IC-phase. While the external electric field is applied, permanent dipoles, which are originated from the charged B5 deep level defects, are aligned in the direction of the applied electric field and the equilibrium polarization can be reached in a relatively short time. When the polarization field is maintained, while cooling the temperature of sample to a sufficiently low degrees, the relaxation times of the aligned dipoles drastically increases. Practically, frozen internal electric field or electrets states remain inside the TlInS{sub 2}:La when the applied bias field is switched off. The influence of deep level defects on TSDC spectra of TlInS{sub 2}:La has been revealed for the first time.« less

Lattice dynamics and vibrational spectra of conformationally disordered polymers: poly(vinylidene fluoride)

NASA Astrophysics Data System (ADS)

Borionetti, Gabriella; Zannoni, Giuseppe; Zerbi, Giuseppe

1990-07-01

Lattice dynamical calculations were performed on poly(vinylidene fluoride) considered in the ideally perfect α and β structures or in a conformationally disordered structure. Head-to-head and GTG' defects were considered. The conformational soliton proposed by Taylor has been also considered as a cooperative large defect and its spectrum has been calculated. From calculations indications were obtained for the idenfication of infrared or Raman bands originating from the "amorphous" part of the material. The problem of the existence of localized or cooperative conformational defects in this material is presented and information obtainable from the vibrational spectra are discussed.

Highly Efficient Defect Emission from ZnO:Zn and ZnO:S Powders

NASA Astrophysics Data System (ADS)

Everitt, Henry

2013-03-01

Bulk Zinc Oxide (ZnO) is a wide band gap semiconductor with an ultraviolet direct band gap energy of 3.4 eV and a broad, defect-related visible wavelength emission band centered near 2 eV. We have shown that the external quantum efficiency can exceed 50% for this nearly white emission band that closely matches the human dark-adapted visual response. To explore the potential of ZnO as a rare earth-free white light phosphor, we investigated the mechanism of efficient defect emission in three types of ZnO powders: unannealed, annealed, and sulfur-doped. Annealing and sulfur-doping of ZnO greatly increase the strength of defect emission while suppressing the UV band edge emission. Continuous wave and ultrafast one- and two-photon excitation spectroscopy are used to examine the defect emission mechanism. Low temperature photoluminescence (PL) and PL excitation (PLE) spectra were measured for all three compounds, and it was found that bound excitons mediate the defect emission. Temperature-dependent PLE spectra for the defect and band edge emission were measured to estimate trapping and activation energies of the bound excitons and clarify the role they play in the defect emission. Time-resolved techniques were used to ascertain the role of exciton diffusion, the effects of reabsorption, and the spatial distributions of radiative and non-radiative traps. In unannealed ZnO we find that defect emission is suppressed and UV band edge emission is inefficient (< 2%) because of reabsorption and non-radiative recombination due to a high density of non-radiative bulk traps. By annealing ZnO, bulk trap densities are reduced, and a high density of defects responsible for the broad visible emission are created near the surface. Interestingly, nearly identical PLE spectra are found for both the band edge and the defect emission, one of many indications that the defect emission is deeply connected to bound excitons. Quantum efficiency, also measured as a function of excitation wavelength, closely mirrors the PLE spectra for both emission bands. Sulfur-doped ZnO exhibits additional PLE and X-ray features indicative of a ZnS-rich surface shell that correlates with even more efficient defect emission. The results presented here offer hope that engineering defects in ZnO materials may significantly improve the quantum efficiency for white light phosphor applications. This work was supported by the Army's in-house laboratory innovative research program.

The Correlation Between Dislocations and Vacancy Defects Using Positron Annihilation Spectroscopy

NASA Astrophysics Data System (ADS)

Pang, Jinbiao; Li, Hui; Zhou, Kai; Wang, Zhu

2012-07-01

An analysis program for positron annihilation lifetime spectra is only applicable to isolated defects, but is of no use in the presence of defective correlations. Such limitations have long caused problems for positron researchers in their studies of complicated defective systems. In order to solve this problem, we aim to take a semiconductor material, for example, to achieve a credible average lifetime of single crystal silicon under plastic deformation at different temperatures using positron life time spectroscopy. By establishing reasonable positron trapping models with defective correlations and sorting out four lifetime components with multiple parameters, as well as their respective intensities, information is obtained on the positron trapping centers, such as the positron trapping rates of defects, the density of the dislocation lines and correlation between the dislocation lines, and the vacancy defects, by fitting with the average lifetime with the aid of Matlab software. These results give strong grounds for the existence of dislocation-vacancy correlation in plastically deformed silicon, and lay a theoretical foundation for the analysis of positron lifetime spectra when the positron trapping model involves dislocation-related defects.

Streaked X Ray Spectra from Polar Direct Drive Capsules with an Equatorial Defect

NASA Astrophysics Data System (ADS)

Murphy, T. J.; Bradley, P. A.; Cobble, J. A.; Hsu, S. C.; Krasheninnikova, N. S.; Magelssen, G. R.; Schmitt, M. J.; Tregillis, I. L.; Wysocki, F. J.

2011-10-01

In the Defect Implosion Experiment (DIME) on Omega, capsules with an equatorial ``trench'' defect have been imploded to study defect-induced mix processes. The capsules contain layers doped with titanium and/or vanadium, with doped layers in contact with the deuterium fill gas on some targets, and separated from the gas by a layer of undoped plastic in others. Streaked x-ray spectra from the capsule implosions provide information on conditions in the mix layer. Polar direct drive was utilized in preparation for experiments planned for the National Ignition Facility in 2012. This work is supported by US DOE/NNSA, performed at LANL, operated by LANS LLC under contract DE-AC52-06NA25396.

Use of Isobestic and Isoemission Points in Absorption and Luminescence Spectra for Study of the Transformation of Radiation Defects in Lithium Fluoride

NASA Astrophysics Data System (ADS)

Voitovich, A. P.; Kalinov, V. S.; Stupak, A. P.; Runets, L. P.

2015-03-01

Isobestic and isoemission points are recorded in the combined absorption and luminescence spectra of two types of radiation defects involved in complex processes consisting of several simultaneous parallel and sequential reactions. These points are observed if a constant sum of two terms, each formed by the product of the concentration of the corresponding defect and a characteristic integral coefficient associated with it, is conserved. The complicated processes involved in the transformation of radiation defects in lithium fluoride are studied using these points. It is found that the ratio of the changes in the concentrations of one of the components and the reaction product remains constant in the course of several simultaneous reactions.

Characterization of point defects in monolayer arsenene

NASA Astrophysics Data System (ADS)

Liang, Xiongyi; Ng, Siu-Pang; Ding, Ning; Wu, Chi-Man Lawrence

2018-06-01

Topological defects that are inevitably found in 2D materials can dramatically affect their properties. Using density functional theory (DFT) calculations and ab initio molecular dynamics (AIMD) method, the structural, thermodynamic, electronic and magnetic properties of six types of typical point defects in arsenene, i.e. the Stone-Wales defect, single and double vacancies and adatoms, were systemically studied. It was found that these defects were all more easily generated in arsenene with lower formation energies than those with graphene and silicene. Stone-Wales defects can be transformed from pristine arsenene by overcoming a barrier of 2.19 eV and single vacancy defects tend to coalesce into double vacancy defects by diffusion. However, a type of adatom defect does not exhibit kinetic stability at room temperature. In addition, SV defects and another type of adatom defect can remarkably affect the electronic and magnetic properties of arsenene, e.g. they can introduce localized states near the Fermi level, as well as a strongly local magnetic moment due to dangling bond and unpaired electron. Furthermore, the simulated scanning tunneling microscopy (STM) and Raman spectroscopy were computed and the types of point defects can be fully characterized by correlating the STM images and Raman spectra to the defective atomistic structures. The results provide significant insights to the effect of defects in arsenene for potential applications, as well as identifications of two helpful tools (STM and Raman spectroscopy) to distinguish the type of defects in arsenene for future experiments.

Interaction of phosphine with Si(100) from core-level photoemission and real-time scanning tunneling microscopy

NASA Astrophysics Data System (ADS)

Lin, Deng-Sung; Ku, Tsai-Shuan; Chen, Ru-Ping

2000-01-01

In this paper, we investigate the interaction of phosphine (PH3) on the Si(100)-2×1 surface at temperatures between 635 and 900 K. The hydrogen desorption, growth mode, surface morphology, and chemical composition and ordering of the surface layer are examined by synchrotron radiation core-level photoemission and real-time high-temperature scanning tunneling microscopy. The P 2p core-level spectra indicate that decomposition of PHn is complete above ~550 K and the maximum P coverage is strongly influenced by the growth temperature, which governs the coverage of H-terminated sites. The scanning tunneling microscopy (STM) images taken at real time during PH3 exposure indicate that a surface phosphorus atom readily and randomly displaces one Si atom from the substrate. The ejected Si diffuses, nucleates, and incorporates itself into islands or step edges, leading to similar growth behavior as that found in Si chemical vapor deposition. Line defects both perpendicular and parallel to the dimer rows are observed on the nearly P-saturated surface. Perpendicular line defects act as a strain relief mechanism. Parallel line defects result from growth kinetics. STM images also indicate that incorporating a small amount of phosphorus eliminates the line defects in the Si(100)-2×n surface.

Semiconductor color-center structure and excitation spectra: Equation-of-motion coupled-cluster description of vacancy and transition-metal defect photoluminescence

NASA Astrophysics Data System (ADS)

Lutz, Jesse J.; Duan, Xiaofeng F.; Burggraf, Larry W.

2018-03-01

Valence excitation spectra are computed for deep-center silicon-vacancy defects in 3C, 4H, and 6H silicon carbide (SiC), and comparisons are made with literature photoluminescence measurements. Optimizations of nuclear geometries surrounding the defect centers are performed within a Gaussian basis-set framework using many-body perturbation theory or density functional theory (DFT) methods, with computational expenses minimized by a QM/MM technique called SIMOMM. Vertical excitation energies are subsequently obtained by applying excitation-energy, electron-attached, and ionized equation-of-motion coupled-cluster (EOMCC) methods, where appropriate, as well as time-dependent (TD) DFT, to small models including only a few atoms adjacent to the defect center. We consider the relative quality of various EOMCC and TD-DFT methods for (i) energy-ordering potential ground states differing incrementally in charge and multiplicity, (ii) accurately reproducing experimentally measured photoluminescence peaks, and (iii) energy-ordering defects of different types occurring within a given polytype. The extensibility of this approach to transition-metal defects is also tested by applying it to silicon-substituted chromium defects in SiC and comparing with measurements. It is demonstrated that, when used in conjunction with SIMOMM-optimized geometries, EOMCC-based methods can provide a reliable prediction of the ground-state charge and multiplicity, while also giving a quantitative description of the photoluminescence spectra, accurate to within 0.1 eV of measurement for all cases considered.

Symmetry breaking and excitonic effects on optical properties of defective nanographenes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Noguchi, Yoshifumi, E-mail: y.noguchi@issp.u-tokyo.ac.jp; Sugino, Osamu

2015-02-14

We investigate optical properties of the nanographene family and predict a defect induced effect by utilizing the all-electron first-principles GW+Bethe-Salpeter equation (BSE) method based on the many-body perturbation theory. As an accuracy check of the GW+BSE, photoabsorption spectra are calculated for a grossly warped nanographene (C{sub 80}H{sub 30}), which was very recently synthesized [Kawasumi et al., Nat. Chem. 5, 739–744 (2013)]. The calculated spectra are found to faithfully reproduce the shape, height, and position of the measured peaks. Then the method is applied to the flat nanographene without defect (C{sub 24}H{sub 12} and C{sub 38}H{sub 16}), the curved ones withmore » single defect (C{sub 20}H{sub 10}, C{sub 28}H{sub 14}, and C{sub 32}H{sub 16}), and fragments of C{sub 80}H{sub 30} with double defect (C{sub 36}H{sub 16} and C{sub 42}H{sub 20}). The existence of the defects significantly changes the optical spectra. In particular, the interaction between the defects is found to break the symmetry of the atomic geometries and enhance the excitonic effect, thereby generating the extra peaks at the lower photon energy side of the main peak. The present results might help explain the origin of the first two peaks experimentally observed for C{sub 80}H{sub 30}.« less

[The NIR spectra based variety discrimination for single soybean seed].

PubMed

Zhu, Da-Zhou; Wang, Kun; Zhou, Guang-Hua; Hou, Rui-Feng; Wang, Cheng

2010-12-01

With the development of soybean producing and processing, the quality breeding becomes more and more important for soybean breeders. Traditional sampling detection methods for soybean quality need to destroy the seed, and does not satisfy the requirement of earlier generation materials sieving for breeding. Near infrared (NIR) spectroscopy has been widely used for soybean quality detection. However, all these applications were referred to mass samples, and they were not suitable for little or single seed detection in breeding procedure. In the present study, the acousto--optic tunable filter (AOTF) NIR spectroscopy was used to measure the single soybean seed. Two varieties of soybean were measured, which contained 60 KENJIANDOU43 seeds and 60 ZHONGHUANG13 seeds. The results showed that NIR spectra combined with soft independent modeling of class analogy (SIMCA) could accurately discriminate the soybean varieties. The classification accuracy for KENJIANDOU43 seeds and ZHONGHUANG13 was 100%. The spectra of single soybean seed were measured at different positions, and it showed that the seed shape has significant influence on the measurement of spectra, therefore, the key point for single seed measurement was how to accurately acquire the spectra and keep their representativeness. The spectra for soybeans with glossy surface had high repeatability, while the spectra of seeds with external defects had significant difference for several measurements. For the fast sieving of earlier generation materials in breeding, one could firstly eliminate the seeds with external defects, then apply NIR spectra for internal quality detection, and in this way the influence of seed shape and external defects could be reduced.

Electrically detected magnetic resonance of carbon dangling bonds at the Si-face 4H-SiC/SiO2 interface

NASA Astrophysics Data System (ADS)

Gruber, G.; Cottom, J.; Meszaros, R.; Koch, M.; Pobegen, G.; Aichinger, T.; Peters, D.; Hadley, P.

2018-04-01

SiC based metal-oxide-semiconductor field-effect transistors (MOSFETs) have gained a significant importance in power electronics applications. However, electrically active defects at the SiC/SiO2 interface degrade the ideal behavior of the devices. The relevant microscopic defects can be identified by electron paramagnetic resonance (EPR) or electrically detected magnetic resonance (EDMR). This helps to decide which changes to the fabrication process will likely lead to further increases of device performance and reliability. EDMR measurements have shown very similar dominant hyperfine (HF) spectra in differently processed MOSFETs although some discrepancies were observed in the measured g-factors. Here, the HF spectra measured of different SiC MOSFETs are compared, and it is argued that the same dominant defect is present in all devices. A comparison of the data with simulated spectra of the C dangling bond (PbC) center and the silicon vacancy (VSi) demonstrates that the PbC center is a more suitable candidate to explain the observed HF spectra.

The electronic and optical properties of amorphous silica with hydrogen defects by ab initio calculations

NASA Astrophysics Data System (ADS)

Ren, Dahua; Xiang, Baoyan; Hu, Cheng; Qian, Kai; Cheng, Xinlu

2018-04-01

Hydrogen can be trapped in the bulk materials in four forms: interstitial molecular H2, interstitial atom H, O‑H+(2Si=O–H)+, Si‑H‑( {{4O}}\\bar \\equiv {{Si&x2212H}})‑ to affect the electronic and optical properties of amorphous silica. Therefore, the electronic and optical properties of defect-free and hydrogen defects in amorphous silica were performed within the scheme of density functional theory. Initially, the negative charged states hydrogen defects introduced new defect level between the valence band top and conduction band bottom. However, the neutral and positive charged state hydrogen defects made both the valence band and conduction band transfer to the lower energy. Subsequently, the optical properties such as absorption spectra, conductivity and loss functions were analyzed. It is indicated that the negative hydrogen defects caused the absorption peak ranging from 0 to 2.0 eV while the positive states produced absorption peaks at lower energy and two strong absorption peaks arose at 6.9 and 9.0 eV. However, the neutral hydrogen defects just improved the intensity of absorption spectrum. This may give insights into understanding the mechanism of laser-induced damage for optical materials. Project supported by the Science and Technology of Hubei Provincial Department of Education (No. B2017098).

Comparison of gain degradation and deep level transient spectroscopy in pnp Si bipolar junction transistors irradiated with different ion species

DOE Office of Scientific and Technical Information (OSTI.GOV)

Aguirre, B. A.; Bielejec, E.; Fleming, R. M.

Here, we studied the effect of light ion and heavy ion irradiations on pnp Si BJTs. A mismatch in DLTS deep peak amplitude for devices with same final gain but irradiated with different ion species was observed. Also, different ions cause different gain degradation when the DLTS spectra are matched. Pre-dosed ion-irradiated samples show that ion induced ionization does not account for the differences in DLTS peak height but isochronal annealing studies suggest that light ions produce more VP defects than heavy ions to compensate for the lack of clusters that heavy ions produce. The creation of defect clusters bymore » heavy ions is evident by the higher content of E4 and V* 2 defects compared to light ions.« less

Comparison of gain degradation and deep level transient spectroscopy in pnp Si bipolar junction transistors irradiated with different ion species

DOE PAGES

Aguirre, B. A.; Bielejec, E.; Fleming, R. M.; ...

2016-12-09

Here, we studied the effect of light ion and heavy ion irradiations on pnp Si BJTs. A mismatch in DLTS deep peak amplitude for devices with same final gain but irradiated with different ion species was observed. Also, different ions cause different gain degradation when the DLTS spectra are matched. Pre-dosed ion-irradiated samples show that ion induced ionization does not account for the differences in DLTS peak height but isochronal annealing studies suggest that light ions produce more VP defects than heavy ions to compensate for the lack of clusters that heavy ions produce. The creation of defect clusters bymore » heavy ions is evident by the higher content of E4 and V* 2 defects compared to light ions.« less

Elementary defects in graphane

NASA Astrophysics Data System (ADS)

Podlivaev, A. I.; Openov, L. A.

2017-07-01

The main zero-dimensional defects in graphane, a completely hydrogenated single-layer graphene, having the chair-type conformation have been numerically simulated. The hydrogen and carbon-hydrogen vacancies, Stone-Wales defect, and "transmutation defect" resulting from the simultaneous hoppings of two hydrogen atoms between the neighboring carbon atoms have been considered. The energies of formations of these defects have been calculated and their effect on the electronic structure, phonon spectra, and Young modulus has been studied.

First-principles study of structural, electronic, and optical properties of surface defects in GaAs(001) - β2(2x4)

NASA Astrophysics Data System (ADS)

Bacuyag, Dhonny; Escaño, Mary Clare Sison; David, Melanie; Tani, Masahiko

2018-06-01

We performed first-principles calculations based on density functional theory (DFT) to investigate the role of point defects in the structural, electronic, and optical properties of the GaAs(001)- β2(2x4). In terms of structural properties, AsGa is the most stable defect structure, consistent with experiments. With respect to the electronic structure, band structures revealed the existence of sub-band and midgap states for all defects. The induced sub-bands and midgap states originated from the redistributions of charges towards these defects and neighboring atoms. The presence of these point defects introduced deep energy levels characteristic of EB3 (0.97 eV), EL4 (0.52 eV), and EL2 (0.82 eV) for AsGa, GaAs, GaV, respectively. The optical properties are found to be strongly related to these induced gap states. The calculated onset values in the absorption spectra, corresponding to the energy gaps, confirmed the absorption below the known bulk band gap of 1.43 eV. These support the possible two-step photoabsorption mediated by midgap states as observed in experiments.

Did BICEP2 see vector modes? First B-mode constraints on cosmic defects.

PubMed

Moss, Adam; Pogosian, Levon

2014-05-02

Scaling networks of cosmic defects, such as strings and textures, actively generate scalar, vector, and tensor metric perturbations throughout the history of the Universe. In particular, vector modes sourced by defects are an efficient source of the cosmic microwave background B-mode polarization. We use the recently released BICEP2 and POLARBEAR B-mode polarization spectra to constrain properties of a wide range of different types of cosmic strings networks. We find that in order for strings to provide a satisfactory fit on their own, the effective interstring distance needs to be extremely large--spectra that fit the data best are more representative of global strings and textures. When a local string contribution is considered together with the inflationary B-mode spectrum, the fit is improved. We discuss implications of these results for theories that predict cosmic defects.

Ultrasensitive ppb-level NO2 gas sensor based on WO3 hollow nanosphers doped with Fe

NASA Astrophysics Data System (ADS)

Zhang, Ziyue; haq, Mahmood; Wen, Zhen; Ye, Zhizhen; Zhu, Liping

2018-03-01

WO3 mesoporous hollow nanospheres doped with Fe synthesized by a facile method have mesoporous hollow nanospherical like morphology, small grain size (10 nm), high crystalline quality and ultrahigh surface area (165 m2/g). XRD spectra and Raman spectra indicate the Fe doping leading to the smaller cell parameters as compared to pure WO3, and the slight distortion in the crystal lattice produces a number of defects, making it a better candidate for gas sensing. XPS analysis shows that Fe-doped WO3 mesoporous hollow nanospheres have more oxygen vacancies than pure WO3, which is beneficial to the adsorption of oxygen and NO2 and its surface reaction. The gas sensor based on Fe-WO3 exhibited excellent low ppb-level (10 ppb) NO2 detecting performance and outstanding selectivity.

Divacancy-tin related defects in irradiated germanium

NASA Astrophysics Data System (ADS)

Khirunenko, L. I.; Sosnin, M. G.; Duvanskii, A. V.; Abrosimov, N. V.; Riemann, H.

2018-04-01

A new absorption spectrum has been detected in the region of 770-805 cm-1 following the annealing of low temperature irradiated Sn-doped Ge. The spectrum develops simultaneously with the disappearance of the V2-related absorption band. The new spectra arise both in p- (doping with gallium) and n- (doping with antimony) type samples and are completely identical to the absorption spectrum of the corresponding dopants. The studies have shown that the defects responsible for the registered spectra have hydrogen-like excited states similar to those observed for hydrogen-like group-III acceptors and group-V donors in Ge. The defects are identified as SnV2Ga and SnV2Sb. The formation of the revealed complexes consists of two stages. During the first stage, the defects are created as a result of the direct interaction of SnV2 diffusing upon the annealing with atoms Ga or Sb. The second stage arises, apparently, due to the participation of SnV2 in the formation of intermediate defects that are optically inactive and transform into the revealed defects at annealing temperatures Tann. > 243 K.

Infrared studies of the evolution of the C{sub i}O{sub i}(Si{sub I}) defect in irradiated Si upon isothermal anneals

DOE Office of Scientific and Technical Information (OSTI.GOV)

Angeletos, T.; Londos, C. A., E-mail: hlontos@phys.uoa.gr; Chroneos, A., E-mail: alexander.chroneos@imperial.ac.uk

2016-03-28

Carbon-oxygen-self-interstitial complexes were investigated in silicon by means of Fourier transform infrared spectroscopy. Upon irradiation, the C{sub i}O{sub i} defect (C{sub 3}) forms which for high doses attract self-interstitials (Si{sub I}s) leading to the formation of the C{sub i}O{sub i}(Si{sub I}) defect (C{sub 4}) with two well-known related bands at 939.6 and 1024 cm{sup −1}. The bands are detectable in the spectra both in room temperature (RT) and liquid helium (LH) temperature. Upon annealing at 150 °C, these bands were transformed to three bands at 725, 952, and 973 cm{sup −1}, detectable only at LH temperatures. Upon annealing at 220 °C, these bands weremore » transformed to three bands at 951, 969.5, and 977 cm{sup −1}, detectable both at RT and LH temperatures. Annealing at 280 °C resulted in the transformation of these bands to two new bands at 973 and 1024 cm{sup −1}. The latter bands disappear from the spectra upon annealing at 315 °C without the emergence of other bands in the spectra. Considering reaction kinetics and defect metastability, we developed a model to describe the experimental results. Annealing at 150 °C triggers the capturing of Si{sub I}s by the C{sub 4} defect leading to the formation of the C{sub i}O{sub i}(Si{sub I}){sub 2} complex. The latter structure appears to be bistable: measuring at LH, the defect is in configuration C{sub i}O{sub i}(Si{sub I}){sub 2} giving rise to the bands at 725, 952, and 973 cm{sup −1}, whereas on measurements at RT, the defect converts to another configuration C{sub i}O{sub i}(Si{sub I}){sub 2}{sup *} without detectable bands in the spectra. Possible structures of the two C{sub i}O{sub i}(Si{sub I}){sub 2} configurations are considered and discussed. Upon annealing at 220 °C, additional Si{sub I}s are captured by the C{sub i}O{sub i}(Si{sub I}){sub 2} defect leading to the formation of the C{sub i}O{sub i}(Si{sub I}){sub 3} complex, which in turn on annealing at 280 °C converts to the C{sub i}O{sub i}(Si{sub I}){sub 4} complex. The latter defect anneals out at 315 °C, without being accompanied in the spectra by the growth of new bands.« less

The V + I defects in diamond: An ab initio investigation of the electronic structure, of the Raman and IR spectra, and of their possible recombination.

PubMed

Salustro, S; Nöel, Y; Zicovich-Wilson, C M; Olivero, P; Dovesi, R

2016-11-14

The double defect in diamond, vacancy (V) plus 〈100〉 self-split-interstitial (V+I), is investigated at the ab initio quantum mechanical level, by considering the vicinal case VI 1 (V is one of the first neighbors of one of the two C atoms constituting the I defect) and the two possible "second neighbors" cases, VI 2 D , VI 2 S , in which a carbon atom is a first neighbor of both V and I. The case in which the two defects are at a larger distance is simulated by considering the two isolated defects separately (VI ∞ ). A 6-21G local Gaussian-type basis set and the B3LYP hybrid functional are used for most of the calculations; richer basis sets and other functionals (a global hybrid as PBE0, a range-separated hybrid as HSE06, LDA, PBE, and Hartree-Fock) have also been used for comparison. With this computational approach we evaluate the energy difference between the various spin states, the location of the corresponding bands in the energy gap of pristine diamond, as well as the defect formation energy of the four defects. The path for the recombination of V and I is explored for the vicinal case, by using the distinguished reaction coordinate strategy. A barrier as high as 0.75 eV is found with B3LYP between VI 1 and the perfect diamond recombined structure; when other hybrids are used, as PBE0 or HSE06, the barrier increases up to 1.01 eV (pure density functional theory produces lower barriers: 0.62 and 0.67 for PBE and LDA, respectively). Such a barrier is lower than the one estimated in a very indirect way through experimental data, ranging from 1.3 to 1.7 eV. It confirms however the evidence of the extremely low recombination rate also at high temperature. The Raman (and IR) spectra of the various defects are generated, which permit one to unambiguously attribute to these defects (thanks also to the graphical animation of the modes) many of the peaks observed in damaged diamond above the dominant peak of perfect bulk. For the residual non-attributed peaks, more complicated aggregations of defects should be explored.

Enhanced gamma ray sensitivity in bismuth triiodide sensors through volumetric defect control

DOE PAGES

Johns, Paul M.; Baciak, James E.; Nino, Juan C.

2016-09-02

In some of the more attractive semiconducting compounds for ambient temperature radiation detector applications are impacted by low charge collection efficiency due to the presence of point and volumetric defects. This has been particularly true in the case of BiI 3, which features very attractive properties (density, atomic number, band gap, etc.) to serve as a gamma ray detector, but has yet to demonstrate its full potential. Here, we show that by applying growth techniques tailored to reduce defects, the spectral performance of this promising semiconductor can be realized. Gamma ray spectra from >100 keV source emissions are now obtainedmore » from high quality Sb:BiI 3 bulk crystals with limited concentrations of defects (point and extended). The spectra acquired in these high quality crystals feature photopeaks with resolution of 2.2% at 662 keV. Infrared microscopy is used to compare the local microstructure between radiation sensitive and non-responsive crystals. Our work demonstrates that BiI 3 can be prepared in melt-grown detector-grade samples with superior quality and can acquire the spectra from a variety of gamma ray sources.« less

Variations in the Infrared Spectra of Wüstite with Defects and Disorder

NASA Astrophysics Data System (ADS)

Koike, C.; Matsuno, J.; Chihara, H.

2017-08-01

The presence of FeO particles in circumstellar space has been suggested based on the observation of a mysterious 21 μm emission band. However, the complete infrared spectra of FeO have not been obtained so far; hence, data of the infrared (IR) spectra of FeO need to be investigated. We prepared synthetic and commercial samples of FeO, which were obtained by crushing bulk samples, annealing iron oxalate dihydrate ({{FeC}}2{{{O}}}4\\cdot 2{{{H}}}2{{O}}), and mechanical milling of a powder mixture comprising (Fe and {{Fe}}2{{{O}}}3) particles with different milling times. We present a new study on the IR spectra of these samples, and show that these spectra changed according to defects and disorders. Furthermore, FeO particles are very sensitive to oxygen fugacity and temperature. The spectra of FeO particles were compared with the unidentified observed feature. It may be difficult for FeO particles to exist alone in the ISM and circumstellar space. This may be connected to the problem of missing iron in the ISM.

High-resolution spectroscopy and quantum-defect model for the gerade triplet np and nf Rydberg states of He{sub 2}

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sprecher, D.; Liu, J.; Krähenmann, T.

2014-02-14

Photoionization spectra and Rydberg-state-resolved threshold-ionization spectra of the gerade triplet np Rydberg states of {sup 4}He{sub 2} located in the vicinity of the X{sup +2}Σ{sub u}{sup +}(ν{sup +} =0) ionization threshold were recorded from the 2sσa{sup 3}Σ{sub u}{sup +} metastable state. An accuracy of 0.01 cm{sup −1} was achieved for the experimental term values of the observed Rydberg states. The data were combined with spectroscopic data on low-lying triplet np and nf Rydberg states from the literature to derive energy- and internuclear-distance-dependent eigenquantum-defect parameters of multichannel quantum-defect theory (MQDT). The MQDT calculations reproduce the experimental data within their experimental uncertainties andmore » enabled the derivation of potential-energy curves for the lowest triplet p Rydberg states (n = 2–5) of He{sub 2}. The eigenquantum-defect parameters describing the p -f interaction were found to be larger than 0.002 at the energies corresponding to the high-n Rydberg states, so that the p -f interaction plays an important role in the autoionization dynamics of np Rydberg states with ν{sup +} = 0. By extrapolating the experimental term values of triplet np Rydberg states of {sup 4}He{sub 2} in the range of principal quantum number n between 87 and 110, the positions of the (ν{sup +} = 0, N{sup +} = 3) and (ν{sup +} = 0, N{sup +} = 5) levels of the ground state of {sup 4}He{sub 2}{sup +} were determined to lie 70.937(3) cm{sup −1} and 198.369(6) cm{sup −1}, respectively, above the (ν{sup +} = 0, N{sup +} = 1) ground rotational level.« less

Nitrogen: Unraveling the Secret to Stable Carbon-Supported Pt-Alloy Electrocatalysts

DTIC Science & Technology

2013-10-01

materials reveal broad N1s spectra, indicative of formation of multiple functionalities including but not limited to pyridinic, graphitic and pyrrolic ...network along with nitrogen substitutional defects, while high-dosage increases vacancy agglomerations and pyridinic and pyrrolic nitrogen defects...Article Online highly oriented pyrolytic graphite (HOPG) surface. Simulated defects included pyridinic (Npyridinic), pyrrolic (Npyrrolic), graphitic

Impurity-defect complexes in non-implanted aluminum

NASA Astrophysics Data System (ADS)

Pedersen, F. T.; Grann, H.; Weyer, G.

1986-02-01

The formation of impurity-defect complexes in ion-implanted aluminum has been studied in the temperature interval 100 400K. Radioactive119In isotopes have been implanted. Mössbauer spectra have been measured for the 24 keV γ-radiation emitted after the decay to119Sn. The spectra could be analysed satisfactorily with two lines, one of which is known to be due to substitutional Sn. A second line, which has a higher isomer shift and lower Debye temperature, is tentatively assigned to vacancy-associated Sn, formed by trapping of thermally mobile (multi-)vacancies. Comparison to similar DPAC experiments suggests that cubic Sn-V4 complexes are formed. Some indication (˜15%) for an athermal formation of impurity defects below 175K is obtained.

Radioluminescence response of germanosilicate optical fibres

NASA Astrophysics Data System (ADS)

Khanlary, M. R.; Townsend, P. D.; Townsend, J. E.

1993-07-01

X-ray irradiation of germanosilicate optical fibres simultaneously produces signals from both the core and substrate and so the radioluminescence spectra record the defect structure of both regions. The data provide evidence for the presence of dopants and trace impurities, as well as intrinsic defects formed by thermal and radiation processing. Examples of the changes in spectra or luminescence sensitivity with radiation dose, the influence of fibre pulling conditions and post irradiation heating are noted. The temperature dependence of the radioluminescence is reported. Whilst most of the intrinsic defects produce broad emission bands, rare earth dopants show line features. However, line features have also been noted for Al doped fibres. Such studies of fibre luminescence offer a sensitive monitor of changes in the structure of the glass network.

Many-Body Theory of Proton-Generated Point Defects for Losses of Electron Energy and Photons in Quantum Wells

NASA Astrophysics Data System (ADS)

Huang, Danhong; Iurov, Andrii; Gao, Fei; Gumbs, Godfrey; Cardimona, D. A.

2018-02-01

The effects of point defects on the loss of either energies of ballistic electron beams or incident photons are studied by using a many-body theory in a multi-quantum-well system. This theory includes the defect-induced vertex correction to a bare polarization function of electrons within the ladder approximation, and the intralayer and interlayer screening of defect-electron interactions is also taken into account in the random-phase approximation. The numerical results of defect effects on both energy-loss and optical-absorption spectra are presented and analyzed for various defect densities, numbers of quantum wells, and wave vectors. The diffusion-reaction equation is employed for calculating distributions of point defects in a layered structure. For completeness, the production rate for Frenkel-pair defects and their initial concentration are obtained based on atomic-level molecular-dynamics simulations. By combining the defect-effect, diffusion-reaction, and molecular-dynamics models with an available space-weather-forecast model, it will be possible in the future to enable specific designing for electronic and optoelectronic quantum devices that will be operated in space with radiation-hardening protection and, therefore, effectively extend the lifetime of these satellite onboard electronic and optoelectronic devices. Specifically, this theory can lead to a better characterization of quantum-well photodetectors not only for high quantum efficiency and low dark current density but also for radiation tolerance or mitigating the effects of the radiation.

Unexpected effects of a trap in CCD echelle spectra of B-type stars

NASA Technical Reports Server (NTRS)

Morrison, Nancy D.; Zimba, Jason R.

1990-01-01

Because of the nature of echelle spectra, cosmetic defects such as traps may mimic real spectral features. An example from spectra taken at CTIO with a GEC CCD is presented, and it is shown how the affected pixels can be eliminated from the reduced spectrum, at a slight cost in signal-to-noise ratio.

Synchrotron Radiation and the Far-Infrared and Mid-Infrared Spectra of Ncncs

NASA Astrophysics Data System (ADS)

Winnewisser, Manfred; Winnewisser, Brenda P.; De Lucia, Frank C.; Tokaryk, Dennis; Ross, Stephen Cary; Billinghurst, Brant E.

2014-06-01

The large-amplitude in-plane bending vibration of NCNCS at 85 wn has a potential energy function which includes a barrier to linearity with a height of about 285 wn. The topology of the surface of the space defined by this two-dimensional potential function exhibits non-trivial monodromy. Therefore an energy/momentum map for a quantum system with its motion determined by such a potential takes the form of a lattice which contains a defect associated with the top of the barrier. In NCNCS, the wavenumber values of the fundamental vibrational excitation and the barrier height mean that easily accessible energy levels allow us to observe 3 bending vibrational levels below and 3 above the barrier, yet still below all of the other vibrational levels, allowing the study of all the levels in the neighborhood of the defect. In three measuring campaigns at the Canadian Light Source in May of the years 2011, 2012, and 2013 we have now obtained 8 of the 9 fundamental vibrational band systems of NCNCS in high resolution, in particular that of the large-amplitude bend in the FIR. So far only a-type spectra have been assigned. Thus we have now determined the Δvb = 1, and ΔKa = 0 vibrational intervals (using bent molecule notation) but do not yet have experimental values for either rotational ΔKa = +/- 1 intervals nor ro-vibrational Δvb = 1, ΔKa = +/- 1 intervals. In May of 2014 we will have our last measuring campaign and hope to observe the more elusive b-type transitions.

Power spectrum analysis for defect screening in integrated circuit devices

DOEpatents

Tangyunyong, Paiboon; Cole Jr., Edward I.; Stein, David J.

2011-12-01

A device sample is screened for defects using its power spectrum in response to a dynamic stimulus. The device sample receives a time-varying electrical signal. The power spectrum of the device sample is measured at one of the pins of the device sample. A defect in the device sample can be identified based on results of comparing the power spectrum with one or more power spectra of the device that have a known defect status.

Structural defects and recombination behavior of excited carriers in Cu(In,Ga)Se2 solar cells

NASA Astrophysics Data System (ADS)

Yang, J.; Du, H. W.; Li, Y.; Gao, M.; Wan, Y. Z.; Xu, F.; Ma, Z. Q.

2016-08-01

The carriers' behavior in neutral region (NTR) and space charged region (SCR) of Cu(In,Ga)Se2 thin film based solar cells has been investigated by temperature dependent photoluminescence (PL-T), electroluminescence (EL-T) and current-voltage (IV-T) from 10 to 300 K. PL-T spectra show that three kinds of defects, namely VSe, InCu and (InCu+VCu), are localized within the band gap of NTR and SCR of CIGS layer, corresponding to the energy levels of EC-0.08, EC-0.20 and EC-0.25 eV, respectively. The InCu and (InCu+VCu) deep level defects are non-radiative recombination centers at room temperature. The IV-T and EL-T analysis reveals that the injection modes of electrons from ZnO conduction band into Cu(In,Ga)Se2 layer are tunneling, thermally-excited tunneling and thermionic emission under 10-40, 60-160, and 180-300 K, respectively. At 10-160 K, the electrons tunnel into (InCu+VCu) and Vse defect levels in band gap of SCR and the drifting is involved in the emission bands at 0.96 and 1.07 eV, which is the direct evidence for a tunneling assisted recombination. At 180-300 K, the electrons are directly injected into the Cu(In,Ga)Se2 conduction band, and the emission of 1.13 eV are ascribed to the transitions from the conduction band to the valence band.

Spatial correlation between chemical and topological defects in vitreous silica: UV-resonance Raman study

DOE Office of Scientific and Technical Information (OSTI.GOV)

Saito, M., E-mail: makina.saito@elettra.eu; D’Amico, F.; Bencivenga, F.

2014-06-28

A spatial correlation between chemical and topological defects in the tetrahedron network in vitreous silica produced by a fusion process of natural quartz crystals was found by synchrotron-based UV resonance Raman experiments. Furthermore, a quantitative correlation between these defects was obtained by comparing visible Raman and UV absorption spectra. These results indicate that in vitreous silica produced by the fusion process the topological defects disturb the surrounding tetrahedral silica network and induce further disorder regions with sub nanometric sizes.

Tunable hybrid optical modes in a bounded cholesteric liquid crystal with a twist defect

NASA Astrophysics Data System (ADS)

Pyatnov, Maxim V.; Vetrov, Stepan Ya.; Timofeev, Ivan V.

2018-03-01

Coupling between the defect mode of a cholesteric liquid crystal and the localized mode of a cholesteric liquid crystal-phase plate-metal structure is theoretically demonstrated. It is shown that the transmittance spectrum can be tuned by changing the twist-defect angle and helix pitch, which are governed by external factors. The spectra for different circular polarizations of the incident light are different; specifically, at the nondiffracting polarization, there is no defect-mode transmittance peak.

Electronic and transport properties of zigzag carbon nanotubes with the presence of periodical antidot and boron/nitride doping defects

NASA Astrophysics Data System (ADS)

Zoghi, Milad; Yazdanpanah Goharrizi, Arash; Mirjalili, Seyed Mohammad; Kabir, M. Z.

2018-06-01

Electronic and transport properties of Carbon nanotubes (CNTs) are affected by the presence of physical or chemical defects in their structures. In this paper, we present novel platforms of defected zigzag CNTs (Z-CNTs) in which two topologies of antidot and Boron/Nitride (BN) doping defects are periodically imposed throughout the length of perfect tubes. Using the tight binding model and the non-equilibrium Green’s function method, it is realized that the quantum confinement of Z-CNTs is modified by the presence of such defects. This new quantum confinement results in the appearance of mini bands and mini gaps in the transmission spectra, as well as a modified band structure and band gap size. The modified band gap could be either larger or smaller than the intrinsic band gap of a perfect tube, which is determined by the category of Z-CNT. The in-depth analysis shows that the size of the modified band gap is the function of several factors consisting of: the radii of tube (D r), the distance between adjacent defects (d d), the utilized defect topology, and the kind of defect (antidot or BN doping). Furthermore, taking advantage of the tunable band gap size of Z-CNT with the presence of periodical defects, new platforms of defect-based Z-CNT resonant tunneling diode (RTD) are proposed for the first time. Our calculations demonstrate the apparition of resonances in transmission spectra and the negative differential resistance in the I-V characteristics for such RTD platforms.

Large-size TlBr single crystal growth and defect study

NASA Astrophysics Data System (ADS)

Zhang, Mingzhi; Zheng, Zhiping; Chen, Zheng; Zhang, Sen; Luo, Wei; Fu, Qiuyun

2018-04-01

Thallium bromide (TlBr) is an attractive semiconductor material for fabrication of radiation detectors due to its high photon stopping power originating from its high atomic number, wide band gap and high resistivity. In this paper the vertical Bridgman method was used for crystal growth and TlBr single crystals with diameter of 15 mm were grown. X-ray diffraction (XRD) was used to identify phase and orientation. Electron backscatter diffraction (EBSD) was used to investigate crystal microstructure and crystallographic orientation. The optical and electric performance of the crystal was characterized by infrared (IR) transmittance spectra and I-V measurement. The types of point defects in the crystals were investigated by thermally stimulated current (TSC) spectra and positron annihilation spectroscopy (PAS). Four types of defects, with ionization energy of each defect fitting as follows: 0.1308, 0.1540, 0.3822 and 0.538 eV, were confirmed from the TSC result. The PAS result showed that there were Tl vacancies in the crystal.

Effect of Cobalt Concentration and Oxygen Vacancy on Magnetism of Co Doped ZnO Nanorods.

PubMed

Li, Congli; Che, Ping; Sun, Changyan; Li, Wenjun

2016-03-01

Zn(1-x)Co(x)O (x = 0-0.07) single-crystalline nanorods were prepared by a modified microemulsion route. The crystalline structure, morphology, optical, and hysteresis loop at low and room temperature of as-prepared materials were characterized by XRD, TEM, PL spectra, and magnetic measurement respectively. The nanorods are 80-250 nm in diameter and about 3 μm in length. X-ray diffraction data, TEM images confirm that the materials synthesized in optimal conditions are ZnO:Co single crystalline solid solution without any impurities related to Co. The PL spectra show that the ferromagnetic samples exhibit strong Zn interstitials and oxygen vacancy emission indicating defects may stabilize ferromagnetic order in the obtained diluted magnetic semiconductors. Magnetic measurements show that the Zn(1-x)Co(x)O nanorods exist obvious ferromagnetic characteristics with T(c) above 300 K. M(s) and coercivities first increase and then decrease with dopant concentration increasing, reaching the highest for 3% doping level. The structural and magnetic properties of these samples support the hypothesis that the FM of DMS nanorods is due to a defect mediated mechanism instead of cobalt nanoclusters and carrier mediated.

Carbon, oxygen and intrinsic defect interactions in germanium-doped silicon

NASA Astrophysics Data System (ADS)

Londos, C. A.; Sgourou, E. N.; Chroneos, A.; Emtsev, V. V.

2011-10-01

Production and annealing of oxygen-vacancy (VO) and oxygen-carbon (CiOi, CiOiI) defects in germanium-doped Czochralski-grown silicon (Cz-Si) containing carbon are investigated. All the samples were irradiated with 2 MeV fast electrons. Radiation-produced defects are studied using infrared spectroscopy by monitoring the relevant bands in optical spectra. For the VO defects, it is established that the doping with Ge affects the thermal stability of VO (830 cm-1) defects as well as their fraction converted to VO2 (888 cm-1) defects. In Ge-free samples containing carbon, it was found that carbon impurity atoms do not affect the thermal stability of VO defects, although they affect the fraction of VO defects that is converted to VO2 complexes. Considering the oxygen-carbon complexes, it is established that the annealing of the 862 cm-1 band associated with the CiOi defects is accompanied with the emergence of the 1048 cm-1 band, which has earlier been assigned to the CsO2i center. The evolution of the CiOiI bands is also traced. Ge doping does not seem to affect the thermal stability of the CiOi and CiOiI defects. Density functional theory (DFT) calculations provide insights into the stability of the defect clusters (VO, CiOi, CiOiI) at an atomic level. Both experimental and theoretical results are consistent with the viewpoint that Ge affects the stability of the VO but does not influence the stability of the oxygen-carbon clusters. DFT calculations demonstrate that C attracts both Oi and VO pairs predominately forming next nearest neighbor clusters in contrast to Ge where the interactions with Oi and VO are more energetically favorable at nearest neighbor configurations.

Identification of ultradilute dopants in ceramics.

PubMed

Tanaka, Isao; Mizoguchi, Teruyasu; Matsui, Masafumi; Yoshioka, Satoru; Adachi, Hirohiko; Yamamoto, Tomoyuki; Okajima, Toshihiro; Umesaki, Masanori; Ching, Wai Yim; Inoue, Yoshiyuki; Mizuno, Masataka; Araki, Hideki; Shirai, Yasuharu

2003-08-01

The properties of ceramic materials are strongly influenced by the presence of ultradilute impurities (dopants). Near-edge X-ray absorption fine structure (NEXAFS) measurements using third-generation synchotron sources can be used to identify ultradilute dopants, provided that a good theoretical tool is available to interpret the spectra. Here, we use NEXAFS analysis and first-principles calculations to study the local environments of Ga dopants at levels of 10 p.p.m in otherwise high-purity MgO. This analysis suggests that the extra charge associated with substitutional Ga on a Mg site is compensated by the formation of a Mg vacancy. This defect model is then confirmed by positron lifetime measurements and plane-wave pseudopotential calculations. This powerful combination of techniques should provide a general method of identifying the defect states of ultradilute dopants in ceramics.

Structure and optical homogeneity of LiNbO{sub 3}:Zn (0.03–4.5 mol.%) crystals

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sidorov, Nikolay, E-mail: sidorov@chemy.kolasc.net.ru, E-mail: tepl-na@chemy.kolasc.net.ru, E-mail: fleischermed@gmail.com, E-mail: Jovial1985@yandex.ru, E-mail: palat-mn@chemy.kolasc.net.ru; Tepljakova, Natalja, E-mail: sidorov@chemy.kolasc.net.ru, E-mail: tepl-na@chemy.kolasc.net.ru, E-mail: fleischermed@gmail.com, E-mail: Jovial1985@yandex.ru, E-mail: palat-mn@chemy.kolasc.net.ru; Gabain, Aleksei, E-mail: sidorov@chemy.kolasc.net.ru, E-mail: tepl-na@chemy.kolasc.net.ru, E-mail: fleischermed@gmail.com, E-mail: Jovial1985@yandex.ru, E-mail: palat-mn@chemy.kolasc.net.ru

2014-11-14

Structure and optical homogeneity of LiNbO{sub 3}:Zn (0.03–4.5 mol.%) crystals were searched by photoinduced light scattering and by Raman spectroscopy. The photorefractive effect depends on Zn{sup 2+} concentration nonmonotonically. Decrease of photorefractive effect is explained by decrease of structure defects with localized electrons. The Zn{sup 2+} cations replace structure defects Nb{sub Li} and Li{sub Nb}, trapping levels appear near the bottom of the conduction band and photo electrons recombine with emission under laser radiation. By the Raman spectra the area of the high structure order is found. In this area the own alternation, the alternation of impurity cations and themore » vacancies along the polar axis is almost perfect.« less

Understanding the effect of surface/bulk defects on the photocatalytic activity of TiO2: anatase versus rutile.

PubMed

Yan, Junqing; Wu, Guangjun; Guan, Naijia; Li, Landong; Li, Zhuoxin; Cao, Xingzhong

2013-07-14

The sole effect of surface/bulk defects of TiO2 samples on their photocatalytic activity was investigated. Nano-sized anatase and rutile TiO2 were prepared by hydrothermal method and their surface/bulk defects were adjusted simply by calcination at different temperatures, i.e. 400-700 °C. High temperature calcinations induced the growth of crystalline sizes and a decrease in the surface areas, while the crystalline phase and the exposed facets were kept unchanged during calcination, as indicated by the characterization results from XRD, Raman, nitrogen adsorption-desorption, TEM and UV-Vis spectra. The existence of surface/bulk defects in calcined TiO2 samples was confirmed by photoluminescence and XPS spectra, and the surface/bulk defect ratio was quantitatively analyzed according to positron annihilation results. The photocatalytic activity of calcined TiO2 samples was evaluated in the photocatalytic reforming of methanol and the photocatalytic oxidation of α-phenethyl alcohol. Based on the characterization and catalytic results, a direct correlation between the surface specific photocatalytic activity and the surface/bulk defect density ratio could be drawn for both anatase TiO2 and rutile TiO2. The surface defects of TiO2, i.e. oxygen vacancy clusters, could promote the separation of electron-hole pairs under irradiation, and therefore, enhance the activity during photocatalytic reaction.

Ab initio theory of point defects in oxide materials: structure, properties, chemical reactivity

NASA Astrophysics Data System (ADS)

Pacchioni, Gianfranco

2000-05-01

Point defects play a fundamental role in determining the physical and chemical properties of inorganic materials. This holds not only for the bulk properties but also for the surface of oxides where several kinds of point defects exist and exhibit a rich and complex chemistry. A particularly important defect in oxides is the oxygen vacancy. Depending on the electronic structure of the material the nature of oxygen vacancies changes dramatically. In this article we provide a rationalization of the very different electronic structure of neutral and charged oxygen vacancies in SiO 2 and MgO, two oxide materials with completely different electronic structure (from very ionic, MgO, to largely covalent, SiO 2). We used methods of ab initio quantum chemistry, from density functional theory (DFT) to configuration interaction (CI), to determine the ground and excited state properties of these defects. The theoretical results are combined with recent spectroscopic measurements. A series of observable properties has been determined in this way: defect formation energies, hyperfine interactions in electron paramagnetic resonance (EPR) spectra of paramagnetic centers, optical spectra, surface chemical reactivity. The interplay between experimental and theoretical information allows one to unambiguously identify the structure of oxygen vacancies in these binary oxides and on their surfaces.

125Te NMR and Seebeck Effect in Bi 2Te 3 Synthesized from Stoichiometric and Te-Rich Melts

DOE PAGES

Levin, E. M.; Iowa State Univ., Ames, IA; Riedemann, T. M.; ...

2016-10-14

Bi 2Te 3 is a well-known thermoelectric material and, as a new form of quantum matter, a topological insulator. Variation of local chemical composition in Bi2Te3 results in formation of several types of atomic defects, including Bi and Te vacancies and Bi and Te antisite defects; these defects can strongly affect material functionality via generation of free electrons and/or holes. Nonuniform distribution of atomic defects produces electronic inhomogeneity, which can be detected by 125Te nuclear magnetic resonance (NMR). Here we report on 125Te NMR and Seebeck effect (heat to electrical energy conversion) for two single crystalline samples: (#1) grown frommore » stoichiometric composition by Bridgman technique and (#2) grown out of Te-rich, high temperature flux. The Seebeck coefficients of these samples show p- and n-type conductivity, respectively, arising from different atomic defects. 125Te NMR spectra and spin–lattice relaxation measurements demonstrate that both Bi 2Te 3 samples are electronically inhomogeneous at the atomic scale, which can be attributed to a different Te environment due to spatial variation of the Bi/Te ratio and formation of atomic defects. In conclusion, correlations between 125Te NMR spectra, spin–lattice relaxation times, the Seebeck coefficients, carrier concentrations, and atomic defects are discussed. Our data demonstrate that 125Te NMR is an effective probe to study antisite defects in Bi 2Te 3.« less

125Te NMR and Seebeck Effect in Bi 2Te 3 Synthesized from Stoichiometric and Te-Rich Melts

DOE Office of Scientific and Technical Information (OSTI.GOV)

Levin, E. M.; Iowa State Univ., Ames, IA; Riedemann, T. M.

Bi 2Te 3 is a well-known thermoelectric material and, as a new form of quantum matter, a topological insulator. Variation of local chemical composition in Bi2Te3 results in formation of several types of atomic defects, including Bi and Te vacancies and Bi and Te antisite defects; these defects can strongly affect material functionality via generation of free electrons and/or holes. Nonuniform distribution of atomic defects produces electronic inhomogeneity, which can be detected by 125Te nuclear magnetic resonance (NMR). Here we report on 125Te NMR and Seebeck effect (heat to electrical energy conversion) for two single crystalline samples: (#1) grown frommore » stoichiometric composition by Bridgman technique and (#2) grown out of Te-rich, high temperature flux. The Seebeck coefficients of these samples show p- and n-type conductivity, respectively, arising from different atomic defects. 125Te NMR spectra and spin–lattice relaxation measurements demonstrate that both Bi 2Te 3 samples are electronically inhomogeneous at the atomic scale, which can be attributed to a different Te environment due to spatial variation of the Bi/Te ratio and formation of atomic defects. In conclusion, correlations between 125Te NMR spectra, spin–lattice relaxation times, the Seebeck coefficients, carrier concentrations, and atomic defects are discussed. Our data demonstrate that 125Te NMR is an effective probe to study antisite defects in Bi 2Te 3.« less

Optical characterization of single-crystal diamond grown by DC arc plasma jet CVD

NASA Astrophysics Data System (ADS)

Hei, Li-fu; Zhao, Yun; Wei, Jun-jun; Liu, Jin-long; Li, Cheng-ming; Lü, Fan-xiu

2017-12-01

Optical centers of single-crystal diamond grown by DC arc plasma jet chemical vapor deposition (CVD) were examined using a low-temperature photoluminescence (PL) technique. The results show that most of the nitrogen-vacancy (NV) complexes are present as NV- centers, although some H2 and H3 centers and B-aggregates are also present in the single-crystal diamond because of nitrogen aggregation resulting from high N2 incorporation and the high mobility of vacancies under growth temperatures of 950-1000°C. Furthermore, emissions of radiation-induced defects were also detected at 389, 467.5, 550, and 588.6 nm in the PL spectra. The reason for the formation of these radiation-induced defects is not clear. Although a Ni-based alloy was used during the diamond growth, Ni-related emissions were not detected in the PL spectra. In addition, the silicon-vacancy (Si-V)-related emission line at 737 nm, which has been observed in the spectra of many previously reported microwave plasma chemical vapor deposition (MPCVD) synthetic diamonds, was absent in the PL spectra of the single-crystal diamond prepared in this work. The high density of NV- centers, along with the absence of Ni-related defects and Si-V centers, makes the single-crystal diamond grown by DC arc plasma jet CVD a promising material for applications in quantum computing.

Fingerprints of spin-orbital polarons and of their disorder in the photoemission spectra of doped Mott insulators with orbital degeneracy

NASA Astrophysics Data System (ADS)

Avella, Adolfo; Oleś, Andrzej M.; Horsch, Peter

2018-04-01

We explore the effects of disordered charged defects on the electronic excitations observed in the photoemission spectra of doped transition metal oxides in the Mott insulating regime by the example of the R1 -xCaxVO3 perovskites, where R = La, ⋯, Lu. A fundamental characteristic of these vanadium d2 compounds with partly filled t2 g valence orbitals is the persistence of spin and orbital order up to high doping, in contrast to the loss of magnetic order in high-Tc cuprates at low defect concentration. We study the disordered electronic structure of such doped Mott-Hubbard insulators within the unrestricted Hartree-Fock approximation and, as a result, manage to explain the spectral features that occur in photoemission and inverse photoemission. In particular, (i) the atomic multiplet excitations in the inverse photoemission spectra and the various defect-related states and satellites are qualitatively well reproduced, (ii) a robust Mott gap survives up to large doping, and (iii) we show that the defect states inside the Mott gap develop a soft gap at the Fermi energy. The soft defect-states gap, which separates the highest occupied from the lowest unoccupied states, can be characterized by a shape and a scale parameter extracted from a Weibull statistical sampling of the density of states near the chemical potential. These parameters provide a criterion and a comprehensive schematization for the insulator-metal transition in disordered systems. Our results provide clear indications that doped holes are bound to charged defects and form small spin-orbital polarons whose internal kinetic energy is responsible for the opening of the soft defect-states gap. We show that this kinetic gap survives disorder fluctuations of defects and is amplified by the long-range electron-electron interactions, whereas we observe a Coulomb singularity in the atomic limit. The small size of spin-orbital polarons is inferred by an analysis of the inverse participation ratio and by means of a complementary many-body polaron theory, which yields a similar robust spin and orbital order as the Hartree-Fock approximation. Using realistic parameters for the vanadium perovskite La1 -xCaxVO3 , we show that its soft gap is reproduced as well as the marginal doping dependence of the position of the chemical potential relative to the center of the lower Hubbard band. The present theory uncovers also the reasons why the d1→d0 satellite excitations, which directly probe the effect of the random defect fields on the polaron state, are not well resolved in the available experimental photoemission spectra for La1 -xCaxVO3 .

Pulse-height defect due to electron interaction in dead layers of Ge/Li/ gamma-ray detectors

NASA Technical Reports Server (NTRS)

Larsen, R. N.; Strauss, M. G.

1969-01-01

Study shows the pulse-height degradation of gamma ray spectra in germanium/lithium detectors to be due to electron interaction in the dead layers that exist in all semiconductor detectors. A pulse shape discrimination technique identifies and eliminates these defective pulses.

Polariton Local States in Periodic Bragg Multiple Quantum Well Structures

NASA Astrophysics Data System (ADS)

Deych, Lev; Yamilov, Alexey; Lisyansky, Alexander

2000-11-01

We analytically study defect polariton states in Bragg MQW structures, and defect induced changes in transmission and reflection spectra. Defect layers can differ from the host layers in three different ways: in the exciton-light coupling strength, in the exciton resonance frequency, and in interwell spacing. We show that a single defect leads to two local polariton modes in the photonic band gap. These modes lead to peculiarities in reflection and transmission spectra. Each type of defect can be reproduced experimentally, and we show that each of them play distinctly different roles in the optical properties of the system. We obtain closed analytical expressions for respective local frequencies, as well as for reflection and transmission coefficients. On the basis of the results obtained, we give practical recommendation for experimental observation of the studied effects in samples used in Refs. [1,2]. [1] M.Hübner, J. Kuhl, T. Stroucken, A. Knorr, S.W. Koch, R. Hey, K. Ploog, Phys. Rev. Lett. 76, 4199 (1996). [2] M.Hübner, J.P. Prineas, C. Ell, P. Brick, E.S. Lee, G. Khitrova, H.M. Gibbs, S.W. Koch, Phys. Rev. Lett. 83, 2841 (1999).

Probing of O2 vacancy defects and correlated magnetic, electrical and photoresponse properties in indium-tin oxide nanostructures by spectroscopic techniques

NASA Astrophysics Data System (ADS)

Ghosh, Shyamsundar; Dev, Bhupendra Nath

2018-05-01

Indium-tin oxide (ITO) 1D nanostructures with tunable morphologies i.e. nanorods, nanocombs and nanowires are grown on c-axis (0 0 0 1) sapphire (Al2O3) substrate in oxygen deficient atmosphere through pulsed laser deposition (PLD) technique and the effect of oxygen vacancies on optical, electrical, magnetic and photoresponse properties is investigated using spectroscopic methods. ITO nanostructures are found to be enriched with significant oxygen vacancy defects as evident from X-ray photoelectron and Raman spectroscopic analysis. Photoluminescence spectra exhibited intense mid-band blue emission at wavelength of region of 400-450 nm due to the electronic transition from conduction band maxima (CBM) to the singly ionized oxygen-vacancy (VO+) defect level within the band-gap. Interestingly, ITO nanostructures exhibited significant room-temperature ferromagnetism (RTFM) and the magnetic moment found proportional to concentration of VO+ defects which indicates VO+ defects are mainly responsible for the observed RTFM in nanostructures. ITO nanowires being enriched with more VO+ defects exhibited strongest RTFM as compared to other morphologies. Current voltage (I-V) characteristics of ITO nanostructures showed an enhancement of current under UV light as compared to dark which indicates such 1D nanostructure can be used as photovoltaic material. Hence, the study shows that there is ample opportunity to tailor the properties of ITOs through proper defect engineering's and such photosensitive ferromagnetic semiconductors might be promising for spintronic and photovoltaic applications.

Photoluminescence properties of Eu3+ doped HfO2 coatings formed by plasma electrolytic oxidation of hafnium

NASA Astrophysics Data System (ADS)

Stojadinović, Stevan; Tadić, Nenad; Ćirić, Aleksandar; Vasilić, Rastko

2018-03-01

Plasma electrolytic oxidation was used for synthesis of Eu3+ doped monoclinic HfO2 coatings on hafnium substrate. Results of photoluminescence (PL) measurements show the existence of two distinct regions: one that is related to the blue emission originating from oxygen vacancy defects in HfO2 and the other one characterized with a series of sharp orange-red emission peaks related to f-f transitions of Eu3+ from excited level 5D0 to lower levels 7FJ (J = 0, 1, 2, 3, and 4). PL peaks appearing in excitation spectra of obtained coatings are attributed either to charge transfer state of Eu3+ or to direct excitation of the Eu3+ ground state 7F0 into higher levels of the 4f-manifold. PL of formed coatings increases with PEO time due to an increase of oxygen vacancy defects and the content of Eu3+. Acquired experimental data suggest that hypersensitive electrical dipole transition is much more intense than the magnetic dipole transition, indicating that Eu3+ ions occupy a non-inversion symmetry sites.

Defect dynamics in Li substituted nanocrystalline ZnO: A spectroscopic analysis

NASA Astrophysics Data System (ADS)

Ghosh, S.; Nambissan, P. M. G.; Thapa, S.; Mandal, K.

2014-12-01

Very recently, vacancy-type defects have been found to play a major role in stabilizing d0 ferromagnetism in various low dimensional ZnO systems. In this context, the evolution of vacancy-type defects within the ZnO nanocrystals due to the doping of ZnO by alkali metal lithium (Li) is investigated using X-ray photoelectron (XPS), photoluminescence (PL) and positron annihilation spectroscopy (PAS). Li-doping is found to have significant effects in modifying the vacancy-type defects, especially the Zn vacancy (VZn) defects within the ZnO lattice. XPS measurement indicated that initially the Li1+ ions substitute at Zn2+ sites, but when Li concentration exceeds 7 at%, excess Li starts to move through the interstitial sites. The increase in positron lifetime components and the lineshape S-parameter obtained from coincident Doppler broadening spectra with Li-doping indicated an enhancement of VZn defect concentration within the doped ZnO lattice. The vacancy type defects, initially of the predominant configuration VZn+O+Zn got reduced to neutral ZnO divacancies due to the partial recombination by the doped Li1+ ions but, when the doping concentration exceeded 7 at% and Li1+ ions started migrating to the interstitials, positron diffusion is partly impeded and this results in reduced probability of annihilation. PL spectra have shown intense green and yellow-orange emission due to the stabilization of a large number of VZn defects and Li substitutional (LiZn) defects respectively. Hence Li can be a very useful dopant in stabilizing and modifying significant amount of Zn vacancy-defects which can play a useful role in determining the material behavior.

Study of cobalt effect on structural and optical properties of Dy doped ZnO nanoparticles

NASA Astrophysics Data System (ADS)

Kumar, Pawan; Pandey, Praveen C.

2018-05-01

The present study has been carried out to investigate the effect of Co doping on structural and optical properties of Dy doped ZnO nanoparticles. We have prepared pure Zinc oxide, Dy (1%) doped ZnO and Dy (1%) doped ZnO co-doped with Co(2%) with the help of simple sol-gel combustion method. The structural analysis carried out using X-ray diffraction spectra (XRD) indicates substitution of Dy and Co at Zn site of ZnO crystal structure and hexagonal crystal structure without any secondary phase formation in all the samples. The surface morphology was analyzed by transmission electron microscopy (TEM). Absorption study indicates that Dy doping causes a small shift in band edge, while Co co-doping results significant change is absorption edge as well as introduce defect level absorption in the visible region. The band gap of samples decreases due to Dy and Co doping, which can be attributed to defect level formation below the conduction band in the system.

The Radio Spectra and - Inertial Defects Behavior of Planar Aromatic Heterocycles

NASA Astrophysics Data System (ADS)

McNaughton, Don; Jahn, Michaela K.; Grabow, Jens-Uwe; Godfrey, Peter; Travers, Michael; Wachsmuth, Dennis

2016-06-01

The simplest tricyclic aromatic nitrogen heterocyclic molecules 5,6 benzoquinoline and 7,8 benzoquinoline are possible candidates for detection of aromatic systems in the interstellar medium. Therefore the pure rotational spectra have been recorded using frequency-scanned Stark modulated, jet-cooled millimetre wave absorption spectroscopy (48-87 GHz) and Fourier Transform Microwave (FT MW) spectroscopy (2-26 GHz) of a supersonic rotationally cold molecular jet. Guided by ab initio molecular orbital predictions, spectral analysis of mm wave spectra, and higher resolution FT MW spectroscopy provided accurate rotational and centrifugal distortion constants together with 14N nuclear quadrupole coupling constants for both species. The determined inertial defects, along with those of similar species are used to develop an empirical formula for calculation of inertial defects of aromatic ring systems. The predictive ability of the formula is shown to be excellent for planar species with a number of pronounced out of plane vibrations. The resultant constants are of sufficient accuracy to be used in potential astrophysical searches. We gratefully acknowledge support from the Deutsche Forschungsgemeinschaft, the Deutsche Akademische Austauschdienst, as well as the Land Niedersachsen (J.-U.G). DMcN also thanks the Royal Society of Chemistry for their generous travel support.

Improved Si0.5Ge0.5/Si interface quality achieved by the process of low energy hydrogen plasma cleaning and investigation of interface quality with positron annihilation spectroscopy

NASA Astrophysics Data System (ADS)

Liao, M.-H.; Chen, C.-H.

2013-04-01

The Positron Annihilation Spectra (PAS), Raman, and Photoluminescence spectroscopy reveal that Si0.5Ge0.5/Si interface quality can be significantly improved by the low energy plasma cleaning process using hydrogen. In the PAS, the particularly small value of lifetime and intensity near the Si0.5Ge0.5/Si interface in the sample with the treatment indicate that the defect concentration is successfully reduced 2.25 times, respectively. Fewer defects existed in the Si0.5Ge0.5/Si interface result in the high compressive strain about 0.36% in the top epi-Si0.5Ge0.5 layer, which can be observed in Raman spectra and stronger radiative recombination rate about 1.39 times for the infrared emission, which can be observed in the photoluminescence spectra. With better Si0.5Ge0.5/Si interface quality, the SiGe-based devices can have better optical and electrical characteristics for more applications in the industry. The PAS is also demonstrated that it is the useful methodology tool to quantify the defect information in the SiGe-based material.

Influence of defect luminescence and structural modification on the electrical properties of Magnesium Doped Zinc Oxide Nanorods

NASA Astrophysics Data System (ADS)

Santoshkumar, B.; Biswas, Amrita; Kalyanaraman, S.; Thangavel, R.; Udayabhanu, G.; Annadurai, G.; Velumani, S.

2017-06-01

Magnesium doped zinc oxide nanorod arrays on zinc oxide seed layers were grown by hydrothermal method. X-ray diffraction (XRD) patterns revealed the growth orientation along the preferential (002) direction. The hexagonal morphology was revealed from the field emission scanning electron microscope (FESEM) images. The elemental composition of the samples was confirmed by energy dispersive x-ray analysis spectra (EDS) and mapping dots. Carrier concentration, resistivity and mobility of the samples were obtained by Hall measurements. I-V characteristic curve confirmed the increase in resistivity upon doping. Photoluminescence (PL) spectra exposed the characteristic of UV emission along with defect mediated visible emission in the samples. Electrochemical impedance spectroscopy and cyclic voltammetry were undertaken to study the charge transport property. Owing to the change in the structural parameters and defect concentration the electrical properties of the doped samples were altered.

Effects of excess oxygen on the 4.5-6.3 eV absorption spectra of oxygen-rich high purity silica

NASA Astrophysics Data System (ADS)

Magruder, R. H.; Robinson, S. J.

2016-05-01

Type III silica samples were implanted with O using a multi-energy process that produced a layer of constant concentration to within ±5% beginning ∼80 nm from the surface and extending to ∼640 nm below the surfaces of the samples. The concentrations of excess oxygen in the layer ranged from 0.035 to ∼2.1at.%. In these samples we show that E‧ centers and NBOHCs, as well as the normal cadre of ODC (II) centers, were suppressed, and the optical absorption from 4.7 to 6.4 eV was primarily due to oxygen excess defects. Using Gaussian fitting techniques to examine the optical difference spectra, we have been able to identify four defect centers that are related to excess oxygen defect bands at 4.76 eV, 5.42 eV, 5.75 eV and 6.25 eV.

The narrow pass band filter of tunable 1D phononic crystals with a dielectric elastomer layer

NASA Astrophysics Data System (ADS)

Wu, Liang-Yu; Wu, Mei-Ling; Chen, Lien-Wen

2009-01-01

In this paper, we study the defect bands of a 1D phononic crystal consisting of aluminum (Al) and polymethyl methacrylate (PMMA) layers with a dielectric elastomer (DE) defect layer. The plane wave expansion (PWE) method and supercell calculation are used to calculate the band structure and the defect bands. The transmission spectra are obtained using the finite element method (FEM). Since the thickness of the dielectric elastomer defect layer is controlled by applying an electric voltage, the frequencies of the defect bands can be tuned. A narrow pass band filter can be developed and designed by using the dielectric elastomer.

Energy conversion of X-ray, ultraviolet and infrared radiation in Gd2O3 crystals doped with Er3+ ions

NASA Astrophysics Data System (ADS)

Trofimova, E. S.; Pustovarov, V. A.; Kuznetsova, Yu. A.; Zatsepin, A. F.

2017-09-01

Spectra of photoluminescence (PL) and X-ray excited luminescence (XRL) in region of 1.5-5.0 eV, PL excitation spectra (2.8-5.8 eV), PL decay kinetics were measured in Gd2O3 crystals doped both with Er3+ and Zn2+ ions. Synchrotron radiation (VEPP-3 storage ring, Novosibirsk, Russia) were used for XRL measurements. PL spectra were studied at room temperature and T= 88 K under excitation with energy Eexc: a) in fundamental absorption region (Eexc≥Eg); b) in intracenter excitation region (Eexc

Magnetically addressable fluorescent Fe3O4/ZnO nanocomposites: Structural, optical and magnetization studies

NASA Astrophysics Data System (ADS)

Roychowdhury, A.; Pati, S. P.; Mishra, A. K.; Kumar, S.; Das, D.

2013-06-01

Fe3O4/ZnO nanocomposites (NCs) are prepared by a wet chemical route. X-ray diffraction, transmission electron microscopy and Fourier transform infrared spectroscopy studies confirm the coexistence of Fe3O4 and ZnO phases in the NCs. The UV-vis absorption spectra show a red shift of the absorption peak with increase in Fe3O4 content indicating a modification of the band structure of ZnO in the NCs. Photoluminescence emission spectra of the NCs display strong excitonic emission in the UV region along with weak emission bands in the visible range caused by electronic transitions involving defect-related energy levels in the band gap of ZnO. Positron annihilation lifetimes indicate that cation vacancies in the ZnO structure are the strong traps for positrons and the overall defect concentration in the NCs decreases with increase in Fe3O4 content. Dc magnetization measurements reveal an anomalous temperature dependence of the coercivity of the NCs that is argued to be due to the anomalous variation of magnetocrystalline anisotropy at lower temperature. The irreversibility observed in the temperature dependent ZFC-FC magnetization points to the presence of a spin-glass phase in the NCs.

Cosmic microwave background constraints for global strings and global monopoles

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lopez-Eiguren, Asier; Lizarraga, Joanes; Urrestilla, Jon

We present the first cosmic microwave background (CMB) power spectra from numerical simulations of the global O( N ) linear σ-model, with N =2,3, which have global strings and monopoles as topological defects. In order to compute the CMB power spectra we compute the unequal time correlators (UETCs) of the energy-momentum tensor, showing that they fall off at high wave number faster than naive estimates based on the geometry of the defects, indicating non-trivial (anti-)correlations between the defects and the surrounding Goldstone boson field. We obtain source functions for Einstein-Boltzmann solvers from the UETCs, using a recently developed method thatmore » improves the modelling at the radiation-matter transition. We show that the interpolation function that mimics the transition is similar to other defect models, but not identical, confirming the non-universality of the interpolation function. The CMB power spectra for global strings and global monopoles have the same overall shape as those obtained using the non-linear σ-model approximation, which is well captured by a large- N calculation. However, the amplitudes are larger than the large- N calculation would naively predict, and in the case of global strings much larger: a factor of 20 at the peak. Finally we compare the CMB power spectra with the latest CMB data in other to put limits on the allowed contribution to the temperature power spectrum at multipole l = 10 of 1.7% for global strings and 2.4% for global monopoles. These limits correspond to symmetry-breaking scales of 2.9× 10{sup 15} GeV (6.3× 10{sup 14} GeV with the expected logarithmic scaling of the effective string tension between the simulation time and decoupling) and 6.4× 10{sup 15} GeV respectively. The bound on global strings is a significant one for the ultra-light axion scenario with axion masses m {sub a} ∼< 10{sup −28} eV . These upper limits indicate that gravitational waves from global topological defects will not be observable at the gravitational wave observatory LISA.« less

Gamma ray irradiated silicon nanowires: An effective model to investigate defects at the interface of Si/SiOx

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yin, Kui; Zhao, Yi; Liu, Liangbin

2014-01-20

The effect of gamma ray irradiation on silicon nanowires was investigated. Here, an additional defect emerged in the gamma-ray-irradiated silicon nanowires and was confirmed with electron spin resonance spectra. {sup 29}Si nuclear magnetic resonance spectroscopy showed that irradiation doses had influence on the Q{sup 4} unit structure. This phenomenon indicated that the unique core/shell structure of silicon nanowires might contribute to induce metastable defects under gamma ray irradiation, which served as a satisfactory model to investigate defects at the interface of Si/SiOx.

Influence of Compensating Defect Formation on the Doping Efficiency and Thermoelectric Properties of Cu2ySe1xBrx

DTIC Science & Technology

2015-09-24

level spectra (Figure S3b). The relative contents of Cu to Se were determined using a Thermo- Finnigan ELEMENT2 single collector sector field...has a wide range of stoichiometric deviation (Cu2−δSe). 17,22,23 Above 410 K, the compound has an average cubic structure with Se ions forming a face...extrinsically, and a single parabolic band model can be applied to analyze the transport and predict the optimum carrier densities for a maximum figure of

Understanding and Calibrating Density-Functional-Theory Calculations Describing the Energy and Spectroscopy of Defect Sites in Hexagonal Boron Nitride.

PubMed

Reimers, Jeffrey R; Sajid, A; Kobayashi, Rika; Ford, Michael J

2018-03-13

Defect states in 2-D materials present many possible uses but both experimental and computational characterization of their spectroscopic properties is difficult. We provide and compare results from 13 DFT and ab initio computational methods for up to 25 excited states of a paradigm system, the V N C B defect in hexagonal boron nitride (h-BN). Studied include: (i) potentially catastrophic effects for computational methods arising from the multireference nature of the closed-shell and open-shell states of the defect, which intrinsically involves broken chemical bonds, (ii) differing results from DFT and time-dependent DFT (TDDFT) calculations, (iii) comparison of cluster models to periodic-slab models of the defect, (iv) the starkly differing effects of nuclear relaxation on the various electronic states that control the widths of photoabsorption and photoemission spectra as broken bonds try to heal, (v) the effect of zero-point energy and entropy on free-energy differences, (vi) defect-localized and conduction/valence-band transition natures, and (vii) strategies needed to ensure that the lowest-energy state of a defect can be computationally identified. Averaged state-energy differences of 0.3 eV are found between CCSD(T) and MRCI energies, with thermal effects on free energies sometimes also being of this order. However, DFT-based methods can perform very poorly. Simple generalized-gradient functionals like PBE fail at the most basic level and should never be applied to defect states. Hybrid functionals like HSE06 work very well for excitations within the triplet manifold of the defect, with an accuracy equivalent to or perhaps exceeding the accuracy of the ab initio methods used. However, HSE06 underestimates triplet-state energies by on average of 0.7 eV compared to closed-shell singlet states, while open-shell singlet states are predicted to be too low in energy by 1.0 eV. This leads to misassignment of the ground state of the V N C B defect. Long-range corrected functionals like CAM-B3LYP are shown to work much better and to represent the current entry level for DFT calculations on defects. As significant differences between cluster and periodic-slab models are also found, the widespread implementation of such functionals in periodic codes is in urgent need.

The Effect of High Pressure - High Temperature Treatment on Neutron Irradiation Induced Defects in Czochralski Silicon

DTIC Science & Technology

2001-01-01

Spectra by the 830cm’ Localized Vibrational Mode (LVM) band. Upon annealing , this defect is converted to the V0 2 defect responsible for a LVM band at...887cmŕ. The purpose of this work is to study the effect of various combinations of HTHP treatment prior to irradiation on the annealing behaviour of...and stacking faults. Keywords: high temperature - high pressure treatments, annealing , neutron irradiation. 1. INTRODUCTION Oxygen is one of the two

The effect of defect emissions on enhancement photocatalytic performance of ZnSe QDs and ZnSe/rGO nanocomposites

NASA Astrophysics Data System (ADS)

Yousefi, Ramin; Azimi, H. R.; Mahmoudian, M. R.; Basirun, Wan Jeffrey

2018-03-01

A systematic study about the origin of defects emission of ZnSe structure was conducted by photoluminescence (PL) spectrometer at room temperature. It was observed that different intermediate energy levels in band-gap space of ZnSe structure were generated by different defects such as Se-, Zn-vacancies, Se-, Zn-interstitials, and surface states. Effects of these defects on the photocatalytic performance of ZnSe quantum dots (QDs) and ZnSe/graphene nanocomposites were investigated. The pristine ZnSe QDs and ZnSe/graphene nanocomposites were synthesized by a co-precipitation method. The PL spectra of the samples showed four emissions from four regions of the visible spectrum such as violet, green, orange, and red emissions. The violet emission was associated with the near-band-edge (NBE) of the ZnSe nanostructures, while, the other emissions were related to different defects of ZnSe structures. Annealing the samples in the H2 atmosphere caused to increase orange emission intensity and indicated that origin of orange emission was a donor-acceptor pair (DAPs) related to singly positively charged Se-vacancies (VSe) to singly negatively charged zinc vacancy (VZn-). Photocatalytic study of the samples to remove the methylene blue (MB) dye showed that the photocatalytic performance of the samples improved by graphene as an additive and increasing the orange emission intensity.

Left-right correlation in coupled F-center defects.

PubMed

Janesko, Benjamin G

2016-08-07

This work explores how left-right correlation, a textbook problem in electronic structure theory, manifests in a textbook example of electrons trapped in crystal defects. I show that adjacent F-center defects in lithium fluoride display symptoms of "strong" left-right correlation, symptoms similar to those seen in stretched H2. Simulations of UV/visible absorption spectra qualitatively fail to reproduce experiment unless left-right correlation is taken into account. This is of interest to both the electronic structure theory and crystal-defect communities. Theorists have a new well-behaved system to test their methods. Crystal-defect groups are cautioned that the approximations that successfully model single F-centers may fail for adjacent F-centers.

Optical transitions in two-dimensional topological insulators with point defects

NASA Astrophysics Data System (ADS)

Sablikov, Vladimir A.; Sukhanov, Aleksei A.

2016-12-01

Nontrivial properties of electronic states in topological insulators are inherent not only to the surface and boundary states, but to bound states localized at structure defects as well. We clarify how the unusual properties of the defect-induced bound states are manifested in optical absorption spectra in two-dimensional topological insulators. The calculations are carried out for defects with short-range potential. We find that the defects give rise to the appearance of specific features in the absorption spectrum, which are an inherent property of topological insulators. They have the form of two or three absorption peaks that are due to intracenter transitions between electron-like and hole-like bound states.

Thermodynamics and Kinetics of Three Mg-H-VN Complexes in Mg:GaN from Combined First-Principles Calculation and Experiment

NASA Astrophysics Data System (ADS)

Lee, Donghwa; Mitchell, Brandon; Fujiwara, Y.; Dierolf, V.

2014-05-01

An understanding of the formation and dissociation process of Mg-H defects in GaN is of paramount importance for high efficient GaN-based solid-state lighting. Through a combination of first-principle calculations and experimental observations, we find the existence of three types of Mg related centers forming different Mg-H-VN complexes in Mg:GaN. Our study shows that the three different arrangements, which differ by the relative position of the H, determine the degree of acceptor passivation by changing their charge state from +3 to +1. The energetic study demonstrates that the relative stability of the defect complexes can vary with the location of the Fermi level, as well as thermal annealing and electron beam irradiation. The inclusion of a VN is shown to produce an additional variance in optical spectra associated with Mg acceptor activation, resulting from changes in the defect configurations and charge states. Our study shows that these three Mg-H-VN complexes are key components for understanding the Mg acceptor activation and passivation processes.

Demonstration of 2-hydroxybenzoylglycine as a drug binding inhibitor in newborn infants.

PubMed Central

Suh, B; Wadsworth, S J; Lichtenwalner, D M

1987-01-01

Newborn infants have drug binding defects that share similarities to those of uremic subjects. Since 2-hydroxybenzoylglycine has been chemically defined to be a major drug binding inhibitor in uremia, a search for the presence of a similar compound in the sera of newborn infants was made. An organic substance that has the characteristics of 2-hydroxybenzoylglycine as supported by the retardation factor values on thin-layer chromatograms, retention times of high performance liquid chromatograms, fluorescence emission spectra, and mass spectrum has been demonstrated to be present in the majority of the neonatal sera studied. A strong positive correlation between the levels of the binding inhibitor and the extent of binding defects for nafcillin has been observed. The substance could effectively reduce the total bilirubin concentration when added to the cord sera specimens. It is concluded that 2-hydroxybenzoylglycine plays an important role in drug binding defects observed in the newborn, and the inhibitor may also play a part in the precipitation of bilirubin-induced neurotoxicity in neonates when the substance is abnormally elevated. Images PMID:3654972

Perspectives on holoprosencephaly: Part III. Spectra, distinctions, continuities, and discontinuities.

PubMed

Cohen, M M

1989-10-01

This paper attempts to balance our knowledge of holoprosencephalic spectra and continuities with important distinctions and discontinuities. Prevalence studies and syndrome delineation are briefly reviewed. The following topics receive detailed coverage: human teratogens, special aspects of forebrain and hindbrain malformations, aprosencephaly/atelencephaly, association with neural tube defects, current assessment of "facial principles," and endocrine abnormalities.

Structural defects and recombination behavior of excited carriers in Cu(In,Ga)Se{sub 2} solar cells

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yang, J.; Du, H. W.; Li, Y.

2016-08-15

The carriers’ behavior in neutral region (NTR) and space charged region (SCR) of Cu(In,Ga)Se{sub 2} thin film based solar cells has been investigated by temperature dependent photoluminescence (PL-T), electroluminescence (EL-T) and current-voltage (IV-T) from 10 to 300 K. PL-T spectra show that three kinds of defects, namely V{sub Se}, In{sub Cu} and (In{sub Cu}+V{sub Cu}), are localized within the band gap of NTR and SCR of CIGS layer, corresponding to the energy levels of E{sub C}-0.08, E{sub C}-0.20 and E{sub C}-0.25 eV, respectively. The In{sub Cu} and (In{sub Cu}+V{sub Cu}) deep level defects are non-radiative recombination centers at room temperature.more » The IV-T and EL-T analysis reveals that the injection modes of electrons from ZnO conduction band into Cu(In,Ga)Se{sub 2} layer are tunneling, thermally-excited tunneling and thermionic emission under 10-40, 60-160, and 180-300 K, respectively. At 10-160 K, the electrons tunnel into (In{sub Cu}+V{sub Cu}) and V{sub se} defect levels in band gap of SCR and the drifting is involved in the emission bands at 0.96 and 1.07 eV, which is the direct evidence for a tunneling assisted recombination. At 180-300 K, the electrons are directly injected into the Cu(In,Ga)Se{sub 2} conduction band, and the emission of 1.13 eV are ascribed to the transitions from the conduction band to the valence band.« less

Reconstruction of structural damage based on reflection intensity spectra of fiber Bragg gratings

NASA Astrophysics Data System (ADS)

Huang, Guojun; Wei, Changben; Chen, Shiyuan; Yang, Guowei

2014-12-01

We present an approach for structural damage reconstruction based on the reflection intensity spectra of fiber Bragg gratings (FBGs). Our approach incorporates the finite element method, transfer matrix (T-matrix), and genetic algorithm to solve the inverse photo-elastic problem of damage reconstruction, i.e. to identify the location, size, and shape of a defect. By introducing a parameterized characterization of the damage information, the inverse photo-elastic problem is reduced to an optimization problem, and a relevant computational scheme was developed. The scheme iteratively searches for the solution to the corresponding direct photo-elastic problem until the simulated and measured (or target) reflection intensity spectra of the FBGs near the defect coincide within a prescribed error. Proof-of-concept validations of our approach were performed numerically and experimentally using both holed and cracked plate samples as typical cases of plane-stress problems. The damage identifiability was simulated by changing the deployment of the FBG sensors, including the total number of sensors and their distance to the defect. Both the numerical and experimental results demonstrate that our approach is effective and promising. It provides us with a photo-elastic method for developing a remote, automatic damage-imaging technique that substantially improves damage identification for structural health monitoring.

Defect center characteristics of silica optical fiber material by gamma ray radiation

NASA Astrophysics Data System (ADS)

Luo, Wenyun; Xiao, Zhongyin; Wen, Jianxiang; Yin, Jianchong; Chen, Zhenyi; Wang, Zihua; Wang, Tingyun

2011-12-01

Defect centers play a major role in the radiation-induced transmission loss for silica optical fibers. We have investigated characteristics of the best known defect centers E' in silica optical fiber material irradiated with γ ray at room temperature, and measured by using electron spin resonance (ESR) and spectrophotometer. The results show that the defect concentrations increase linearly with radiation doses from 1kGy to 50kGy. We have established the mechanism models of radiation induced defect centers' formation. We have also studied the influences of thermal annealing on defect centers. The radiation induced defect centers can be efficiently decreased by thermal annealing. Particularly, the defect concentration is less than the initial one when the temperature of thermal annealing is over 500°C for our silica samples. These phenomena can also be explained by the optical absorption spectra we have obtained.

Luminescence Properties of Surface Radiation-Induced Defects in Lithium Fluoride

NASA Astrophysics Data System (ADS)

Voitovich, A. P.; Kalinov, V. S.; Martynovich, E. F.; Novikov, A. N.; Runets, L. P.; Stupak, A. P.

2013-11-01

Luminescence and luminescence excitation spectra are recorded for surface radiation-induced defects in lithium fluoride at temperatures of 77 and 293 K. The presence of three bands with relatively small intensity differences is a distinctive feature of the excitation spectrum. These bands are found to belong to the same type of defects. The positions of the peaks and the widths of the absorption and luminescence bands for these defects are determined. The luminescence decay time is measured. All the measured characteristics of these surface defects differ from those of previously known defects induced by radiation in the bulk of the crystals. It is found that the luminescence of surface defects in an ensemble of nanocrystals with different orientations is not polarized. The number of anion vacancies in the surface defects is estimated using the polarization measurements. It is shown that radiative scattering distorts the intensity ratios of the luminescence excitation bands located in different spectral regions.

Photoluminescence enhancement from GaN by beryllium doping

NASA Astrophysics Data System (ADS)

García-Gutiérrez, R.; Ramos-Carrazco, A.; Berman-Mendoza, D.; Hirata, G. A.; Contreras, O. E.; Barboza-Flores, M.

2016-10-01

High quality Be-doped (Be = 0.19 at.%) GaN powder has been grown by reacting high purity Ga diluted alloys (Be-Ga) with ultra high purity ammonia in a horizontal quartz tube reactor at 1200 °C. An initial low-temperature treatment to dissolve ammonia into the Ga melt produced GaN powders with 100% reaction efficiency. Doping was achieved by dissolving beryllium into the gallium metal. The powders synthesized by this method regularly consist of two particle size distributions: large hollow columns with lengths between 5 and 10 μm and small platelets in a range of diameters among 1 and 3 μm. The GaN:Be powders present a high quality polycrystalline profile with preferential growth on the [10 1 bar 1] plane, observed by means of X-ray diffraction. The three characteristics growth planes of the GaN crystalline phase were found by using high resolution TEM microscopy. The optical enhancing of the emission in the GaN powder is attributed to defects created with the beryllium doping. The room temperature photoluminescence emission spectra of GaN:Be powders, revealed the presence of beryllium on a shoulder peak at 3.39 eV and an unusual Y6 emission at 3.32eV related to surface donor-acceptor pairs. Also, a donor-acceptor-pair transition at 3.17 eV and a phonon replica transition at 3.1 eV were observed at low temperature (10 K). The well-known yellow luminescence band coming from defects was observed in both spectra at room and low temperature. Cathodoluminescence emission from GaN:Be powders presents two main peaks associated with an ultraviolet band emission and the yellow emission known from defects. To study the trapping levels related with the defects formed in the GaN:Be, thermoluminescence glow curves were obtained using UV and β radiation in the range of 50 and 150 °C.

NARROW LINE ABSORPTION IN CACO3.

DTIC Science & Technology

CARBONATES), (*CALCIUM COMPOUNDS, (*ABSORPTION SPECTRA, CALCITE), (*CALCITE, RADIATION EFFECTS), ELECTRON PARAMAGNETIC RESONANCE, SINGLE CRYSTALS , NEUTRONS, X RAYS, GAMMA RAYS, IONS, CRYSTAL DEFECTS, PARAMAGNETIC RESONANCE.

Specific features of the circular dichroism of a chiral photonic crystal with a defect layer inside in the presence of a gain

NASA Astrophysics Data System (ADS)

Gevorgyan, A. H.

2017-01-01

The specific features of the circular dichroism (CD) spectra of a cholesteric liquid crystal (CLC) layer with a defect layer inside in the presence of gain have been investigated. The features of the dependence of CD on the parameter characterizing the gain on the defect mode are analyzed for two cases: (i) gain is present in the defect layer and is absent in the CLC sublayers and (ii) gain is absent in the defect layer but is present in the CLC sublayers. It is shown that these dependences significantly differ in the two aforementioned cases. The dependences of the reflection, transmission, and absorption on the defect mode on the gain parameter have been investigated for incident light with both circular polarizations.

Donor-acceptor pair recombination luminescence from monoclinic Cu{sub 2}SnS{sub 3} thin film

DOE Office of Scientific and Technical Information (OSTI.GOV)

Aihara, Naoya; Tanaka, Kunihiko, E-mail: tanaka@vos.nagaokaut.ac.jp; Uchiki, Hisao

2015-07-20

The defect levels in Cu{sub 2}SnS{sub 3} (CTS) were investigated using photoluminescence (PL) spectroscopy. A CTS thin film was prepared on a soda-lime glass/molybdenum substrate by thermal co-evaporation and sulfurization. The crystal structure was determined to be monoclinic, and the compositional ratios of Cu/Sn and S/Metal were determined to be 1.8 and 1.2, respectively. The photon energy of the PL spectra observed from the CTS thin film was lower than that previously reported. All fitted PL peaks were associated with defect related luminescence. The PL peaks observed at 0.843 and 0.867 eV were assigned to donor-acceptor pair recombination luminescence, the thermalmore » activation energies of which were determined to be 22.9 and 24.8 meV, respectively.« less

Defect properties of Sn- and Ge-doped ZnTe: suitability for intermediate-band solar cells

NASA Astrophysics Data System (ADS)

Flores, Mauricio A.

2018-01-01

We investigate the electronic structure and defect properties of Sn- and Ge- doped ZnTe by first-principles calculations within the DFT+GW formalism. We find that ({{{Sn}}}{{Zn}}) and ({{{Ge}}}{{Zn}}) introduce isolated energy levels deep in the band gap of ZnTe, derived from Sn-5s and Ge-4s states, respectively. Moreover, the incorporation of Sn and Ge on the Zn site is favored in p-type ZnTe, in both Zn-rich and Te-rich environments. The optical absorption spectra obtained by solving the Bethe-Salpeter equation reveals that sub-bandgap absorptance is greatly enhanced due to the formation of the intermediate band. Our results suggest that Sn- and Ge-doped ZnTe would be a suitable material for the development of intermediate-band solar cells, which have the potential to achieve efficiencies beyond the single-junction limit.

Absolute band structure determination on naturally occurring rutile with complex chemistry: Implications for mineral photocatalysis on both Earth and Mars

NASA Astrophysics Data System (ADS)

Li, Yan; Xu, Xiaoming; Li, Yanzhang; Ding, Cong; Wu, Jing; Lu, Anhuai; Ding, Hongrui; Qin, Shan; Wang, Changqiu

2018-05-01

Rutile is the most common and stable form of TiO2 that ubiquitously existing on Earth and other terrestrial planets like Mars. Semiconducting mineral such as rutile-based photoredox reactions have been considered to play important roles in geological times. However, due to the inherent complexity in chemistry, the precision determination on band structure of natural rutile and the theoretical explanation on its solar-driven photochemistry have been hardly seen yet. Considering the multiple minor and trace elements in natural rutile, we firstly obtained the single-crystal crystallography, mineralogical composition and defects characteristic of the rutile sample by using both powder and single crystal X-ray diffraction, electron microprobe analysis and X-ray photoelectron spectroscopy. Then, the band gap was accurately determined by synchrotron-based O K-edge X-ray absorption and emission spectra, which was firstly applied to natural rutile due to its robustness on compositions and defects. The absolute band edges of the rutile sample was calculated by considering the electronegativity of the atoms, band gap and point of zero charge. Besides, after detecting the defect energy levels by photoluminescence spectra, we drew the schematic band structure of natural rutile. The band gap (2.7 eV) of natural rutile was narrower than that of synthetic rutile (3.0 eV), and the conduction and valence band edges of natural rutile at pH = pHPZC were determined to be -0.04 V and 2.66 V (vs. NHE), respectively. The defect energy levels located at nearly the middle position of the forbidden band. Further, we used theoretical calculations to verify the isomorphous substitution of Fe and V for Ti gave rise to the distortion of TiO6 octahedron and created vacancy defects in natural rutile. Based on density functional theory, the narrowed band gap was interpreted to the contribution of Fe-3d and V-3d orbits, and the defect energy state was formed by hybridization of O-2p and Fe/V/Ti-3d orbits in the forbidden band. Therefore, excitons can be created under visible light. The conduction band electrons and valence band holes enabled the photoreduction of CO2 to organic molecules (e.g., acetic acid and CH4) and photooxidative generation of oxidants (e.g., radOH, O2 and ClO4-) via rutile photocatalysis, respectively. This study underlies the capability of natural semiconducting minerals in solar energy utilization and the implications of their photocatalysis in both the origin of primitive life on Earth and formation of modern environments on Mars.

Biochemical changes on the repair of surgical bone defects grafted with biphasic synthetic micro-granular HA + β-tricalcium phosphate induced by laser and LED phototherapies assessed by Raman spectroscopy

NASA Astrophysics Data System (ADS)

Pinheiro, Antonio Luiz B.; Soares, Luiz Guilherme P.; Marques, Aparecida Maria C.; Silveira, Landulfo

2016-03-01

This work aimed the assessment of the biochemical changes during bone mineralization induced by laser and LED irradiation in an animal model of bone repair using a spectral model based on Raman spectroscopy. Six groups were studied: Clot, Laser (λ780 nm, 70 mW), LED (λ850 nm +/- 10 nm, 150 mW), Biomaterial (biphasic synthetic microgranular hydroxyapatite (HA) + β-tricalcium phosphate), Laser + Biomaterial and LED + Biomaterial. When indicated, defects were further irradiated at 48 h interval during 2 wks, 20 J/cm2 per session. At 15th and 30th days, femurs were dissected and spectra of the defects were collected. Raman spectra were submitted to a model to estimate the relative amount of collagen, phosphate HA and carbonate HA, by using spectra of pure collagen, biomaterial and basal bone, respectively. At 15th days, the use of biomaterial associated to phototherapy reduced the collagen formation, whereas the amount of carbonate HA was not different in all groups. The phosphate HA was higher in the groups that received biomaterial grafts. At 30th days, it was observed an increase of collagen for the group Laser + Biomaterial, and a reduction in the carbonate HA for the LED + Biomaterial. The phosphate HA was higher for the groups LED + Biomaterial and Laser + Biomaterial, while decreased for the group Biomaterial. These results indicated that the use of Laser and LED phototherapies improved the repair of bone defects grafted with the biomaterial by increasing the collagen deposition and phosphate HA.

Raman spectroscopy, "big data", and local heterogeneity of solid state synthesized lithium titanate

NASA Astrophysics Data System (ADS)

Pelegov, Dmitry V.; Slautin, Boris N.; Gorshkov, Vadim S.; Zelenovskiy, Pavel S.; Kiselev, Evgeny A.; Kholkin, Andrei L.; Shur, Vladimir Ya.

2017-04-01

Existence of defects is an inherent property of real materials. Due to an explicit correlation between defects concentration and conductivity, it is important to understand the level and origins of the structural heterogeneity for any particulate electrode material. Poor conductive lithium titanate Li4Ti5O12 (LTO), widely used in batteries for grids and electric buses, needs it like no one else. In this work, structural heterogeneity of compacted lithium titanate is measured locally in 100 different points by conventional micro-Raman technique, characterized in terms of variation of Raman spectra parameters and interpreted using our version of "big data" analysis. This very simple approach with automated measurement and treatment has allowed us to demonstrate inherent heterogeneity of solid-state synthesized LTO and attribute it to the existence of lithium and oxygen vacancies. The proposed approach can be used as a fast, convenient, and cost-effective defects-probing tool for a wide range of materials with defects-sensitive properties. In case of LTO, such an approach can be used to increase its charge/discharge rates by synthesis of materials with controlled nonstoichiometry. New approaches to solid state synthesis of LTO, suitable for high-power applications, will help to significantly reduce the costs of batteries for heavy-duty electric vehicles and smart-grids.

Detection of magnetic circular dichroism in amorphous materials utilizing a single-crystalline overlayer

DOE PAGES

Lin, J.; Zhong, X. Y.; Song, C.; ...

2017-12-27

Physicists are fascinated with topological defects in solid-state materials, because by breaking the translational symmetry they offer emerging properties that are not present in their parental phases. For example, edge dislocations—the 2π phase-winding topological defects—in antiferromagnetic NiO crystals can exhibit ferromagnetic behaviors. Herein, we study how these defects could give rise to exotic topological orders when they interact with a high energy electron beam. To probe this interaction, we formed a coherent electron nanobeam in a scanning transmission electron microscope and recorded the far-field transmitted patterns as the beam steps through the edge dislocation core in [001] NiO. Surprisingly, wemore » found the amplitude patterns of the <020> Bragg disks evolve in a similar manner to the evolution of an annular solar eclipse. Using the ptychographic technique, we recovered the missing phase information in the diffraction plane and revealed the topological phase vortices in the diffracted beams. Through atomic topological defects, the wave function of electrons can be converted from plane wave to electron vortex. This approach provides a new perspective for boosting the collection efficiency of magnetic circular dichroism spectra with high spatial resolution and understanding the relationship between symmetry breaking and exotic property of individual topological defect at atomic level.« less

Detection of magnetic circular dichroism in amorphous materials utilizing a single-crystalline overlayer

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lin, J.; Zhong, X. Y.; Song, C.

Physicists are fascinated with topological defects in solid-state materials, because by breaking the translational symmetry they offer emerging properties that are not present in their parental phases. For example, edge dislocations—the 2π phase-winding topological defects—in antiferromagnetic NiO crystals can exhibit ferromagnetic behaviors. Herein, we study how these defects could give rise to exotic topological orders when they interact with a high energy electron beam. To probe this interaction, we formed a coherent electron nanobeam in a scanning transmission electron microscope and recorded the far-field transmitted patterns as the beam steps through the edge dislocation core in [001] NiO. Surprisingly, wemore » found the amplitude patterns of the <020> Bragg disks evolve in a similar manner to the evolution of an annular solar eclipse. Using the ptychographic technique, we recovered the missing phase information in the diffraction plane and revealed the topological phase vortices in the diffracted beams. Through atomic topological defects, the wave function of electrons can be converted from plane wave to electron vortex. This approach provides a new perspective for boosting the collection efficiency of magnetic circular dichroism spectra with high spatial resolution and understanding the relationship between symmetry breaking and exotic property of individual topological defect at atomic level.« less

EPR and photoluminescence study of irradiated anion-defective alumina single crystals

NASA Astrophysics Data System (ADS)

Kortov, V. S.; Ananchenko, D. V.; Konev, S. F.; Pustovarov, V. A.

2017-09-01

Electron paramagnetic resonance (EPR) and photoluminescence (PL) spectra of anion-defective alumina single crystals were measured. Exposure to a dose 10 Gy-1 kGy causes isotropic EPR signal of a complex form, this signal contains narrow and broad components. At the same time, in the PL spectrum alongside with a band of F+-centers (3.8 eV) an additional emission band with the maximum of 2.25 eV is registered. This band corresponds to aggregate F22+-centers which were create under irradiation. By comparing measurements in EPR and PL spectra with further stepped annealing in the temperature range of 773-1473 K of the samples exposed to the same doses, we were able to conclude that a narrow component of isotropic EPR signal is associated with the formation of paramagnetic F22+-centers under irradiation. A wide component can be caused by deep hole traps which are created by a complex defect (VAl2- - F+) with a localized hole.

New Data On The Cathodoluminescence Of White Marbles: Interpretation Of Peaks And Relationships To Weathering

NASA Astrophysics Data System (ADS)

Garcia-Guinea, J.; Crespo-Feo, E.; Correcher, V.; Iordanidis, A.; Charalampides, G.; Karamitrou-Mentessidi, G.

This work focus on the Thermoluminescence (TL), the Spatially Resolved Spectral Cathodoluminescence (CL) and Raman spectroscopy (Raman) of white marble specimens collected from the archaeological park of Aiani (Greece) and from patterns of Iceland calcite and Macael marble for comparison purposes. The spectra CL were measured with a high sensitivity cathodoluminescence spectrometer MonoCL3 of Gatan (UK) attached to an FEI-ESEM microscope (CL-ESEM). The experimental set of spectra CL curves of Aiani white marbles suggest that the blue band is more resistant to weathering in comparison with the red band which drops down easily under weathering. The comparison among CL spectra of CaCO3 patterns give a slight difference between the small 330 nm peak, detected in marble and not observed in the monocrystal pattern of Iceland calcite. The Backscattering Electron Dispersed (BSED) images of the white marble are similar to the CL monochromatic plots at 330 nm which highlight the surfaces with remarkable clarity, suggesting a CL emission-defect associated to the marble crystal interfaces, such as protons or hydroxyls. Conversely, the 395 nm monochromatic mapping depicts a CL image emitting from bulk and not from interfaces attributable to point defects or cationic activators in Ca2+ positions. The blue band of the spectra luminescence of marble is composed by several peaks associated to very different types of luminescent defects. This statement is not inconsequential since in archaeological TL dating of marbles the regenerated luminescence in the blue region of the spectrum is a serious difficulty and further research on this topic is necessary.

Tuning optical properties of opal photonic crystals by structural defects engineering

NASA Astrophysics Data System (ADS)

di Stasio, F.; Cucini, M.; Berti, L.; Comoretto, D.; Abbotto, A.; Bellotto, L.; Manfredi, N.; Marinzi, C.

2009-06-01

We report on the preparation and optical characterization of three dimensional colloidal photonic crystal (PhC) containing an engineered planar defect embedding photoactive push-pull dyes. Free standing polystyrene films having thickness between 0.6 and 3 mm doped with different dipolar chromophores were prepared. These films were sandwiched between two artificial opals creating a PhC structure with planar defect. The system was characterized by reflectance at normal incidence angle (R), variable angle transmittance (T) and photoluminescence spectroscopy (PL) Evidence of defect states were observed in T and R spectra which allow the light to propagate for selected frequencies within the pseudogap (stop band).

A qualitative study of spin polarization effect in defect tuned Co/graphene/Co nanostructures

NASA Astrophysics Data System (ADS)

Mandal, Sumit; Saha, Shyamal K.

2014-10-01

Theoretical reports predict that in contact with a ferromagnetic giant spin, spin polarization evolves in defective graphene since defects in graphene act as local spin moments. We have synthesized different Co/graphene/Co nano spin valve like structures tuning the degree of defect applying ultrasonic vibration and characterized them by Raman spectroscopy. Initially with increasing ID/IG ratio in Raman spectra, antiferromagnetic coupling between the Co nanosheets on either sides of graphene enhances leading to betterment in spin transport through graphene. But for highest ID/IG, a totally new phenomenon called antiferro quadrupolar ordering (AFQ) takes place which eventually reduces the spin polarization effect.

Chemical characterisation of non-defective and defective green arabica and robusta coffees by electrospray ionization-mass spectrometry (ESI-MS).

PubMed

Mendonça, Juliana C F; Franca, Adriana S; Oliveira, Leandro S; Nunes, Marcella

2008-11-15

The coffee roasted in Brazil is considered to be of low quality, due to the presence of defective coffee beans that depreciate the beverage quality. These beans, although being separated from the non-defective ones prior to roasting, are still commercialized in the coffee trading market. Thus, it was the aim of this work to verify the feasibility of employing ESI-MS to identify chemical characteristics that will allow the discrimination of Arabica and Robusta species and also of defective and non-defective coffees. Aqueous extracts of green (raw) defective and non-defective coffee beans were analyzed by direct infusion electrospray ionization mass spectrometry (ESI-MS) and this technique provided characteristic fingerprinting mass spectra that not only allowed for discrimination of species but also between defective and non-defective coffee beans. ESI-MS profiles in the positive mode (ESI(+)-MS) provided separation between defective and non-defective coffees within a given species, whereas ESI-MS profiles in the negative mode (ESI(-)-MS) provided separation between Arabica and Robusta coffees. Copyright © 2008 Elsevier Ltd. All rights reserved.

EPR and transient capacitance studies on electron-irradiated silicon solar cells

NASA Technical Reports Server (NTRS)

Lee, Y. H.; Cheng, L. J.; Mooney, P. M.; Corbett, J. W.

1977-01-01

One and two ohm-cm solar cells irradiated with 1 MeV electrons at 30 C were studied using both EPR and transient capacitance techniques. In 2 ohm-cm cells, Si-G6 and Si-G15 EPR spectra and majority carrier trapping levels at (E sub V + 0.23) eV and (E sub V + 0.38) eV were observed, each of which corresponded to the divacancy and the carbon-oxygen-vacancy complex, respectively. In addition, a boron-associated defect with a minority carrier trapping level at (E sub C -0.27) eV was observed. In 1 ohm-cm cells, the G15 spectrum and majority carrier trap at (E sub V + 0.38) eV were absent and an isotropic EPR line appeared at g = 1.9988 (+ or - 0.0003); additionally, a majority carrier trapping center at (E sub V + 0.32) eV, was found which could be associated with impurity lithium. The formation mechanisms of these defects are discussed according to isochronal annealing data in electron-irradiated p-type silicon.

Deep level transient spectroscopy signatures of majority traps in GaN p-n diodes grown by metal-organic vapor-phase epitaxy technique on GaN substrates

NASA Astrophysics Data System (ADS)

PŁaczek-Popko, E.; Trzmiel, J.; Zielony, E.; Grzanka, S.; Czernecki, R.; Suski, T.

2009-12-01

In this study, we present the results of investigation on p-n GaN diodes by means of deep level transient spectroscopy (DLTS) within the temperature range of 77-350 K. Si-doped GaN layers were grown by metal-organic vapor-phase epitaxy technique (MOVPE) on the free-standing GaN substrates. Subsequently Mg-doped GaN layers were grown. To perform DLTS measurements Ni/Au contacts to p-type material and Ti/Au contacts to n-type material were processed. DLTS signal spectra revealed the presence of two majority traps of activation energies obtained from Arrhenius plots equal to E1=0.22 eV and E2=0.65 eV. In present work we show that the trap E1 is linked with the extended defects whereas the trap E2 is the point defect related. Its capture cross section is thermally activated with energy barrier for capture equal to 0.2 eV.

Vacancy-fluorine complexes and their impact on the properties of metal-oxide transistors with high-k gate dielectrics studied using monoenergetic positron beams

NASA Astrophysics Data System (ADS)

Uedono, A.; Inumiya, S.; Matsuki, T.; Aoyama, T.; Nara, Y.; Ishibashi, S.; Ohdaira, T.; Suzuki, R.; Miyazaki, S.; Yamada, K.

2007-09-01

Vacancy-fluorine complexes in metal-oxide semiconductors (MOS) with high-k gate dielectrics were studied using a positron annihilation technique. F+ ions were implanted into Si substrates before the deposition of gate dielectrics (HfSiON). The shift of threshold voltage (Vth) in MOS capacitors and an increase in Fermi level position below the HfSiON/Si interface were observed after F+ implantation. Doppler broadening spectra of the annihilation radiation and positron lifetimes were measured before and after HfSiON fabrication processes. From a comparison between Doppler broadening spectra and those obtained by first-principles calculation, the major defect species in Si substrates after annealing treatment (1050 °C, 5 s) was identified as vacancy-fluorine complexes (V3F2). The origin of the Vth shift in the MOS capacitors was attributed to V3F2 located in channel regions.

Raman Scattering by Crystals of Rare-Earth Hexaborides with Different Isotopes of Boron

NASA Astrophysics Data System (ADS)

Markov, Yu. F.; Gurin, V. N.; Ponkratov, K. V.

2018-04-01

Monocrystals of lanthanum hexaboride LaB6 containing both natural boron and its isotopes 10B and 11B have been produced using the solution-melt method. Polyelement hexaboride rare-earths have been grown and the corresponding ceramics have been synthesized for the first time. All these crystals have been studied by means of various techniques, generally using Raman scattering. The Raman spectra attributed to various spectral lines corresponding to nonanalyzable representations have been obtained and interpreted. Frequencies and half-widths of spectral lines have been obtained, the removal of degeneracy and the development of respective splitting of degenerate oscillations induced by defects, mainly by boron isotope inclusions, have been identified. The influence of defects on the Raman spectra has been determined.

Synchrotron VUV-UV and positron lifetime spectroscopy study of vacancy-type defects in reactor neutron-irradiated MgO.nAl2O3 (n = 2)

NASA Astrophysics Data System (ADS)

Rahman, Abu Zayed Mohammad Saliqur; Cao, Xingzhong; Wang, Baoyi; Evslin, Jarah; Xu, Qiu; Atobe, Kozo

2016-12-01

We investigated neutron-irradiation-induced point defects in spinel single crystals using a synchrotron VUV-UV source and positron lifetime spectroscopy. Photoexcitation (PE) spectra near 230 nm and their corresponding photoluminescence (PL) spectra at 475 nm were attributed to F-centers. With increasing irradiation temperature and fluence, PE efficiency and PL intensity decreased dramatically. Positron lifetimes (PLT) of neutron-irradiated and non-irradiated samples were measured to identify the cation vacancies. A PLT measurement of 250 ps was obtained in a neutron-irradiated (20 K) sample which is tentatively attributed to an aluminum monovacancy. Decreasing PLT with higher irradiation indicates the formation of oxygen-vacancy complex centers.

Left-right correlation in coupled F-center defects

DOE Office of Scientific and Technical Information (OSTI.GOV)

Janesko, Benjamin G., E-mail: b.janesko@tcu.edu

This work explores how left-right correlation, a textbook problem in electronic structure theory, manifests in a textbook example of electrons trapped in crystal defects. I show that adjacent F-center defects in lithium fluoride display symptoms of “strong” left-right correlation, symptoms similar to those seen in stretched H{sub 2}. Simulations of UV/visible absorption spectra qualitatively fail to reproduce experiment unless left-right correlation is taken into account. This is of interest to both the electronic structure theory and crystal-defect communities. Theorists have a new well-behaved system to test their methods. Crystal-defect groups are cautioned that the approximations that successfully model single F-centersmore » may fail for adjacent F-centers.« less

The effect of defect types on the electronic and optical properties of graphene nanoflakes physisorbed by ionic liquids.

PubMed

Shakourian-Fard, Mehdi; Kamath, Ganesh

2017-02-08

Defect engineering and non-covalent interaction strategies allow for dramatically tuning the optoelectronic properties of graphene. Using ab initio density functional theory (M06-2X/cc-pVDZ), we find that the nature of defects on the graphene nanoflakes (GNFs) and the size of defective GNF (DGNF) surfaces affect the binding energy (ΔE b ) of ionic liquids (ILs) and the UV-Vis absorption spectra of DGNFIL complexes. Further, our results indicate that increasing the size of DGNFs affects the geometrical structure of the surfaces and increases the binding energy of ILs by about 10%. Analysis based on AIM and EDA shows that the interactions between ILs and DGNFs are non-covalent in nature (dispersion energy being dominant) and associated with charge transfer between the IL and nanoflakes. A comparison between the ΔE b values of ILs on DGNFs, GNFs, and h-BN nanoflakes (h-BNNF) shows that the presence of defects on the GNF surfaces increases the binding energy values as follows: DGNFIL > pristine GNFIL > h-BNNFIL. Our calculations indicate that increasing the size of DGNF surfaces leads to a decrease in the HOMO-LUMO energy gap (E g ) of the DGNF surfaces. Orbital energy and density of state calculations show that the E g of DV(SW)-GNFs decreases upon IL adsorption and their Fermi energy level is shifted depending on the type of IL, thus enabling better conductivity. Reactivity descriptors generally indicate that the chemical potential (μ) and chemical hardness (η) of nanoflakes decrease upon IL adsorption, whereas the electrophilicity index (ω) increases. The UV-Vis absorption spectrum of DV-GNF and SW-GNF shows four bands in the visible spectrum which correspond to π → π* transitions with the absorption bands of SW-GNF appearing at higher wavelengths than those of DV-GNF. The most intense absorption bands in DV-GNF (λ = 348 nm) and SW-GNF (λ = 375 nm) are associated with electronic transitions HOMO-1 → LUMO+2 and HOMO → LUMO+1, respectively. In addition, these absorption bands undergo a red-shift by both increasing the size of the DV(SW)-GNF surfaces and IL adsorption. We also observe that the energy gaps and absorption spectra can be altered by varying the defect types and the type of IL adsorbate, where the defect types affect the spectral shapes of the bands and adsorbates at the first absorption peak, thus having potential application for light-emitting devices.

Identification of urinary metabolites of imperatorin with a single run on an LC/Triple TOF system based on multiple mass defect filter data acquisition and multiple data mining techniques.

PubMed

Qiao, Shi; Shi, Xiaowei; Shi, Rui; Liu, Man; Liu, Ting; Zhang, Kerong; Wang, Qiao; Yao, Meicun; Zhang, Lantong

2013-08-01

The detection of drug metabolites, especially for minor metabolites, continues to be a challenge because of the complexity of biological samples. Imperatorin (IMP) is an active natural furocoumarin component originating from many traditional Chinese herbal medicines and is expected to be pursued as a new vasorelaxant agent. In the present study, a generic and efficient approach was developed for the in vivo screening and identification of IMP metabolites using liquid chromatography-Triple TOF mass spectrometry. In this approach, a novel on-line data acquisition method mutiple mass defect filter (MMDF) combined with dynamic background subtraction was developed to trace all probable urinary metabolites of IMP. Comparing with the traditionally intensity-dependent data acquisition method, MMDF method could give the information of low-level metabolites masked by background noise and endogenous components. Thus, the minor metabolites in complex biological matrices could be detected. Then, the sensitive and specific multiple data-mining techniques extracted ion chromatography, mass defect filter, product ion filter, and neutral loss filter were used for the discovery of IMP metabolites. Based on the proposed strategy, 44 phase I and 7 phase II metabolites were identified in rat urine after oral administration of IMP. The results indicated that oxidization was the main metabolic pathway and that different oxidized substituent positions had a significant influence on the fragmentation of the metabolites. Two types of characteristic ions at m/z 203 and 219 can be observed in the MS/MS spectra. This is the first study of IMP metabolism in vivo. The interpretation of the MS/MS spectra of these metabolites and the proposed metabolite pathway provide essential data for further pharmacological studies of other linear-type furocoumarins.

Defect modes in a stacked structure of chiral photonic crystals.

PubMed

Chen, Jiun-Yeu; Chen, Lien-Wen

2005-06-01

An optical propagation simulation is carried out for the study of photonic defect modes in a stacked structure of cholesteric liquid crystal films with spatially varying pitch. The defects are introduced by a pitch jump and a phase jump in the cholesteric helix. The effect of a finite sample thickness on transmission of the defect mode and on the required polarization of incident light to create the defect mode is discussed. For normal and near-normal incidence of circularly polarized light with the same handedness as structure, the defect caused by a pitch jump results in discrete peaks within a forbidden band in the transmission. The particular spectrum is similar to the feature of a Fabry-Pérot interferometer. By introducing an additional phase jump, linear blueshifts of the defect modes in transmission spectra are correlated with an increase in the twist angle.

Noise and degradation of amorphous silicon devices

NASA Astrophysics Data System (ADS)

Bakker, J. P. R.

2003-10-01

Electrical noise measurements are reported on two devices of the disordered semiconductor hydrogenated amorphous silicon (a-Si:H). The material is applied in sandwich structures and in thin-film transistors (TFTs). In a sandwich configuration of an intrinsic layer and two thin doped layers, the observed 1/f resistance noise can be attributed to a distribution of energy levels in the system. Two candidates which eventually could explain the origin of the energy distribution are investigated: generation-recombination noise and long-range potential fluctuations. A simulation program was applied to fit the current-voltage characteristics and resolves the defect density, the energy position and width of the Gaussian distributions of deep defects. Generation-recombination (g-r) is calculated for a one-dimensional semiconductor device with traps, taking the transport of local fluctuations into account. Although the times characterizing capture and emission for deep defects are in the right (ms) range, the calculated noise intensity is five to six orders of magnitude below the measured noise level. Another noise source must cause the 1/f noise in a-Si:H. The alternative is provided by the theory of long-range potential fluctuations. The timescale of the fluctuations is again the capture or emission time for deep defects. When an electron is emitted or captured, the charge state of a deep defect fluctuates. As a result, the potential around that defect will fluctuate, being screened by the surrounding defects. Free electrons will instantaneously adjust to the local potential. The adjustment causes a resistance fluctuation, which is measured as a voltage fluctuation in presence of a constant current. The theory predicts the noise intensity accurately, without any adjustable parameters. Unlike the intensity, the spectral shape is fitted by adjustment of two parameters of the potential landscape. The complete temperature dependence of the noise spectra is consistently described by a Gaussian distribution of potential barriers, located 0.27 eV above the conduction band edge, with a halfwidth of 0.09 eV. A large number of experiments is explained by the theory of long-range potential fluctuations: the thickness dependence, the absence of an isotope effect and the analogous results for oppositely doped devices. From these experiments, it is concluded that a universal potential landscape exists in undoped a-Si:H. Further, the relation between degradation upon prolonged light-soaking and noise is studied. After degradation, the curvature of noise spectra is unaffected, while the intensity increases slightly. These observations are consistent with the theoretical predictions using the observed increase of the defect density. It seems that the potential landscape does not change significantly upon degradation. Noise measurements in the sub-threshold regime of a-Si:H TFTs turn out to yield diffusion noise. Diffusion of electrons through the one-dimensional channel is identified as the source of the noise. The drift mobility extracted from the combined noise and conduction data is below the value that characterizes the on-state. The number of free electrons as determined from combined noise and conduction measurements are in quantitative agreement with an alternative determination from conduction measurements only.

Biochemical changes on the repair of surgical bone defects grafted with biphasic synthetic micro-granular HA + β-tricalcium phosphate induced by laser and LED phototherapies and assessed by Raman spectroscopy.

PubMed

Pinheiro, Antônio Luiz Barbosa; Soares, Luiz Guilherme Pinheiro; Marques, Aparecida Maria Cordeiro; Cangussú, Maria Cristina Teixeira; Pacheco, Marcos Tadeu Tavares; Silveira, Landulfo

2017-04-01

This work aimed the assessment of biochemical changes induced by laser or LED irradiation during mineralization of a bone defect in an animal model using a spectral model based on Raman spectroscopy. Six groups were studied: clot, laser (λ = 780 nm; 70 mW), LED (λ = 850 ± 10 nm; 150 mW), biomaterial (biphasic synthetic micro-granular hydroxyapatite (HA) + β-tricalcium phosphate), biomaterial + laser, and biomaterial + LED. When indicated, defects were further irradiated at a 48-h interval during 2 weeks (20 J/cm 2 per session). At the 15th and 30th days, femurs were dissected and spectra of the defects were collected. Raman spectra were submitted to a model to estimate the relative amount of collagen, phosphate HA, and carbonate HA by using the spectra of pure collagen and biomaterials composed of phosphate and carbonate HA, respectively. The use of the biomaterial associated to phototherapy did not change the collagen formation at both 15 and 30 days. The amount of carbonate HA was not different in all groups at the 15th day. However, at the 30th day, there was a significant difference (ANOVA, p = 0.01), with lower carbonate HA for the group biomaterial + LED compared to biomaterial (p < 0.05). The phosphate HA was higher in the groups that received biomaterial grafts at the 15th day compared to clot (significant for the biomaterial; p < 0.01). At the 30th day, the phosphate HA was higher for the group biomaterial + laser, while this was lower for all the other groups. These results indicated that the use of laser phototherapy improved the repair of bone defects grafted with the biomaterial by increasing the deposition of phosphate HA.

Different valence Sn doping - A simple way to detect oxygen concentration variation of ZnO quantum dots synthesized under ultrasonic irradiation.

PubMed

Yang, Weimin; Zhang, Bing; Zhang, Qitu; Wang, Lixi; Song, Bo; Wu, Fan; Wong, C P

2017-09-01

An ultrasonic method is employed to synthesize the Sn doped Zn 0.95 Sn 0.05 O quantum dots with green light emission. Sn 2+ and Sn 4+ ions are used to create different optical defects inside Zn 0.95 Sn 0.05 O quantum dots and the changing trend of oxygen concentration under different ultrasonic irradiation power are investigated. The photoluminescence spectra are employed to characterize the optical defects of Zn 0.95 Sn 0.05 O quantum dots. The UV-vis spectra are used to study the band gap of Zn 0.95 Sn 0.05 O quantum dots, which is influenced by their sizes. The results indicate that ultrasonic power would influence the size of Zn 0.95 Sn 0.05 O quantum dots as well as the type and quantity of defects in ZnO quantum dots. Changing trends in size of Sn 2+ and Sn 4+ doped Zn 0.95 Sn 0.05 O quantum dots are quite similar with each other, while the changing trends in optical defects types and concentration of Sn 2+ and Sn 4+ doped Zn 0.95 Sn 0.05 O quantum dots are different. The difference of the optical defects concentration changing between Sn 2+ doped Zn 0.95 Sn 0.05 O quantum dots (V O defects) and Sn 4+ doped Zn 0.95 Sn 0.05 O quantum dots (O Zn and O i defects) shows that the formation process of ZnO under ultrasonic irradiation wiped oxygen out. Copyright © 2017 Elsevier B.V. All rights reserved.

Charge transfer and surface defect healing within ZnO nanoparticle decorated graphene hybrid materials

NASA Astrophysics Data System (ADS)

Pham, Chuyen V.; Repp, Sergej; Thomann, Ralf; Krueger, Michael; Weber, Stefan; Erdem, Emre

2016-05-01

To harness the unique properties of graphene and ZnO nanoparticles (NPs) for novel applications, the development of graphene-ZnO nanoparticle hybrid materials has attracted great attention and is the subject of ongoing research. For this contribution, graphene-oxide-ZnO (GO-ZnO) and thiol-functionalized reduced graphene oxide-ZnO (TrGO-ZnO) nanohybrid materials were prepared by novel self-assembly processes. Based on electron paramagnetic resonance (EPR) and photoluminescence (PL) investigations on bare ZnO NPs, GO-ZnO and TrGO-ZnO hybrid materials, we found that several physical phenomena were occurring when ZnO NPs were hybridized with GO and TrGO. The electrons trapped in Zn vacancy defects (VZn-) within the core of ZnO NPs vanished by transfer to GO and TrGO in the hybrid materials, thus leading to the disappearance of the core signals in the EPR spectra of ZnO NPs. The thiol groups of TrGO and sulfur can effectively ``heal'' the oxygen vacancy (VO+) related surface defects of ZnO NPs while oxygen-containing functionalities have low healing ability at a synthesis temperature of 100 °C. Photoexcited electron transfer from the conduction band of ZnO NPs to graphene leads to photoluminescence (PL) quenching of near band gap emission (NBE) of both GO-ZnO and TrGO-ZnO. Simultaneously, electron transfer from graphene to defect states of ZnO NPs is the origin of enhanced green defect emission from GO-ZnO. This observation is consistent with the energy level diagram model of hybrid materials.To harness the unique properties of graphene and ZnO nanoparticles (NPs) for novel applications, the development of graphene-ZnO nanoparticle hybrid materials has attracted great attention and is the subject of ongoing research. For this contribution, graphene-oxide-ZnO (GO-ZnO) and thiol-functionalized reduced graphene oxide-ZnO (TrGO-ZnO) nanohybrid materials were prepared by novel self-assembly processes. Based on electron paramagnetic resonance (EPR) and photoluminescence (PL) investigations on bare ZnO NPs, GO-ZnO and TrGO-ZnO hybrid materials, we found that several physical phenomena were occurring when ZnO NPs were hybridized with GO and TrGO. The electrons trapped in Zn vacancy defects (VZn-) within the core of ZnO NPs vanished by transfer to GO and TrGO in the hybrid materials, thus leading to the disappearance of the core signals in the EPR spectra of ZnO NPs. The thiol groups of TrGO and sulfur can effectively ``heal'' the oxygen vacancy (VO+) related surface defects of ZnO NPs while oxygen-containing functionalities have low healing ability at a synthesis temperature of 100 °C. Photoexcited electron transfer from the conduction band of ZnO NPs to graphene leads to photoluminescence (PL) quenching of near band gap emission (NBE) of both GO-ZnO and TrGO-ZnO. Simultaneously, electron transfer from graphene to defect states of ZnO NPs is the origin of enhanced green defect emission from GO-ZnO. This observation is consistent with the energy level diagram model of hybrid materials. Electronic supplementary information (ESI) available. See DOI: 10.1039/c6nr00393a

Magnetic resonance tracking of fluorescent nanodiamond fabrication

NASA Astrophysics Data System (ADS)

Shames, A. I.; Osipov, V. Yu; Boudou, J. P.; Panich, A. M.; von Bardeleben, H. J.; Treussart, F.; Vul', A. Ya

2015-04-01

Magnetic resonance techniques (electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR)) are used for tracking the multi-stage process of the fabrication of fluorescent nanodiamonds (NDs) produced by high-energy electron irradiation, annealing, and subsequent nano-milling. Pristine commercial high pressure and high temperature microdiamonds (MDs) with mean size 150 μm contain ~5  ×  1018 spins/g of singlet (S = 1/2) substitutional nitrogen defects P1, as well as sp3 C-C dangling bonds in the crystalline lattice. The half-field X-band EPR clearly shows (by the appearance of the intense ‘forbidden’ g = 4.26 line) that high-energy electron irradiation and annealing of MDs induce a large amount (~5  ×  1017 spins/g) of triplet (S = 1) magnetic centers, which are identified as negatively charged nitrogen vacancy defects (NV-). This is supported by EPR observations of the ‘allowed’ transitions between Zeeman sublevels of the triplet state. After progressive milling of the fluorescent MDs down to an ultrasubmicron scale (≤100 nm), the relative abundance of EPR active NV- defects in the resulting fluorescent NDs (FND) substantially decreases and, vice versa, the content of C-inherited singlet defects correlatively increases. In the fraction of the finest FNDs (mean particle size <20 nm), which are contained in the dried supernatant of ultracentrifuged aqueous dispersion of FNDs, the NV- content is found to be reduced by one order of magnitude whereas the singlet defects content increases up to ~2  ×  1019 spins/g. In addition, another triplet-type defect, which is characterized by the g = 4.00 ‘forbidden’ line, appears. On reduction of the particle size below the 20 nm limit, the ‘allowed’ EPR lines become practically unobservable, whereas the ‘forbidden’ lines remain as a reliable fingerprint of the presence of NV- centers in small ND systems. The same size reduction causes the disappearance of the characteristic hyperfine satellites in the spectra of the P1 centers. We discuss the mechanisms that cause both the strong reduction of the peak intensity of the ‘allowed’ lines in EPR spectra of triplet defects and the transformation of the P1 spectra.

The preparation and characterization of La doped TiO 2 nanoparticles and their photocatalytic activity

NASA Astrophysics Data System (ADS)

Liqiang, Jing; Xiaojun, Sun; Baifu, Xin; Baiqi, Wang; Weimin, Cai; Honggang, Fu

2004-10-01

In this paper, pure and La doped TiO2 nanoparticles with different La content were prepared by a sol-gel process using Ti (OC4H9)4 as raw material, and also were characterized by XRD, TG-DTA, TEM, XPS, DRS and Photoluminescence (PL) spectra. We mainly investigated the effects of calcining temperature and La content on the properties and the photocatalytic activity for degrading phenol of as-prepared TiO2 samples, and also discussed the relationships between PL spectra and photocatalytic activity as well as the mechanisms of La doping on TiO2 phase transformation. The results showed that La3+ did not enter into the crystal lattices of TiO2 and was uniformly dispersed onto TiO2 as the form of La2O3 particles with small size, which possibly made La dopant have a great inhibition on TiO2 phase transformation; La dopant did not give rise to a new PL signal, but it could improve the intensity of PL spectra with a appropriate La content, which was possibly attributed to the increase in the content of surface oxygen vacancies and defects after doping La; La doped TiO2 nanoparticles calcined at 600°C exhibited higher photocatalytic activity, indicating that 600°C was an appropriate calcination temperature. The order of photocatalytic activity of La doped TiO2 samples with different La content was as following: 1>1.5>3>0.5>5>0 mol%, which was the same as the order of their PL intensity, namely, the stronger the PL intensity, the higher the photocatalytic activity, demonstrating that there were certain relationships between PL spectra and photocatalytic activity. This could be explained by the points that PL spectra mainly resulted from surface oxygen vacancies and defects during the process of PL, while surface oxygen vacancies and defects could be favorable in capturing the photoinduced electrons during the process of photocatalytic reactions.

Studies of surface states in zinc oxide nanopowders

NASA Astrophysics Data System (ADS)

Peters, Raul Mugabe

The surface of ZnO semiconductor nanosystems is a key performance-defining factor in numerous applications. In this work we present experimental results for the surface defect-related properties of ZnO nanoscale systems. Surface photovoltage spectroscopy was used to determine the defect level energies within the band gap, the conduction vs. valence band nature of the defect-related transitions, and to probe key dynamic parameters of the surface on a number of commercially available ZnO nanopowders. In our experimental setup, surface photovoltage characterization is conducted in high vacuum in tandem with in situ oxygen remote plasma treatments. Surface photovoltage investigations of the as-received and plasma-processed samples revealed a number of common spectral features related to surface states. Furthermore, we observed significant plasma-induced changes in the surface defect properties. Ex situ positron annihilation and photoluminescence measurements were performed on the studied samples and correlated with surface photovoltage results. The average positron lifetimes were found to be substantially longer than in a bulk single crystalline sample, which is consistent with the model of grains with defect-rich surface and subsurface layers. Compression of the powders into pellets yielded reduction of the average positron lifetimes. Surface photovoltage, positron annihilation, and photoluminescence spectra consistently showed sample-to-sample differences due to the variation in the overall quality of the nanopowders, which partially obscures observation of the scaling effects. However, the results demonstrated that our approach is efficient in detecting specific surface states in nanoscale ZnO specimens and in elucidating their nature.

Elastic dependence of defect modes in one-dimensional photonic crystals with a cholesteric elastomer slab

NASA Astrophysics Data System (ADS)

Avendanño, Carlos G.; Martínez, Daniel

2018-07-01

We studied the transmission spectra in a one-dimensional dielectric multilayer photonic structure containing a cholesteric liquid crystal elastomer layer as a defect. For circularly polarized incident electromagnetic waves, we analyzed the optical defect modes induced in the band gap spectrum as a function of the incident angle and the axial strain applied along the same axis as the periodic medium. The physical parameters of the structure were chosen in such a way the photonic band gap of the cholesteric elastomer lies inside that of the multilayer. We found that, in addition to the defect modes associated with the thickness of the defect layer and the anisotropy of the elastic polymer, two new defect modes appear at both band edges of the cholesteric structure, whose amplitudes and spectral positions can be elastically tuned. Particularly, we showed that, at normal incidence, the defect modes shift toward the long-wavelength region with the strain; whereas, for constant elongation, such defects move toward larger frequencies with the incidence angle.

Defect identification for the As Ga family

NASA Astrophysics Data System (ADS)

Overhof, H.; Spaeth, J.-M.

2003-12-01

The AsGa family consists of at least four distinctly different point defects including the technologically important EL2 defect. While the different members are easily distinguished from their MCDA spectra, the differences of the hf and shf interactions as derived from ODEPR and ODENDOR are rather small. We present ab initio calculations using the LMTO-ASA Green's function method for a variety of defect models that might be relevant for the identification of AsGa-related defects. We confirm the identification of the isolated AsGa and show that the {AsGa-X2} defect must be identified with the nearest-neighbor antistructure pair rather than with the {AsGa-VAs} pair. For the {AsGa-X1} defect a distant antistructure pair is a likely candidate. For the EL2, the most important member of the AsGa family, we have not found a conclusive defect model. The recent ODENDOR data are similar to those of the distant orthorhombic {AsGa-VGa} pair, which, however is a triple acceptor and not a donor.

Fast synthesize ZnO quantum dots via ultrasonic method.

PubMed

Yang, Weimin; Zhang, Bing; Ding, Nan; Ding, Wenhao; Wang, Lixi; Yu, Mingxun; Zhang, Qitu

2016-05-01

Green emission ZnO quantum dots were synthesized by an ultrasonic sol-gel method. The ZnO quantum dots were synthesized in various ultrasonic temperature and time. Photoluminescence properties of these ZnO quantum dots were measured. Time-resolved photoluminescence decay spectra were also taken to discover the change of defects amount during the reaction. Both ultrasonic temperature and time could affect the type and amount of defects in ZnO quantum dots. Total defects of ZnO quantum dots decreased with the increasing of ultrasonic temperature and time. The dangling bonds defects disappeared faster than the optical defects. Types of optical defects first changed from oxygen interstitial defects to oxygen vacancy and zinc interstitial defects. Then transformed back to oxygen interstitial defects again. The sizes of ZnO quantum dots would be controlled by both ultrasonic temperature and time as well. That is, with the increasing of ultrasonic temperature and time, the sizes of ZnO quantum dots first decreased then increased. Moreover, concentrated raw materials solution brought larger sizes and more optical defects of ZnO quantum dots. Copyright © 2015 Elsevier B.V. All rights reserved.

Structural, electronic and photocatalytic properties of atomic defective BiI3 monolayers

NASA Astrophysics Data System (ADS)

Yan, Huang; Ziyu, Hu; Xu, Gong; Xiaohong, Shao

2018-01-01

The structural, electronic and photocatalytic properties of five vacancy-containing 2D BiI3 monolayers are investigated by the first-principle calculations. The electronic structures show that the five structures are stable and have comparable binding energies to that of the pristine BiI3 monolayer, and the defects can tune the band gaps. Optical spectra indicate that the five structures retain high absorption capacity for visible light. The spin-orbit coupling (SOC) effect is found to play an important role in the band edge of defective structures, and the VBi and VBi-I3 defective BiI3 monolayers can make absolute band edges straddle water redox potentials more easily.

Photoluminescent properties of electrochemically synthetized ZnO nanotubes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gracia Jiménez, J.M.

ZnO nanotubes were prepared by a sequential combination of electrochemical deposition, chemical attack and regeneration. ZnO nanocolumns were initially electrodeposited on conductive substrates and then converted into nanotubes by a process involving chemical etching and subsequent regrowth. The morphology of these ZnO nanocolumns and derived nanotubes was monitored by Scanning Electron Microscopy and their optical properties was studied by photoluminescence spectroscopy. Photoluminescence were measured as a function of temperature, from 6 to 300 K, for both nanocolumns and nanotubes. In order to study the behaviour of induced intrinsic defect all ZnO films were annealed in air at 400 °C andmore » their photoluminescent properties were also registered before and after annealing. The behaviour of photoluminescence is explained taking into account the contribution of different point defects. A band energy diagram related to intrinsic defects was proposed to describe the behaviour of photoluminescence spectra. - Highlights: •ZnO nanotubes were obtained after etching and regrowth of electrodeposited ZnO films. •Photoluminescence spectra contain two parts involving excitonic and defects transitions. •Annealing produces a blue shift in the PL peaks in both ZnO nanocolumns and nanotubes. •Etching causes a blue shift in PL peaks due to confinement effect in nanotubes walls.« less

Defect-induced ferromagnetism in ZnO nanoparticles prepared by mechanical milling

NASA Astrophysics Data System (ADS)

Phan, The-Long; Zhang, Y. D.; Yang, D. S.; Nghia, N. X.; Thanh, T. D.; Yu, S. C.

2013-02-01

Though ZnO is known as a diamagnetic material, recent studies have revealed that its nanostructures can be ferromagnetic (FM). The FM origin has been ascribed to intrinsic defects. This work shines light on an alternate method based on mechanical milling to induce defect-related ferromagnetism in ZnO nanoparticles (NPs) from initial diamagnetic ZnO powders. Our idea is motivated by the fact that mechanical milling introduces more defects to a ground material. We point out that the FM order increases with increasing the density of defects in ZnO NPs. The experimental results obtained from analyzing X-ray absorption, electron spin resonance, and Raman scattering spectra demonstrate that the ferromagnetism in ZnO NPs is due to intrinsic defects mainly related to oxygen and zinc vacancies. Among these, zinc vacancies play a decisive role in introducing a high FM order in ZnO NPs.

Anomalous satellite inductive peaks in alternating current response of defective carbon nanotubes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hirai, Daisuke; Watanabe, Satoshi; Yamamoto, Takahiro

2014-05-07

AC response of defective metallic carbon nanotubes is investigated from first principles. We found that capacitive peaks appear at electron scattering states. Moreover, we show that satellite inductive peaks are seen adjacent to a main capacitive peak, which is in contrast to the conductance spectra having no satellite features. The appearance of satellite inductive peaks seems to depend on the scattering states. Our analysis with a simple resonant scattering model reveals that the origin of the satellite inductive peaks can be understood by just one parameter, i.e., the lifetime of electrons at a defect state.

Evaluation of induced color changes in chicken breast meat during simulation of pink color defect.

PubMed

Holownia, K; Chinnan, M S; Reynolds, A E; Koehler, P E

2003-06-01

The objective of the study was to establish a pink threshold and simulate the pink defect in cooked chicken breast meat with treatment combinations that would induce significant changes in the color of raw and cooked meat. The subjective pink threshold used in judging pink discoloration was established at a* = 3.8. Samples of three color groups (normal, lighter than normal, and darker than normal) of boneless, skinless chicken breast muscles were selected based on instrumental color values. The in situ changes were induced using sodium chloride, sodium tripolyphosphate, sodium erythorbate, and sodium nitrite at two levels: present and not present. Fillets in all treatments were subjected to individual injections, followed by tumbling, cooking, and chilling. Samples were analyzed for color [lightness (L*), red/green axis (a*), yellow/blue axis (b*)] and reflectance spectra. Simulation of the pink defect was achieved in eight of the 16 treatment combinations when sodium nitrite was present and in an additional two treatment combinations when it was absent. Pinking in cooked samples was affected (P < 0.05) by L* of raw meat color. Results confirmed that it was possible to simulate the undesired pinking in cooked chicken white meat when in situ conditions were induced by sodium chloride, sodium tripolyphosphate, and sodium nitrite. The continuation of the simulation study can aid in developing alternative processing methods to eliminate potential pink defects.

A new photoluminescence emission peak of ZnO SiO2 nanocomposites and its energy transfer to Eu3+ ions

NASA Astrophysics Data System (ADS)

Hong, Jian-He; Cong, Chang-Jie; Zhang, Zhi-Guo; Zhang, Ke-Li

2007-07-01

This work reports a new photoluminescence (PL) emission peak at about 402 nm from amorphous ZnO nanoparticles in a silica matrix, and the energy transfer from it to Eu3+ ions. The amorphous ZnO SiO2 nanocomposites were prepared by the sol gel method, which is verified by X-ray diffraction (XRD) profiles and FT IR spectra. The luminescence emission spectra are fitted by four Gauss profiles, two of which at longer wavelength are due to the defects of the material and the others to amorphous ZnO nanoparticles and the Zn O Si interface state. With the reduction of Zn/Si ratio and diethanolamine, the relative intensities of visible emission decrease. The weak visible emission is due to the reduction of defects after calcined at high temperature. The new energy state at the Zn O Si interface results in strong emission at about 402 nm. When Eu3+ ions are co-doped, weak energy transfer from ZnO SiO2 nanocomposites to Eu3+ emission are observed in the excitation spectra.

Theoretical surface core-level shifts for Be(0001)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Feibelman, P.J.

1994-05-15

Core-ionization potentials (CIP's) are computed for Be(0001). Three core features are observed in corresponding photoelectron spectra, with CIP's shifted relative to the bulk core level by [minus]0.825, [minus]0.570, and [minus]0.265 eV. The computed CIP shifts for the outer and subsurface layers, [minus]0.60 and [minus]0.29 eV, respectively, agree with the latter two of these. It is surmised that the [minus]0.825-eV shift is associated with a surface defect. The negative signs of the Be(0001) surface core-level shifts do not fit into the thermochemical picture widely used to explain CIP shifts. The reason is that a core-ionized Be atom is too small tomore » bond effectively to the remainder of the unrelaxed Be lattice.« less

Ligand field splittings in core level transitions for transition metal (TM) oxides: Tanabe-Sugano diagrams and (TM) dangling bonds in vacated O-atom defects

NASA Astrophysics Data System (ADS)

Lucovsky, Gerry; Wu, Kun; Pappas, Brian; Whitten, Jerry

2013-04-01

Defect states in the forbidden band-gap below the conduction band edge are active as electron traps in nano-grain high-) transition metal (TM) oxides with thickness >0.3 nm, e.g., ZrO2 and HfO2. These oxides have received considerable attention as gate-dielectrics in complementary metal oxide semiconductor (CMOS) devices, and more recently are emerging as candidates for charge storage and memory devices. To provide a theoretical basis for device functionality, ab-initio many-electron theory is combined with X-ray absorption spectroscopy (XAS) to study O K edge and TM core level transitions. These studies identify ligand field splittings (ΔLF) for defect state features,. When compared with those obtained from O-atom and TM-atom core spectroscopic transitions, this provides direct information about defect state sun-nm bonding arrangements. comparisons are made for (i) elemental TiO2 and Ti2O3 with different formal ionic charges, Ti4+ and Ti3+ and for (ii) Magneli Phase alloys, TinO2n-1, n is an integer 9>=n>3, and (TiO2)x(HfO2)1-x alloys. The alloys display multi-valent behavior from (i) different ionic-charge states, (ii} local bond-strain, and (iii) metallic hopping transport. The intrinsic bonding defects in TM oxides are identified as pairs of singly occupied dangling bonds. For 6-fold coordinated Ti-oxides defect excited states in 2nd derivative O K pre-edge spectra are essentially the same as single Ti-atom d2 transitions in Tanabe-Sugano (T-S) diagrams. O-vacated site defects in 8-fold coordinated ZrO2 and HfO2 are described by d8 T-S diagrams. T-S defect state ordering and splittings are functions of the coordination and symmetry of vacated site bordering TM atoms. ΔLF values from the analysis of T-S diagrams indicate medium range order (MRO) extending to 3rd and 4th nearest-neighbor (NN) TM-atoms. Values are different for 6-fold Ti, and 8-fold ZrO2 and HfO2, and scale inversely with differences in respective formal ionic radii. O-vacated site bonding defects in TM nano-grain oxides are qualitatively similar to vacant-site defects in non-crystalline SiO2 and GeO2 for ulta-thin films, < 0.2 nm thick, and yield similar performance in MOSCAPs on Ge substrates heralding applications in aggressively-scale CMOS devices.

Microscopic origin of lattice contraction and expansion in undoped rutile TiO2 nanostructures

NASA Astrophysics Data System (ADS)

Santara, Batakrushna; Giri, P. K.; Imakita, Kenji; Fujii, Minoru

2014-05-01

We have investigated the microscopic origin of lattice expansion and contraction in undoped rutile TiO2 nanostructures by employing several structural and optical spectroscopic tools. Rutile TiO2 nanostructures with morphologies such as nanorods, nanopillars and nanoflowers, depending upon the growth conditions, are synthesized by an acid-hydrothermal process. Depending on the growth conditions and post-growth annealing, lattice contraction and expansion are observed in the nanostructures and it is found to correlate with the nature and density of intrinsic defects in rutile TiO2. The change in lattice volume correlates well with the optical bandgap energy. Irrespective of growth conditions, theTiO2 nanostructures exhibit strong near infrared (NIR) photoluminescence (PL) at 1.43 eV and a weak visible PL, which are attributed to the Ti interstitials and O vacancies, respectively, in rutile TiO2 nanostructures. Further, ESR study reveals the presence of singly ionized oxygen vacancy defects. It is observed that lattice distortion depends systematically on the relative concentration and type of defects such as oxygen vacancies and Ti interstitials. XPS analyses revealed a downshift in energy for both Ti 2p and O 1s core level spectra for various growth conditions, which is believed to arise from the lattice distortions. It is proposed that the Ti4+ interstitial and F+ oxygen vacancy defects are primarily responsible for lattice expansion, whereas the electrostatic attraction between Ti4+ interstitial and O2- interstitial defects causes the lattice contraction in the undoped TiO2 nanostructures. The control of lattice parameters through the intrinsic defects may provide new routes to achieving novel functionalities in advanced materials that can be tailored for future technological applications.

AVIRIS spectra of California wetlands

NASA Technical Reports Server (NTRS)

Gross, Michael F.; Ustin, Susan L.; Klemas, Vytautas

1988-01-01

Spectral data gathered by the AVIRIS from wetlands in the Suisun Bay area of California on 13 October 1987 were analyzed. Spectra representing stands of numerous vegetation types (including Sesuvium verrucosum, Scirpus acutus and Scirpus californicus, Xanthium strumarium, Cynadon dactylon, and Distichlis spicata) and soil were isolated. Despite some defects in the data, it was possible to detect vegetation features such as differences in the location of the chlorophyll red absorption maximum. Also, differences in cover type spectra were evident in other spectral regions. It was not possible to determine if the observed features represent noise, variability in canopy architecture, or chemical constituents of leaves.

Influence of the morphology of ZnO nanostructures on luminescent and photovoltaic properties

NASA Astrophysics Data System (ADS)

Ibrayev, N. Kh.; Ilyassov, B. R.; Afanasyev, D. A.

2017-03-01

Arrays of ZnO nanorods and nanoplates are synthesized by the hydrothermal and electrochemical methods, respectively. The photoluminescence spectra indicate that the nanoplates have a more defective structure than the nanorods. The obtained ZnO nanostructures are used as the basis to construct dye-sensitized solar cells. The influence of morphology and defectiveness of ZnO nanostructures on the luminescent and photovoltaic properties of the cells is studied.

Corroded surface roughness of copper analyzed by Fourier transform infrared mapping microscopy and optical profilometric study.

PubMed

Kasperek, J; Lefez, B; Beucher, E

2004-02-01

This study shows the effects of roughness on infrared spectra shapes of thin corrosion products on metallic substrates. The calculated spectra show that the baseline is mainly affected by increasing roughness and that such effects do not shift the position of the absorption bands. The model obtained has been used to extract data of artificial patina on a copper surface. Surface defects of copper substrates can be distinguished on the whole surface, from the morphological and chemical points of view, using optical profilometry and infrared microspectroscopy. An homogeneous layer of cuprite covers the surface except in the linear defects. Fourier transform infrared (FT-IR) analysis indicates that a mixture of atacamite and clinoatacamite is mainly located in these scratches. The width of these particular areas is in good agreement with profilometric observations.

Extended x-ray absorption fine structure spectroscopy and x-ray absorption near edge spectroscopy study of aliovalent doped ceria to correlate local structural changes with oxygen vacancies clustering

DOE Office of Scientific and Technical Information (OSTI.GOV)

Shirbhate, S. C.; Acharya, S. A., E-mail: saha275@yahoo.com; Yadav, A. K.

2016-04-04

This study provides atomic scale insight to understand the role of aliovalent dopants on oxygen vacancies clustering and dissociation mechanism in ceria system in order to enhance the performance of oxy-ion conductor. Dopants induced microscale changes in ceria are probed by extended X-ray absorption fine structure spectroscopy, X-ray absorption near edge spectra, and Raman spectroscopy. The results are explored to establish a correlation between atomic level structural changes (coordination number, interatomic spacing) → formation of dimer and trimer type cation-oxygen vacancies defect complex (intrinsic and extrinsic) → dissociation of oxygen vacancies from defect cluster → ionic conductivity temperature. It ismore » a strategic approach to understand key physics of ionic conductivity mechanism in order to reduce operating temperature of electrolytes for intermediate temperature (300–450 °C) electrochemical devices for the first time.« less

Spatial luminescence imaging of dopant incorporation in CdTe Films

DOE PAGES

Guthrey, Harvey; Moseley, John; Colegrove, Eric; ...

2017-01-25

State-of-the-art cathodoluminescence (CL) spectrum imaging with spectrum-per-pixel CL emission mapping is applied to spatially profile how dopant elements are incorporated into Cadmium telluride (CdTe). Emission spectra and intensity monitor the spatial distribution of additional charge carriers through characteristic variations in the CL emission based on computational modeling. Our results show that grain boundaries play a role in incorporating dopants in CdTe exposed to copper, phosphorus, and intrinsic point defects in CdTe. Furthermore, the image analysis provides critical, unique feedback to understand dopant incorporation and activation in the inhomogeneous CdTe material, which has struggled to reach high levels of hole density.

Fabrication and characterization of n-ZnO nanonails array/p(+)-GaN heterojunction diode.

PubMed

Zhu, G Y; Chen, G F; Li, J T; Shi, Z L; Lin, Y; Ding, T; Xu, X Y; Dai, J; Xu, C X

2012-10-01

A novel heterojunctional structure of n-ZnO nanonails array/p(+)-GaN light-emitting diode was fabricated by Chemical Vapor Deposition method. A broad electroluminescence spectrum shows two peaks centered at 435 nm and 478 nm at room temperature, respectively. By comparing the photoluminescence and electroluminescence spectra, together with analyzing the energy band structure of heterojunction light emitting diode, it suggested that the electroluminescence peak located at 435 nm originates from Mg acceptor level of p(+)-GaN layer, whereas the electroluminescence peak located at 478 nm originates from the defects of n-ZnO nanonails array.

Charge transfer and surface defect healing within ZnO nanoparticle decorated graphene hybrid materials.

PubMed

Pham, Chuyen V; Repp, Sergej; Thomann, Ralf; Krueger, Michael; Weber, Stefan; Erdem, Emre

2016-05-05

To harness the unique properties of graphene and ZnO nanoparticles (NPs) for novel applications, the development of graphene-ZnO nanoparticle hybrid materials has attracted great attention and is the subject of ongoing research. For this contribution, graphene-oxide-ZnO (GO-ZnO) and thiol-functionalized reduced graphene oxide-ZnO (TrGO-ZnO) nanohybrid materials were prepared by novel self-assembly processes. Based on electron paramagnetic resonance (EPR) and photoluminescence (PL) investigations on bare ZnO NPs, GO-ZnO and TrGO-ZnO hybrid materials, we found that several physical phenomena were occurring when ZnO NPs were hybridized with GO and TrGO. The electrons trapped in Zn vacancy defects (VZn(-)) within the core of ZnO NPs vanished by transfer to GO and TrGO in the hybrid materials, thus leading to the disappearance of the core signals in the EPR spectra of ZnO NPs. The thiol groups of TrGO and sulfur can effectively "heal" the oxygen vacancy (VO(+)) related surface defects of ZnO NPs while oxygen-containing functionalities have low healing ability at a synthesis temperature of 100 °C. Photoexcited electron transfer from the conduction band of ZnO NPs to graphene leads to photoluminescence (PL) quenching of near band gap emission (NBE) of both GO-ZnO and TrGO-ZnO. Simultaneously, electron transfer from graphene to defect states of ZnO NPs is the origin of enhanced green defect emission from GO-ZnO. This observation is consistent with the energy level diagram model of hybrid materials.

Vacancy-Rich Monolayer BiO 2-x as a Highly Efficient UV, Visible, and Near-Infrared Responsive Photocatalyst

DOE PAGES

Li, Jun; Wu, Xiaoyong; Pan, Wenfeng; ...

2017-09-08

Here in this paper, a full-spectrum responsive vacancy-rich monolayer BiO 2-x has been synthesized. The increased density of states at the conduction band (CB) minimum in the monolayer BiO 2-x is responsible for the enhanced photon response and photo-absorption, which were confirmed by UV/Vis-NIR diffuse reflectance spectra (DRS) and photocurrent measurements. Compared to bulk BiO 2-x, monolayer BiO 2-x has exhibited enhanced photocatalytic performance for rhodamine B and phenol removal under UV, visible, and near-infrared light (NIR) irradiation, which can be attributed to the vacancy VBi-O"' as confirmed by the positron annihilation spectra. The presence of V Bi-O"' defects inmore » monolayer BiO 2-x promoted the separation of electrons and holes. This finding provides an atomic level understanding for developing highly efficient UV, visible, and NIR light responsive photocatalysts.« less

Vacancy-Rich Monolayer BiO 2-x as a Highly Efficient UV, Visible, and Near-Infrared Responsive Photocatalyst

DOE Office of Scientific and Technical Information (OSTI.GOV)

Li, Jun; Wu, Xiaoyong; Pan, Wenfeng

Here in this paper, a full-spectrum responsive vacancy-rich monolayer BiO 2-x has been synthesized. The increased density of states at the conduction band (CB) minimum in the monolayer BiO 2-x is responsible for the enhanced photon response and photo-absorption, which were confirmed by UV/Vis-NIR diffuse reflectance spectra (DRS) and photocurrent measurements. Compared to bulk BiO 2-x, monolayer BiO 2-x has exhibited enhanced photocatalytic performance for rhodamine B and phenol removal under UV, visible, and near-infrared light (NIR) irradiation, which can be attributed to the vacancy VBi-O"' as confirmed by the positron annihilation spectra. The presence of V Bi-O"' defects inmore » monolayer BiO 2-x promoted the separation of electrons and holes. This finding provides an atomic level understanding for developing highly efficient UV, visible, and NIR light responsive photocatalysts.« less

Fluorescence spectral properties of stomach tissues with pathology

NASA Astrophysics Data System (ADS)

Giraev, K. M.; Ashurbekov, N. A.; Lahina, M. A.

2012-05-01

Steady-state fluorescence and diffuse reflection spectra are measured for in vivo normal and pathological (chronic atrophic and ulcerating defects, malignant neoplasms) stomach mucous lining tissues. The degree of distortion of the fluorescence spectra is estimated taking light scattering and absorption into account. A combination of Gauss and Lorentz functions is used to decompose the fluorescence spectra. Potential groups of fluorophores are determined and indices are introduced to characterize the dynamics of their contributions to the resultant spectra as pathologies develop. Reabsorption is found to quench the fluorescence of structural proteins by as much as a factor of 3, while scattering of the light can increase the fluorescence intensity of flavin and prophyrin groups by as much as a factor of 2.

Investigation of Structural Re-ordering of Hydrogen Bonds in LiNbO3:Mg Crystals Around the Threshold Concentration of Magnesium

NASA Astrophysics Data System (ADS)

Sidorov, N. V.; Teplyakova, N. A.; Palatnikov, M. N.; Bobreva, L. A.

2017-09-01

Crystals of LiNbO3congr and LiNbO3:Mg (0.19-5.91 mole %) were studied by IR and Raman spectroscopy. It was found that the intensities of the bands corresponding to the stretching vibrations of the OH groups in the IR spectra of LiNbO3:Mg crystals change and components of the bands disappear with increase of the Mg content. This was explained by disappearance of the OH groups close to {Nb}_{Li}^{4+}-{V}_{Li}- defects as a result of displacement of NbLi defects by Mg cations. In the Raman spectra of the LiNbO3:Mg (5.1 mole %) compared with the congruent crystal the lines corresponding to the vibrations of oxygen atoms in the oxygen octahedra and the stretching bridge vibrations of the oxygen atoms along the polar axis become broader, and new low-intensity lines that may correspond to pseudoscalar vibrations of A2-type symmetry also appear. The broadening of the lines is due to deformation of the oxygen octahedra caused both by increase of the Mg content in the crystal structure and by change in the localization of the protons. Suppression of the photorefraction effect in the LiNbO3:Mg crystals with Mg contents above the threshold level can be explained by change in the localization of the protons in the structure and by screening of the space charge field.

Quantification of irradiation defects in beta-silicon carbide using Raman spectroscopy

DOE PAGES

Koyanagi, T.; Lance, M. J.; Katoh, Y.

2016-08-11

Raman spectra from polycrystalline beta-silicon carbide (SiC) were collected following neutron irradiation at 380–1180 °C to 0.011–1.87 displacement per atom. The longitudinal optical (LO) peak shifted to a lower frequency and broadened as a result of the irradiation. The changes observed in the LO phonon line shape and position in neutron-irradiated SiC are explained by a combination of changes in the lattice constant and Young's modulus, and the phonon confinement effect. The phonon confinement model reasonably estimates the defect-defect distance in the irradiated SiC, which is consistent with results from previous experimental studies and simulations.

Defect modes in photonic crystal slabs studied using terahertz time-domain spectroscopy.

PubMed

Jian, Zhongping; Pearce, Jeremy; Mittleman, Daniel M

2004-09-01

We describe broadband coherent transmission studies of two-dimensional photonic crystals consisting of a hexagonal array of air holes in a dielectric slab in a planar waveguide. By filling several of the air holes in the photonic crystal slab, we observe the signature of a defect mode within the stop band, in both the amplitude and phase spectra. The experimental results are in reasonable agreement with theoretical calculations using the transfer matrix method.

Detection and quantification of creep strain using process compensated resonance testing (PCRT) sorting modules trained with modeled resonance spectra

NASA Astrophysics Data System (ADS)

Heffernan, Julieanne; Biedermann, Eric; Mayes, Alexander; Livings, Richard; Jauriqui, Leanne; Goodlet, Brent; Aldrin, John C.; Mazdiyasni, Siamack

2018-04-01

Process Compensated Resonant Testing (PCRT) is a full-body nondestructive testing (NDT) method that measures the resonance frequencies of a part and correlates them to the part's material and/or damage state. PCRT testing is used in the automotive, aerospace, and power generation industries via automated PASS/FAIL inspections to distinguish parts with nominal process variation from those with the defect(s) of interest. Traditional PCRT tests are created through the statistical analysis of populations of "good" and "bad" parts. However, gathering a statistically significant number of parts can be costly and time-consuming, and the availability of defective parts may be limited. This work uses virtual databases of good and bad parts to create two targeted PCRT inspections for single crystal (SX) nickel-based superalloy turbine blades. Using finite element (FE) models, populations were modeled to include variations in geometric dimensions, material properties, crystallographic orientation, and creep damage. Model results were verified by comparing the frequency variation in the modeled populations with the measured frequency variations of several physical blade populations. Additionally, creep modeling results were verified through the experimental evaluation of coupon geometries. A virtual database of resonance spectra was created from the model data. The virtual database was used to create PCRT inspections to detect crystallographic defects and creep strain. Quantification of creep strain values using the PCRT inspection results was also demonstrated.

Observation of the origin of d0 magnetism in ZnO nanostructures using X-ray-based microscopic and spectroscopic techniques

NASA Astrophysics Data System (ADS)

Singh, Shashi B.; Wang, Yu-Fu; Shao, Yu-Cheng; Lai, Hsuan-Yu; Hsieh, Shang-Hsien; Limaye, Mukta V.; Chuang, Chen-Hao; Hsueh, Hung-Chung; Wang, Hsaiotsu; Chiou, Jau-Wern; Tsai, Hung-Ming; Pao, Chih-Wen; Chen, Chia-Hao; Lin, Hong-Ji; Lee, Jyh-Fu; Wu, Chun-Te; Wu, Jih-Jen; Pong, Way-Faung; Ohigashi, Takuji; Kosugi, Nobuhiro; Wang, Jian; Zhou, Jigang; Regier, Tom; Sham, Tsun-Kong

2014-07-01

Efforts have been made to elucidate the origin of d0 magnetism in ZnO nanocactuses (NCs) and nanowires (NWs) using X-ray-based microscopic and spectroscopic techniques. The photoluminescence and O K-edge and Zn L3,2-edge X-ray-excited optical luminescence spectra showed that ZnO NCs contain more defects than NWs do and that in ZnO NCs, more defects are present at the O sites than at the Zn sites. Specifically, the results of O K-edge scanning transmission X-ray microscopy (STXM) and the corresponding X-ray-absorption near-edge structure (XANES) spectroscopy demonstrated that the impurity (non-stoichiometric) region in ZnO NCs contains a greater defect population than the thick region. The intensity of O K-edge STXM-XANES in the impurity region is more predominant in ZnO NCs than in NWs. The increase in the unoccupied (occupied) density of states at/above (at/below) the conduction-band minimum (valence-band maximum) or the Fermi level is related to the population of defects at the O sites, as revealed by comparing the ZnO NCs to the NWs. The results of O K-edge and Zn L3,2-edge X-ray magnetic circular dichroism demonstrated that the origin of magnetization is attributable to the O 2p orbitals rather than the Zn d orbitals. Further, the local density approximation (LDA) + U verified that vacancies in the form of dangling or unpaired 2p states (due to Zn vacancies) induced a significant local spin moment in the nearest-neighboring O atoms to the defect center, which was determined from the uneven local spin density by analyzing the partial density of states of O 2p in ZnO.Efforts have been made to elucidate the origin of d0 magnetism in ZnO nanocactuses (NCs) and nanowires (NWs) using X-ray-based microscopic and spectroscopic techniques. The photoluminescence and O K-edge and Zn L3,2-edge X-ray-excited optical luminescence spectra showed that ZnO NCs contain more defects than NWs do and that in ZnO NCs, more defects are present at the O sites than at the Zn sites. Specifically, the results of O K-edge scanning transmission X-ray microscopy (STXM) and the corresponding X-ray-absorption near-edge structure (XANES) spectroscopy demonstrated that the impurity (non-stoichiometric) region in ZnO NCs contains a greater defect population than the thick region. The intensity of O K-edge STXM-XANES in the impurity region is more predominant in ZnO NCs than in NWs. The increase in the unoccupied (occupied) density of states at/above (at/below) the conduction-band minimum (valence-band maximum) or the Fermi level is related to the population of defects at the O sites, as revealed by comparing the ZnO NCs to the NWs. The results of O K-edge and Zn L3,2-edge X-ray magnetic circular dichroism demonstrated that the origin of magnetization is attributable to the O 2p orbitals rather than the Zn d orbitals. Further, the local density approximation (LDA) + U verified that vacancies in the form of dangling or unpaired 2p states (due to Zn vacancies) induced a significant local spin moment in the nearest-neighboring O atoms to the defect center, which was determined from the uneven local spin density by analyzing the partial density of states of O 2p in ZnO. Electronic supplementary information (ESI) available: Scanning photoelectron microscopy (SPEM) results of ZnO NCs and NWs. Computational details and calculated total and partial density of states (PDOS) of bulk wurtzite ZnO with oxygen anion vacancies (VO). See DOI: 10.1039/c4nr01961j

Radiation-acoustic treatment of gallium phosphide light diodes

NASA Astrophysics Data System (ADS)

Tartachnik, Volodimir P.; Gontaruk, Olexsandr M.; Vernydub, Roman M.; Kryvutenko, Anatoly M.; Olikh, Yaroslav M.; Opilat, Vitalij Y.; Petrenko, Igor V.; Pinkovska, Myroslava B.

1999-11-01

The ultrasound influence on the defects of technological and radiation origin of GaP light diodes has been investigated. GaP light diodes were treated by ultrasound wave in different operating modes. Electroluminescence spectra were measured at room and low temperatures, integrated luminosity of devices was checked by solar cell. In order to find out the radiation field influence on non-equilibrium defects of acoustic origin samples were irradiated at room temperature by gamma rays of Co60. It has been discovered that in GaP light diodes treated by ultrasound unstable at room temperature dislocation networks occur at the volume of crystal. Ultrasound dose increase causes the creation of complex defects with high relaxation time and appearing of a part of more mobile defect,s which relax quicker. The nature of effects discovered has been discussed. The method of the emissive capacity restoring of samples degraded after irradiation have been proposed.

First-principles study of the covalently functionalized graphene

NASA Astrophysics Data System (ADS)

Jha, Sanjiv Kumar

Theoretical investigations of nanoscale systems, such as functionalized graphene, present major challenges to conventional computational methods employed in quantum chemistry and solid state physics. The properties of graphene can be affected by chemical functionalization. The surface functionalization of graphene offers a promising way to increase the solubility and reactivity of graphene for use in nanocomposites and chemical sensors. Covalent functionalization is an efficient way to open band-gap in graphene for applications in nanoelectronics. We apply ab initio computational methods based on density functional theory to study the covalent functionalization of graphene with benzyne (C6H4), tetracyanoethylene oxide (TCNEO), and carboxyl (COOH) groups. Our calculations are carried out using the SIESTA and Quantum-ESPRESSO electronic structure codes combined with the generalized gradient (GGA) and local density approximations (LDA) for the exchange correlation functionals and norm-conserving Troullier-Martins pseudopotentials. Calculated binding energies, densities of states (DOS), band structures, and vibrational spectra of functionalized graphene are analyzed in comparison with the available experimental data. Our calculations show that the reactions of [2 + 2] and [2 + 4] cycloaddition of C6H4 to the surface of pristine graphene are exothermic, with binding energies of --0.73 eV and --0.58 eV, respectively. Calculated band structures indicate that the [2 + 2] and [2 + 4] attachments of benzyne results in opening small band gap in graphene. The study of graphene--TCNEO interactions suggests that the reaction of cycloaddition of TCNEO to the surface of pristine graphene is endothermic. On the other hand, the reaction of cycloaddition of TCNEO is found to be exothermic for the edge of an H-terminated graphene sheet. Simulated Raman and infrared spectra of graphene functionalized with TCNEO are consistent with experimental results. The Raman (non-resonant) and infrared (IR) spectra of graphene functionalized with carboxyl (COON) groups are studied in graphene with no surface defects, di-vacancies (DV), and Stone-Wales (SW) defects. Simulated Raman and IR spectra of carboxylated graphene are consistent with available experimental results. Computed vibrational spectra of carboxylated graphene show that the presence of point defects near the functionalization site affect the Raman and IR spectroscopic signatures of the functionalized graphene.

Qualitative properties of roasting defect beans and development of its classification methods by hyperspectral imaging technology.

PubMed

Cho, Jeong-Seok; Bae, Hyung-Jin; Cho, Byoung-Kwan; Moon, Kwang-Deog

2017-04-01

Qualitative properties of roasting defect coffee beans and their classification methods were studied using hyperspectral imaging (HSI). The roasting defect beans were divided into 5 groups: medium roasting (Cont), under developed (RD-1), over roasting (RD-2), interior under developed (RD-3), and interior scorching (RD-4). The following qualitative properties were assayed: browning index (BI), moisture content (MC), chlorogenic acid (CA), trigonelline (TG), and caffeine (CF) content. Their HSI spectra (1000-1700nm) were also analysed to develop the classification methods of roasting defect beans. RD-2 showed the highest BI and the lowest MC, CA, and TG content. The accuracy of classification model of partial least-squares discriminant was 86.2%. The most powerful wavelength to classify the defective beans was approximately 1420nm (related to OH bond). The HSI reflectance values at 1420nm showed similar tendency with MC, enabling the use of this technology to classify the roasting defect beans. Copyright © 2016. Published by Elsevier Ltd.

Ion irradiation of electronic-type-separated single wall carbon nanotubes: A model for radiation effects in nanostructured carbon

NASA Astrophysics Data System (ADS)

Rossi, Jamie E.; Cress, Cory D.; Helenic, Alysha R.; Schauerman, Chris M.; DiLeo, Roberta A.; Cox, Nathanael D.; Messenger, Scott R.; Weaver, Brad D.; Hubbard, Seth M.; Landi, Brian J.

2012-08-01

The structural and electrical properties of electronic-type-separated (metallic and semiconducting) single wall carbon nanotube (SWCNT) thin-films have been investigated after irradiation with 150 keV 11B+ and 150 keV 31P+ with fluences ranging from 1012 to 1015 ions/cm2. Raman spectroscopy results indicate that the ratio of the Raman D to G' band peak intensities (D/G') is a more sensitive indicator of SWCNT structural modification induced by ion irradiation by one order of magnitude compared to the ratio of the Raman D to G band peak intensities (D/G). The increase in sheet resistance (Rs) of the thin-films follows a similar trend as the D/G' ratio, suggesting that the radiation induced variation in bulk electrical transport for both electronic-types is equal and related to localized defect generation. The characterization results for the various samples are compared based on the displacement damage dose (DDD) imparted to the sample, which is material and damage source independent. Therefore, it is possible to extend the analysis to include data from irradiation of transferred CVD-graphene films on SiO2/Si substrates using 35 keV C+ ions, and compare the observed changes at equivalent levels of ion irradiation-induced damage to that observed in the SWCNT thin-film samples. Ultimately, a model is developed for the prediction of the radiation response of nanostructured carbon materials based on the DDD for any incident ion with low-energy recoil spectra. The model is also related to the defect concentration, and subsequently the effective defect-to-defect length, and yields a maximum defect concentration (minimum defect-to-defect length) above which the bulk electrical transport properties in SWCNT thin-films and large graphene-based electronic devices rapidly degrade when exposed to harsh environments.

Multi-resonance frequency spin dependent charge pumping and spin dependent recombination - applied to the 4H-SiC/SiO2 interface

NASA Astrophysics Data System (ADS)

Anders, M. A.; Lenahan, P. M.; Lelis, A. J.

2017-12-01

We report on a new electrically detected magnetic resonance (EDMR) approach involving spin dependent charge pumping (SDCP) and spin dependent recombination (SDR) at high (K band, about 16 GHz) and ultra-low (360 and 85 MHz) magnetic resonance frequencies to investigate the dielectric/semiconductor interface in 4H-SiC metal-oxide-semiconductor field-effect transistors (MOSFETs). A comparison of SDCP and SDR allows for a comparison of deep level defects and defects with energy levels throughout most of the bandgap. Additionally, a comparison of high frequency and ultra-low frequency measurements allows for (1) the partial separation of spin-orbit coupling and hyperfine effects on magnetic resonance spectra, (2) the observation of otherwise forbidden half-field effects, which make EDMR, at least, in principle, quantitative, and (3) the observation of Breit-Rabi shifts in superhyperfine measurements. (Observation of the Breit-Rabi shift helps in both the assignment and the measurement of superhyperfine parameters.) We find that, as earlier work also indicates, the SiC silicon vacancy is the dominating defect in n-MOSFETs with as-grown oxides and that post-oxidation NO anneals significantly reduce their population. In addition, we provide strong evidence that NO anneals result in the presence of nitrogen very close to a large fraction of the silicon vacancies. The results indicate that the presence of nearby nitrogen significantly shifts the silicon vacancy energy levels. Our results also show that the introduction of nitrogen introduces a disorder at the interface. This nitrogen induced disorder may provide at least a partial explanation for the relatively modest improvement in mobility after the NO anneals. Finally, we compare the charge pumping and SDCP response as a function of gate amplitude and charge pumping frequency.

Electronically active defects in the Cu2ZnSn(Se,S)4 alloys as revealed by transient photocapacitance spectroscopy

NASA Astrophysics Data System (ADS)

Miller, D. Westley; Warren, Charles W.; Gunawan, Oki; Gokmen, Tayfun; Mitzi, David B.; Cohen, J. David

2012-10-01

Transient photocapacitance (TPC) spectra were obtained on a series of Cu2ZnSn(Se,S)4 absorber devices with varying Se:S ratios, providing bandgaps (Eg) between 1 eV and 1.5 eV. Efficiencies varied between 8.3% and 9.3% for devices with Eg ≤ 1.2 eV and were near 6.5% for devices with Eg ≥ 1.4 eV. The TPC spectra revealed a band-tail region with Urbach energies at or below 18 meV for the first group, but in the 25-30 meV range for the higher band-gap samples. A deeper defect band centered near 0.8 eV was also observed in most samples. We identified a correlation between the Urbach energies and the voltage deficit in these devices.

The Chemical Capacitance as a Fingerprint of Defect Chemistry in Mixed Conducting Oxides.

PubMed

Fleig, Juergen; Schmid, Alexander; Rupp, Ghislain M; Slouka, Christoph; Navickas, Edvinas; Andrejs, Lukas; Hutter, Herbert; Volgger, Lukas; Nenning, Andreas

2016-01-01

The oxygen stoichiometry of mixed conducting oxides depends on the oxygen chemical potential and thus on the oxygen partial pressure in the gas phase. Also voltages may change the local oxygen stoichiometry and the amount to which such changes take place is quantified by the chemical capacitance of the sample. Impedance spectroscopy can be used to probe this chemical capacitance. Impedance measurements on different oxides ((La,Sr)FeO3-δ = LSF, Sr(Ti,Fe)O3-δ = STF, and Pb(Zr,Ti)O3 = PZT) are presented, and demonstrate how the chemical capacitance may affect impedance spectra in different types of electrochemical cells. A quantitative analysis of the spectra is based on generalized equivalent circuits developed for mixed conducting oxides by J. Jamnik and J. Maier. It is discussed how defect chemical information can be deduced from the chemical capacitance.

Orientation and temperature dependent adsorption of H 2S on GaAs: Valence band photoemission

NASA Astrophysics Data System (ADS)

Ranke, W.; Kuhr, H. J.; Finster, J.

A cylindrically shaped GaAs single crystal was used to study the adsorption of H 2S on the six inequivalent orientations (001), (113), (111), (110), (111) and (113) by angle resolved valence band photoelectron spectroscopy and surface dipole measurements. Adsorption at 150 K on the surface prepared by molecular beam epitaxy (MBE) yields similar adsorbate induced emission on all orientations which were ascribed to SH radicals. On (110), where preferential adsorption occurs additional features from molecular H 2S are observed. The adsorbate spectra at 720 K are ascribed to atomic sulphur. On the surface prepared by ion bombardment and annealing, defect enhanced adsorption occurs in the range (111)-(113). The adsorbate spectra are very similar to those on the MBE surface at 720 K. Thus, no new species are adsorbed on defects but only sticking probability and penetration capability are increased.

2D XANES-XEOL mapping: observation of enhanced band gap emission from ZnO nanowire arrays

NASA Astrophysics Data System (ADS)

Wang, Zhiqiang; Guo, Xiaoxuan; Sham, Tsun-Kong

2014-05-01

Using 2D XANES-XEOL spectroscopy, it is found that the band gap emission of ZnO nanowire arrays is substantially enhanced i.e. that the intensity ratio between the band gap and defect emissions increases by more than an order of magnitude when the excitation energy is scanned across the O K-edge. Possible mechanisms are discussed.Using 2D XANES-XEOL spectroscopy, it is found that the band gap emission of ZnO nanowire arrays is substantially enhanced i.e. that the intensity ratio between the band gap and defect emissions increases by more than an order of magnitude when the excitation energy is scanned across the O K-edge. Possible mechanisms are discussed. Electronic supplementary information (ESI) available: XEOL spectra with different excitation energies. X-ray attenuation length vs. photon energy. Details of surface defects in ZnO NWs. The second O K-edge and Zn L-edge 2D XANES-XEOL maps. Comparison of the first and second TEY at O K-edge and Zn L-edge scans, respectively. Raman spectra of the ZnO NWs with different IBGE/IDE ratios. See DOI: 10.1039/c4nr01049c

Evolution of Defect Structures and Deep Subgap States during Annealing of Amorphous In-Ga-Zn Oxide for Thin-Film Transistors

NASA Astrophysics Data System (ADS)

Jia, Junjun; Suko, Ayaka; Shigesato, Yuzo; Okajima, Toshihiro; Inoue, Keiko; Hosomi, Hiroyuki

2018-01-01

We investigate the evolution behavior of defect structures and the subgap states in In-Ga-Zn oxide (IGZO) films with increasing postannealing temperature by means of extended x-ray absorption fine-structure (EXAFS) measurements, positron annihilation lifetime spectroscopy (PALS), and cathodoluminescence (CL) spectroscopy, aiming to understand the relationship between defect structures and subgap states. EXAFS measurements reveal the varied oxygen coordination numbers around cations during postannealing and confirm two types of point defects, namely, excess oxygen around Ga atoms and oxygen deficiency around In and/or Zn atoms. PALS suggests the existence of cation-vacancy (VM )-related clusters with neutral or negative charge in both amorphous and polycrystalline IGZO films. CL spectra show a main emission band at approximately 1.85 eV for IGZO films, and a distinct shoulder located at about 2.15 eV for IGZO films postannealed above 600 °C . These two emission bands are assigned to a recombination between the electrons in the conduction band and/or in the shallow donor levels near the conduction band and the acceptors trapped above the valence-band maximum. The shallow donors are attributed to the oxygen deficiency, and the acceptors are thought to possibly arise from the excess oxygen or the VM-related clusters. These results open up an alternative route for understanding the device instability of amorphous IGZO-based thin-film transistors, especially the presence of the neutral or negatively charged VM-related clusters in amorphous IGZO films.

[Preparation and luminescent properties of CaMoO4:Tb3+].

PubMed

Wang, Xi-gui; Bo, Su-ling; Na, Mi-la; Qi, Xia

2010-01-01

The precursor of the sample CaMoO4:Tb3+ was prepared by the coprecipitation method. TG-DTA spectra show that there is, at 850 degrees C, an energy absorption peak, suggesting that the sample reaches the activation spot of its response. The XRD pattern of the roasted sample shows that CaMoO4:Tb3+, in the single phase, is a representative scheelite structure of CaMoO4, but the peaks shift toward right, implying that tiny crystal defect in the crystal is produced. The defect is likely to be formed by the formation of the holes as two Tb3+ replace three Ca2+ in a cell. The excitation and emission spectra of the sample were investigated and revealed that the defect structure of the sample is in favor of the energy transfer of the characteristic peak (488 nm) of the MoO4(2-) effectively to Tb3+, and makes the 4f electrons of the Tb3+ transit, especially the (7)F6-->(5)D4 electronic transition (488 nm) of the Tb3+, to be greatly strengthened. As a result, the emission spectra with lamdaex=488 nm show that the emission intensity of the spontaneously activated fluorescence MoO4(2-) is greatly weakened, while the green light luminescence intensity of the (5)D4-->(7)F5 transition (544 nm) of the Tb3+ is greatly enhanced. This suggests that the sample CaMoO4:Tb3+ will become the luminescence material with potentially great application value.

Production and evolution of A-centers in n-type Si1-xGex

NASA Astrophysics Data System (ADS)

Sgourou, E. N.; Andrianakis, A.; Londos, C. A.; Chroneos, A.

2013-03-01

The vacancy-oxygen pair (VO or A-center) in n-type Si1-xGex crystals (x = 0, 0.025, 0.055) has been studied using infrared (IR) spectroscopy. It is determined that the VO production is suppressed in the case of n-type Si1-xGex as compared to Si. It is observed that the annealing temperature of the VO defect in Si1-xGex is substantially lower as compared to Si. The decay of the VO (830 cm-1) band, in the course of 20 min isochronal anneals, shows two stages: The onset of the first stage is at ˜180 °C and the decrease of the VO signal is accompanied in the spectra by the increase of the intensity of two bands at ˜834 and 839 cm-1. These bands appear in the spectra immediately after irradiation and were previously correlated with (VO-Ge) structures. The onset of the second stage occurs at ˜250 °C were the 830 cm-1 band of VO and the above two bands of (VO-Ge) decrease together in the spectra accompanied by the simultaneous growth of the 885 cm-1 band of the VO2 defect. Interestingly, the percentage of the VO pairs that are converted to VO2 defects is larger in the Si1-xGex samples with intermediate Ge content (x = 0.025) as compared with Si (x = 0) and with the high Ge content samples (x = 0.055). The results are discussed in view of the association of VO pairs with Ge.

Er3+ -doped anatase TiO2 nanocrystals: crystal-field levels, excited-state dynamics, upconversion, and defect luminescence.

PubMed

Luo, Wenqin; Fu, Chengyu; Li, Renfu; Liu, Yongsheng; Zhu, Haomiao; Chen, Xueyuan

2011-11-04

A comprehensive survey of electronic structure and optical properties of rare-earth ions embedded in semiconductor nanocrystals (NCs) is of vital importance for their potential applications in areas as diverse as luminescent bioprobes, lighting, and displays. Er3+ -doped anatase TiO2 NCs, synthesized via a facile sol-gel solvothermal method, exhibit intense and well-resolved intra-4f emissions of Er3+ . Crystal-field (CF) spectra of Er3+ in TiO2 NCs are systematically studied by means of high-resolution emission and excitation spectra at 10-300 K. The CF analysis of Er3+ assuming a site symmetry of C(2v) yields a small root-mean-square deviation of 25.1 cm(-1) and reveals the relatively large CF strength (549 cm(-1) ) of Er3+, thus verifying the rationality of the C(2v) symmetry assignment of Er3+ in anatase TiO2 NCs. Based on a simplified thermalization model for the temperature-dependent photoluminescence (PL) dynamics from (4) S(3/2) , the intrinsic radiative luminescence lifetimes of (4) S(3/2) and (2) H(11/2) are experimentally determined to be 3.70 and 1.73 μs, respectively. Green and red upconversion (UC) luminescence of Er3+ can be achieved upon laser excitation at 974.5 nm. The UC intensity of Er3+ in Yb/Er-codoped NCs is found to be about five times higher than that of Er-singly-doped counterparts as a result of efficient Yb3+ sensitization and energy transfer upconversion (ETU) evidenced by its distinct UC luminescence dynamics. Furthermore, the origin of defect luminescence is revealed based on the temperature-dependent PL spectra upon excitation above the TiO2 bandgap at 325 nm. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Temperature dependence of photoluminescence peaks of porous silicon structures

NASA Astrophysics Data System (ADS)

Brunner, Róbert; Pinčík, Emil; Kučera, Michal; Greguš, Ján; Vojtek, Pavel; Zábudlá, Zuzana

2017-12-01

Evaluation of photoluminescence spectra of porous silicon (PS) samples prepared by electrochemical etching is presented. The samples were measured at temperatures 30, 70 and 150 K. Peak parameters (energy, intensity and width) were calculated. The PL spectrum was approximated by a set of Gaussian peaks. Their parameters were fixed using fitting a procedure in which the optimal number of peeks included into the model was estimated using the residuum of the approximation. The weak thermal dependence of the spectra indicates the strong influence of active defects.

Photonic spectra of a Bragg microresonator with a ferroelectric resonator layer

NASA Astrophysics Data System (ADS)

Fedorova, Irina V.; Eliseeva, Svetlana V.; Sementsov, Dmitrij I.

2018-05-01

Transmission spectra of a photonic crystal resonator structure have been obtained where the Bragg dielectric mirrors contain a finite number of periods with an inverted order of layers and the resonator layer is made of a ferroelectric with a permittivity many times exceeding the permittivity of the layers in Bragg mirrors. Almost a complete transmission suppression was detected not only in the photonic band gap (except for a narrow region of the defect mode), but also outside the forbidden band.

Application of hybrid SiO2-coated CdTe nanocrystals for sensitive sensing of Cu2+ and Ag+ ions.

PubMed

Cao, Yongqiang; Zhang, Aiyu; Ma, Qian; Liu, Ning; Yang, Ping

2013-01-01

A new ion sensor based on hybrid SiO2 -coated CdTe nanocrystals (NCs) was prepared and applied for sensitive sensing of Cu(2+) and Ag(+) for the selective quenching of photoluminescence (PL) of NCs in the presence of ions. As shown by ion detection experiments conducted in pure water rather than buffer solution, PL responses of NCs were linearly proportional to concentrations of Cu(2+) and Ag(+) ions < 3 and 7 uM, respectively. Much lower detection limits of 42.37 nM for Cu(2+) and 39.40 nM for Ag(+) were also observed. In addition, the NC quenching mechanism was discussed in terms of the characterization of static and transient optical spectra. The transfer and trapping of photoinduced charges in NCs by surface energy levels of CuS and Ag2 S clusters as well as surface defects generated by the exchange of Cu(2+) and Ag(+) ions with Cd(2+) ion in NCs, resulted in PL quenching and other optical spectra changes, including steady-state absorption and transient PL spectra. It is our hope that these results will be helpful in the future preparation of new ion sensors. Copyright © 2012 John Wiley & Sons, Ltd.

The functionalization of carbon nanosheets

NASA Astrophysics Data System (ADS)

Quinlan, Ronald A.

Carbon nanosheets are a novel two-dimensional nanostructure made up of 2-20 graphene atomic planes oriented with their in-plane axis perpendicular to the growth substrate. Previous efforts in developing nanosheet technology have focused on the characterization of the system and their development as an electron source due to the high atomic enhancement factor (beta) and low turn on field. Further investigation of nanosheets as high surface area electrodes revealed poor wetting by polymeric material and extreme hydrophobic behavior. Because nanosheet technology has promise as a high surface area electrode material, this thesis research has focused on three areas of interest: the enhancement of nanosheets through chemical modification, the incorporation of the nanosheets into a polymeric composite and the delivery of a proof of concept measurement. We have successfully introduced defects into the graphene lattice of the nanosheet system via an acid treatment. Inspection of these defects by x-ray absorption near-edge spectroscopy (XANES) shows the introduction of two features in the spectra assigned to C=O pi* and C-O sigma* transitions. Thermal desorption spectroscopy (TDS) was used to identify the oxygen containing groups created during the functionalization as carboxylic and hydroxyl functional groups. These groups were identified through the combination of carboxylic, hydroxyl, anhydride and lactone peaks in the CO2, CO and H 2O TDS spectra. Deconvolution of the TDS spectra using 1st and 2nd order Polanyi-Wigner equations enables the calculation of desorption energy values for individual features and for the estimation of the number of atoms desorbing from the surface during a particular event. Identification of the exact nature of the functional groups was attempted through high resolution x-ray photoelectron spectroscopy (XPS) of the C(1s) and O(1s) peaks. Though the pairing of sub-peaks with specific functionalities of the system was not possible due to the complexities of the spectra, the trends observed in the data support the data gathered via the XANES and TDS experiments. Also, a procedure for the classification of defect density and exact functionality was outlined. Deconvolution of the TDS spectra using 1 st and 2nd order Polanyi-Wigner equations enabled the calculation of desorption energy values for individual features and for the estimation of the number of atoms desorbing from the surface during a particular event. This information along with the changing sub-peak areas from dedicated and calibrated XPS system would allow for not only a more accurate estimation of defect density, but also for the identification of sub-peaks in the C(1s) and O(1s) spectra. Finally, photoluminescence measurements of poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) and MEH-PPV/nanosheet systems showed a quenching of three orders of magnitude for the MEH-PPV/nanosheet system suggesting that nanosheets are a viable option for excition separation in organic photovoltaics.

Defects and anharmonicity induced electron spectra of YBa2Cu3O7-δ superconductors

NASA Astrophysics Data System (ADS)

Singh, Anu; Indu, B. D.

2018-05-01

The effects of defects and anharmonicities on the electron density of states (EDOS) have been studied in high-temperature superconductors (HTS) adopting the many body quantum dynamical theory of electron Green's functions via a generalized Hamiltonian that includes the effects of electron-phonon interactions, anharmonicities and point impurities. The automatic emergence of pairons and temperature dependence of EDOS are appear as special feature of the theory. The results thus obtained and their numerical analysis for YBa2Cu3O7-δ superconductors clearly demonstrate that the presence of defects, anharmonicities and electron-phonon interactions modifies the behavior of EDOS over a wide range of temperature.

Optical study of plasma sprayed hydroxyapatite coatings deposited at different spray distance

NASA Astrophysics Data System (ADS)

Belka, R.; Kowalski, S.; Żórawski, W.

2017-08-01

Series of hydroxyapatite (HA) coatings deposited on titanium substrate at different spray (plasma gun to workpiece) distance were investigated. The optical methods as dark field confocal microscopy, Raman/PL and UV-VIS spectroscopy were used for study the influence of deposition process on structural degradation of HA precursor. The hydroxyl group concentration was investigated by study the OH mode intensity in the Raman spectra. Optical absorption coefficients at near UV region were analyzed by Diffuse Reflectance Spectroscopy. PL intensity observed during Raman measurement was also considered as relation to defects concentration and degradation level. It was confirmed the different gunsubstrate distance has a great impact on structure of deposited HA ceramics.

Surface defect assisted broad spectra emission from CdSe quantum dots for white LED application

NASA Astrophysics Data System (ADS)

Samuel, Boni; Mathew, S.; Anand, V. R.; Correya, Adrine Antony; Nampoori, V. P. N.; Mujeeb, A.

2018-02-01

This paper reports, broadband photoluminescence from CdSe quantum dots (QDs) under the excitation of 403 nm using fluorimeter and 403 nm CW laser excitation. The broad spectrum obtained from the colloidal quantum dots was ranges from 450 nm to 800 nm. The broadness of the spectra was attributed to the merging of band edge and defect driven emissions from the QDs. Six different sizes of particles were prepared via kinetic growth method by using CdO and elemental Se as sources of Cd and Se respectively. The particle sizes were measured from TEM images. The size dependent effect on broad emission was also studied and the defect state emission was found to be predominant in very small QDs. The defect driven emission was also observed to be redshifted, similar to the band edge emission, due to quantum confinement effect. The emission corresponding to different laser power was also studied and a linear relation was obtained. In order to study the colour characteristics of the emission, CIE chromaticity coordinate, CRI and CCT of the prepared samples were measured. It is observed that, these values were tunable by the addition of suitable intensity of blue light from the excitation source to yield white light of various colour temperatures. The broad photoluminescence spectrum of the QDs, were compared with that of a commercially available white LED. It was found that the prepared QDs are good alternatives for the phosphor in phosphor converted white LEDs, to provide good spectral tunability.

Understanding lattice defects to influence ferromagnetic order of ZnO nanoparticles by Ni, Cu, Ce ions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Verma, Kuldeep Chand, E-mail: dkuldeep.physics@gmail.com; Kotnala, R.K., E-mail: rkkotnala@gmail.com

Future spintronics technologies based on diluted magnetic semiconductors (DMS) will rely heavily on a sound understanding of the microscopic origins of ferromagnetism in such materials. It remains unclear, however, whether the ferromagnetism in DMS is intrinsic - a precondition for spintronics - or due to dopant clustering. For this, we include a simultaneous doping from transition metal (Ni, Cu) and rare earth (Ce) ions in ZnO nanoparticles that increase the antiferromagnetic ordering to achieve high-T{sub c} ferromagnetism. Rietveld refinement of XRD patterns indicate that the dopant ions in ZnO had a wurtzite structure and the dopants, Ni{sup 2+}, Cu{sup 2+},more » Ce{sup 3+} ions, are highly influenced the lattice constants to induce lattice defects. The Ni, Cu, Ce ions in ZnO have nanoparticles formation than nanorods was observed in pure sample. FTIR involve some organic groups to induce lattice defects and the metal-oxygen bonding of Zn, Ni, Cu, Ce and O atoms to confirm wurtzite structure. Raman analysis evaluates the crystalline quality, structural disorder and defects in ZnO lattice with doping. Photoluminescence spectra have strong near-band-edge emission and visible emission bands responsible for defects due to oxygen vacancies. The energy band gap is calculated using Tauc relation. Room temperature ferromagnetism has been described due to bound magnetic polarons formation with Ni{sup 2+}, Cu{sup 2+}, Ce{sup 3+} ions in ZnO via oxygen vacancies. The zero field and field cooling SQUID measurement confirm the strength of antiferromagnetism in ZnO. The field cooling magnetization is studied by Curie-Weiss law that include antiferromagnetic interactions up to low temperature. The XPS spectra have involve +3/+4 oxidation states of Ce ions to influence the observed ferromagnetism. - Graphical abstract: The lattice defects/vacancies attributed by Ni and Ce ions in the wurtzite ZnO structure are responsible in high T{sub c} -ferromagnetism due to long-range magnetic interactions with cluster and spin-glass type growth. - Highlights: • Lattice defects/vacancies attributed high T{sub c} –ferromagnetism. • Transition metal and rare earth ions deform the wurtzite ZnO lattice to induce defects. • Oxygen vacancies are more favorable than Zn with Ni, Cu, Ce into ZnO. • Defects assisted long-range ferromagnetism of doped ZnO include cluster and spin-glass growth.« less

Wind Turbine Bearing Diagnostics Based on Vibration Monitoring

NASA Astrophysics Data System (ADS)

Kadhim, H. T.; Mahmood, F. H.; Resen, A. K.

2018-05-01

Reliability maintenance can be considered as an accurate condition monitoring system which increasing beneficial and decreasing the cost production of wind energy. Supporting low friction of wind turbine rotating shaft is the main task of rolling element bearing and it is the main part that suffers from failure. The rolling failures elements have an economic impact and may lead to malfunctions and catastrophic failures. This paper concentrates on the vibration monitoring as a Non-Destructive Technique for assessing and demonstrates the feasibility of vibration monitoring for small wind turbine bearing defects based on LabVIEW software. Many bearings defects were created, such as inner race defect, outer race defect, and ball spin defect. The spectra data were recorded and compared with the theoretical results. The accelerometer with 4331 NI USB DAQ was utilized to acquiring, analyzed, and recorded. The experimental results were showed the vibration technique is suitable for diagnostic the defects that will be occurred in the small wind turbine bearings and developing a fault in the bearing which leads to increasing the vibration amplitude or peaks in the spectrum.

The application of an optical Fourier spectrum analyzer on detecting defects in mass-produced satellite photographs

NASA Technical Reports Server (NTRS)

Athale, R.; Lee, S. H.

1976-01-01

Various defects in mass-produced pictures transmitted to earth from a satellite are investigated. It is found that the following defects are readily detectable via Fourier spectrum analysis: (1) bit slip, (2) breakup causing loss of image, and (3) disabled track at the top of the imagery. The scratches made on the film during mass production, which are difficult to detect by visual observation, also show themselves readily in Fourier spectrum analysis. A relation is established between the number of scratches, their width and depth and the intensity of their Fourier spectra. Other defects that are found to be equally suitable for Fourier spectrum analysis or visual (image analysis) detection are synchronous loss without blurring of image, and density variation in gray scale. However, the Fourier spectrum analysis is found to be unsuitable for detection of such defects as pin holes, annotation error, synchronous loss with blurring of images, and missing image in the beginning of the work order. The design of an automated, real time system, which will reject defective films, is treated.

Identification of the spatial location of deep trap states in AlGaN/GaN heterostructures by surface photovoltage spectroscopy

NASA Astrophysics Data System (ADS)

Jana, Dipankar; Porwal, S.; Sharma, T. K.

2017-12-01

Spatial and spectral origin of deep level defects in molecular beam epitaxy grown AlGaN/GaN heterostructures are investigated by using surface photovoltage spectroscopy (SPS) and pump-probe SPS techniques. A deep trap center ∼1 eV above the valence band is observed in SPS measurements which is correlated with the yellow luminescence feature in GaN. Capture of electrons and holes is resolved by performing temperature dependent SPS and pump-probe SPS measurements. It is found that the deep trap states are distributed throughout the sample while their dominance in SPS spectra depends on the density, occupation probability of deep trap states and the background electron density of GaN channel layer. Dynamics of deep trap states associated with GaN channel layer is investigated by performing frequency dependent photoluminescence (PL) and SPS measurements. A time constant of few millisecond is estimated for the deep defects which might limit the dynamic performance of AlGaN/GaN based devices.

Core level line shapes and surface band structures of Sn/Ge(111) and Sn/Si(111)

NASA Astrophysics Data System (ADS)

Uhrberg, Roger

2001-03-01

We have performed a comparative study of Sn/Ge(111) and the closely related Sn/Si(111) system, using photoelectron spectroscopy (PES) and low energy electron diffraction (LEED). In similarity with the results from the Sn/Ge(111) surface, the Sn 4d spectra from Sn/Si(111) exhibit two major components and the valence band spectra show two surface state bands at both room temperature (RT) and 70 K. These features, which have been associated with the low temperature 3x3 phase in the case of Sn/Ge(111), are not expected for the rt3xrt3 RT surfaces. In contrast to Sn/Ge(111), we do not observe any transition to a 3x3 phase in LEED for Sn/Si(111) at temperatures down to 70 K (the lowest temperature in this study). Despite the absence of a 3x3 phase for Sn/Si(111) the core-level and the valence band data are very similar to those of Sn/Ge(111). The Sn 4d spectra show, however, one interesting difference. The intensity ratio of the two Sn 4d components is reversed for the Sn/Si(111) surface compared the Ge counterpart. This and the other PES results will be discussed in terms of the two different types of 3x3 periodicities that have been reported to be induced by substitutional defects on the Sn/Ge(111) surface [1]. [1] A.V. Melechko et al., Phys. Rev. B61, 2235 (2000)

Structure and luminescence properties of 10-BN sheets

NASA Astrophysics Data System (ADS)

Han, Wei-Qiang; Liu, Lijia; Sham, T. K.; Liu, Zhenxian

2012-10-01

Isotopic 10BN sheets were first prepared using graphene sheets as templates to react with 10B2O3. The edge-areas of BN sheets have much higher oxygen-doping ratios compared to other areas. The emission peak of X-ray excited optical luminescence spectra of the 10BN-sheets is broader and red-shifted because of the isotopic effect. A broad violet-blue emission at a wavelength centered at ~400 nm is assigned to the defect emission due to oxygen-doping and defects in the BN network.

Pressure-induced structural modifications of rare-earth hafnate pyrochlore

NASA Astrophysics Data System (ADS)

Turner, Katlyn M.; Rittman, Dylan R.; Heymach, Rachel A.; Tracy, Cameron L.; Turner, Madison L.; Fuentes, Antonio F.; Mao, Wendy L.; Ewing, Rodney C.

2017-06-01

Complex oxides with the pyrochlore (A2B2O7) and defect-fluorite ((A,B)4O7) structure-types undergo structural transformations under high-pressure. Rare-earth hafnates (A2Hf2O7) form the pyrochlore structure for A  =  La-Tb and the defect-fluorite structure for A  =  Dy-Lu. High-pressure transformations in A2Hf2O7 pyrochlore (A  =  Sm, Eu, Gd) and defect-fluorite (A  =  Dy, Y, Yb) were investigated up to ~50 GPa and characterized by in situ Raman spectroscopy and synchrotron x-ray diffraction (XRD). Raman spectra at ambient pressure revealed that all compositions, including the defect-fluorites, have some pyrochlore-type short-range order. In situ high-pressure synchrotron XRD showed that all of the rare earth hafnates investigated undergo a pressure-induced phase transition to a cotunnite-like (orthorhombic) structure that begins between 18 and 25 GPa. The phase transition to the cotunnite-like structure is not complete at 50 GPa, and upon release of pressure, the hafnates transform to defect-fluorite with an amorphous component. For all compositions, in situ Raman spectroscopy showed that disordering occurs gradually with increasing pressure. Pyrochlore-structured hafnates retain their short-range order to a higher pressure (30 GPa vs.  <10 GPa) than defect-fluorite-structured hafnates. Rare earth hafnates quenched from 50 GPa show Raman spectra consistent with weberite-type structures, as also reported for irradiated rare-earth stannates. The second-order Birch-Murnaghan equation of state fit gives a bulk modulus of ~250 GPa for hafnates with the pyrochlore structure, and ~400 GPa for hafnates with the defect-fluorite structure. Dy2Hf2O7 is intermediate in its response, with some pyrochlore-type ordering, based on Raman spectroscopy and the equation of state, with a bulk modulus of ~300 GPa. As predicted based on the similar ionic radius of Zr4+ and Hf4+, rare-earth hafnates show similar behavior to that reported for rare earth zirconates at high pressure.

Pressure-induced structural modifications of rare-earth hafnate pyrochlore.

PubMed

Turner, Katlyn M; Rittman, Dylan R; Heymach, Rachel A; Tracy, Cameron L; Turner, Madison L; Fuentes, Antonio F; Mao, Wendy L; Ewing, Rodney C

2017-06-28

Complex oxides with the pyrochlore (A 2 B 2 O 7 ) and defect-fluorite ((A,B) 4 O 7 ) structure-types undergo structural transformations under high-pressure. Rare-earth hafnates (A 2 Hf 2 O 7 ) form the pyrochlore structure for A  =  La-Tb and the defect-fluorite structure for A  =  Dy-Lu. High-pressure transformations in A 2 Hf 2 O 7 pyrochlore (A  =  Sm, Eu, Gd) and defect-fluorite (A  =  Dy, Y, Yb) were investigated up to ~50 GPa and characterized by in situ Raman spectroscopy and synchrotron x-ray diffraction (XRD). Raman spectra at ambient pressure revealed that all compositions, including the defect-fluorites, have some pyrochlore-type short-range order. In situ high-pressure synchrotron XRD showed that all of the rare earth hafnates investigated undergo a pressure-induced phase transition to a cotunnite-like (orthorhombic) structure that begins between 18 and 25 GPa. The phase transition to the cotunnite-like structure is not complete at 50 GPa, and upon release of pressure, the hafnates transform to defect-fluorite with an amorphous component. For all compositions, in situ Raman spectroscopy showed that disordering occurs gradually with increasing pressure. Pyrochlore-structured hafnates retain their short-range order to a higher pressure (30 GPa vs.  <10 GPa) than defect-fluorite-structured hafnates. Rare earth hafnates quenched from 50 GPa show Raman spectra consistent with weberite-type structures, as also reported for irradiated rare-earth stannates. The second-order Birch-Murnaghan equation of state fit gives a bulk modulus of ~250 GPa for hafnates with the pyrochlore structure, and ~400 GPa for hafnates with the defect-fluorite structure. Dy 2 Hf 2 O 7 is intermediate in its response, with some pyrochlore-type ordering, based on Raman spectroscopy and the equation of state, with a bulk modulus of ~300 GPa. As predicted based on the similar ionic radius of Zr 4+ and Hf 4+ , rare-earth hafnates show similar behavior to that reported for rare earth zirconates at high pressure.

DFT calculations of graphene monolayer in presence of Fe dopant and vacancy

NASA Astrophysics Data System (ADS)

Ostovari, Fatemeh; Hasanpoori, Marziyeh; Abbasnejad, Mohaddeseh; Salehi, Mohammad Ali

2018-07-01

In the present work, the effects of Fe doping and vacancies on the electronic, magnetic and optical properties of graphene are studied by density functional theory based calculations. The conductive behavior is revealed for the various defected graphene by means of electronic density of states. However, defected structures show different magnetic and optical properties compared to those of pure one. The ferromagnetic phase is the most probable phase by substituting Fe atoms and vacancies at AA sublattice of graphene. The optical properties of impure graphene differ from pure graphene under illumination with parallel polarization of electric field, whereas for perpendicular polarization it remains unchanged. In presence of defect and under parallel polarization of light, the static dielectric constant rises strongly and the maximum peak of Im ε(ω) shows red shift relative to pure graphene. Moreover, the maximum absorption peak gets broaden in the visible to infrared region at the same condition and the magnitude and related energy of peaks shift to higher value in the EELS spectra. Furthermore, the results show that the maximum values of refractive index and reflectivity spectra increase rapidly and represent the red and blue shifts; respectively. Generally; substituting the C atom with Fe has more effect on magnetic and optical properties relative to the C vacancies.

Minority carrier diffusion and defects in InGaAsN grown by molecular beam epitaxy

NASA Astrophysics Data System (ADS)

Kurtz, Steven R.; Klem, J. F.; Allerman, A. A.; Sieg, R. M.; Seager, C. H.; Jones, E. D.

2002-02-01

To gain insight into the nitrogen-related defects of InGaAsN, nitrogen vibrational mode spectra, Hall mobilities, and minority carrier diffusion lengths are examined for InGaAsN (1.1 eV band gap) grown by molecular beam epitaxy (MBE). Annealing promotes the formation of In-N bonding, and lateral carrier transport is limited by large scale (≫mean free path) material inhomogeneities. Comparing solar cell quantum efficiencies with our earlier results for devices grown by metalorganic chemical vapor deposition (MOCVD), we find significant electron diffusion in the MBE material (reversed from the hole diffusion in MOCVD material), and minority carrier diffusion in InGaAsN cannot be explained by a "universal," nitrogen-related defect.

The Role of Pressure to Quantify the Defects and its Effect on the Morphology of Graphene Layers

NASA Astrophysics Data System (ADS)

Amit, Kumar; Sharma, Rishi

The work reports the traces of graphene synthesized by Thermal-CVD technique over oxidized silicon substrates coated with nickel and cobalt catalysts under different pressures. Other process parameters like temperature, gas composition and time are kept constant during the growth. Effect of pressure on the nature of defects and structure of the graphene has been analyzed by Raman spectra. Effect of pressure on morphology has also been studied. It has been observed that the variation of pressure is responsible for any change in the structure and morphology of the graphene for a given catalyst. Formation of graphene and its defects has been explained with the help of two-step diffusion process.

Photocatalytic characteristics of single phase Fe-doped anatase TiO{sub 2} nanoparticles sensitized with vitamin B{sub 12}

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gharagozlou, Mehrnaz, E-mail: gharagozlou@icrc.ac.ir; Bayati, R.

Highlights: • Anatase TiO{sub 2}/B{sub 12} hybrid nanostructured catalyst was successfully synthesized by sol–gel technique. • The nanoparticle catalyst was doped with iron at several concentrations. • Nanoparticles were characterized in detail by XRD, Raman, TEM, EDS, and spectroscopy techniques. • The formation mechanism and role of point defects on photocatalytic properties were discussed. • A structure-property-processing correlation was established. - Abstract: We report a processing-structure-property correlation in B{sub 12}-anatase titania hybrid catalysts doped with several concentrations of iron. Our results clearly show that low-level iron doping alters structure, defect content, and photocatalytic characteristics of TiO{sub 2}. XRD and Ramanmore » studies revealed formation of a single-phase anatase TiO{sub 2} where no iron based segregation in particular iron oxide, was detected. FT-IR spectra clearly confirmed sensitization of TiO{sub 2} nanoparticles with vitamin B{sub 12}. TEM micrographs and diffraction patterns confirmed crystallization of anatase nanoparticles with a radius of 15–20 nm. Both XRD and Raman signals showed a peak shift and a peak broadening which are surmised to originate from creation of point defects, namely oxygen vacancy and titanium interstitial. The doped samples revealed a narrower band gap as compared to undoped samples. Photocatalytic activity of the samples was assessed through measuring the decomposition rate of rhodamine B. It was found that sensitization with vitamin B{sub 12} and Fe-doping significantly enhances the photocatalytic efficiency of the anatase nanoparticles. We also showed that there is an optimum Fe-doping level where the maximum photocatalytic activity is achieved. The boost of photocatalytic activity was qualitatively understood to originate from a more effective use of the light photons, formation of point defects, which enhance the charge separation, higher carrier mobility.« less

Selective extraction of halogenated compounds from data measured by comprehensive multidimensional gas chromatography/high resolution time-of-flight mass spectrometry for non-target analysis of environmental and biological samples.

PubMed

Hashimoto, Shunji; Zushi, Yasuyuki; Fushimi, Akihiro; Takazawa, Yoshikatsu; Tanabe, Kiyoshi; Shibata, Yasuyuki

2013-03-22

We developed a method that selectively extracts a subset from comprehensive 2D gas chromatography (GC×GC) and high-resolution time-of-flight mass spectrometry (HRTOFMS) data to detect and identify trace levels of organohalogens. The data were obtained by measuring several environmental and biological samples, namely fly ash, soil, sediment, the atmosphere, and human urine. For global analysis, some samples were measured without purification. By using our novel software, the mass spectra of organochlorines or organobromines were then extracted into a data subset under high mass accuracy conditions that were approximately equivalent to a mass resolution of 6000 for some samples. Mass defect filtering as pre-screening for the data extraction was very effective in removing the mass spectra of hydrocarbons. Those results showed that data obtained with HRTOFMS are valuable for global analysis of organohalogens, and probably of other compounds if specific data extraction methods can be devised. Copyright © 2013 Elsevier B.V. All rights reserved.

Vacancy-Rich Monolayer BiO2-x as a Highly Efficient UV, Visible, and Near-Infrared Responsive Photocatalyst.

PubMed

Li, Jun; Wu, Xiaoyong; Pan, Wenfeng; Zhang, Gaoke; Chen, Hong

2018-01-08

Vacancy-rich layered materials with good electron-transfer property are of great interest. Herein, a full-spectrum responsive vacancy-rich monolayer BiO 2-x has been synthesized. The increased density of states at the conduction band (CB) minimum in the monolayer BiO 2-x is responsible for the enhanced photon response and photo-absorption, which were confirmed by UV/Vis-NIR diffuse reflectance spectra (DRS) and photocurrent measurements. Compared to bulk BiO 2-x , monolayer BiO 2-x has exhibited enhanced photocatalytic performance for rhodamine B and phenol removal under UV, visible, and near-infrared light (NIR) irradiation, which can be attributed to the vacancy V Bi-O ''' as confirmed by the positron annihilation spectra. The presence of V Bi-O ''' defects in monolayer BiO 2-x promoted the separation of electrons and holes. This finding provides an atomic level understanding for developing highly efficient UV, visible, and NIR light responsive photocatalysts. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Structural and electronic properties of ultrathin FeSe films grown on Bi2Se3(0 0 0 1) studied by STM/STS

NASA Astrophysics Data System (ADS)

Singh, Udai Raj; Warmuth, Jonas; Markmann, Verena; Wiebe, Jens; Wiesendanger, Roland

2017-01-01

We report scanning tunnelling microscopy and spectroscopy (STM/STS) studies on one and two unit cell (UC) high FeSe thin films grown on Bi2Se3(0 0 0 1). In our thin films, we find the tetragonal phase of FeSe and dumb-bell shaped defects oriented along Se-Se bond directions. In addition, we observe striped moiré patterns with a periodicity of (7.3  ±  0.1) nm generated by the mismatch between the FeSe lattice and the Bi2Se3 lattice. We could not find any signature of a superconducting gap in the tunneling spectra measured on the surface of one and two UC thick islands of FeSe down to 6.5 K. The spectra rather show an asymmetric behavior across and a finite density of states at the Fermi level (E F) resembling those taken in the normal state of bulk FeSe.

Magnetic state and phase composition of carbon-encapsulated Co@C nanoparticles according to 59Co, 13C NMR data and Raman spectroscopy

NASA Astrophysics Data System (ADS)

Mikhalev, K. N.; Germov, A. Yu; Uimin, M. A.; Yermakov, A. E.; Konev, A. S.; Novikov, S. I.; Gaviko, V. S.; Ponosov, Yu S.

2018-05-01

59Co, 13C NMR spectra, magnetization and Raman spectra of Co@C nanoparticles encapsulated in carbon have been analyzed. It has been shown that the cores of the nanoparticles consist of metallic cobalt with FCC structure and perhaps the carbide of cobalt Co3C. Carbon shell have been characterized as a highly defective structure similar to amorphous or glassy-like carbon, however, it may include a small amount of the carbon nanotubes.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Marimuthu, T.; Anandhan, N., E-mail: anandhan-kn@rediffmail.com; Mummoorthi, M.

Zinc oxide (ZnO) and zinc oxide/eosin yellow (ZnO/EY) thin films were potentiostatically deposited onto fluorine doped tin oxide (FTO) glass substrate. Effect of eosin yellow dye on structural, morphological and optical properties was studied. X-ray diffraction patterns, micro Raman spectra and photoluminescence (PL) spectra reveal hexagonal wurtzite structure with less atomic defects in 101 plane orientation of the ZnO/EY film. Scanning electron microscopy (SEM) images show flower for ZnO and porous like structure for ZnO/EY thin film, respectively. DSSC was constructed and evaluated by measuring the current density verses voltage curve.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Butenko, A.; Zion, E.; Richter, V.

The influence of long-term ageing (about one year) on the Raman scattering (RS) spectra and the temperature dependence of conductivity has been studied in two series of monolayer graphene samples irradiated by different doses of C{sup +} and Xe{sup +} ions. It is shown that the main result of ageing consists of changes in the intensity and position of D- and G- and 2D-lines in RS spectra and in an increase of the conductivity. The observed effects are explained in terms of an increase of the radius of the “activated” area around structural defects.

Structural phase transitions in niobium oxide nanocrystals

NASA Astrophysics Data System (ADS)

Yuvakkumar, R.; Hong, Sun Ig

2015-09-01

Niobium oxide nanocrystals were successfully synthesized employing the green synthesis method. Phase formation, microstructure and compositional properties of 1, 4 and 7 days incubation treated samples after calcinations at 450 °C were examined using X-ray diffraction, Raman, photoluminescence (PL), infrared, X-ray photoelectron spectra and transmission electron microscopic characterizations. It was observed that phase formation of Nb2O5 nanocrystals was dependent upon the incubation period required to form stable metal oxides. The characteristic results clearly revealed that with increasing incubation and aging, the transformation of cubic, orthorhombic and monoclinic phases were observed. The uniform heating at room temperature (32 °C) and the ligation of niobium atoms due to higher phenolic constituents of utilized rambutan during aging processing plays a vital role in structural phase transitions in niobium oxide nanocrystals. The defects over a period of incubation and the intensities of the PL spectra changing over a period of aging were related to the amount of the defects induced by the phase transition.

SrAl2O4:Eu2+ (1%) luminescence under UV, VUV and electron beam excitation

NASA Astrophysics Data System (ADS)

Nazarov, M.; Mammadova, S.; Spassky, D.; Vielhauer, S.; Abdullayeva, S.; Huseynov, A.; Jabbarov, R.

2018-01-01

This paper reports the luminescence properties of nanosized SrAl2O4:Eu2+ (1%) phosphors. The samples were prepared by combustion method at 600 °C, followed by annealing of the resultant combustion ash at 1000 °C in a reductive (Ar + H2) atmosphere. X-ray diffraction (XRD), photo luminescence (PL) and cathodoluminescence (CL) analysis and thermal stimulated luminescence (TSL) method were applied to characterize the phosphor. For the first time a peak at 375 nm was observed in CL spectra of SrAl2O4:Eu2+ (1%) nanophosphors. Luminescence excitation spectra analysis have shown that this peak is related to crystal defects. Also in TSL curve one strong peak was observed in the region above room temperature (T = 325 K), which is attributed to lattice defects, namely oxygen vacancies. A green LED was fabricated by the combination of the SrAl2O4:Eu2+ (1%) nanosized phosphor and a 365 nm UV InGaN chip.

Resonant Raman and FTIR spectra of carbon doped GaN

NASA Astrophysics Data System (ADS)

Ito, S.; Kobayashi, H.; Araki, K.; Suzuki, K.; Sawaki, N.; Yamashita, K.; Honda, Y.; Amano, H.

2015-03-01

Intentionally carbon (C) doped (0 0 0 1)GaN was grown using C2H2 on a sapphire substrate by metalorganic vapor phase epitaxy. Optical spectra of the heavily doped samples were investigated at room temperature. In Raman spectra excited by the 325 nm line of a He-Cd laser, multiple LO phonon scattering signals up to 7th order were observed, and the A1(LO) phonon energy was determined to be 737.5 cm-1 (91.45 meV). In infrared reflectance spectra, on the other hand, a local vibration mode was found at 777.5 cm-1, which is attributed to a Ga-C bond in the GaN matrix suggesting that the C sits on an N site (CN). In spite of the strong suggestion of CN, the samples did not show p-type conduction. Possible origin of the carrier compensation is discussed in relation to the enhancement of defect related yellow luminescence in the photoluminescence spectra.

X-Band Rapid-Scan Electron Paramagnetic Resonance of Radiation-Induced Defects in Tooth Enamel

PubMed Central

Yu, Zhelin; Romanyukha, Alexander; Eaton, Sandra S.; Eaton, Gareth R.

2015-01-01

X-band rapid-scan electron paramagnetic resonance (EPR) spectra from tooth enamel samples irradiated with doses of 0.5, 1 and 10 Gy had substantially improved signal-to-noise relative to conventional continuous wave EPR. The radiation-induced signal in 60 mg of a tooth enamel sample irradiated with a 0.5 Gy dose was readily characterized in spectra recorded with 34 min data acquisition times. The coefficient of variance of the calculated dose for a 1 Gy irradiated sample, based on simulation of the first-derivative spectra for three replicates as the sum of native and radiation-induced signals, was 3.9% for continuous wave and 0.4% for rapid scan. PMID:26207683

Landau quantization of Dirac fermions in graphene and its multilayers

NASA Astrophysics Data System (ADS)

Yin, Long-Jing; Bai, Ke-Ke; Wang, Wen-Xiao; Li, Si-Yu; Zhang, Yu; He, Lin

2017-08-01

When electrons are confined in a two-dimensional (2D) system, typical quantum-mechanical phenomena such as Landau quantization can be detected. Graphene systems, including the single atomic layer and few-layer stacked crystals, are ideal 2D materials for studying a variety of quantum-mechanical problems. In this article, we review the experimental progress in the unusual Landau quantized behaviors of Dirac fermions in monolayer and multilayer graphene by using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). Through STS measurement of the strong magnetic fields, distinct Landau-level spectra and rich level-splitting phenomena are observed in different graphene layers. These unique properties provide an effective method for identifying the number of layers, as well as the stacking orders, and investigating the fundamentally physical phenomena of graphene. Moreover, in the presence of a strain and charged defects, the Landau quantization of graphene can be significantly modified, leading to unusual spectroscopic and electronic properties.

Vertical growth of ZnO nanorods on ZnO seeded FTO substrate for dye sensitized solar cells

NASA Astrophysics Data System (ADS)

Marimuthu, T.; Anandhan, N.

2018-04-01

Zinc oxide (ZnO) nanorods (NRs) were electrochemically grown on fluorine doped tin oxide (FTO) and ZnO seeded FTO substrates. X-ray diffraction (XRD) patterns, Raman spectra and photoluminescence (PL) spectra reveal that the hexagonal wurtzite structured ZnO grown on a seeded FTO substrate has a high crystallinity, crystal quality and less atomic defects. Felid emission scanning electron microscope (FE-SEM) images display a high growth density of NRs grown on seeded FTO substrate compared to NRs grown on FTO substrate. The efficiency of the DSSCs based on NRs grown on FTO and seeded FTO substrates is 0.85 and 1.52 %, respectively. UV-Vis absorption spectra and electrochemical impedance spectra depict that the NRs grown on seeded FTO photoanode have higher dye absorption and charge recombination resistance than that of the NRs grown on FTO substrate.

Bias-polarity-dependent UV/visible transferable electroluminescence from ZnO nanorod array LED with graphene oxide electrode supporting layer

NASA Astrophysics Data System (ADS)

Liu, Weizhen; Wang, Wei; Xu, Haiyang; Li, Xinghua; Yang, Liu; Ma, Jiangang; Liu, Yichun

2015-09-01

A simple top electrode preparation process, employing continuous graphene oxide films as electrode supporting layers, was adopted to fabricate a ZnO nanorod array/p-GaN heterojunction LED. The achieved LED demonstrated different electroluminescence behaviors under forward and reverse biases: a yellow-red emission band was observed under forward bias, whereas a blue-UV emission peak was obtained under reverse bias. Electroluminescence spectra under different currents and temperatures, as well as heterojunction energy-band alignments, reveal that the yellow-red emission under forward bias originates from recombinations related to heterointerface defects, whereas the blue-UV electroluminescence under reverse bias is ascribed to transitions from near-band-edge and Mg-acceptor levels in p-GaN.

First principles calculation of material properties of group IV elements and III-V compounds

NASA Astrophysics Data System (ADS)

Malone, Brad Dean

This thesis presents first principles calculations on the properties of group IV elements and group III-V compounds. It includes investigations into what structure a material is likely to form in, and given that structure, what are its electronic, optical, and lattice dynamical properties as well as what are the properties of defects that might be introduced into the sample. The thesis is divided as follows: • Chapter 1 contains some of the conceptual foundations used in the present work. These involve the major approximations which allow us to approach the problem of systems with huge numbers of interacting electrons and atomic cores. • Then, in Chapter 2, we discuss one of the major limitations to the DFT formalism introduced in Chapter 1, namely its inability to predict the quasiparticle spectra of materials and in particular the band gap of a semiconductor. We introduce a Green's function approach to the electron self-energy Sigma known as the GW approximation and use it to compute the quasiparticle band structures of a number of group IV and III-V semiconductors. • In Chapter 3 we present a first-principles study of a number of high-pressure metastable phases of Si with tetrahedral bonding. The phases studied include all experimentally determined phases that result from decompression from the metallic beta-Sn phase, specifically the BC8 (Si-III), hexagonal diamond (Si-IV), and R8 (Si-XII). In addition to these, we also study the hypothetical ST12 structure found upon decompression from beta-Sn in germanium. • Our attention is then turned to the first principles calculations of optical properties in Chapter 4. The Bethe-Salpeter equation is then solved to obtain the optical spectrum of this material including electron-hole interactions. The calculated optical spectrum is compared with experimental data for other forms of silicon commonly used in photovoltaic devices, namely the cubic, polycrystalline, and amorphous forms. • In Chapter 5 we present first principles calculations of the quasiparticle and optical excitation spectra of recently predicted silicon and germanium polytypes in the body-centered-tetragonal (bct) structure. The quasiparticle spectra calculated within the GW approximation predict that both silicon and germanium in the bct structure are small band gap materials. The optical spectra are then evaluated by solving the Bethe-Salpeter equation taking into account. • We examine the low-pressure phases of Ge in Chapter 6 by performing first principles calculations of the electronic structure and lattice dynamics of the R8, BC8, ST12, and hexagonal diamond structures of Ge. To aid future experimental investigation, we include predictions of the Raman-active frequencies of these phases as well as present the full phonon dispersion throughout the zone. • In Chapter 7 we demonstrate how first principles calculations can be used to predict new structures. In a study aimed at finding new useful forms of silicon, we use an ab initio random structure searching (AIRSS) method to identify a new phase of silicon in the Ibamstructure. The Ibam phase is found to be semimetallic within density functional theory with a small band overlap, and it is expected that quasiparticle corrections using the GW approximation would yield a semiconducting state with a small band gap. • We present a first-principles study of boron and phosphorus substitutional defects in Si-XII in Chapter 8. Recent result from nanoindentation experiments reveal that the Si-XII phase is semiconducting and has the interesting property that it can be doped n- and p-type at room temperature without an annealing step. Using the hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE), we examine the formation energies of the B and P defects at the two distinct atomic sites in Si-XII to find on which site the substitutional defects are more easily accommodated. We also estimate the thermodynamic transition levels of each defect in its relevant charge states. (Abstract shortened by UMI.).

Investigation of the Photocurrent in Hot-Wall-Epitaxy-Grown BaIn2S4 Layers

NASA Astrophysics Data System (ADS)

You, S. H.; Hong, K. J.; Jeong, T. S.; Youn, C. J.

2015-12-01

The photocurrent (PC) of hot-wall-epitaxy-grown BaIn2S4 layers was studied at different temperatures and for different photoresponse intensities. With increasing temperature, the position of the PC spectra tended to shift toward longer wavelength. These PC peaks corresponded to band-to-band transitions caused by intrinsic transitions from the valence band states to the conduction band states. Also, the bandgap variations were well matched by the equation E g( T) = E g(0) - 3.79 × 10-3 T 2/( T + 499), where E g(0) was estimated to be 3.0597 eV, 3.2301 eV, and 3.2606 eV for transitions corresponding to the valence band states Γ 4(z), Γ 5(x), and Γ 5(y), respectively. By use of the selection rule and results from the PC spectroscopy, the crystal field and the spin-orbit splitting were found to be 0.1703 and 0.0306 eV, respectively. Thus, the PC intensity gradually decreased with decreasing temperature. The decrease of PC intensity was caused by the presence of trapping centers associated with native defects in the BaIn2S4 layers. The trap level was found to be a shallow donor-level type of 20.4 meV, 1.6 meV below the conduction band. Consequently, these trap levels, which are related to native defects in BaIn2S4 layers, are believed to limit PC intensity with decreasing temperature.

Synthesis and influence of ultrasonic treatment on luminescence of Mn incorporated ZnS nanoparticles

NASA Astrophysics Data System (ADS)

Cadis, A.-I.; Muresan, L. E.; Perhaita, I.; Munteanu, V.; Karabulut, Y.; Garcia Guinea, J.; Canimoglu, A.; Ayvacikli, M.; Can, N.

2017-10-01

Manganese (Mn) doping of ZnS phosphors was achieved by precipitation method using different ultrasound (US) maturation times. The structural and luminescence properties of the samples were carried out by means of X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), photoluminescence (PL), and cathodoluminescence (CL). The real amount of manganese incorporated in ZnS lattice was calculated based on ICP-OES results. According with XRD patterns, the phase structure of ZnS:Mn samples is cubic. EDS spectra reveal deviations of the Mn dopant concentration from the target composition. Both 300 K PL and CL emission spectra of the Mn doped ZnS phosphors display intense orange emission at 590 and 600 nm, respectively, which is characteristic emission of Mn ion corresponding to a 4T1→6A1 transition. Both PL and CL spectra confirmed manganese is substitutionally incorporated into the ZnS host as Mn2+. However, it is suggested that the origin of broad blue emission around 400 nm appeared in CL is due to the radiative recombination at deep level defect states in the ZnS. The ultrasound treatment at first enhances the luminescent intensity by ∼3 times in comparison with samples prepared by classical way. This study gives rise to an optimization guideline, which is extremely demanded for the development of new luminescent materials.

Effects of thread interruptions on tool pins in friction stir welding of AA6061

DOE PAGES

Reza-E-Rabby, Md.; Tang, Wei; Reynolds, Anthony P.

2017-06-21

In this paper, effects of pin thread and thread interruptions (flats) on weld quality and process response parameters during friction stir welding (FSW) of 6061 aluminium alloy were quantified. Otherwise, identical smooth and threaded pins with zero to four flats were adopted for FSW. Weldability and process response variables were examined. Results showed that threads with flats significantly improved weld quality and reduced in-plane forces. A three-flat threaded pin led to production of defect-free welds under all examined welding conditions. Spectral analyses of in-plane forces and weld cross-sectional analysis were performed to establish correlation among pin flats, force dynamics andmore » defect formation. Finally, the lowest in-plane force spectra amplitudes were consistently observed for defect-free welds.« less

Effects of thread interruptions on tool pins in friction stir welding of AA6061

DOE Office of Scientific and Technical Information (OSTI.GOV)

Reza-E-Rabby, Md.; Tang, Wei; Reynolds, Anthony P.

In this paper, effects of pin thread and thread interruptions (flats) on weld quality and process response parameters during friction stir welding (FSW) of 6061 aluminium alloy were quantified. Otherwise, identical smooth and threaded pins with zero to four flats were adopted for FSW. Weldability and process response variables were examined. Results showed that threads with flats significantly improved weld quality and reduced in-plane forces. A three-flat threaded pin led to production of defect-free welds under all examined welding conditions. Spectral analyses of in-plane forces and weld cross-sectional analysis were performed to establish correlation among pin flats, force dynamics andmore » defect formation. Finally, the lowest in-plane force spectra amplitudes were consistently observed for defect-free welds.« less

Vacancy-type defects in Mg-doped GaN grown by ammonia-based molecular beam epitaxy probed using a monoenergetic positron beam

DOE Office of Scientific and Technical Information (OSTI.GOV)

Uedono, Akira; Malinverni, Marco; Martin, Denis

Vacancy-type defects in Mg-doped GaN were probed using a monoenergetic positron beam. GaN films with a thickness of 0.5–0.7 μm were grown on GaN/sapphire templates using ammonia-based molecular beam epitaxy and characterized by measuring Doppler broadening spectra. Although no vacancies were detected in samples with a Mg concentration [Mg] below 7 × 10{sup 19 }cm{sup −3}, vacancy-type defects were introduced starting at above [Mg] = 1 × 10{sup 20 }cm{sup −3}. The major defect species was identified as a complex between Ga vacancy (V{sub Ga}) and multiple nitrogen vacancies (V{sub N}s). The introduction of vacancy complexes was found to correlate with a decreasemore » in the net acceptor concentration, suggesting that the defect introduction is closely related to the carrier compensation. We also investigated Mg-doped GaN layers grown using In as the surfactant. The formation of vacancy complexes was suppressed in the subsurface region (≤80 nm). The observed depth distribution of defects was attributed to the thermal instability of the defects, which resulted in the introduction of vacancy complexes during the deposition process.« less

Direct Observation of Defect Range and Evolution in Ion-Irradiated Single Crystalline Ni and Ni Binary Alloys.

PubMed

Lu, Chenyang; Jin, Ke; Béland, Laurent K; Zhang, Feifei; Yang, Taini; Qiao, Liang; Zhang, Yanwen; Bei, Hongbin; Christen, Hans M; Stoller, Roger E; Wang, Lumin

2016-02-01

Energetic ions have been widely used to evaluate the irradiation tolerance of structural materials for nuclear power applications and to modify material properties. It is important to understand the defect production, annihilation and migration mechanisms during and after collision cascades. In this study, single crystalline pure nickel metal and single-phase concentrated solid solution alloys of 50%Ni50%Co (NiCo) and 50%Ni50%Fe (NiFe) without apparent preexisting defect sinks were employed to study defect dynamics under ion irradiation. Both cross-sectional transmission electron microscopy characterization (TEM) and Rutherford backscattering spectrometry channeling (RBS-C) spectra show that the range of radiation-induced defect clusters far exceed the theoretically predicted depth in all materials after high-dose irradiation. Defects in nickel migrate faster than in NiCo and NiFe. Both vacancy-type stacking fault tetrahedra (SFT) and interstitial loops coexist in the same region, which is consistent with molecular dynamics simulations. Kinetic activation relaxation technique (k-ART) simulations for nickel showed that small vacancy clusters, such as di-vacancies and tri-vacancies, created by collision cascades are highly mobile, even at room temperature. The slower migration of defects in the alloy along with more localized energy dissipation of the displacement cascade may lead to enhanced radiation tolerance.

Direct Observation of Defect Range and Evolution in Ion-Irradiated Single Crystalline Ni and Ni Binary Alloys

PubMed Central

Lu, Chenyang; Jin, Ke; Béland, Laurent K.; Zhang, Feifei; Yang, Taini; Qiao, Liang; Zhang, Yanwen; Bei, Hongbin; Christen, Hans M.; Stoller, Roger E.; Wang, Lumin

2016-01-01

Energetic ions have been widely used to evaluate the irradiation tolerance of structural materials for nuclear power applications and to modify material properties. It is important to understand the defect production, annihilation and migration mechanisms during and after collision cascades. In this study, single crystalline pure nickel metal and single-phase concentrated solid solution alloys of 50%Ni50%Co (NiCo) and 50%Ni50%Fe (NiFe) without apparent preexisting defect sinks were employed to study defect dynamics under ion irradiation. Both cross-sectional transmission electron microscopy characterization (TEM) and Rutherford backscattering spectrometry channeling (RBS-C) spectra show that the range of radiation-induced defect clusters far exceed the theoretically predicted depth in all materials after high-dose irradiation. Defects in nickel migrate faster than in NiCo and NiFe. Both vacancy-type stacking fault tetrahedra (SFT) and interstitial loops coexist in the same region, which is consistent with molecular dynamics simulations. Kinetic activation relaxation technique (k-ART) simulations for nickel showed that small vacancy clusters, such as di-vacancies and tri-vacancies, created by collision cascades are highly mobile, even at room temperature. The slower migration of defects in the alloy along with more localized energy dissipation of the displacement cascade may lead to enhanced radiation tolerance. PMID:26829570

Grand unified theories, topological defects, and ultrahigh-energy cosmic rays

NASA Technical Reports Server (NTRS)

Bhattacharjee, Pijushpani; Hill, Christopher T.; Schramm, David N.

1992-01-01

The ultrahigh-energy (UHE) proton and neutrino spectra resulting from collapse or annihilations of topological defects surviving from the GUT era are calculated. Irrespective of the specific process under consideration (which determines the overall normalization of the spectrum), the UHE proton spectrum always 'recovers' at approximately 1.8 x 10 exp 11 GeV after a partial Greisen-Zatsepin-Kuz'min 'cutoff' at approximately 5 x 10 exp 10 GeV and continues to a GUT-scale energy with a universal shape determined by the physics of hadronic jet fragmentation. Implications of the results are discussed.

Acceptors in ZnO nanocrystals: A reinterpretation

NASA Astrophysics Data System (ADS)

Gehlhoff, W.; Hoffmann, A.

2012-12-01

In a recent article, Teklemichael et al. reported on the identification of an uncompensated acceptor in ZnO nanocrystals using infrared spectroscopy and electron paramagnetic resonance (EPR) in the dark and under illumination. Most of their conclusions, interpretations, and suggestions turned out to be erroneous. The observed EPR signals were interpreted to originate from axial and nonaxial VZn-H defects. We show that the given interpretation of the EPR results is based on misinterpretations of EPR spectra arising from defects in nanocrystals. The explanation of the infrared absorption lines is in conflict with recent results of valence band ordering and valence band splitting.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Koyanagi, T.; Lance, M. J.; Katoh, Y.

Raman spectra from polycrystalline beta-silicon carbide (SiC) were collected following neutron irradiation at 380–1180 °C to 0.011–1.87 displacement per atom. The longitudinal optical (LO) peak shifted to a lower frequency and broadened as a result of the irradiation. The changes observed in the LO phonon line shape and position in neutron-irradiated SiC are explained by a combination of changes in the lattice constant and Young's modulus, and the phonon confinement effect. The phonon confinement model reasonably estimates the defect-defect distance in the irradiated SiC, which is consistent with results from previous experimental studies and simulations.

Grand unified theories, topological defects and ultrahigh-energy cosmic rays

NASA Technical Reports Server (NTRS)

Bhattacharjee, Pijushpani; Hill, Christopher T.; Schramm, David N.

1991-01-01

The ultrahigh-energy (UHE) proton and neutrino spectra resulting from collapse or annihilations of topological defects surviving from the GUT era are calculated. Irrespective of the specific process under consideration (which determines the overall normalization of the spectrum), the UHE proton spectrum always 'recovers' at approximately 1.8 x 10 exp 11 GeV after a partial Greisen-Zatsepin-Kuz'min 'cutoff' at approximately 5 x 10 exp 10 GeV and continues to a GUT-scale energy with a universal shape determined by the physics of hadronic jet fragmentation. Implications of our results are discussed.

New researches on teratogenic effects of singulair using optical methods

NASA Astrophysics Data System (ADS)

Valiulis, A.; Gaidelis, P.; Januskevicius, A.

2005-11-01

It was proved that singulair does not cause macroscopical and histological alterations on the animals, but it is not known its influence on the molecular level. The purpose of our work is to explore teratogenic effects of singulair on the animals by using our non-standard method (which is preciser than standard method, because it showes not only macroscopical, but also and molecular alterations in the damage tissue), hoping, that it would be the help for the pregnant women to decide can they take this drug or not. We injected singulair (25 mg/kg of rat) to one group of pregnant rats in rareripe period (from the first to the eight day) of embryogenesis. Another group of pregnant rats got injections of teratogenic substance ochratoxin A (25 pg/kg of rat) in the same period of embryogenesis. The third group got injections of physiological solution. There were taken 10 rats to everyone group. In the end of rareripe period we took a blood sample from each rat of the rats and wrote a spectrum of everyone sample by using spectrophotometer. We got, that spectra of solutions from the first and the third group were very similar (there were no statistical difference) and spectra of solutions from the second group were very similar (there were no statistical difference), but statistical difference was between the first and the third group together and the second group in the zone of 200-350 nm. At the twentieth day of pregnancy we made Caesarean section to all of the rats, compared embryos interpendent and did not find any defects in anyone of them through all the first and the third group, but everyone embryo from the second group had a lot of defects.

Surface Defects Enhanced Visible Light Photocatalytic H2 Production for Zn-Cd-S Solid Solution.

PubMed

Zhang, Xiaoyan; Zhao, Zhao; Zhang, Wanwan; Zhang, Guoqiang; Qu, Dan; Miao, Xiang; Sun, Shaorui; Sun, Zaicheng

2016-02-10

In order to investigate the defect effect on photocatalytic performance of the visible light photocatalyst, Zn-Cd-S solid solution with surface defects is prepared in the hydrazine hydrate. X-ray photoelectron spectra and photoluminescence results confirm the existence of defects, such as sulfur vacancies, interstitial metal, and Zn and Cd in the low valence state on the top surface of solid solutions. The surface defects can be effectively removed by treating with sulfur vapor. The solid solution with surface defect exhibits a narrower band gap, wider light absorption range, and better photocatalytic perfomance. The optimized solid solution with defects exhibits 571 μmol h(-1) for 50 mg photocatalyst without loading Pt as cocatalyst under visible light irradiation, which is fourfold better than that of sulfur vapor treated samples. The wavelength dependence of photocatalytic activity discloses that the enhancement happens at each wavelength within the whole absorption range. The theoretical calculation shows that the surface defects induce the conduction band minimum and valence band maximum shift downward and upward, respectively. This constructs a type I junction between bulk and surface of solid solution, which promotes the migration of photogenerated charges toward the surface of nanostructure and leads to enhanced photocatalytic activity. Thus a new method to construct highly efficient visible light photocatalysts is opened. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The effects of anode material type on the optoelectronic properties of electroplated CdTe thin films and the implications for photovoltaic application

NASA Astrophysics Data System (ADS)

Echendu, O. K.; Dejene, B. F.; Dharmadasa, I. M.

2018-03-01

The effects of the type of anode material on the properties of electrodeposited CdTe thin films for photovoltaic application have been studied. Cathodic electrodeposition of two sets of CdTe thin films on glass/fluorine-doped tin oxide (FTO) was carried out in two-electrode configuration using graphite and platinum anodes. Optical absorption spectra of films grown with graphite anode displayed significant spread across the deposition potentials compared to those grown with platinum anode. Photoelectrochemical cell result shows that the CdTe grown with graphite anode became p-type after post-deposition annealing with prior CdCl2 treatment, as a result of carbon incorporation into the films, while those grown with platinum anode remained n-type after annealing. A review of recent photoluminescence characterization of some of these CdTe films reveals the persistence of a defect level at (0.97-0.99) eV below the conduction band in the bandgap of CdTe grown with graphite anode after annealing while films grown with platinum anode showed the absence of this defect level. This confirms the impact of carbon incorporation into CdTe. Solar cell made with CdTe grown with platinum anode produced better conversion efficiency compared to that made with CdTe grown using graphite anode, underlining the impact of anode type in electrodeposition.

Electron paramagnetic resonance of deep boron in silicon carbide

NASA Astrophysics Data System (ADS)

Baranov, P. G.; Mokhov, E. N.

1996-04-01

In this article we report the first EPR observation of deep boron centres in silicon carbide. A direct identification of the boron atom involved in the defect centre, considered as deep boron, has been established by the presence of a hyperfine interaction with 0268-1242/11/4/005/img1 and 0268-1242/11/4/005/img2 nuclei in isotope-enriched 6H-SiC:B crystals. Deep boron centres were shown from EPR spectra to have axial symmetry along the hexagonal axis. A correspondence between the EPR spectra and the luminescence, ODMR and DLTS spectra of deep boron centres has been indicated. The structural model for a deep boron centre as a boron - vacancy pair is presented and the evidence for bistable behaviour of deep boron centres is discussed.

Ion channeling study of defects in compound crystals using Monte Carlo simulations

NASA Astrophysics Data System (ADS)

Turos, A.; Jozwik, P.; Nowicki, L.; Sathish, N.

2014-08-01

Ion channeling is a well-established technique for determination of structural properties of crystalline materials. Defect depth profiles have been usually determined basing on the two-beam model developed by Bøgh (1968) [1]. As long as the main research interest was focused on single element crystals it was considered as sufficiently accurate. New challenge emerged with growing technological importance of compound single crystals and epitaxial heterostructures. Overlap of partial spectra due to different sublattices and formation of complicated defect structures makes the two beam method hardly applicable. The solution is provided by Monte Carlo computer simulations. Our paper reviews principal aspects of this approach and the recent developments in the McChasy simulation code. The latter made it possible to distinguish between randomly displaced atoms (RDA) and extended defects (dislocations, loops, etc.). Hence, complex defect structures can be characterized by the relative content of these two components. The next refinement of the code consists of detailed parameterization of dislocations and dislocation loops. Defect profiles for variety of compound crystals (GaN, ZnO, SrTiO3) have been measured and evaluated using the McChasy code. Damage accumulation curves for RDA and extended defects revealed non monotonous defect buildup with some characteristic steps. Transition to each stage is governed by the different driving force. As shown by the complementary high resolution XRD measurements lattice strain plays here the crucial role and can be correlated with the concentration of extended defects.

The pressure-induced structural response of rare earth hafnate and stannate pyrochlore from 0.1-50 GPa

NASA Astrophysics Data System (ADS)

Turner, K. M.; Rittman, D.; Heymach, R.; Turner, M.; Tracy, C.; Mao, W. L.; Ewing, R. C.

2017-12-01

Complex oxides with the pyrochlore (A2B2O7) and defect-fluorite ((A,B)4O7) structure-types undergo structural transformations under high-pressure. These compounds are under consideration for applications including as a proposed waste-form for actinides generated in the nuclear fuel cycle. High-pressure transformations in rare earth hafnates (A2Hf2O7, A=Sm, Eu, Gd, Dy, Y, Yb) and stannates (A2Sn2O7, A=Nd, Gd, Er) were investigated to 50 GPa by in situ Raman spectroscopy and synchrotron x-ray diffraction (XRD). Rare-earth hafnates form the pyrochlore structure for A=La-Tb and the defect-fluorite structure for A=Dy-Lu. Lanthanide stannates form the pyrochlore structure. Raman spectra revealed that at ambient pressure all compositions have pyrochlore-type short-range order. Stannate compositions show a larger degree of pyrochlore-type short-range ordering relative to hafnates. In situ high-pressure synchrotron XRD showed that rare earth hafnates and stannates underwent a pressure-induced phase transition to a cotunnite-like (Pnma) structure that begins between 18-25 GPa in hafnates and between 30-33 GPa in stannates. The phase transition is not complete at 50 GPa, and upon decompression, XRD indicates that all compositions transform to defect-fluorite with an amorphous component. In situ Raman spectroscopy showed that disordering in stannates and hafnates occurs gradually upon compression. Pyrochlore-structured hafnates retain short-range order to a higher pressure (30 GPa vs. <10 GPa) than defect-fluorite-structured hafnates. Hafnates and stannates decompressed from 50 GPa show Raman spectra consistent with weberite-type structures, also reported in irradiated stannates. The second-order Birch-Murnaghan equation of state fit gives a bulk modulus of 250 GPa for hafnate compositions with the pyrochlore structure, and 400 GPa for hafnate compositions with the defect-fluorite structure. Stannates have a lower bulk modulus relative to hafnates (between 80-150 GPa). Stannate and hafnate pyrochlore compositions taken to high pressure show structural transformations consistent with irradiated pyrochlore, and compositionally disordered pyrochlore: a long-range structure best described by defect-fluorite, and a short-range structure best described by weberite.

On the validity of the amphoteric-defect model in gallium arsenide and a criterion for Fermi-level pinning by defects

NASA Astrophysics Data System (ADS)

Chen, C.-H.; Tan, T. Y.

1995-10-01

Using the theoretically calculated point-defect total-energy values of Baraff and Schlüter in GaAs, an amphoteric-defect model has been proposed by Walukiewicz to explain a large number of experimental results. The suggested amphoteric-defect system consists of two point-defect species capable of transforming into each other: the doubly negatively charged Ga vacancy V {Ga/2-} and the triply positively charged defect complex (ASGa+ V As)3+, with AsGa being the antisite defect of an As atom occupying a Ga site and V As being an As vacancy. When present in sufficiently high concentrations, the amphoteric defect system V {Ga/2-}/(AsGa+ V As)3+ is supposed to be able to pin the GaAs Fermi level at approximately the E v +0.6 eV level position, which requires that the net free energy of the V Ga/(AsGa+ V As) defect system to be minimum at the same Fermi-level position. We have carried out a quantitative study of the net energy of this defect system in accordance with the individual point-defect total-energy results of Baraff and Schlüter, and found that the minimum net defect-system-energy position is located at about the E v +1.2 eV level position instead of the needed E v +0.6 eV position. Therefore, the validity of the amphoteric-defect model is in doubt. We have proposed a simple criterion for determining the Fermi-level pinning position in the deeper part of the GaAs band gap due to two oppositely charged point-defect species, which should be useful in the future.

Formation and evolution of oxygen-vacancy clusters in lead and tin doped silicon

NASA Astrophysics Data System (ADS)

Londos, C. A.; Aliprantis, D.; Sgourou, E. N.; Chroneos, A.; Pochet, P.

2012-06-01

Infrared spectroscopy (IR) measurements were used to investigate the effect of lead (Pb), tin (Sn), and (Pb, Sn) codoping on electron radiation-induced defects in silicon (Si). The study was mainly focused on oxygen-vacancy (VOn) clusters and in particular their formation and evolution upon annealing. It was determined that Pb causes a larger reduction in the production of the VO defect than Sn. In (Pb, Sn) co-doped Si isochronal anneals revealed that the evolution of VO increases substantially at ˜170 °C. This is attributed to the release of V from the SnV pair. Interestingly, in the corresponding evolution curves of VO in the Sn- and the Pb-doped samples, this inverse annealing stage is also present for the former while it is not present for the latter. This is attributed to the formation of PbV pairs that do not dissociate below 280 °C. The partial capture of V by Sn in co-doped samples is rationalized through the higher compressive local strain around Pb atoms that leads to a retardation of vacancy diffusion. The conversion of VO to the VO2 defect is substantially reduced in the Pb-doped sample. The evolution curves of VO and VO2 clusters in the isovalent doped Si samples hint the production of VO2 from other mechanisms (i.e., besides VO + Oi → VO2). For larger VOn clusters (n = 3,4), the signals are very weak in the Pb-doped sample, whereas for n ≥ 5, they are not present in the spectra. Conversely, bands related with the VO5 and VOnCs defects are present in the spectra of the Sn-doped and (Pb, Sn) codoped Si.

Investigation on tip enhanced Raman spectra of graphene

NASA Astrophysics Data System (ADS)

Li, Xinjuan; Liu, Yanqi; Zeng, Zhuo; Wang, Peijie; Fang, Yan; Zhang, Lisheng

2018-02-01

Tip-enhanced Raman scattering (TERS) is a promising analytical approach for some two-dimensional materials and offers the possibility to correlate imaging and chemical data. Tip-enhanced Raman spectra of graphene are discussed in some details, including substrate, gap between tip-apex and sample surface as well as Ag-nanowire. The TERS spectra give special emphasis to the possibility of TERS tip to induce a large number of defects only while got the tip attached to sample surface. Then the dependence of the TERS spectra of graphene and gap between the probe tip and sample surface was studied, and distribution features of electromagnetic (EM) field around tip were also simulated by finite-difference time-domain (FDTD). The Raman signal enhancement of graphene was further discussed with respect to experimental data. Furthermore, the Ag-nanowire as a nano-antenna could significantly enhance the weak Raman signal of D-band of monolayer graphene is shown, and the TERS spectra of graphene with regard to different regions of Ag-nanowires (endpoints, body) were obtained toward investigating into the distribution of electromagnetic field.

Activation of CO and CO2 on homonuclear boron bonds of fullerene-like BN cages: first principles study

NASA Astrophysics Data System (ADS)

Sinthika, S.; Kumar, E. Mathan; Surya, V. J.; Kawazoe, Y.; Park, Noejung; Iyakutti, K.; Thapa, Ranjit

2015-12-01

Using density functional theory we investigate the electronic and atomic structure of fullerene-like boron nitride cage structures. The pentagonal ring leads to the formation of homonuclear bonds. The homonuclear bonds are also found in other BN structures having pentagon line defect. The calculated thermodynamics and vibrational spectra indicated that, among various stable configurations of BN-60 cages, the higher number of homonuclear N-N bonds and lower B:N ratio can result in the more stable structure. The homonuclear bonds bestow the system with salient catalytic properties that can be tuned by modifying the B atom bonding environment. We show that homonuclear B-B (B2) bonds can anchor both oxygen and CO molecules making the cage to be potential candidates as catalyst for CO oxidation via Langmuir-Hinshelwood (LH) mechanism. Moreover, the B-B-B (B3) bonds are reactive enough to capture, activate and hydrogenate CO2 molecules to formic acid. The observed trend in reactivity, viz B3 > B2 > B1 is explained in terms of the position of the boron defect state relative to the Fermi level.

Activation of CO and CO2 on homonuclear boron bonds of fullerene-like BN cages: first principles study

PubMed Central

Sinthika, S.; Kumar, E. Mathan; Surya, V. J.; Kawazoe, Y.; Park, Noejung; Iyakutti, K.; Thapa, Ranjit

2015-01-01

Using density functional theory we investigate the electronic and atomic structure of fullerene-like boron nitride cage structures. The pentagonal ring leads to the formation of homonuclear bonds. The homonuclear bonds are also found in other BN structures having pentagon line defect. The calculated thermodynamics and vibrational spectra indicated that, among various stable configurations of BN-60 cages, the higher number of homonuclear N-N bonds and lower B:N ratio can result in the more stable structure. The homonuclear bonds bestow the system with salient catalytic properties that can be tuned by modifying the B atom bonding environment. We show that homonuclear B-B (B2) bonds can anchor both oxygen and CO molecules making the cage to be potential candidates as catalyst for CO oxidation via Langmuir–Hinshelwood (LH) mechanism. Moreover, the B-B-B (B3) bonds are reactive enough to capture, activate and hydrogenate CO2 molecules to formic acid. The observed trend in reactivity, viz B3 > B2 > B1 is explained in terms of the position of the boron defect state relative to the Fermi level. PMID:26626147

Activation of CO and CO2 on homonuclear boron bonds of fullerene-like BN cages: first principles study.

PubMed

Sinthika, S; Kumar, E Mathan; Surya, V J; Kawazoe, Y; Park, Noejung; Iyakutti, K; Thapa, Ranjit

2015-12-02

Using density functional theory we investigate the electronic and atomic structure of fullerene-like boron nitride cage structures. The pentagonal ring leads to the formation of homonuclear bonds. The homonuclear bonds are also found in other BN structures having pentagon line defect. The calculated thermodynamics and vibrational spectra indicated that, among various stable configurations of BN-60 cages, the higher number of homonuclear N-N bonds and lower B:N ratio can result in the more stable structure. The homonuclear bonds bestow the system with salient catalytic properties that can be tuned by modifying the B atom bonding environment. We show that homonuclear B-B (B2) bonds can anchor both oxygen and CO molecules making the cage to be potential candidates as catalyst for CO oxidation via Langmuir-Hinshelwood (LH) mechanism. Moreover, the B-B-B (B3) bonds are reactive enough to capture, activate and hydrogenate CO2 molecules to formic acid. The observed trend in reactivity, viz B3 > B2 > B1 is explained in terms of the position of the boron defect state relative to the Fermi level.

Defects, Tunneling, and EPR Spectra of Single-Molecule Magnets

DTIC Science & Technology

2003-01-01

Caranin, Phys. Rev. Lett. 87, 187203 (2001); Phys. Rev. B 65, 094423 (2002). 19. A. Cornia, R. Sessoli, L. Sorace, D. Gatteschi , A. L. Barra, and C...Phys. Rev. B 64, 184426 (2001). 25. A. Mukhin, B. Gorshunov, M. Dressel, C. Sangregorio. and D. Gatteschi , Phys. Rev. B 63, 214411 (2001). 26. W

The application of hypserspectral imaging analysis to fresh food safety inspection

USDA-ARS?s Scientific Manuscript database

Line-scan hyperspectral images of fresh matured tomatoes were collected for image analysis. Algorithms were developed, based on spectral analysis, to detect defect of cracks on fresh produce. Four wavebands of 569 nm, 645 nm, 702 nm and 887 nm were selected from spectra analysis to use the relative...

The effect of reaction temperature on the room temperature ferromagnetic property of sol-gel derived tin oxide nanocrystal

NASA Astrophysics Data System (ADS)

Sakthiraj, K.; Hema, M.; Balachandra Kumar, K.

2018-06-01

In the present study, nanocrystalline tin oxide materials were prepared using sol-gel method with different reaction temperatures (25 °C, 50 °C, 75 °C & 90 °C) and the relation between the room temperature ferromagnetic property of the sample with processing temperature has been analysed. The X-ray diffraction pattern and infrared absorption spectra of the as-prepared samples confirm the purity of the samples. Transmission electron microscopy images visualize the particle size variation with respect to reaction temperature. The photoluminescence spectra of the samples demonstrate that luminescence process in materials is originated due to the electron transition mediated by defect centres. The room temperature ferromagnetic property is observed in all the samples with different amount, which was confirmed using vibrating sample magnetometer measurements. The saturation magnetization value of the as-prepared samples is increased with increasing the reaction temperature. From the photoluminescence & magnetic measurements we accomplished that, more amount of surface defects like oxygen vacancy and tin interstitial are created due to the increase in reaction temperature and it controls the ferromagnetic property of the samples.

Interaction between confined phonons and photons in periodic silicon resonators

NASA Astrophysics Data System (ADS)

Iskandar, A.; Gwiazda, A.; Younes, J.; Kazan, M.; Bruyant, A.; Tabbal, M.; Lerondel, G.

2018-03-01

In this paper, we demonstrate that phonons and photons of different momenta can be confined and interact with each other within the same nanostructure. The interaction between confined phonons and confined photons in silicon resonator arrays is observed by means of Raman scattering. The Raman spectra from large arrays of dielectric silicon resonators exhibited Raman enhancement accompanied with a downshift and broadening. The analysis of the Raman intensity and line shape using finite-difference time-domain simulations and a spatial correlation model demonstrated an interaction between photons confined in the resonators and phonons confined in highly defective regions prompted by the structuring process. It was shown that the Raman enhancement is due to collective lattice resonance inducing field confinement in the resonators, while the spectra downshift and broadening are signatures of the relaxation of the phonon wave vector due to phonon confinement in defective regions located in the surface layer of the Si resonators. We found that as the resonators increase in height and their shape becomes cylindrical, the amplitude of their coherent oscillation increases and hence their ability to confine the incoming electric field increases.

Doping β-Ga2O3 with europium: influence of the implantation and annealing temperature

NASA Astrophysics Data System (ADS)

Peres, M.; Lorenz, K.; Alves, E.; Nogales, E.; Méndez, B.; Biquard, X.; Daudin, B.; Víllora, E. G.; Shimamura, K.

2017-08-01

β-Ga2O3 bulk single crystals were doped by ion implantation at temperatures from room temperature to 1000 °C, using a 300 keV Europium beam with a fluence of 1  ×  1015 at cm-2. Rising the implantation temperature from room temperature to 400-600 °C resulted in a significant increase of the substitutional Eu fraction and of the number of Eu ions in the 3+  charge state as well as in a considerable decrease of implantation damage. Eu is found in both charge states 2+  and 3+  and their relative fractions are critically dependent on the implantation and annealing temperature, suggesting that defects play an important role in stabilizing one of the charge states. The damage recovery during post-implant annealing is a complex process and typically defect levels first increase for intermediate annealing temperatures and a significant recovery of the crystal only starts around 1000 °C. Cathodoluminescence spectra are dominated by the sharp Eu3+ related intra-ionic 4f transition lines in the red spectral region. They show a strong increase of the emission intensity with increasing annealing temperature, in particular for samples implanted at elevated temperature, indicating the optical activation of Eu3+ ions. However, no direct correlation of emission intensity and Eu3+ fraction was found, again pointing to the important role of defects on the physical properties of these luminescent materials.

A comparison of the doppler-broadened positron annihilation spectra of neutron irradiated Al 2O 3 and MgAl 2O 3

NASA Astrophysics Data System (ADS)

Jones, P. L.; Schaffer, J. P.; Cocks, F. H.; Clinard, F. W.; Hurley, G. F.

1985-01-01

Radiation damage studies of oxides and ceramics have become of increasing importance due to the projected use of these materials in thermonuclear fusion reactors as electronic insulators and first wall materials. In addition these materials are important in RAD waste disposal. As part of a study of the defect structure in radiation damaged ceramics Doppler-broadened positron annihilation spectra have been obtained for a series of single crystal sapphire (α-Al 2O 3) and polycrystal (1:1) and (1:2) magnesium aluminate spinel (MgO·Al 2O 3 and MgO-2Al 2O 3) samples. These samples were irradiated in EBR-II to a fluence of 3 × 10 25 n/m 2 (E > 0.1 MeV) at 740°C, and 2 × 10 26 n/m 2 (E > 0.1 MeV) at ~ 550°C respectively. Positron annihilation spectra lineshapes for the irradiated, annealed, and as-received samples of both materials were compared using S parameter analysis. These calculations were made on deconvoluted gamma ray spectra that were free of any instrumental broadening effects. In this way, absolute S parameter changes could be calculated. The observed changes in the S parameter are consistent with independent volume swelling measurements for both the α-A1 2O 3 and the (1:2) MgAl 2O 4 samples. However, the change in S parameter measured for the (1:1) spinel is contrary to the measured volume change. This apparent anomaly indicates a predominence of interstitial as opposed to vacancy type defects in this material.

Li-Like CX

NASA Astrophysics Data System (ADS)

Dorsey, Zachary

2018-06-01

A fantastic discovery in 1996 caused an epiphany when it was realized that the charge exchange (CX) process was an important source of X-ray emission from comets. Over the last two decades, CX induced X-ray emission has been detected in a wide variety of solar and extra-solar sources. Recently, our group developed the Kronos database which contains CX cross sections and emission spectra for all H-like and most He-Like ions up to iron, based primarily on multichannel Landau-Zener calculations. Here we present preliminary results for extending Kronos to Li-like ions. Due to the incompleteness of highly-charged Li-like atomic structure data, we estimate missing energy levels and transition probabilities with a quantum defect approach and the package Autostructure.This work was support by NASA grants NNX13AF31G.

Native Defect Related Optical Properties of ZnGeP2

NASA Technical Reports Server (NTRS)

Dietz, N.; Tsveybak, I.; Ruderman, W.; Wood, G.; Bachmann, K. J.

1994-01-01

We present photoluminescence, photoconductivity, and optical absorption spectra for ZnGeP2 crystals grown from the melt by gradient freezing and from the vapor phase by high pressure physical vapor transport (HPVT). A model of donor and acceptor related subbands in the energy gap of ZnGeP2 is introduced that explains the experimental results. The emission with peak position at 1.2 eV is attributed to residual disorder on the cation sublattice. The lower absorption upon annealing is interpreted in terms of both the reduction of the disorder on the cation sublattice and changes in the Fermi level position. The n-type conductivity of ZnGeP2 Crystals grown under Ge-deficient conditions by the HPVT is related to the presence of additional donor states.

Effect of simvastatin versus low level laser therapy (LLLT) on bone regeneration in rabbit's tibia

NASA Astrophysics Data System (ADS)

Gheith, Mostafa E.; Khairy, Maggie A.

2014-02-01

Simvastatin is a cholesterol lowering drug which proved effective on promoting bone healing. Recently low level laser therapy (LLLT) proved its effect as a biostimulator promoting bone regeneration. This study aims to compare the effect of both Simvastatin versus low level laser on bone healing in surgically created bone defects in rabbit's tibia. Material and methods: The study included 12 New Zealand white rabbits. Three successive 3mm defects were created in rabbits tibia first defect was left as control, second defect was filled with Simvastatin while the third defect was acted on with Low Level Laser (optical fiber 320micrometer). Rabbits were sacrificed after 48 hours, 1 week and 2 weeks intervals. Histopathology was conducted on the three defects Results: The histopathologic studies showed that the bony defects treated with the Low Level Laser showed superior healing patterns and bone regeneration than those treated with Simvastatin. While the control defect showed the least healing pattern.

Photoluminescence and contactless electroreflectance characterization of BexCd1-xSe alloys

NASA Astrophysics Data System (ADS)

Huang, P. J.; Huang, Y. S.; Firszt, F.; Meczynska, H.; Maksimov, O.; Tamargo, M. C.; Tiong, K. K.

2007-01-01

A detailed optical characterization of a Bridgman-grown wurtzite- (WZ-) type Be0.075Cd0.925Se mixed crystal and three zinc-blende (ZB) BexCd1-xSe epilayers grown by MBE on InP substrates has been carried out via photoluminescence (PL) and contactless electroreflectance (CER) in the temperature range of 15-400 K. The PL spectrum of the WZ-BeCdSe at low temperature consists of an exciton line, an edge emission feature due to recombination of donor-acceptor pairs, and a broad band related to recombination through deep-level defects, while the PL emission peaks of the ZB-BeCdSe epilayers show an asymmetric shape with a tail on the low-energy side. Various interband transitions, originating from the band edge and spin-orbit splitting critical points, of the samples have been observed in the CER spectra. The peak positions of the exciton emission lines in the PL spectra correspond quite well to the energies of the fundamental transitions determined from electromodulation data. The parameters that describe the temperature dependence of the fundamental and spin split-off bandgaps and the broadening function of the band-edge exciton are evaluated and discussed.

[Fluorescence spectra and quantum yield of TiO2 nanocrystals synthesized by alcohothermal method].

PubMed

Song, Cui-Hong; Li, Yan-Ting; Li, Jing; Wei, Yong-Ju; Hu, Yu-Zhu; Wei, Yu

2008-01-01

Fluorescence spectra and fluorescence quantum yield of TiO2 nanocrystals were studied. Using tetra n-butyl titanate as a starting material, a facile alcohothermal technique was used to synthesize TiO2 nanocrystals. As can be seen from the transmittance electron microscopy (TEM) image, TiO2 nanocrystals with a relatively uniform particle size distribution of < 10 nm are present in the transparent sol. The transparent sol presents a strong stable fluorescence emission with a maximum at 450 nm, which is greatly dependent on the size quantization effects, defect energy level and the surface state of TiO2 nanocrystals. The quantum yield (gamma) of TiO2 was determined by the relative comparison procedure, using freshly prepared analytical purity quinine sulfate in 0.05 mol x L(-1) H2SO4 as a relative quantum yield standard. The emission quantum yield of TiO2 nanocrystals prepared in alcoholic media was calculated to be about 0.20 at wavelengths ranging from 330 to 370 nm, which was much higher than the values reported in previous works. So, it is supposed that nano-TiO2 will be applied as a potential quantum dots fluorescence probe in biological analysis.

Effect of temperature on the spectral properties of InP/ZnS nanocrystals

NASA Astrophysics Data System (ADS)

Savchenko, S. S.; Vokhmintsev, A. S.; Weinstein, I. A.

2018-01-01

Optical absorption (OA) and photoluminescence (PL) spectra of InP/ZnS core/shell nanocrystals with 2.3 nm average size were investigated in the temperature range of 6.5-296 K. Using second derivative spectrophotometry technique energies of the OA transitions at 296 K in quantum dot (QD) solutions and films are evaluated to be E 1 = 2.37, E 2 = 4.10 and E 3 = 4.68 eV. Temperature shifts of the E 1 and E 2 levels are found to result from interaction with effective phonons of 59 and 37 meV energies, respectively. Herewith the 370 meV half-width of the first exciton absorption peak remains constant due to the dominance of inhomogeneous broadening effects caused by QD parameters distribution. Measured PL spectra have a complex structure and can be described in 6.5-296 K range by two independent Gaussian components associated with exciton and defect-related states. In addition, Stokes shift of 320 meV is observed to decrease at T > 200 K. PL thermal quenching analysis in frame of Mott mechanism points to presence of non-radiative relaxation channel with an activation energy of 74 meV.

A photoluminescence, thermoluminescence and electron paramagnetic resonance study of EFG grown europium doped lithium fluoride (LiF) crystals

NASA Astrophysics Data System (ADS)

Seth, Pooja; Swati, G.; Haranath, D.; Rao, S. M. D.; Aggarwal, Shruti

2018-07-01

Europium (Eu) doped LiF crystals have been grown by the Edge-defined film fed growth (EFG) technique. The designing and installation of the furnace used for the growth of the crystals have been discussed in detail. In the present study, Eu (Eu2O3) has been doped in LiF in different concentration (0.02-0.2 wt %). X-ray diffractometry (XRD) and Energy Dispersive X-ray (EDX) spectroscopy confirms the incorporation of Eu in LiF. The influence of Eu on LiF has been investigated through photoluminescence (PL), thermoluminescence (TL) and electron paramagnetic resonance (EPR) in as-grown and annealed crystals. PL emission spectra shows the presence of both Eu3+ and Eu2+ form in the as-grown crystals which is confirmed by EPR results. Whereas, in annealed crystals, Eu is present predominantly as Eu2+ form. This suggests that growing crystals at high temperature (∼900 °C) in argon gas atmosphere through EFG technique favours the reduction of Eu3+ → Eu2+. This reduction phenomenon has been explained on the basis of charge compensation model. TL study of the LiF: Eu (0.02-0.2 wt %) crystals has been done after irradiation with Co60 gamma rays. In this study, it has been observed that the TL intensity as well as glow curve structure of LiF: Eu crystals are a strong function of Eu concentration. The maximum TL is observed at Eu concentration of 0.05 wt% at which a well defined glow curve structure with a prominent peak at 185 °C and a small peak at 253 °C. Beyond this concentration (0.05 wt %), TL intensity decreases due to aggregation of defects in the host. The peak at 185 °C in LiF: Eu (0.05 wt %) is certainly due to the presence of Eu2+ associated defects which is also supported by the PL spectra. It has been observed that Eu doping have a key role in creation of more defect levels which lead to the increased number of electron and hole traps. Further, trapping parameters are analysed using glow curve deconvolution method to have an insight study of TL phenomena. Further, TL glow curve structure of as-grown and annealed crystal are distinct which may be attributed to the nature of defect traps formed inside the LiF.

Raman effect in icosahedral boron-rich solids

PubMed Central

Werheit, Helmut; Filipov, Volodymyr; Kuhlmann, Udo; Schwarz, Ulrich; Armbrüster, Marc; Leithe-Jasper, Andreas; Tanaka, Takaho; Higashi, Iwami; Lundström, Torsten; Gurin, Vladimir N; Korsukova, Maria M

2010-01-01

We present Raman spectra of numerous icosahedral boron-rich solids having the structure of α-rhombohedral, β-rhombohedral, α-tetragonal, β-tetragonal, YB66, orthorhombic or amorphous boron. The spectra were newly measured and, in some cases, compared with reported data and discussed. We emphasize the importance of a high signal-to-noise ratio in the Raman spectra for detecting weak effects evoked by the modification of compounds, accommodation of interstitial atoms and other structural defects. Vibrations of the icosahedra, occurring in all the spectra, are interpreted using the description of modes in α-rhombohedral boron by Beckel et al. The Raman spectrum of boron carbide is largely clarified. Relative intra- and inter-icosahedral bonding forces are estimated for the different structural groups and for vanadium-doped β-rhombohedral boron. The validity of Badger's rule is demonstrated for the force constants of inter-icosahedral B–B bonds, whereas the agreement is less satisfactory for the intra-icosahedral B–B bonds. PMID:27877328

Microstructure of thermally grown and deposited alumina films probed with positrons

NASA Astrophysics Data System (ADS)

Somieski, Bertram; Hulett, Lester D.; Xu, Jun; Pint, Bruce A.; Tortorelli, Peter F.; Nielsen, Bent; Asoka-Kumar, Palakkal; Suzuki, Ryoichi; Ohdaira, Toshiyuki

1999-03-01

Aluminum oxide films used for corrosion protection of iron and nickel aluminides were generated by substrate oxidation as well as plasma and physical vapor depositions. The films grown by oxidation were crystalline. The others were amorphous. Defect structures of the films were studied by positron spectroscopy techniques. Lifetimes of the positrons, and Doppler broadening of the γ photons generated by their annihilation, were measured as functions of the energies with which they were injected. In this manner, densities and sizes of the defects were determined as functions of depths from the outer surfaces of the films. Alumina films generated by oxidation had high densities of open volume defects, mainly consisting of a few aggregated vacancies. In the outer regions of the films the structures of the defects did not depend on substrate compositions. Positron lifetime measurements, and the S and W parameters extracted from Doppler broadening spectra, showed uniform distributions of defects in the crystalline Al2O3 films grown on nickel aluminide substrates, but these data indicated intermediate layers of higher defect contents at the film/substrate interfaces of oxides grown on iron aluminide substrates. Amorphous films generated by plasma and physical vapor deposition had much larger open volume defects, which caused the average lifetimes of the injected positrons to be significantly longer. The plasma deposited film exhibited a high density of large cavities.

Direct Observation of Defect Range and Evolution in Ion-Irradiated Single Crystalline Ni and Ni Binary Alloys

DOE PAGES

Lu, Chenyang; Jin, Ke; Béland, Laurent K.; ...

2016-02-01

We report that energetic ions have been widely used to evaluate the irradiation tolerance of structural materials for nuclear power applications and to modify material properties. It is important to understand the defect production, annihilation and migration mechanisms during and after collision cascades. In this study, single crystalline pure nickel metal and single-phase concentrated solid solution alloys of 50%Ni50%Co (NiCo) and 50%Ni50%Fe (NiFe) without apparent preexisting defect sinks were employed to study defect dynamics under ion irradiation. Both cross-sectional transmission electron microscopy characterization (TEM) and Rutherford backscattering spectrometry channeling (RBS-C) spectra show that the range of radiation-induced defect clusters farmore » exceed the theoretically predicted depth in all materials after high-dose irradiation. Defects in nickel migrate faster than in NiCo and NiFe. Both vacancy-type stacking fault tetrahedra (SFT) and interstitial loops coexist in the same region, which is consistent with molecular dynamics simulations. Kinetic activation relaxation technique (k-ART) simulations for nickel showed that small vacancy clusters, such as di-vacancies and tri-vacancies, created by collision cascades are highly mobile, even at room temperature. The slower migration of defects in the alloy along with more localized energy dissipation of the displacement cascade may lead to enhanced radiation tolerance.« less

Light Emission Mechanisms in CuInS 2 Quantum Dots Evaluated by Spectral Electrochemistry

DOE Office of Scientific and Technical Information (OSTI.GOV)

Fuhr, Addis S.; Yun, Hyeong Jin; Makarov, Nikolay S.

Luminescent CuInS 2 (CIS) quantum dots (QDs) exhibit highly efficient intragap emission and long, hundreds-of-nanoseconds radiative lifetimes. These spectral properties, distinct from structurally similar II–VI QDs, can be explained by the involvement of intragap defect states containing a localized hole capable of coupling with a conduction band electron for a radiative transition. However, the absolute energies of the intragap and band-edge states, the structure of the emissive defect(s), and the role and origin of nonemissive decay channels still remain poorly understood. Here, we address these questions by applying methods of spectral electrochemistry. Cyclic voltammetry measurements reveal a well-defined intragap statemore » whose redox potential is close to that of the Cu x defect state (where x = 1+ or 2+). The energy offset of this state from the valence band accounts well for the apparent photoluminescence Stokes shift observed in optical spectra. These results provide direct evidence that Cu-related defects serve as emission centers responsible for strong intragap emission from CIS QDs. We then use in situ spectroelectrochemistry to reveal two distinct emission pathways based on the differing oxidation states of Cu defects, which can be controlled by altering QD stoichiometry (1+ for stoichiometric QDs and 2+ for Cu-deficient QDs).« less

Exome Sequencing in 32 Patients with Anophthalmia/Microphthalmia and Developmental Eye Defects

PubMed Central

Slavotinek, Anne M.; Garcia, Sarah T.; Chandratillake, Gemma; Bardakjian, Tanya; Ullah, Ehsan; Wu, Di; Umeda, Kyle; Lao, Richard; Tang, Paul Ling-Fung; Wan, Eunice; Madireddy, Lohith; Lyalina, Svetlana; Mendelsohn, Bryce A.; Dugan, Sarah; Tirch, Jean; Tischler, Reana; Harris, Jason; Clark, Michael J.; Chervitz, Stephen; Patwardhan, Anil; West, John M.; Ursell, Phillip; de Alba Campomanes, Alejandra; Schneider, Adele; Kwok, Pui-yan; Baranzini, Sergio; Chen, Richard O.

2014-01-01

Anophthalmia/microphthalmia (A/M) is a genetically heterogeneous birth defect for which the etiology is unknown in more than 50% of patients. We used exome sequencing with the ACE Exome™ (Personalis, Inc; 18 cases) and UCSF Genomics Core (21 cases) to sequence 28 patients with A/M and four patients with varied developmental eye defects. In the 28 patients with A/M, we identified de novo mutations in three patients (OTX2, p.(Gln91His), RARB, p.Arg387Cys and GDF6, p.Ala249Glu) and inherited mutations in STRA6 in two patients. In patients with developmental eye defects, a female with cataracts and cardiomyopathy had a de novo COL4A1 mutation, p.(Gly773Arg), expanding the phenotype associated with COL4A1 to include cardiomyopathy. A male with a chorioretinal defect, microcephaly, seizures and sensorineural deafness had two PNPT1 mutations, p.(Ala507Ser) and c.401-1G>A, and we describe eye defects associated with this gene for the first time. Exome sequencing was efficient for identifying mutations in pathogenic genes for which there is no clinical testing available and for identifying cases that expand phenotypic spectra, such as the PNPT1 and COL4A1-associated disorders described here. PMID:25457163

Growth of defect-free GaAsSbN axial nanowires via self-catalyzed molecular beam epitaxy

NASA Astrophysics Data System (ADS)

Sharma, Manish; Deshmukh, Prithviraj; Kasanaboina, Pavan; Reynolds, C. Lewis, Jr.; Liu, Yang; Iyer, Shanthi

2017-12-01

Bandgap reduction of 10% by incorporation of a dilute amount of N is reported for the first time, in axial GaAsSb nanowires (NWs) grown on Si (111) via Ga-assisted molecular beam epitaxy. Impact of N incorporation on the surface morphology, NW growth kinetics, and their structural and optical properties were examined. Dilute nitride NWs with Sb composition of 7 at% did not exhibit any noticeable planar defects, as revealed by the absence of satellite twin peaks in the selected-area diffraction pattern and high-resolution transmission electron microscopy imaging. Point defects were also minimal in as-grown dilute nitride NWs, as ascertained from the comparison of low-temperature photoluminescence spectra as well as the shape and shift of Raman modes, with in situ annealed NWs in different ambients. Evidence of enhanced incorporation of N was found in the NWs in situ annealed in N ambient, but with deteriorated optical quality due to simultaneous creation of N-induced defects. The lack of any noticeable defects in the as-grown GaAsSbN NWs demonstrates the advantage of the vapor-liquid-solid mechanism responsible for growth of axial configuration over the vapor-solid growth mechanism for core-shell NWs as well as their thin film counterpart, which commonly exhibit N-induced point defects.

Light Emission Mechanisms in CuInS 2 Quantum Dots Evaluated by Spectral Electrochemistry

DOE PAGES

Fuhr, Addis S.; Yun, Hyeong Jin; Makarov, Nikolay S.; ...

2017-09-07

Luminescent CuInS 2 (CIS) quantum dots (QDs) exhibit highly efficient intragap emission and long, hundreds-of-nanoseconds radiative lifetimes. These spectral properties, distinct from structurally similar II–VI QDs, can be explained by the involvement of intragap defect states containing a localized hole capable of coupling with a conduction band electron for a radiative transition. However, the absolute energies of the intragap and band-edge states, the structure of the emissive defect(s), and the role and origin of nonemissive decay channels still remain poorly understood. Here, we address these questions by applying methods of spectral electrochemistry. Cyclic voltammetry measurements reveal a well-defined intragap statemore » whose redox potential is close to that of the Cu x defect state (where x = 1+ or 2+). The energy offset of this state from the valence band accounts well for the apparent photoluminescence Stokes shift observed in optical spectra. These results provide direct evidence that Cu-related defects serve as emission centers responsible for strong intragap emission from CIS QDs. We then use in situ spectroelectrochemistry to reveal two distinct emission pathways based on the differing oxidation states of Cu defects, which can be controlled by altering QD stoichiometry (1+ for stoichiometric QDs and 2+ for Cu-deficient QDs).« less

Low defect densities in molecular beam epitaxial GaAs achieved by isoelectronic In doping

NASA Technical Reports Server (NTRS)

Bhattacharya, P. K.; Dhar, S.; Berger, P.; Juang, F.-Y.

1986-01-01

A study has been made of the effects of adding small amounts of In (0.2-1.2 pct) to GaAs grown by molecular beam epitaxy. The density of four electron traps decreases in concentration by an order of magnitude, and the peak intensities of prominent emissions in the excitonic spectra are reduced with increase in In content. Based on the higher surface migration rate of In, compared to Ga, at the growth temperatures it is apparent that the traps and the excitonic transitions are related to point defects. This agrees with earlier observations by Briones and Collins (1982) and Skromme et al. (1985).

Diameter Control and Photoluminescence of ZnO Nanorods from Trialkylamines

DOE PAGES

Andelman, Tamar; Gong, Yinyan; Neumark, Gertrude; ...

2007-01-01

A novel solution method to control the diameter of ZnO nanorods is reported. Small diameter (2-3 nm) nanorods were synthesized from trihexylamine, and large diameter (50–80 nm) nanorods were synthesized by increasing the alkyl chain length to tridodecylamine. The defect (green) emission of the photoluminescence (PL) spectra of the nanorods varies with diameter, and can thus be controlled by the diameter control. The small ZnO nanorods have strong green emission, while the large diameter nanorods exhibit a remarkably suppressed green band. We show that this observation supports surface oxygen vacancies as the defect that gives rise to the green emission.

Radiation stability of SiO2 micro- and nanopowders under electron and proton exposure

NASA Astrophysics Data System (ADS)

Li, Chundong; Mikhailov, M. M.; Neshchimenko, V. V.

2014-01-01

The effects of proton and electron (E = 100 keV, F = 5 × 1015 сm-2) exposure on the reflective spectra of SiO2 micro- and nanopowders in wavelength range from 250 to 2500 nm have been investigated. It has been established that the reflectance and radiation stability of nanopowders is less than that of micropowders. This effect is caused by the high concentration of radiation defects, which act as surface absorption centers (Es‧ centers) near the energies 5.47 and 4.45 eV, and peroxide silicon defects (tbnd Sisbnd Osbnd Osbnd Sitbnd) near the energy 3.84 eV.

Radiation damage buildup by athermal defect reactions in nickel and concentrated nickel alloys

DOE PAGES

Zhang, S.; Nordlund, K.; Djurabekova, F.; ...

2017-04-12

We develop a new method using binary collision approximation simulating the Rutherford backscattering spectrometry in channeling conditions (RBS/C) from molecular dynamics atom coordinates of irradiated cells. The approach allows comparing experimental and simulated RBS/C signals as a function of depth without fitting parameters. The simulated RBS/C spectra of irradiated Ni and concentrated solid solution alloys (CSAs, NiFe and NiCoCr) show a good agreement with the experimental results. The good agreement indicates the damage evolution under damage overlap conditions in Ni and CSAs at room temperature is dominated by defect recombination and migration induced by irradiation rather than activated thermally.

Application of positron annihilation lifetime technique to the study of deep level transients in semiconductors

NASA Astrophysics Data System (ADS)

Deng, A. H.; Shan, Y. Y.; Fung, S.; Beling, C. D.

2002-03-01

Unlike its conventional applications in lattice defect characterization, positron annihilation lifetime technique was applied to study temperature-dependent deep level transients in semiconductors. Defect levels in the band gap can be determined as they are determined by conventional deep level transient spectroscopy (DLTS) studies. The promising advantage of this application of positron annihilation over the conventional DLTS is that it could further extract extra microstructure information of deep-level defects, such as whether a deep level defect is vacancy related or not. A demonstration of EL2 defect level transient study in GaAs was shown and the EL2 level of 0.82±0.02 eV was obtained by a standard Arrhenius analysis, similar to that in conventional DLTS studies.

Defect quasi Fermi level control-based CN reduction in GaN: Evidence for the role of minority carriers

NASA Astrophysics Data System (ADS)

Reddy, Pramod; Kaess, Felix; Tweedie, James; Kirste, Ronny; Mita, Seiji; Collazo, Ramon; Sitar, Zlatko

2017-10-01

Compensating point defect reduction in wide bandgap semiconductors is possible by above bandgap illumination based defect quasi Fermi level (dQFL) control. The point defect control technique employs excess minority carriers that influence the dQFL of the compensator, increase the corresponding defect formation energy, and consequently are responsible for point defect reduction. Previous studies on various defects in GaN and AlGaN have shown good agreement with the theoretical model, but no direct evidence for the role of minority carriers was provided. In this work, we provide direct evidence for the role of minority carriers in reducing point defects by studying the predicted increase in work done against defect (CN-1) formation with the decrease in the Fermi level (free carrier concentration) in Si doped GaN at a constant illumination intensity. Comparative defect photoluminescence measurements on illuminated and dark regions of GaN show an excellent quantitative agreement with the theory by exhibiting a greater reduction in yellow luminescence attributed to CN-1 at lower doping, thereby providing conclusive evidence for the role of the minority carriers in Fermi level control-based point defect reduction.

Ion-beam-induced damage formation in CdTe

DOE Office of Scientific and Technical Information (OSTI.GOV)

Rischau, C. W.; Schnohr, C. S.; Wendler, E.

2011-06-01

Damage formation in <111>- and <112>-oriented CdTe single crystals irradiated at room temperature and 15 K with 270 keV Ar or 730 keV Sb ions was investigated in situ using Rutherford backscattering spectroscopy (RBS) in channeling configuration. Defect profiles were calculated from the RBS spectra using the computer code DICADA and additional energy-dependent RBS measurements were performed to identify the type of defects. At both temperatures no formation of a buried amorphous layer was detected even after prolonged irradiation with several 10{sup 16} ions/cm{sup 2}. The fact that CdTe is not rendered amorphous even at 15 K suggests that themore » high resistance to amorphization is caused by the high ionicity of CdTe rather than thermal effects. The calculated defect profiles show the formation of a broad defect distribution that extends much deeper into the crystal than the projected range of the implanted ions at both temperatures. The post-range defects in CdTe thus do not seem to be of thermal origin either, but are instead believed to result from migration driven by the electronic energy loss.« less

Radiation-induced defect centers in glass ceramics

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tsai, T.E.; Friebele, E.J.; Griscom, D.L.

1989-01-15

Electron spin resonance (ESR) was used to characterize the radiation-induced defect centers in low-thermal-expansion glass ceramics, including two types of Zerodur and Astrositall. The observed ESR spectra can be associated with different types of defect centers: a Zn/sup +/ center, several types of oxygen hole centers (OHCs), an aluminum-oxygen hole center (Al-OHC), an Fe/sup 3 +/ center, Ti/sup 3 +/ and Zr/sup 3 +/ centers, and three types of As centers. An Sb/sup 4 +/ center, which is not observed in Zerodur, is tentatively identified in Astrositall. From the effect of crystallization on the observed defect concentrations in Zerodur andmore » computer simulation of the spectral lines of some of the centers, we infer that among the nine defect centers observed in the Zerodurs, the As-associated centers are located in the glassy phase and/or at the interface between the glassy and crystalline phases, Zn/sup +/ and Al-OHC are in the crystalline phase, and the rest (including most of the OHCs) are in the glassy phase. Radiation-induced compaction in these materials appears to be related to the generation of OHCs in the glass phase.« less

Unraveling energy conversion modeling in the intrinsic persistent upconverted luminescence of solids: a study of native point defects in antiferromagnetic Er2O3.

PubMed

Huang, Bolong

2016-05-11

We investigated the mechanism of the intrinsic persistent luminescence of Er2O3 in the A-type lattice based on first-principles calculations. We found that the native point defects were engaged in mutual subtle interactions in the form of chemical reactions between different charge states. The release of energy related to lattice distortion facilitates the conversion of energy for electrons to be transported between the valence band and the trap levels or even between the deep trap levels so as to generate persistent luminescence. The defect transitions that take place along the zero-phonon line release energy to enable optical transitions, with the exact amount of negative effective correlation energy determined by the lattice distortions. Our calculations on the thermodynamic transition levels confirm that both the visible and NIR experimentally observed intrinsic persistent luminescence (phosphor or afterglow) are related to the thermodynamic transition levels of oxygen-related defects, and the thermodynamic transition levels within different charge states for these defects are independent of the chemical potentials of the given species. Lattice distortion defects such as anion Frenkel (a-Fr) pair defects play an important role in transporting O-related defects between different lattice sites. To obtain red persistent luminescence that matches the biological therapeutic window, it is suggested to increase the electron transition levels between high-coordinated O vacancies and related metastable a-Fr defects; a close-packed core-shell structure is required to quench low-coordinated O-related defects so as to reduce the green band luminescence. We further established a conversed chain reaction (CCR) model to interpret the energy conversion process of persistent luminescence in terms of the inter-reactions of native point defects between different charge states. It is advantageous to use the study of defect levels combined with formation energies to suggest limits to doping energy and explain photostimulated luminescence in terms of native point defects.

Defect interactions in GaAs single crystals

NASA Technical Reports Server (NTRS)

Gatos, H. C.; Lagowski, J.

1984-01-01

The two-sublattice structural configuration of GaAs and deviations from stoichiometry render the generation and interaction of electrically active point defects (and point defect complexes) critically important for device applications and very complex. Of the defect-induced energy levels, those lying deep into the energy band are very effective lifetime ""killers". The level 0.82 eV below the condition band, commonly referred to as EL2, is a major deep level, particularly in melt-grown GaAs. This level is associated with an antisite defect complex (AsGa - VAS). Possible mechanisms of its formation and its annihilation were further developed.

Distinctive distribution of defects in CdZnTe:In ingots and their effects on the photoelectric properties

NASA Astrophysics Data System (ADS)

Fu, Xu; Wang, Fang-Bao; Zuo, Xi-Ran; Wang, Ze-Jian; Wang, Qian-Ru; Wang, Ke-Qin; Xu, Ling-Yan; Xu, Ya-Dong; Guo, Rong-Rong; Yu, Hui; Jie, Wan-Qi

2018-03-01

Photoelectric properties of CdZnTe:In samples with distinctive defect distributions are investigated using various techniques. Samples cut from the head (T04) and tail (W02) regions of a crystal ingot show distinct differences in Te inclusion distribution. Obvious difference is not observed in Fourier transform infrared (FTIR) spectra, UV–Vis–NIR transmittance spectra, and I–V measurements. However, carrier mobility of the tip sample is higher than that of the tail according to the laser beam induced current (LBIC) measurements. Low temperature photoluminescence (PL) measurement presents sharp emission peaks of D0X and A0X, and relatively large peak of D0X (or A0X) / Dcomplex for T04, indicating a better crystalline quality. Thermally stimulated current (TSC) spectrum shows higher density of shallow point defects, i.e., Cd vacancies, {In}}{{Cd}}+, etc., in W02 sample, which could be responsible for the deterioration of electron mobility. Project supported by the National Natural Science Foundations of China (Grant Nos. 51502244, 51702271, U1631116, and 51372205), the National Key Research and Development Program of China (Grant Nos. 2016YFF0101301 and 2016YFE0115200), the Fund of the State Key Laboratory of Solidification Processing in Northwestern Polytechnical University, China (Grant No. SKLSP201741) the Natural Science Basic Research Plan in Shaanxi Province, China (Grant No. 2016KJXX-09), and the Fundamental Research Funds for the Central Universities, China (Grant No. 3102015BJ(II)ZS014).

F + centre generation in MgO crystals at high density of excitation by accelerated electrons of subthreshold energy

NASA Astrophysics Data System (ADS)

Annenkov, Y. M.; Surzhikov, A. P.; Surzhikov, V. P.; Pogrebnjak, A. D.

1981-07-01

Optical absorption spectra and the angular distribution of annihilated positrons in MgO crystals irradiated by subtreshold superdense electron pulses are measured. The experimental results obtained show the effective contribution of the creation mechanism of non-impact radiation defects in MgO crystals at the highest electron irradiation densities.

Band-bending induced by charged defects and edges of atomically thin transition metal dichalcogenide films

NASA Astrophysics Data System (ADS)

Le Quang, T.; Nogajewski, K.; Potemski, M.; Dau, M. T.; Jamet, M.; Mallet, P.; Veuillen, J.-Y.

2018-07-01

We report scanning tunneling microscopy/spectroscopy (STM/STS) investigations of the band-bending in the vicinity of charged point defects and edges of monolayer MoSe2 and mono- and trilayer WSe2 films deposited on graphitized silicon carbide substrates. By tracing the spatial evolution of the structures of the STS spectra, we evaluate the magnitude and the extent of the band-bending to be equal to few hundreds milielectronvolts and several nanometres, respectively. With the aid of a simple electrostatic model, we show that the spatial variation of the Coulomb potential close to the film edges can be well reproduced by taking into account the metallic screening by graphene. Additionally, the analysis of our data for trilayer WSe2 provides reasonable estimations of its dielectric constant () and of the magnitude of the charge trapped at the defect site (Q  =  +e).

Connecting defects and amorphization in UiO-66 and MIL-140 metal–organic frameworks: a combined experimental and computational study.

PubMed

Bennett, Thomas D; Todorova, Tanya K; Baxter, Emma F; Reid, David G; Gervais, Christel; Bueken, Bart; Van de Voorde, B; De Vos, Dirk; Keen, David A; Mellot-Draznieks, Caroline

2016-01-21

The mechanism and products of the structural collapse of the metal–organic frameworks (MOFs) UiO-66, MIL-140B and MIL-140C upon ball-milling are investigated through solid state 13C NMR and pair distribution function (PDF) studies, finding amorphization to proceed by the breaking of a fraction of metal–ligand bonding in each case. The amorphous products contain inorganic–organic bonding motifs reminiscent of the crystalline phases. Whilst the inorganic Zr6O4(OH)4 clusters of UiO-66 remain intact upon structural collapse, the ZrO backbone of the MIL-140 frameworks undergoes substantial distortion. Density functional theory calculations have been performed to investigate defective models of MIL-140B and show, through comparison of calculated and experimental 13C NMR spectra, that amorphization and defects in the materials are linked.

Quality of Heusler single crystals examined by depth-dependent positron annihilation techniques

NASA Astrophysics Data System (ADS)

Hugenschmidt, C.; Bauer, A.; Böni, P.; Ceeh, H.; Eijt, S. W. H.; Gigl, T.; Pfleiderer, C.; Piochacz, C.; Neubauer, A.; Reiner, M.; Schut, H.; Weber, J.

2015-06-01

Heusler compounds exhibit a wide range of different electronic ground states and are hence expected to be applicable as functional materials in novel electronic and spintronic devices. Since the growth of large and defect-free Heusler crystals is still challenging, single crystals of Fe2TiSn and Cu2MnAl were grown by the optical floating zone technique. Two positron annihilation techniques—angular correlation of annihilation radiation and Doppler broadening spectroscopy (DBS)—were applied in order to study both the electronic structure and lattice defects. Recently, we succeeded to observe clearly the anisotropy of the Fermi surface of Cu2MnAl, whereas the spectra of Fe2TiSn were disturbed by foreign phases. In order to estimate the defect concentration in different samples of Heusler compounds, the positron diffusion length was determined by DBS using a monoenergetic positron beam.

Marshak Lectureship: Vibrational properties of isolated color centers in diamond

NASA Astrophysics Data System (ADS)

Alkauskas, Audrius

In this talk we review our recent work on first-principles calculations of vibrational properties of isolated defect spin qubits and single photon emitters in diamond. These properties include local vibrational spectra, luminescence lineshapes, and electron-phonon coupling. They are key in understanding physical mechanisms behind spin-selective optical initialization and read-out, quantum efficiency of single-photon emitters, as well as in the experimental identification of as yet unknown centers. We first present the methodology to calculate and analyze vibrational properties of effectively isolated defect centers. We then apply the methodology to the nitrogen-vacancy and the silicon-vacancy centers in diamond. First-principles calculations yield important new insights about these important defects. Work performed in collaboration with M. W. Doherty, A. Gali, E. Londero, L. Razinkovas, and C. G. Van de Walle. Supported by the Research Council of Lithuania (Grant M-ERA.NET-1/2015).

Surface optical properties calculated from first principles: The influence of defects, self-energy and excitonic effects

NASA Astrophysics Data System (ADS)

Gero Schmidt, Wolf

2002-03-01

Optical spectroscopies are emerging as powerful tools to probe surfaces, since they allow for the real-time monitoring under challenging conditions as may be encountered, e.g., during material growth. However, their full potential can only be realised if it becomes possible to calculate surface optical spectra accurately and with true predictive power. Such calculations have been difficult, however, due to the large numerical expense involved. Based on a massively parallel, real-space multigrid implementation of DFT-LDA we have calculated reflectance anisotropy spectra for a wide range of group-IV materials and III-V compounds. Transitions between surface states give rise to specific, fingerprint-like spectral features. In addition, the anisotropic surface potential, the electric field at the surface of the sample and, to some extent, surface induced strain and relaxation may cause optical anisotropies in the layers underneath the surface. Surface defects have to be taken into account in order to explain some experimental results. Our DFT-LDA results explain very well the stoichiometric trends and qualitative features of the measured spectra. Quantitative agreement with the measured data is achieved by taking many-body effects into account. We include electronic self-energy corrections in the GW approximation using a model dielectric function to describe the screening. An efficient algorithm for solving the Bethe-Salpeter equation allows us to study the influence of electron-hole attraction and local-field effects on the surface optical properties.

Skull defect reconstruction based on a new hybrid level set.

PubMed

Zhang, Ziqun; Zhang, Ran; Song, Zhijian

2014-01-01

Skull defect reconstruction is an important aspect of surgical repair. Historically, a skull defect prosthesis was created by the mirroring technique, surface fitting, or formed templates. These methods are not based on the anatomy of the individual patient's skull, and therefore, the prosthesis cannot precisely correct the defect. This study presented a new hybrid level set model, taking into account both the global optimization region information and the local accuracy edge information, while avoiding re-initialization during the evolution of the level set function. Based on the new method, a skull defect was reconstructed, and the skull prosthesis was produced by rapid prototyping technology. This resulted in a skull defect prosthesis that well matched the skull defect with excellent individual adaptation.

Raman spectroscopy characterization of actinide oxides (U 1-yPu y)O 2: Resistance to oxidation by the laser beam and examination of defects

NASA Astrophysics Data System (ADS)

Jégou, C.; Caraballo, R.; Peuget, S.; Roudil, D.; Desgranges, L.; Magnin, M.

2010-10-01

Structural changes in four (U 1-yPu y)O 2 materials with very different plutonium concentrations (0 ⩽ y ⩽ 1) and damage levels (up to 110 dpa) were studied by Raman spectroscopy. The novel experimental approach developed for this purpose consisted in using a laser beam as a heat source to assess the reactivity and structural changes of these materials according to the power supplied locally by the laser. The experiments were carried out in air and in water with or without hydrogen peroxide. As expected, the material response to oxidation in air depends on the plutonium content of the test oxide. At the highest power levels U 3O 8 generally forms with UO 2 whereas no significant change in the spectra indicating oxidation is observed for samples with high plutonium content ( 239PuO 2). Samples containing 25 wt.% plutonium exhibit intermediate behavior, typified mainly by a higher-intensity 632 cm -1 peak and the disappearance of the 1LO peak at 575 cm -1. This can be attributed to the presence of anion sublattice defects without any formation of higher oxides. The range of materials examined also allowed us to distinguish partly the chemical effects of alpha self-irradiation. The results obtained with water and hydrogen peroxide (a water radiolysis product) on a severely damaged 238PuO 2 specimen highlight a specific behavior, observed for the first time.

Gold atoms and dimers on amorphous SiO(2): calculation of optical properties and cavity ringdown spectroscopy measurements.

PubMed

Del Vitto, Annalisa; Pacchioni, Gianfranco; Lim, Kok Hwa; Rösch, Notker; Antonietti, Jean-Marie; Michalski, Marcin; Heiz, Ulrich; Jones, Harold

2005-10-27

We report on the optical absorption spectra of gold atoms and dimers deposited on amorphous silica in size-selected fashion. Experimental spectra were obtained by cavity ringdown spectroscopy. Issues on soft-landing, fragmentation, and thermal diffusion are discussed on the basis of the experimental results. In parallel, cluster and periodic supercell density functional theory (DFT) calculations were performed to model atoms and dimers trapped on various defect sites of amorphous silica. Optically allowed electronic transitions were calculated, and comparisons with the experimental spectra show that silicon dangling bonds [[triple bond]Si(.-)], nonbridging oxygen [[triple bond]Si-O(.-)], and the silanolate group [[triple bond]Si-O(-)] act as trapping centers for the gold particles. The results are not only important for understanding the chemical bonding of atoms and clusters on oxide surfaces, but they will also be of fundamental interest for photochemical studies of size-selected clusters on surfaces.

The quantum Ising chain with a generalized defect

NASA Astrophysics Data System (ADS)

Grimm, Uwe

1990-08-01

The finite-size scaling properties of the quantum Ising chain with different types of generalized defects are studied. This not only means an alteration of the coupling constant as previously examined, but also an additional arbitrary transformation in the algebra of observables at one site of the chain. One can distinguish between two classes of generalized defects: on the one hand those which do not affect the finite-size integrability of the Ising chain, and on the other hand those that destroy this property. In this context, finite-size integrability is always understood as a synonym for the possibility to write the hamiltonian of the finite chain as a bilinear expression in fermionic operators by means of a Jordan-Wigner transformation. Concerning the first type of defect, an exact solution for the scaling spectrum is obtained for the most universal defect that preserves the global Z2 symmetry of the chain. It is shown that in the continuum limit this yields the same result as for one properly chosen ordinary defect, that is changing the coupling constant only, and thus the finite-size scaling spectra can be described by irreps of a shifted u(1) Kac-Moody algebra. The other type of defect is examined by means of numerical finite-size calculations. In contrast to the first case, these calculations suggest a non-continuous dependence of the scaling dimensions on the defect parameters. A conjecture for the operator content involving only one primary field of a Virasoro algebra with central charge c= {1}/{2} is given.

Nickel enhanced graphene growth directly on dielectric substrates by molecular beam epitaxy

NASA Astrophysics Data System (ADS)

Wofford, Joseph M.; Speck, Florian; Seyller, Thomas; Lopes, Joao Marcelo J.; Riechert, Henning

2016-07-01

The efficacy of Ni as a surfactant to improve the crystalline quality of graphene grown directly on dielectric Al2O3(0001) substrates by molecular beam epitaxy is examined. Simultaneously exposing the substrate to a Ni flux throughout C deposition at 950 °C led to improved charge carrier mobility and a Raman spectrum indicating less structural disorder in the resulting nanocrystalline graphene film. X-ray photoelectron spectroscopy confirmed that no residual Ni could be detected in the film and showed a decrease in the intensity of the defect-related component of the C1s level. Similar improvements were not observed when a lower substrate temperature (850 °C) was used. A close examination of the Raman spectra suggests that Ni reduces the concentration of lattice vacancies in the film, possibly by catalytically assisting adatom incorporation.

Effect of doping ions on the structural defect and the electrical behavior of CaCu{sub 3}Ti{sub 4}O{sub 12} ceramics

DOE Office of Scientific and Technical Information (OSTI.GOV)

Xue, Renzhong; Department of Technology and Physics, Zhengzhou University of Light Industry, Zhengzhou 450002; Zhao, Gaoyang, E-mail: zhaogy@xaut.edu.cn

Graphical abstract: The dielectric constant decreases with Ta doping, increases with Y doping and keeps almost constant with Zr doping compared with that of pure CCTO. - Highlights: • Y and Ta doping cause different defect types and concentration. • Defect influences the grain boundary mobility and results in different grain size. • Y doping increases the dielectric constant and decreases the nonlinear property. • Ta doping decreases the dielectric constant and enhances the nonlinear property. • Zr doped sample has nearly the defect type and dielectric properties as CaCu{sub 3}Ti{sub 4}O{sub 12}. - Abstract: The microstructure, dielectric and electricalmore » properties of CaCu{sub 3}Ti{sub 4−x}R{sub x}O{sub 12} (R = Y, Zr, Ta; x = 0 and 0.005) ceramics were investigated by XRD, Raman spectra, SEM and dielectric spectrum measurements. Positron annihilation measurements have been performed to investigate the influence of doping on the defects. The results show that all samples form a single crystalline phase. Y and Ta doping cause different defect types and increase the defect size and concentration, which influence the mobility of grain boundary and result in the different grain size. Y doping increases the dielectric constant and decreases the nonlinear property while Ta doping lead to an inverse result. Zr-doped sample has nearly the defect type, grain morphology and dielectric properties as pure CaCu{sub 3}Ti{sub 4}O{sub 12}. The effects of microstructure including the grain morphology and the vacancy defects on the mechanism of the dielectric and electric properties by doping are discussed.« less

Angular shaping of fluorescence from synthetic opal-based photonic crystal.

PubMed

Boiko, Vitalii; Dovbeshko, Galyna; Dolgov, Leonid; Kiisk, Valter; Sildos, Ilmo; Loot, Ardi; Gorelik, Vladimir

2015-01-01

Spectral, angular, and temporal distributions of fluorescence as well as specular reflection were investigated for silica-based artificial opals. Periodic arrangement of nanosized silica globules in the opal causes a specific dip in the defect-related fluorescence spectra and a peak in the reflectance spectrum. The spectral position of the dip coincides with the photonic stop band. The latter is dependent on the size of silica globules and the angle of observation. The spectral shape and intensity of defect-related fluorescence can be controlled by variation of detection angle. Fluorescence intensity increases up to two times at the edges of the spectral dip. Partial photobleaching of fluorescence was observed. Photonic origin of the observed effects is discussed.

F-aggregate centers formation in BaLiF3 crystals

NASA Astrophysics Data System (ADS)

Prado, L.; Gomes, L.; Baldochi, S. L.; Morato, S. P.; Vieira, N. D.

The kinetics of F-aggregate centers formation is investigated in the inverted fluoroperovskite of BaLiF3 submitted to electron-irradiation. By studies of the changes in the absorption spectra during storage of samples in the dark, at room temperature, it was possible to verify a surprising and interesting dependence on defect formation with the crystal growth direction. In spite of its cubic structure, crystals grown in the <100> and <111> directions and submitted to the same conditions of irradiation, showed in particular, to enhance the production of a defect absorbing at 630 nm in <100> crystals which we believe to correspond to F+2-centers in BaLiF3

Positron annihilation study of vacancy-type defects in fast-neutron-irradiated MgO·nAl2O3

NASA Astrophysics Data System (ADS)

Rahman, Abu Zayed Mohammad Saliqur; Li, Zhuoxin; Cao, Xingzhong; Wang, Baoyi; Wei, Long; Xu, Qiu; Atobe, Kozo

2014-09-01

The positron lifetimes of fast-neutron-irradiated MgO·nAl2O3 single crystals were measured to investigate the formation of cation vacancies. Al monovacancy was possibly observed in samples irradiated by fast neutrons at ultra-low temperatures. Additionally, vacancy-oxygen complex centers were possibly observed in samples irradiated at higher temperatures and fast neutron fluences. Coincidence Doppler broadening (CDB) spectra were measured to obtain information regarding the vicinity of vacancy-type defects. A peak at approximately 11 × 10-3 m0c was observed, which may be due to the presence of oxygen atoms in the neighborhood of the vacancies.

Design of a terahertz photonic crystal transmission filter containing ferroelectric material.

PubMed

King, Tzu-Chyang; Chen, Jian-Jie; Chang, Kai-Chun; Wu, Chien-Jang

2016-10-10

The ferroelectric material KTaO₃ (KTO) has a very high refractive index, which is advantageous to the photonic crystal (PC) design. KTO polycrystalline crystal has a high extinction coefficient. In this work, we perform a theoretical study of the transmission properties of a PC bandpass filter made of polycrystalline KTO at terahertz (THz) frequencies. Our results show that the defect modes of usual PC narrowband filters no longer exist because of the existence of the high loss. We provide a new PC structure for the high-extinction materials and show that it has defect modes in its transmittance spectra, providing a possible bandpass filter design in the THz region.

Donor and double-donor transitions of the carbon vacancy related EH{sub 6∕7} deep level in 4H-SiC

DOE Office of Scientific and Technical Information (OSTI.GOV)

Booker, I. D., E-mail: ianbo@ifm.liu.se; Janzén, E., E-mail: erija@ifm.liu.se; Son, N. T.

Using medium- and high-resolution multi-spectra fitting of deep level transient spectroscopy (DLTS), minority carrier transient spectroscopy (MCTS), optical O-DLTS and optical-electrical (OE)-MCTS measurements, we show that the EH{sub 6∕7} deep level in 4H-SiC is composed of two strongly overlapping, two electron emission processes with thermal activation energies of 1.49 eV and 1.58 eV for EH{sub 6} and 1.48 eV and 1.66 eV for EH{sub 7}. The electron emission peaks of EH{sub 7} completely overlap while the emission peaks of EH{sub 6} occur offset at slightly different temperatures in the spectra. OE-MCTS measurements of the hole capture cross section σ{sub p0}(T) in p-type samples revealmore » a trap-Auger process, whereby hole capture into the defect occupied by two electrons leads to a recombination event and the ejection of the second electron into the conduction band. Values of the hole and electron capture cross sections σ{sub n}(T) and σ{sub p}(T) differ strongly due to the donor like nature of the deep levels and while all σ{sub n}(T) have a negative temperature dependence, the σ{sub p}(T) appear to be temperature independent. Average values at the DLTS measurement temperature (∼600 K) are σ{sub n2+}(T) ≈ 1 × 10{sup −14} cm{sup 2}, σ{sub n+}(T) ≈ 1 × 10{sup −14} cm{sup 2}, and σ{sub p0}(T) ≈ 9 × 10{sup −18} cm{sup 2} for EH{sub 6} and σ{sub n2+}(T) ≈ 2 × 10{sup −14} cm{sup 2}, σ{sub n+}(T) ≈ 2 × 10{sup −14} cm{sup 2}, σ{sub p0}(T) ≈ 1 × 10{sup −20} cm{sup 2} for EH{sub 7}. Since EH{sub 7} has already been identified as a donor transition of the carbon vacancy, we propose that the EH{sub 6∕7} center in total represents the overlapping first and second donor transitions of the carbon vacancy defects on both inequivalent lattice sites.« less

Effects of Excess Carriers on Charged Defect Concentrations in Wide Bandgap Semiconductors

DOE Office of Scientific and Technical Information (OSTI.GOV)

Alberi, Kirstin M; Scarpulla, Michael A.

Unintentional doping and doping limits in semiconductors are typically caused by compensating defects with low formation energies. Since the formation enthalpy of a charged defect depends linearly on the Fermi level, doping limits can be especially pronounced in wide bandgap semiconductors where the Fermi level can vary substantially. Introduction of non-equilibrium carrier concentrations during growth or processing alters the chemical potentials of band carriers and allows populations of charged defects to be modified in ways impossible at thermal equilibrium. We demonstrate that in the presence of excess carriers, the rates of carrier capture and emission involving a defect charge transitionmore » level determine the admixture of electron and hole quasi-Fermi levels involved in the formation enthalpy of non-zero charge defect states. To understand the range of possible responses, we investigate the behavior of a single donor-like defect as functions of extrinsic doping and charge transition level energy. We find that that excess carriers will increase the formation enthalpy of compensating defects for most values of the charge transition level in the bandgap. Thus, it may be possible to use non-equilibrium carrier concentrations to overcome limitations on doping imposed by native defects. Cases also exist in which the concentration of defects with the same charge polarity as the majority dopant is either left unchanged or actually increases. This surprising effect arises when emission rates are suppressed relative to the capture rates and is most pronounced in wide bandgap semiconductors. We provide guidelines for carrying out experimental tests of this model.« less

Effects of excess carriers on charged defect concentrations in wide bandgap semiconductors

NASA Astrophysics Data System (ADS)

Alberi, Kirstin; Scarpulla, Michael A.

2018-05-01

Unintentional doping and doping limits in semiconductors are typically caused by compensating defects with low formation energies. Since the formation enthalpy of a charged defect depends linearly on the Fermi level, doping limits can be especially pronounced in wide bandgap semiconductors where the Fermi level can vary substantially. Introduction of non-equilibrium carrier concentrations during growth or processing alters the chemical potentials of band carriers and allows populations of charged defects to be modified in ways impossible at thermal equilibrium. We demonstrate that in the presence of excess carriers, the rates of carrier capture and emission involving a defect charge transition level determine the admixture of electron and hole quasi-Fermi levels involved in the formation enthalpy of non-zero charge defect states. To understand the range of possible responses, we investigate the behavior of a single donor-like defect as functions of extrinsic doping and charge transition level energy. We find that that excess carriers will increase the formation enthalpy of compensating defects for most values of the charge transition level in the bandgap. Thus, it may be possible to use non-equilibrium carrier concentrations to overcome limitations on doping imposed by native defects. Cases also exist in which the concentration of defects with the same charge polarity as the majority dopant is either left unchanged or actually increases. This surprising effect arises when emission rates are suppressed relative to the capture rates and is most pronounced in wide bandgap semiconductors. We provide guidelines for carrying out experimental tests of this model.

Exome sequencing in 32 patients with anophthalmia/microphthalmia and developmental eye defects.

PubMed

Slavotinek, A M; Garcia, S T; Chandratillake, G; Bardakjian, T; Ullah, E; Wu, D; Umeda, K; Lao, R; Tang, P L-F; Wan, E; Madireddy, L; Lyalina, S; Mendelsohn, B A; Dugan, S; Tirch, J; Tischler, R; Harris, J; Clark, M J; Chervitz, S; Patwardhan, A; West, J M; Ursell, P; de Alba Campomanes, A; Schneider, A; Kwok, P-Y; Baranzini, S; Chen, R O

2015-11-01

Anophthalmia/microphthalmia (A/M) is a genetically heterogeneous birth defect for which the etiology is unknown in more than 50% of patients. We used exome sequencing with the ACE Exome(TM) (Personalis, Inc; 18 cases) and UCSF Genomics Core (21 cases) to sequence 28 patients with A/M and four patients with varied developmental eye defects. In the 28 patients with A/M, we identified de novo mutations in three patients (OTX2, p.(Gln91His), RARB, p.Arg387Cys and GDF6, p.Ala249Glu) and inherited mutations in STRA6 in two patients. In patients with developmental eye defects, a female with cataracts and cardiomyopathy had a de novo COL4A1 mutation, p.(Gly773Arg), expanding the phenotype associated with COL4A1 to include cardiomyopathy. A male with a chorioretinal defect, microcephaly, seizures and sensorineural deafness had two PNPT1 mutations, p.(Ala507Ser) and c.401-1G>A, and we describe eye defects associated with this gene for the first time. Exome sequencing was efficient for identifying mutations in pathogenic genes for which there is no clinical testing available and for identifying cases that expand phenotypic spectra, such as the PNPT1 and COL4A1-associated disorders described here. © 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.

Multi-Scale Stochastic Resonance Spectrogram for fault diagnosis of rolling element bearings

NASA Astrophysics Data System (ADS)

He, Qingbo; Wu, Enhao; Pan, Yuanyuan

2018-04-01

It is not easy to identify incipient defect of a rolling element bearing by analyzing the vibration data because of the disturbance of background noise. The weak and unrecognizable transient fault signal of a mechanical system can be enhanced by the stochastic resonance (SR) technique that utilizes the noise in the system. However, it is challenging for the SR technique to identify sensitive fault information in non-stationary signals. This paper proposes a new method called multi-scale SR spectrogram (MSSRS) for bearing defect diagnosis. The new method considers the non-stationary property of the defective bearing vibration signals, and treats every scale of the time-frequency distribution (TFD) as a modulation system. Then the SR technique is utilized on each modulation system according to each frequencies in the TFD. The SR results are sensitive to the defect information because the energy of transient vibration is distributed in a limited frequency band in the TFD. Collecting the spectra of the SR outputs at all frequency scales then generates the MSSRS. The proposed MSSRS is able to well deal with the non-stationary transient signal, and can highlight the defect-induced frequency component corresponding to the impulse information. Experimental results with practical defective bearing vibration data have shown that the proposed method outperforms the former SR methods and exhibits a good application prospect in rolling element bearing fault diagnosis.

Gadolinium substitution induced defect restructuring in multiferroic BiFeO3: case study by positron annihilation spectroscopy

NASA Astrophysics Data System (ADS)

Mukherjee, A.; Banerjee, M.; Basu, S.; Nambissan, P. M. G.; Pal, M.

2013-12-01

Positron annihilation spectroscopy (PAS) comprising of the measurements of positron lifetime and coincidence Doppler broadening spectra has been carried out to understand and monitor the evolution of the vacancy-type defects arising from the ionic deficiencies at lattice points of the multiferroic perovskite bismuth ferrite (BiFeO3) doped with 1, 5 and 10 at% gadolinium (Gd3+) ions. Negatively charged defects in the form of Bi3+ monovacancies (V_{Bi}^{3-} ) were present in the undoped nanocrystallites, which strongly trapped positrons. During the successive doping by Gd3+ ions, the positron trapping efficiency decreased while the doped ions combined with the vacancies to form complexes, which became neutral. A fraction of the positrons got annihilated at the crystallite surfaces too, being evident from the very large positron lifetimes obtained and confirming the nano-size-specific characteristics of the samples. Further, the intercrystallite regions provided favourable sites for orthopositronium formation, although in minute concentrations. The dopant ion-complex formation was also depicted clearly by the defect characteristic S-W plot. Also, the large change of electrical resistivity with Gd concentration has been explained nicely by invoking the defect information from the PAS study. The study has demonstrated the usefulness of an excellent method of defect identification in such a novel material system, which is vital information for exploiting them for further technological applications.

First-principles engineering of charged defects for two-dimensional quantum technologies

NASA Astrophysics Data System (ADS)

Wu, Feng; Galatas, Andrew; Sundararaman, Ravishankar; Rocca, Dario; Ping, Yuan

2017-12-01

Charged defects in two-dimensional (2D) materials have emerging applications in quantum technologies such as quantum emitters and quantum computation. The advancement of these technologies requires a rational design of ideal defect centers, demanding reliable computation methods for the quantitatively accurate prediction of defect properties. We present an accurate, parameter-free, and efficient procedure to evaluate the quasiparticle defect states and thermodynamic charge transition levels of defects in 2D materials. Importantly, we solve critical issues that stem from the strongly anisotropic screening in 2D materials, that have so far precluded the accurate prediction of charge transition levels in these materials. Using this procedure, we investigate various defects in monolayer hexagonal boron nitride (h -BN ) for their charge transition levels, stable spin states, and optical excitations. We identify CBVN (nitrogen vacancy adjacent to carbon substitution of boron) to be the most promising defect candidate for scalable quantum bit and emitter applications.

Suppress carrier recombination by introducing defects. The case of Si solar cell

DOE PAGES

Liu, Yuanyue; Stradins, Paul; Deng, Huixiong; ...

2016-01-11

Deep level defects are usually harmful to solar cells. Here we show that incorporation of selected deep level defects in the carrier-collecting region, however, can be utilized to improve the efficiency of optoelectronic devices. The designed defects can help the transport of the majority carriers by creating defect levels that is resonant with the band edge state, and/or reduce the concentration of minority carriers through Coulomb repulsion, thus suppressing the recombination at the carrier-collecting region. The selection process is demonstrated by using Si solar cell as an example. In conclusion, our work enriches the understanding and utilization of the semiconductormore » defects.« less

Spectra Optia granulocyte apheresis collections result in higher collection efficiency of viable, functional neutrophils in a randomized, crossover, multicenter trial.

PubMed

Cancelas, Jose A; Padmanabhan, Anand; Le, Tuan; Ambruso, Daniel R; Rugg, Neeta; Worsham, D Nicole; Pinkard, Susan L; Graminske, Sharon; Buck, Jennifer; Goldberg, Julie; Bill, Jerry

2015-04-01

Granulocyte transfusion from healthy donors is used in the treatment of patients with granulocyte function defects, or transient neutropenia and severe bacterial or fungal infections resistant to maximal antimicrobial treatment. This study evaluated the performance and safety of the newly developed granulocyte collection protocol of the Spectra Optia in a prospective, multicenter, open-label, randomized, paired crossover trial compared with the COBE Spectra apheresis system in a population of 32 evaluable healthy subjects. All subjects received granulocyte-colony-stimulating factor and dexamethasone before collection. Granulocyte procedures from Spectra Optia apheresis procedures had an approximately 23% higher polymorphonuclear (PMN) collection efficiency (CE) than the COBE Spectra collections (mean, 53.7% vs. 43.2%; p < 0.01), while the platelet CE (10.9% vs. 10.8%, respectively) and hematocrit (7.4% vs. 7.4%) were comparable between collections on both devices. Spectra Optia collections generated a higher total PMN yield per liter of blood processed than those produced by the COBE Spectra (with means of 8.6 × 10(10) vs. 6.8 × 10(10) , respectively). Granulocyte viability was more than 99% with both devices, and chemotaxic and bacterial killing activities of circulating versus collected granulocytes were similarly preserved. Fewer operator adjustments were required with Spectra Optia and there was no significant difference in the number or intensity of adverse events between instruments. CE of the granulocyte collection procedure with the Spectra Optia was approximately 10 percentage points higher than with the COBE Spectra, required less operator involvement, and is safe for clinical implementation. © 2014 AABB.

Photoluminescence Spectra From The Direct Energy Gap of a-SiQDs

NASA Astrophysics Data System (ADS)

Abdul-Ameer, Nidhal M.; Abdulrida, Moafak C.; Abdul-Hakeem, Shatha M.

2018-05-01

A theoretical model for radiative recombination in amorphous silicon quantum dots (a-SiQDs) was developed. In this model, for the first time, the coexistence of both spatial and quantum confinements were considered. Also, it is found that the photoluminescence exhibits significant size dependence in the range (1-4) nm of the quantum dots. a-SiQDs show visible light emission peak energies and high radiative quantum efficiency at room temperature,in contrast to bulk a-Si structures. The quantum efficiency is sensitive to any change in defect density (the volume nonradiative centers density and/or the surface nonradiative centers density) but, with small dots sizes, the quantum efficiency is insensitive to such defects. Our analysis shows that the photoluminescence intensity increases or decreases by the effect of radiative quantum efficiency. By controlling the size of a-SiQDs, we note that the energy of emission can be tuned. The blue shift is attributed to quantum confinement effect. Meanwhile, the spatial confinement effect is clearly observed in red shift in emission spectra. we found a good agreement with the experimental published data. Therefore, we assert that a-SiQDs material is a promising candidate for visible, tunable, and high performance devices of light emitting.

Damage and annealing recovery of boron-implanted ultra-shallow junction: The correlation between beam current and surface configuration

NASA Astrophysics Data System (ADS)

Chang, Feng-Ming; Wu, Zong-Zhe; Lin, Yen-Fu; Kao, Li-Chi; Wu, Cheng-Ta; JangJian, Shiu-Ko; Chen, Yuan-Nian; Lo, Kuang Yao

2018-03-01

The condition of the beam current in the implantation process is a key issue in the damage rate and structural evolution in the sequent annealing process, especially for ultra-shallow layers. In this work, we develop a compensative optical method combined with UV Raman, X-ray photoelectron spectroscopy (XPS), and X-ray absorption near edge spectroscopy (XANES) to inspect the influence of the beam current in the implantation process. The optima condition of the beam current in the implantation process is determined by higher effective Si-B bond portion in UV Raman spectra and less the peak of B-B bond in XPS spectra which is caused by B cluster defects. Results of XANES indicate that the B oxide layer is formed on the surface of the ultra-shallow junction. The defects in the ultra-shallow junction after annealing are analyzed by novel optical analyses, which cannot be inspected by a traditional thermal wave and resistance measurement. This work exhibits the structural variation of the ultra-shallow junction via a variant beam current and provides a valuable metrology in examining the chemical states and the effective activation in the implantation technology.

Low frequency noise peak near magnon emission energy in magnetic tunnel junctions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Liu, Liang; Xiang, Li; Guo, Huiqiang

2014-12-15

We report on the low frequency (LF) noise measurements in magnetic tunnel junctions (MTJs) below 4 K and at low bias, where the transport is strongly affected by scattering with magnons emitted by hot tunnelling electrons, as thermal activation of magnons from the environment is suppressed. For both CoFeB/MgO/CoFeB and CoFeB/AlO{sub x}/CoFeB MTJs, enhanced LF noise is observed at bias voltage around magnon emission energy, forming a peak in the bias dependence of noise power spectra density, independent of magnetic configurations. The noise peak is much higher and broader for unannealed AlO{sub x}-based MTJ, and besides Lorentzian shape noise spectramore » in the frequency domain, random telegraph noise (RTN) is visible in the time traces. During repeated measurements the noise peak reduces and the RTN becomes difficult to resolve, suggesting defects being annealed. The Lorentzian shape noise spectra can be fitted with bias-dependent activation of RTN, with the attempt frequency in the MHz range, consistent with magnon dynamics. These findings suggest magnon-assisted activation of defects as the origin of the enhanced LF noise.« less

Design and development of a LabVIEW-based LED-induced fluorescence spectroscopy system with applications in non-destructive quality assessment of agricultural products

NASA Astrophysics Data System (ADS)

Abbasi, Hamed; Nazeri, Majid; Mireei, Seyed Ahmad

2016-01-01

Over the past several years, the demand for high quality agricultural products has been remarkably increased. Thus, it is important to use non-destructive methods for product quality monitoring. LED-induced fluorescence spectroscopy has proved its potential for nondestructive detection of some defects in agricultural products, such as tissue browning and bruising. Due to such defects, changes in the polyphenol and chlorophyll contents occur which can be considered as the visible marks of decreasing fruit quality. In the present work, a fluorescence spectrometer (spectrofluorometer) controlled by LabVIEW software was designed and developed. In this spectrometer, a consumer-grade webcam was used as an imaging sensor. The spectrometer was able to measure the fluorescence spectra directly from the fruit and vegetable surface in the desired regions. To do so, the spectrometer was equipped with a suitable fiber-optic probe. The hardware solution was based on data acquisition working on the USB platform and controlled by the application running on the PC. In this system, light emitting diodes with different wavelengths were used as the excitation sources for inducing fluorescence spectra of some famous fruits and vegetables.

Low Temperature Photoluminescence of PVT Grown ZnSe and ZnSeTe

NASA Technical Reports Server (NTRS)

Wang, Ling Jun; Su, Ching-Hua; Lehoczky, S. L.

1999-01-01

ZnSe and ZnSeTe single crystals were grown by physical vapor transport (PVT) technique horizontally and vertically. The grown ZnSe and ZnSeTe single crystals were characterized by low temperature photoluminescence at 5 to 10 K using the 3.4 eV emission of an argon laser. The intensity of the sharp near band edge defect lines at 2.799, 2.783 eV and the intrinsic free exciton line at 2.802 eV were mapped on various crystal surfaces with different orientations to the gravitational field. The results show the effects of gravity vector orientation on the defect segregation. Comparison of the photoluminescence spectra of the ZeSe crystal before and after annealing in the Zn vapor shows that the 2.783 eV line of ZnSe crystal is related to the zinc vacancy. The photoluminescence spectra of the ternary ZnSeTe crystal were characterized by a single broad band from 2.2 to 2.4 eV, with a Full Width at Half Maximum (FWHM) of about 100 meV. The temperature dependence of the peak position and intensity were determined from 7 to 150 K.

Defect levels of semi-insulating CdMnTe:In crystals

NASA Astrophysics Data System (ADS)

Kim, K. H.; Bolotinikov, A. E.; Camarda, G. S.; Hossain, A.; Gul, R.; Yang, G.; Cui, Y.; Prochazka, J.; Franc, J.; Hong, J.; James, R. B.

2011-06-01

Using photoluminescence (PL) and current deep-level transient spectroscopy (I-DLTS), we investigated the electronic defects of indium-doped detector-grade CdMnTe:In (CMT:In) crystals grown by the vertical Bridgman method. We similarly analyzed CdZnTe:In (CZT:In) and undoped CdMnTe (CMT) crystals grown under the amount of same level of excess Te and/or indium doping level to detail the fundamental properties of the electronic defect structure more readily. Extended defects, existing in all the samples, were revealed by synchrotron white beam x-ray diffraction topography and scanning electron microscopy. The electronic structure of CMT is very similar to that of CZT, with shallow traps, A-centers, Cd vacancies, deep levels, and Te antisites. The 1.1-eV deep level, revealed by PL in earlier studies of CZT and CdTe, were attributed to dislocation-induced defects. In our I-DLTS measurements, the 1.1-eV traps showed different activation energies with applied bias voltage and an exponential dependence on the trap-filling time, which are typical characteristics of dislocation-induced defects. We propose a new defect-trap model for indium-doped CMT crystals.

Band-like transport in highly crystalline graphene films from defective graphene oxides.

PubMed

Negishi, R; Akabori, M; Ito, T; Watanabe, Y; Kobayashi, Y

2016-07-01

The electrical transport property of the reduced graphene oxide (rGO) thin-films synthesized from defective GO through thermal treatment in a reactive ethanol environment at high temperature above 1000 °C shows a band-like transport with small thermal activation energy (Ea~10 meV) that occurs during high carrier mobility (~210 cm(2)/Vs). Electrical and structural analysis using X-ray absorption fine structure, the valence band photo-electron, Raman spectra and transmission electron microscopy indicate that a high temperature process above 1000 °C in the ethanol environment leads to an extraordinary expansion of the conjugated π-electron system in rGO due to the efficient restoration of the graphitic structure. We reveal that Ea decreases with the increasing density of states near the Fermi level due to the expansion of the conjugated π-electron system in the rGO. This means that Ea corresponds to the energy gap between the top of the valence band and the bottom of the conduction band. The origin of the band-like transport can be explained by the carriers, which are more easily excited into the conduction band due to the decreasing energy gap with the expansion of the conjugated π-electron system in the rGO.

Band-like transport in highly crystalline graphene films from defective graphene oxides

NASA Astrophysics Data System (ADS)

Negishi, R.; Akabori, M.; Ito, T.; Watanabe, Y.; Kobayashi, Y.

2016-07-01

The electrical transport property of the reduced graphene oxide (rGO) thin-films synthesized from defective GO through thermal treatment in a reactive ethanol environment at high temperature above 1000 °C shows a band-like transport with small thermal activation energy (Ea~10 meV) that occurs during high carrier mobility (~210 cm2/Vs). Electrical and structural analysis using X-ray absorption fine structure, the valence band photo-electron, Raman spectra and transmission electron microscopy indicate that a high temperature process above 1000 °C in the ethanol environment leads to an extraordinary expansion of the conjugated π-electron system in rGO due to the efficient restoration of the graphitic structure. We reveal that Ea decreases with the increasing density of states near the Fermi level due to the expansion of the conjugated π-electron system in the rGO. This means that Ea corresponds to the energy gap between the top of the valence band and the bottom of the conduction band. The origin of the band-like transport can be explained by the carriers, which are more easily excited into the conduction band due to the decreasing energy gap with the expansion of the conjugated π-electron system in the rGO.

A comparative study of silicon detector degradation under irradiation by heavy ions and relativistic protons

NASA Astrophysics Data System (ADS)

Eremin, V.; Mitina, D.; Fomichev, A.; Kiselev, O.; Egorov, N.; Eremin, I.; Shepelev, A.; Verbitskaya, E.

2018-01-01

Silicon detectors irradiated by 40Ar ions with the energy of 1.62 GeV were studied with the goal to find the parameters of radiation damage induced by ions. The measurements of the I-V characteristics, temperature dependences of the detector bulk current, deep level spectra and current pulse response were carried out for detectors irradiated within the fluence range 5×1010-2.3×1013 ion/cm2 and the obtained results were compared with the corresponding data for detectors irradiated by 23 GeV protons. It is shown that the processes of defect introduction by ions and overall radiation damage are similar to those induced by 23 GeV protons, while the introduction rates of radiation defects and current generation centers are about ten times higher for irradiation by 40Ar ions. The fact that these processes have much in common gives grounds to use the physical models and characteristic parametrization such as those developed earlier for detectors irradiated by protons and neutrons to build the long-term scenario of Si detector operation in the Time-Of-Flight diagnostic system of Super FRagment Separator designed at GSI for the future Facility for Antiproton and Ion Research, FAIR.

Solvothermal synthesis of well-dispersed MF2 (M = Ca,Sr,Ba) nanocrystals and their optical properties

NASA Astrophysics Data System (ADS)

Zhang, Xiaoming; Quan, Zewei; Yang, Jun; Yang, Piaoping; Lian, Hongzhou; Lin, Jun

2008-02-01

MF2 (M = Ca,Sr,Ba) nanocrystals (NCs) were synthesized via a solvothermal process in the presence of oleic acid and characterized by x-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectra, UV/vis absorption spectra, photoluminescence (PL) excitation and emission spectra, and lifetimes, respectively. In the synthetic process, oleic acid as a surfactant played a crucial role in confining the growth and solubility of the MF2 NCs. The as-prepared CaF2, SrF2 and BaF2 NCs present morphologies of truncated octahedron, cube and sheet in a narrow distribution, respectively. Possible growth mechanisms were proposed to explain these results. The as-prepared NCs are highly crystalline and can be well dispersed in cyclohexane to form stable and clear colloidal solutions, which demonstrate strong emission bands centred at 400 nm in photoluminescence (PL) spectra compared with the cyclohexane solvent. The PL properties of the colloidal solutions of the as-prepared NCs can be ascribed to the trap states of surface defects.

Electronic characterization of defects in narrow gap semiconductors

NASA Technical Reports Server (NTRS)

Patterson, James D.

1993-01-01

The study of point defects in semiconductors has a long and honorable history. In particular, the detailed understanding of shallow defects in common semiconductors traces back to the classic work of Kohn and Luttinger. However, the study of defects in narrow gap semiconductors represents a much less clear story. Here, both shallow defects (caused by long range potentials) and deep defects (from short range potentials) are far from being completely understood. In this study, all results are calculational and our focus is on the chemical trend of deep levels in narrow gap semiconductors. We study substitutional (including antisite), interstitial and ideal vacancy defects. For substitutional and interstitial impurities, the efects of relaxation are included. For materials like Hg(1-x)Cd(x)Te, we study how the deep levels vary with x, of particular interest is what substitutional and interstitial atoms yield energy levels in the gap i.e. actually produce deep ionized levels. Also, since the main technique utilized is Green's functions, we include some summary of that method.

Influence of Atomic Hydrogen, Band Bending, and Defects in the Top Few Nanometers of Hydrothermally Prepared Zinc Oxide Nanorods

NASA Astrophysics Data System (ADS)

Al-Saadi, Mubarak J.; Al-Harthi, Salim H.; Kyaw, Htet H.; Myint, Myo T. Z.; Bora, Tanujjal; Laxman, Karthik; Al-Hinai, Ashraf; Dutta, Joydeep

2017-01-01

We report on the surface, sub-surface (top few nanometers) and bulk properties of hydrothermally grown zinc oxide (ZnO) nanorods (NRs) prior to and after hydrogen treatment. Upon treating with atomic hydrogen (H*), upward and downward band bending is observed depending on the availability of molecular H2O within the structure of the NRs. In the absence of H2O, the H* treatment demonstrated a cleaning effect of the nanorods, leading to a 0.51 eV upward band bending. In addition, enhancement in the intensity of room temperature photoluminescence (PL) signals due to the creation of new surface defects could be observed. The defects enhanced the visible light activity of the ZnO NRs which were subsequently used to photocatalytically degrade aqueous phenol under simulated sunlight. On the contrary, in the presence of H2O, H* treatment created an electronic accumulation layer inducing downward band bending of 0.45 eV ( 1/7th of the bulk ZnO band gap) along with the weakening of the defect signals as observed from room temperature photoluminescence spectra. The results suggest a plausible way of tailoring the band bending and defects of the ZnO NRs through control of H2O/H* species.

Fundamental limits on the electron mobility of β-Ga2O3

NASA Astrophysics Data System (ADS)

Kang, Youngho; Krishnaswamy, Karthik; Peelaers, Hartwin; Van de Walle, Chris G.

2017-06-01

We perform first-principles calculations to investigate the electronic and vibrational spectra and the electron mobility of β-Ga2O3. We calculate the electron-phonon scattering rate of the polar optical phonon modes using the Vogl model in conjunction with Fermi’s golden rule; this enables us to fully take the anisotropic phonon spectra of the monoclinic lattice of β-Ga2O3 into account. We also examine the scattering rate due to ionized impurities or defects using a Yukawa-potential-based model. We consider scattering due to donor impurities, as well as the possibility of compensation by acceptors such as Ga vacancies. We then calculate the room-temperature mobility of β-Ga2O3 using the Boltzmann transport equation within the relaxation time approximation, for carrier densities in the range from 1017 to 1020 cm-3. We find that the electron-phonon interaction dominates the mobility for carrier densities of up to 1019 cm-3. We also find that the intrinsic anisotropy in the mobility is small; experimental findings of large anisotropy must therefore be attributed to other factors. We attribute the experimentally observed reduction of the mobility with increasing carrier density to increasing levels of compensation, which significantly affect the mobility.

Fundamental limits on the electron mobility of β-Ga2O3.

PubMed

Kang, Youngho; Krishnaswamy, Karthik; Peelaers, Hartwin; Van de Walle, Chris G

2017-06-14

We perform first-principles calculations to investigate the electronic and vibrational spectra and the electron mobility of β-Ga 2 O 3 . We calculate the electron-phonon scattering rate of the polar optical phonon modes using the Vogl model in conjunction with Fermi's golden rule; this enables us to fully take the anisotropic phonon spectra of the monoclinic lattice of β-Ga 2 O 3 into account. We also examine the scattering rate due to ionized impurities or defects using a Yukawa-potential-based model. We consider scattering due to donor impurities, as well as the possibility of compensation by acceptors such as Ga vacancies. We then calculate the room-temperature mobility of β-Ga 2 O 3 using the Boltzmann transport equation within the relaxation time approximation, for carrier densities in the range from 10 17 to 10 20 cm -3 . We find that the electron-phonon interaction dominates the mobility for carrier densities of up to 10 19 cm -3 . We also find that the intrinsic anisotropy in the mobility is small; experimental findings of large anisotropy must therefore be attributed to other factors. We attribute the experimentally observed reduction of the mobility with increasing carrier density to increasing levels of compensation, which significantly affect the mobility.

Comparative study of Monte Carlo particle transport code PHITS and nuclear data processing code NJOY for recoil cross section spectra under neutron irradiation

NASA Astrophysics Data System (ADS)

Iwamoto, Yosuke; Ogawa, Tatsuhiko

2017-04-01

Because primary knock-on atoms (PKAs) create point defects and clusters in materials that are irradiated with neutrons, it is important to validate the calculations of recoil cross section spectra that are used to estimate radiation damage in materials. Here, the recoil cross section spectra of fission- and fusion-relevant materials were calculated using the Event Generator Mode (EGM) of the Particle and Heavy Ion Transport code System (PHITS) and also using the data processing code NJOY2012 with the nuclear data libraries TENDL2015, ENDF/BVII.1, and JEFF3.2. The heating number, which is the integral of the recoil cross section spectra, was also calculated using PHITS-EGM and compared with data extracted from the ACE files of TENDL2015, ENDF/BVII.1, and JENDL4.0. In general, only a small difference was found between the PKA spectra of PHITS + TENDL2015 and NJOY + TENDL2015. From analyzing the recoil cross section spectra extracted from the nuclear data libraries using NJOY2012, we found that the recoil cross section spectra were incorrect for 72Ge, 75As, 89Y, and 109Ag in the ENDF/B-VII.1 library, and for 90Zr and 55Mn in the JEFF3.2 library. From analyzing the heating number, we found that the data extracted from the ACE file of TENDL2015 for all nuclides were problematic in the neutron capture region because of incorrect data regarding the emitted gamma energy. However, PHITS + TENDL2015 can calculate PKA spectra and heating numbers correctly.

New Fe i Level Energies and Line Identifications from Stellar Spectra. II. Initial Results from New Ultraviolet Spectra of Metal-poor Stars

DOE Office of Scientific and Technical Information (OSTI.GOV)

Peterson, Ruth C.; Kurucz, Robert L.; Ayres, Thomas R., E-mail: peterson@ucolick.org

2017-04-01

The Fe i spectrum is critical to many areas of astrophysics, yet many of the high-lying levels remain uncharacterized. To remedy this deficiency, Peterson and Kurucz identified Fe i lines in archival ultraviolet and optical spectra of metal-poor stars, whose warm temperatures favor moderate Fe i excitation. Sixty-five new levels were recovered, with 1500 detectable lines, including several bound levels in the ionization continuum of Fe i. Here, we extend the previous work by identifying 59 additional levels, with 1400 detectable lines, by incorporating new high-resolution UV spectra of warm metal-poor stars recently obtained by the Hubble Space Telescope Imagingmore » Spectrograph. We provide gf values for these transitions, both computed as well as adjusted to fit the stellar spectra. We also expand our spectral calculations to the infrared, confirming three levels by matching high-quality spectra of the Sun and two cool stars in the H -band. The predicted gf values suggest that an additional 3700 Fe i lines should be detectable in existing solar infrared spectra. Extending the empirical line identification work to the infrared would help confirm additional Fe i levels, as would new high-resolution UV spectra of metal-poor turnoff stars below 1900 Å.« less

New Fe I Level Energies and Line Identifications from Stellar Spectra. II. Initial Results from New Ultraviolet Spectra of Metal-poor Stars

NASA Astrophysics Data System (ADS)

Peterson, Ruth C.; Kurucz, Robert L.; Ayres, Thomas R.

2017-04-01

The Fe I spectrum is critical to many areas of astrophysics, yet many of the high-lying levels remain uncharacterized. To remedy this deficiency, Peterson & Kurucz identified Fe I lines in archival ultraviolet and optical spectra of metal-poor stars, whose warm temperatures favor moderate Fe I excitation. Sixty-five new levels were recovered, with 1500 detectable lines, including several bound levels in the ionization continuum of Fe I. Here, we extend the previous work by identifying 59 additional levels, with 1400 detectable lines, by incorporating new high-resolution UV spectra of warm metal-poor stars recently obtained by the Hubble Space Telescope Imaging Spectrograph. We provide gf values for these transitions, both computed as well as adjusted to fit the stellar spectra. We also expand our spectral calculations to the infrared, confirming three levels by matching high-quality spectra of the Sun and two cool stars in the H-band. The predicted gf values suggest that an additional 3700 Fe I lines should be detectable in existing solar infrared spectra. Extending the empirical line identification work to the infrared would help confirm additional Fe I levels, as would new high-resolution UV spectra of metal-poor turnoff stars below 1900 Å.

21 CFR 110.110 - Natural or unavoidable defects in food for human use that present no health hazard.

Code of Federal Regulations, 2010 CFR

2010-04-01

... 21 Food and Drugs 2 2010-04-01 2010-04-01 false Natural or unavoidable defects in food for human... PRACTICE IN MANUFACTURING, PACKING, OR HOLDING HUMAN FOOD Defect Action Levels § 110.110 Natural or... natural or unavoidable defects to the lowest level currently feasible. (d) The mixing of a food containing...

Oxygen-related 1-platinum defects in silicon: An electron paramagnetic resonance study

NASA Astrophysics Data System (ADS)

Juda, U.; Scheerer, O.; Höhne, M.; Riemann, H.; Schilling, H.-J.; Donecker, J.; Gerhardt, A.

1996-09-01

A monoclinic 1-platinum defect recently detected was investigated more thoroughly by electron paramagnetic resonance (EPR). The defect is one of the dominating defects in platinum doped silicon. With a perfect reproducibility it is observed in samples prepared from n-type silicon as well as from p-type silicon, in float zone (FZ) silicon as well as in Czochralski (Cz) silicon. Its concentration varies with the conditions of preparation and nearly reaches that of isolated substitutional platinum in Cz silicon annealed for 2 h at 540 °C after quenching from the temperature of platinum diffusion. Because of its concentration which in Cz-Si exceeds that in FZ-Si the defect is assumed to be oxygen-related though a hyperfine structure with 17O could not be resolved. The defect causes a level close to the valence band. This is concluded from variations of the Fermi level and from a discussion of the spin Hamiltonian parameters. In photo-EPR experiments the defect is coupled to recently detected acceptorlike self-interstitial related defects (SIRDs); their level position turns out to be near-midgap. These defects belong to the lifetime limiting defects in Pt-doped Si.

Intrinsic and extrinsic defects in a family of coal-derived graphene quantum dots

NASA Astrophysics Data System (ADS)

Singamaneni, Srinivasa Rao; van Tol, Johan; Ye, Ruquan; Tour, James M.

2015-11-01

In this letter, we report on the high frequency (239.2 and 336 GHz) electron spin resonance (ESR) studies performed on graphene quantum dots (GQDs), prepared through a wet chemistry route from three types of coal: (a) bituminous, (b) anthracite, and (c) coke; and from non-coal derived GQDs. The microwave frequency-, power-, and temperature-dependent ESR spectra coupled with computer-aided simulations reveal four distinct magnetic defect centers. In bituminous- and anthracite-derived GQDs, we have identified two of them as intrinsic carbon-centered magnetic defect centers (a broad signal of peak to peak width = 697 (10-4 T), g = 2.0023; and a narrow signal of peak to peak width = 60 (10-4 T), g = 2.003). The third defect center is Mn2+ (6S5/2, 3d5) (signal width = 61 (10-4 T), g = 2.0023, Aiso = 93(10-4 T)), and the fourth defect is identified as Cu2+ (2D5/2, 3d9) (g⊥ = 2.048 and g‖ = 2.279), previously undetected. Coke-derived and non-coal derived GQDs show Mn2+ and two-carbon related signals, and no Cu2+ signal. The extrinsic impurities most likely originate from the starting coal. Furthermore, Raman, photoluminescence, and ESR measurements detected no noticeable changes in the properties of the bituminous GQDs after one year. This study highlights the importance of employing high frequency ESR spectroscopy in identifying the (magnetic) defects, which are roadblocks for spin relaxation times of graphene-based materials. These defects would not have been possible to probe by other spin transport measurements.

Defect-Induced Photoluminescence Enhancement and Corresponding Transport Degradation in Individual Suspended Carbon Nanotubes

NASA Astrophysics Data System (ADS)

Wang, Bo; Shen, Lang; Yang, Sisi; Chen, Jihan; Echternach, Juliana; Dhall, Rohan; Kang, DaeJin; Cronin, Stephen

2018-05-01

This paper is a contribution to the Physical Review Applied collection in memory of Mildred S. Dresselhaus. The utilization of defects in carbon nanotubes to improve their photoluminescence efficiency has become a widespread study of the realization of efficient light-emitting devices. Here, we report a detailed comparison of the defects in nanotubes (quantified by Raman spectroscopy) and photoluminescence (PL) intensity of individual suspended carbon nanotubes (CNTs). We also evaluate the impact of these defects on the electron or hole transport in the nanotubes, which is crucial for the ultimate realization of optoelectronic devices. We find that brightly luminescent nanotubes exhibit a pronounced D-band in their Raman spectra, and vice versa, dimly luminescent nanotubes exhibit almost no D-band. Here, defects are advantageous for light emission by trapping excitons, which extend their lifetimes. We quantify this behavior by plotting the PL intensity as a function of the ID /IG -band Raman intensity ratio, which exhibits a Lorentzian distribution peaked at ID /IG=0.17 . For CNTs with a ID /IG ratio >0.25 , the PL intensity decreases, indicating that above some critical density, nonradiative recombination at defect sites dominates over the advantages of exciton trapping. In an attempt to fabricate optoelectronic devices based on these brightly luminescent CNTs, we transfer these suspended CNTs to platinum electrodes and find that the brightly photoluminescent nanotubes exhibit nearly infinite resistance due to these defects, while those without bright photoluminescence exhibit finite resistance. These findings indicate a potential limitation in the use of brightly luminescent CNTs for optoelectronic applications.

Metastable defect response in CZTSSe from admittance spectroscopy

DOE PAGES

Koeper, Mark J.; Hages, Charles J.; Li, Jian V.; ...

2017-10-02

Admittance spectroscopy is a useful tool used to study defects in semiconductor materials. However, metastable defect responses in non-ideal semiconductors can greatly impact the measurement and therefore the interpretation of results. Here, admittance spectroscopy was performed on Cu2ZnSn(S,Se) 4 where metastable defect response is illustrated due to the trapping of injected carriers into a deep defect state. To investigate the metastable response, admittance measurements were performed under electrically and optically relaxed conditions in comparison to a device following a low level carrier-injection pretreatment. The relaxed measurement demonstrates a single capacitance signature while two capacitance signatures are observed for the devicemore » measured following carrier-injection. The deeper level signature, typically reported for kesterites, is activated by charge trapping following carrier injection. Both signatures are attributed to bulk level defects. The significant metastable response observed on kesterites due to charge trapping obscures accurate interpretation of defect levels from admittance spectroscopy and indicates that great care must be taken when performing and interpreting this measurement on non-ideal devices.« less

Investigation of deep-level defects in Cu(In,Ga)Se2 thin films by two-wavelength excitation photo-capacitance spectroscopy

NASA Astrophysics Data System (ADS)

Hu, Xiaobo; Gupta, Amit; Sakurai, Takeaki; Yamada, Akimasa; Ishizuka, Shogo; Niki, Shigeru; Akimoto, Katsuhiro

2013-10-01

The properties of the defect level located 0.8 eV above the valence band in Cu(In1-x,Gax)Se2 thin films were investigated by a photo-capacitance method using a monochromatic probe light with an energy of 0.7 to 1.8 eV. In addition to the probe light, laser light with a wavelength of 1.55 μm, corresponding to 0.8 eV, was also used to study the saturation effect of the defect level at 0.8 eV. A suppression of electron-hole recombination due to saturation of the defect level was observed at room temperature while no saturation effect was observed at 140 K. The results suggest that the defect level at 0.8 eV acts as a recombination center at least at room temperature.

Distribution function of random strains in an elastically anisotropic continuum and defect strengths of T m3 + impurity ions in crystals with zircon structure

NASA Astrophysics Data System (ADS)

Malkin, B. Z.; Abishev, N. M.; Baibekov, E. I.; Pytalev, D. S.; Boldyrev, K. N.; Popova, M. N.; Bettinelli, M.

2017-07-01

We construct a distribution function of the strain-tensor components induced by point defects in an elastically anisotropic continuum, which can be used to account quantitatively for many effects observed in different branches of condensed matter physics. Parameters of the derived six-dimensional generalized Lorentz distribution are expressed through the integrals computed over the array of strains. The distribution functions for the cubic diamond and elpasolite crystals and tetragonal crystals with the zircon and scheelite structures are presented. Our theoretical approach is supported by a successful modeling of specific line shapes of singlet-doublet transitions of the T m3 + ions doped into AB O4 (A =Y , Lu; B =P , V) crystals with zircon structure, observed in high-resolution optical spectra. The values of the defect strengths of impurity T m3 + ions in the oxygen surroundings, obtained as a result of this modeling, can be used in future studies of random strains in different rare-earth oxides.

Negative differential resistance and effect of defects and deformations in MoS{sub 2} armchair nanoribbon metal-oxide-semiconductor field effect transistor

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sengupta, Amretashis, E-mail: amretashis@dese.iisc.ernet.in; Mahapatra, Santanu

In this work, we present a study on the negative differential resistance (NDR) behavior and the impact of various deformations (like ripple, twist, wrap) and defects like vacancies and edge roughness on the electronic properties of short-channel MoS{sub 2} armchair nanoribbon MOSFETs. The effect of deformation (3°–7° twist or wrap and 0.3–0.7 Å ripple amplitude) and defects on a 10 nm MoS2 ANR FET is evaluated by the density functional tight binding theory and the non-equilibrium Green's function approach. We study the channel density of states, transmission spectra, and the I{sub D}–V{sub D} characteristics of such devices under the varying conditions, withmore » focus on the NDR behavior. Our results show significant change in the NDR peak to valley ratio and the NDR window with such minor intrinsic deformations, especially with the ripple.« less

Temperature control of the ultra-short laser pulse compression in a one-dimensional photonic band gap structure with nematic liquid crystal as a defect layer

NASA Astrophysics Data System (ADS)

Shiri, Ramin; Safari, Ebrahim; Bananej, Alireza

2018-04-01

We investigate numerically the controllable chirped pulse compression in a one-dimensional photonic structure containing a nematic liquid crystal defect layer using the temperature dependent refractive index of the liquid crystal. We consider the structure under irradiation by near-infrared ultra-short laser pulses polarized parallel to the liquid crystal director at a normal angle of incidence. It is found that the dispersion behaviour and consequently the compression ability of the system can be changed in a controlled manner due to the variation in the defect temperature. When the temperature increased from 290 to 305 K, the transmitted pulse duration decreased from 75 to 42 fs in the middle of the structure, correspondingly. As a result, a novel low-loss tunable pulse compressor with a really compact size and high compression factor is achieved. The so-called transfer matrix method is utilized for numerical simulations of the band structure and reflection/transmission spectra of the structure under investigation.

The effect of sputter temperature on vacancy island behavior on Ni(111) measured by photoemission of adsorbed xenon

NASA Astrophysics Data System (ADS)

Malafsky, Geoffrey P.

1994-04-01

The temperature dependence of vacancy coalescence on an ion bombarded Ni(111) surface is measured by photoemission of adsorbed xenon (PAX). The Ni(111) crystal is sputtered by a low fluence (0.06 ML incident ions) Ar + ion beam with incident kinetic energies of 500-3000 eV. The Xe coverage decreases rapidly with increasing temperature between 88 and 375 K with little additional change from 375 to 775 K. The PAX spectra are acquired with a Xe chamber pressure of 8 × 10 -10 Torr and at a temperature of 88 K. Under these conditions, the Xe is selectively adsorbed at defect sites which would make the Xe coverage proportional to the surface defect density on simple defect structures but the large size of the Xe atom relative to the Ni atom prevents the direct relationship of Xe coverage to the defect density when complex and varying defect structures are present. The decrease in Xe coverage is not attributed to the loss of defect sites by adatom-vacancy recombination but the changing vacancy island shape and size with temperature which alters the ratio of adsorbed Xe atoms to surface vacancy sites. This ratio decreases with increasing temperature as the vacancy islands progress from small and irregularly shaped islands to larger and hexagonally shaped islands. This transition is seen in Monte Carlo simulations of the kinetically driven atomic diffusion on the sputtered surface.

Resonances in photoabsorption: Predissociation line shapes in the 3pπD{sup 1}Π{sup +}{sub u} ← Χ{sup 1}Σ{sub g}{sup +} system in H{sub 2}

DOE Office of Scientific and Technical Information (OSTI.GOV)

Mezei, J. Zs.; Laboratoire Ondes et Milieux Complexes, UMR-6294 CNRS and Université du Havre, 25, rue Philippe Lebon, BP 540, 76058, Le Havre France; Schneider, I. F.

2014-08-14

The predissociation of the 3pπD{sup 1}Π{sub u}{sup +},v≥3,N=1, N = 2, and N = 3 levels of diatomic hydrogen is calculated by ab initio multichannel quantum defect theory combined with a R-matrix type approach that accounts for interfering predissociation and autoionization. The theory yields absorption line widths and shapes that are in good agreement with those observed in the high-resolution synchrotron vacuum-ultraviolet absorption spectra obtained by Dickenson et al. [J. Chem. Phys. 133, 144317 (2010)] at the DESIRS beamline of the SOLEIL synchrotron. The theory predicts further that many of the D state resonances with v ⩾ 6 exhibit amore » complex fine structure which cannot be modeled by the Fano profile formula and which has not yet been observed experimentally.« less

Relative efficacy of the argon green, argon blue-green, and krypton red lasers for 10-0 nylon subconjunctival laser suture lysis.

PubMed

Mudgil, A V; To, K W; Balachandran, R M; Janigian, R H; Tsiaras, W G

1999-01-01

To determine the optimal wavelength for subconjunctival laser suture lysis. 130 black monofilament 10-0 nylon sutures were sewn subconjunctivally into the bare sclera of enucleated rabbit globes. The lowest energy levels facilitating laser suture lysis were determined for the argon green (514.5 NM), argon blue-green (488.0 NM, 514.5 NM), and krypton red (647.1 NM) wavelengths. In addition, absorption spectroscopy was performed on the suture material and conjunctiva using the Perkin Elmer W/VIS Lambda 2 spectrometer. Krypton red produced the fewest buttonhole defects, and it was also the most efficient energy source for suture lysis (P = 0.0001) under nontenectomized conjunctiva. Absorbance spectra studies revealed peak absorbance at 628 NM for the 10-0 nylon suture material. Based on animal and absorption spectroscopy studies, krypton red may be a safer and more efficient wavelength for subconjunctival laser suture lysis.

Photoluminescence-detected magnetic-resonance study of fullerene-doped π-conjugated polymers

NASA Astrophysics Data System (ADS)

Lane, P. A.; Shinar, J.; Yoshino, K.

1996-10-01

X-band photoluminescence (PL)-detected magnetic resonance (PLDMR) spectra of C60- and C70-doped 2,5-dihexoxy poly(p-phenylenevinylene) (DHO-PPV), 2,5-dibutoxy poly(p-phenylene ethynylene) (DBO-PPE), and poly(3-dodecylthiophene) (P3DT) are described and discussed. While light doping of DHO-PPV by both fullerenes quenches the PL, both the polaron and triplet exciton resonances are dramatically enhanced. This is attributed to the creation of conformational defects which enhance the fission of 11Bu singlet excitons to polaron pairs and intersystem crossing to 13Bu triplet excitons. The triplet resonance in all polymers is quenched at relatively low doping levels of C60 and C70, which is attributed to quenching of triplets by positive polarons injected onto the polymer chain. Increased doping by C60, but not C70, quenches the polaron resonance, also due to photoinduced charge transfer.

Non-Radiative Carrier Recombination Enhanced by Two-Level Process: A First-Principles Study

NASA Astrophysics Data System (ADS)

Yang, Ji-Hui; Shi, Lin; Wang, Lin-Wang; Wei, Su-Huai

2016-02-01

Non-radiative recombination plays an important role in the performance of optoelectronic semiconductor devices such as solar cells and light-emitting diodes. Most textbook examples assume that the recombination process occurs through a single defect level, where one electron and one hole are captured and recombined. Based on this simple picture, conventional wisdom is that only defect levels near the center of the bandgap can be effective recombination centers. Here, we present a new two-level recombination mechanism: first, one type of carrier is captured through a defect level forming a metastable state; then the local defect configuration rapidly changes to a stable state, where the other type of carrier is captured and recombined through another defect level. This novel mechanism is applied to the recombination center in CdTe. We show that this two-level process can significantly increase the recombination rate (by three orders of magnitude) in agreement with experiments. We expect that this two-level recombination process can exist in a wide range of semiconductors, so its effect should be carefully examined in characterizing optoelectronic materials.

Simultaneous observation of free and defect-bound excitons in CH3NH3PbI3 using four-wave mixing spectroscopy

NASA Astrophysics Data System (ADS)

March, Samuel A.; Clegg, Charlotte; Riley, Drew B.; Webber, Daniel; Hill, Ian G.; Hall, Kimberley C.

2016-12-01

Solar cells incorporating organic-inorganic perovskite, which may be fabricated using low-cost solution-based processing, have witnessed a dramatic rise in efficiencies yet their fundamental photophysical properties are not well understood. The exciton binding energy, central to the charge collection process, has been the subject of considerable controversy due to subtleties in extracting it from conventional linear spectroscopy techniques due to strong broadening tied to disorder. Here we report the simultaneous observation of free and defect-bound excitons in CH3NH3PbI3 films using four-wave mixing (FWM) spectroscopy. Due to the high sensitivity of FWM to excitons, tied to their longer coherence decay times than unbound electron- hole pairs, we show that the exciton resonance energies can be directly observed from the nonlinear optical spectra. Our results indicate low-temperature binding energies of 13 meV (29 meV) for the free (defect-bound) exciton, with the 16 meV localization energy for excitons attributed to binding to point defects. Our findings shed light on the wide range of binding energies (2-55 meV) reported in recent years.

Effect of irradiation with MeV protons and electrons on the conductivity compensation and photoluminescence of moderately doped p-4H-SiC (CVD)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kozlovski, V. V.; Lebedev, A. A., E-mail: shura.lebe@mail.ioffe.ru; Bogdanova, E. V.

The compensation of moderately doped p-4H-SiC samples grown by the chemical vapor deposition (CVD) method under irradiation with 0.9-MeV electrons and 15-MeV protons is studied. The experimentally measured carrier removal rates are 1.2–1.6 cm{sup –1} for electrons and 240–260 cm{sup –1} for protons. The dependence of the concentration of uncompensated acceptors and donors, measured in the study, demonstrates a linear decrease with increasing irradiation dose to the point of complete compensation. This run of the dependence shows that compensation of the samples is due to the transition of carriers to deep centers formed by primary radiation-induced defects. It is demonstratedmore » that, in contrast to n-SiC (CVD), primary defects in the carbon sublattice of moderately doped p-SiC (CVD) only cannot account for the compensation process. In p-SiC, either primary defects in the silicon sublattice, or defects in both sublattices are responsible for conductivity compensation. Also, photoluminescence spectra are examined in relation to the irradiation dose.« less

Advances in low-defect multilayers for EUVL mask blanks

NASA Astrophysics Data System (ADS)

Folta, James A.; Davidson, J. Courtney; Larson, Cindy C.; Walton, Christopher C.; Kearney, Patrick A.

2002-07-01

Low-defect multilayer coatings are required to fabricate mask blanks for Extreme Ultraviolet Lithography (EUVL). The mask blanks consist of high reflectance EUV multilayers on low thermal expansion substrates. A defect density of 0.0025 printable defects/cm2 for both the mask substrate and the multilayer is required to provide a mask blank yield of 60 percent. Current low defect multilayer coating technology allows repeated coating-added defect levels of 0.05/cm2 for defects greater than 90 nm polystyrene latex sphere (PSL) equivalent size for lots of 20 substrates. Extended clean operation of the coating system at levels below 0.08/cm2 for 3 months of operation has also been achieved. Two substrates with zero added defects in the quality area have been fabricated, providing an existence proof that ultra low defect coatings are possible. Increasing the ion source-to-target distance from 410 to 560 mm to reduce undesired coating of the ion source caused the defect density to increase to 0.2/cm2. Deposition and etching diagnostic witness substrates and deposition pinhole cameras showed a much higher level of ion beam spillover (ions missing the sputter target) than expected. Future work will quantify beam spillover, and test designs to reduce spillover, if it is confirmed to be the cause of the increased defect level. The LDD system will also be upgraded to allow clean coating of standard format mask substrates. The upgrade will confirm that the low defect process developed on Si wafers is compatible with the standard mask format 152 mm square substrates, and will provide a clean supply of EUVL mask blanks needed to support development of EUVL mask patterning processes and clean mask handling technologies.

Carrier providers or killers: The case of Cu defects in CdTe

DOE PAGES

Yang, Ji -Hui; Metzger, Wyatt K.; Wei, Su -Huai

2017-07-24

Defects play important roles in semiconductors for optoelectronic applications. Common intuition is that defects with shallow levels act as carrier providers and defects with deep levels are carrier killers. Here, taking the Cu defects in CdTe as an example, we show that relatively shallow defects can play both roles. Using first-principles calculation methods combined with thermodynamic simulations, we study the dialectic effects of Cu-related defects on hole density and lifetime in bulk CdTe. Because CuCd can form a relatively shallow acceptor, we find that increased Cu incorporation into CdTe indeed can help achieve high hole density; however, too much Cumore » can cause significant non-radiative recombination. We discuss strategies to balance the contradictory effects of Cu defects based on the calculated impact of Cd chemical potential, copper defect concentrations, and annealing temperature on lifetime and hole density. Lastly, these findings advance the understanding of the potential complex defect behaviors of relatively shallow defect states in semiconductors.« less

Carrier providers or killers: The case of Cu defects in CdTe

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yang, Ji -Hui; Metzger, Wyatt K.; Wei, Su -Huai

Defects play important roles in semiconductors for optoelectronic applications. Common intuition is that defects with shallow levels act as carrier providers and defects with deep levels are carrier killers. Here, taking the Cu defects in CdTe as an example, we show that relatively shallow defects can play both roles. Using first-principles calculation methods combined with thermodynamic simulations, we study the dialectic effects of Cu-related defects on hole density and lifetime in bulk CdTe. Because CuCd can form a relatively shallow acceptor, we find that increased Cu incorporation into CdTe indeed can help achieve high hole density; however, too much Cumore » can cause significant non-radiative recombination. We discuss strategies to balance the contradictory effects of Cu defects based on the calculated impact of Cd chemical potential, copper defect concentrations, and annealing temperature on lifetime and hole density. Lastly, these findings advance the understanding of the potential complex defect behaviors of relatively shallow defect states in semiconductors.« less

Intrinsic ferromagnetism in nanocrystalline Mn-doped ZnO depending on Mn concentration.

PubMed

Subramanian, Munisamy; Tanemura, Masaki; Hihara, Takehiko; Soga, Tetsuo; Jimbo, Takashi

2011-04-01

The physical properties of Zn(1-x)Mn(x)O nanoparticles synthesized by thermal decomposition are extensively investigated by X-ray diffraction (XRD), Transmission Electron Microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Raman light scattering and Hysteresis measurements. XRD and XPS spectra reveal the absence of secondary phase in nanocrystalline ZnO doped with 5% or less Mn; and, later confirms that the valance state of Mn to be 2+ for all the samples. Raman spectra exhibit a peak at 660 cm(-1) which we attribute to the intrinsic lattice defects of ZnO with increasing Mn concentration. Overall, our results demonstrate that ferromagnetic properties can be realized while Mn-doped ZnO obtained in the nanocrystalline form.

Effects of dry etching processes on exciton and polariton characteristics in ZnTe

NASA Astrophysics Data System (ADS)

Sun, J. H.; Xie, W. B.; Shen, W. Z.; Ogawa, H.; Guo, Q. X.

2003-12-01

We have employed temperature-dependent reflection spectra to study the effects of reactive ion etching (RIE) on the exciton and polariton characteristics in ZnTe crystals exposed to CH4/H2 gases under different rf plasma powers. Classic exciton-polariton theory has been used to calculate the reflection spectra. By comparing with an as-grown ZnTe crystal and the temperature-dependent behavior, we are able to identify the excitons and RIE-induced polariton structures in these dry etched ZnTe crystals. An increase of the rf plasma power will lead to an increase of defect density in the surface damage layers, resulting in a decrease of the photon energies of the observed exciton and polariton structures.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kadria-Vili, Yara; Bachilo, Sergei M.; Blackburn, Jeffrey L.

Photoluminescence spectra of single-walled carbon nanotubes (SWCNTs) have been recorded and analyzed for selected individual nanotubes and structurally sorted bulk samples to clarify the nature of secondary emission features. Room temperature spectra show, in addition to the main peak arising from the E 11 bright exciton, three features at lower frequency, which are identified here (in descending order of energy difference from E 11 emission) as G 1, X 1, and Y 1. The weakest (G 1) is interpreted as a vibrational satellite of E 11 involving excitation of the ~1600 cm -1 G mode. The X 1 feature, althoughmore » more intense than G 1, has a peak amplitude only ~3% of E 11. X 1 emission was found to be polarized parallel to E 11 and to be separated from that peak by 1068 cm -1 in SWCNTs with natural isotopic abundance. The separation remained unchanged for several ( n,m) species, different nanotube environments, and various levels of induced axial strain. In 13C SWCNTs, the spectral separation decreased to 1023 cm -1. The measured isotopic shift points to a phonon-assisted transition that excites the D vibration. This supports prior interpretations of the X 1 band as emission from the dark K-momentum exciton, whose energy we find to be ~230 cm -1 above E 11. The remaining sideband, Y 1, is red-shifted ~300 cm -1 from E 11 and varies in relative intensity among and within individual SWCNTs. We assign it as defect-induced emission, either from an extrinsic state or from a brightened triplet state. In contrast to single-nanotube spectra, bulk samples show asymmetric zero-phonon E 11 peaks, with widths inversely related to SWCNT diameter. As a result, an empirical expression for this dependence is presented to aid the simulation of overlapped emission spectra during quantitative fluorimetric analysis of bulk SWCNT samples.« less

Photoluminescence side band spectroscopy of individual single-walled carbon nanotubes

DOE PAGES

Kadria-Vili, Yara; Bachilo, Sergei M.; Blackburn, Jeffrey L.; ...

2016-09-27

Photoluminescence spectra of single-walled carbon nanotubes (SWCNTs) have been recorded and analyzed for selected individual nanotubes and structurally sorted bulk samples to clarify the nature of secondary emission features. Room temperature spectra show, in addition to the main peak arising from the E 11 bright exciton, three features at lower frequency, which are identified here (in descending order of energy difference from E 11 emission) as G 1, X 1, and Y 1. The weakest (G 1) is interpreted as a vibrational satellite of E 11 involving excitation of the ~1600 cm -1 G mode. The X 1 feature, althoughmore » more intense than G 1, has a peak amplitude only ~3% of E 11. X 1 emission was found to be polarized parallel to E 11 and to be separated from that peak by 1068 cm -1 in SWCNTs with natural isotopic abundance. The separation remained unchanged for several ( n,m) species, different nanotube environments, and various levels of induced axial strain. In 13C SWCNTs, the spectral separation decreased to 1023 cm -1. The measured isotopic shift points to a phonon-assisted transition that excites the D vibration. This supports prior interpretations of the X 1 band as emission from the dark K-momentum exciton, whose energy we find to be ~230 cm -1 above E 11. The remaining sideband, Y 1, is red-shifted ~300 cm -1 from E 11 and varies in relative intensity among and within individual SWCNTs. We assign it as defect-induced emission, either from an extrinsic state or from a brightened triplet state. In contrast to single-nanotube spectra, bulk samples show asymmetric zero-phonon E 11 peaks, with widths inversely related to SWCNT diameter. As a result, an empirical expression for this dependence is presented to aid the simulation of overlapped emission spectra during quantitative fluorimetric analysis of bulk SWCNT samples.« less

Electronic properties of deep-level defects in proton irradiated AlGaAs-GaAs solar cells

NASA Technical Reports Server (NTRS)

Li, S. S.

1981-01-01

Deep level transient spectroscopy and capacitance voltage techniques as well as analysis of the forward current voltage (I-V) characteristics and SEM-EIC data were carried out for proton irradiated GaAs solar cells over a wide range of proton energies and proton fluences. Defect and recombination parameters such as defect energy levels and density, carrier capture cross sections and lifetimes as well as diffusion lengths in the undoped n-GaAs LPE layers were determined. Good correlation between these defect parameters and solar cell performance parameters was obtained for GaAs solar cells irradiated by 200 and 290 KeV protons. It was found that 200 to 290 KeV protons will produce the most defects and damages to the GaAs solar cell structure used. The influence of the low temperature (200 to 400 C) periodic thermal annealing on the deep level defects and the performance of the 200 KeV proton irradiated cells is discussed.

Density Functional Theory Calculations of Activation Energies for Non-radiative Carrier Capture by Deep Defect Levels in Semiconductors.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Modine, Normand Arthur; Wright, Alan F.; Lee, Stephen R.

Carrier recombination due to defects can have a major impact on device performance. The rate of defect-induced carrier recombination is determined by both defect levels and carrier capture cross-sections. Kohn-Sham density functional theory (DFT) has been widely and successfully used to predict defect levels in semiconductors and insulators, but only recently has work begun to focus on using DFT to determine carrier capture cross-sections. Lang and Henry worked out the fundamental theory of carrier-capture cross-sections in the 1970s and showed that, in most cases, room temperature carrier-capture cross-sections differ between defects primarily due to differences in the carrier capture activationmore » energies. Here, we present an approach to using DFT to calculate carrier capture activation energies that does not depend on perturbation theory or an assumed configuration coordinate, and we demonstrate this approach for the -3/-2 level of the Ga vacancy in wurtzite GaN.« less

Radiation damage and defect behavior in proton irradiated lithium-counterdoped n+p silicon solar cells

NASA Technical Reports Server (NTRS)

Stupica, John; Goradia, Chandra; Swartz, Clifford K.; Weinberg, Irving

1987-01-01

Two lithium-counterdoped n+p silicon solar cells with different lithium concentrations were irradiated by 10-MeV protons. Cell performance was measured as a function of fluence, and it was found that the cell with the highest concentration of lithium had the highest radiation resistance. Deep level transient spectroscopy which showed two deep level defects that were lithium related. Relating the defect energy levels obtained from this study with those from earlier work using 1-MeV electron irradiation shows no correlation of the defect energy levels. There is one marked similarity: the absence of the boron-interstitial-oxygen-interstitial defect. This consistency strengthens the belief that lithium interacts with oxygen to prevent the formation of the boron interstitial-oxygen interstitial defect. The results indicate that, in general, addition of lithium in small amounts to the p-base of a boron doped silicon solar cell such that the base remains p-type, tends to increase the radiation resistance of the cell.

Admittance spectroscopy or deep level transient spectroscopy: A contrasting juxtaposition

NASA Astrophysics Data System (ADS)

Bollmann, Joachim; Venter, Andre

2018-04-01

A comprehensive understanding of defects in semiconductors remains of primary importance. In this paper the effectiveness of two of the most commonly used semiconductor defect spectroscopy techniques, viz. deep level transient spectroscopy (DLTS) and admittance spectroscopy (AS) are reviewed. The analysis of defects present in commercially available SiC diodes shows that admittance spectroscopy allows the identification of deep traps with reduced measurement effort compared to deep Level Transient Spectroscopy (DLTS). Besides the N-donor, well-studied intrinsic defects were detected in these diodes. Determination of their activation energy and defect density, using the two techniques, confirm that the sensitivity of AS is comparable to that of DLTS while, due to its well defined peak shape, the spectroscopic resolution is superior. Additionally, admittance spectroscopy can analyze faster emission processes which make the study of shallow defects more practical and even that of shallow dopant levels, possible. A comparative summary for the relevant spectroscopic features of the two capacitance methods are presented.

Studies of Diamonds Using Electron Paramagnetic Resonance and Other Techniques

NASA Astrophysics Data System (ADS)

Zhang, Shigang

Studies of impurities/defects in diamonds grown with the high-temperature high-pressure technique (HTHP) and B- and P-doped diamond films using fast ion implantation and chemical evaporation have been carried out. The main technique employed in the study is electron paramagnetic resonance (EPR). Raman, laser and X-ray fluorescence are also used to characterize the samples. While other commonly used techniques such as infrared (IR) spectroscopy detect no nitrogen in an isotopically enriched ^ {12}C diamond, the clear EPR spectrum consistently measures a nitrogen concentration of about 0.05ppm by calibration against a few standards. The ^{12}C diamond is evaluated to be ideal for optical window application and studies of diamond properties. Neither the EPR lineshape nor the second moment supports a random nitrogen distribution in the ^{12}C diamond. Instead, the average nitrogen distance is found to be larger than the of the random nitrogen distribution. The g-tensor for substitutional nitrogen is found to be axially symmetric along the (111) direction with g_| - g_| = 0.00002(5). In the study of a HTHP IIb blue semiconducting diamond, neutral N is measured with a concentration of 0.02ppm. The result is not well understood since neutral nitrogen is expected to lose its extra electron to boron due to electron-hole recombination. Further studies are suggested to better understand this result. EPR studies of two sets of P-doped diamond films grown using fast ion implantation and chemical incorporation reveal that defect levels caused by diamond doping are still too high for semiconductor applications. As expected, P doping causes a defect level two orders of magnitude higher than B doping, which can be explained by the relatively larger size of P than B. The theoretical analysis based on EPR hyperfine interaction suggest that P forms a shallow donor in diamond and that the electron density at the P site is |psi(0)|^2 = 0.27 times 10^{24} cm^ {-3}. This is consistent with the temperature dependent EPR experimental results. The EPR spectra for all diamond samples I have studied are compared, revealing that the HTHP diamonds show no defect related spectrum, which are commonly observed in natural IIa and IIb diamonds. This result indicates that HTHP diamond has superior quality compared to other diamonds.

Carrier removal and defect behavior in p-type InP

NASA Technical Reports Server (NTRS)

Weinberg, I.; Swartz, C. K.; Drevinsky, P. J.

1992-01-01

A simple expression, obtained from the rate equation for defect production, was used to relate carrier removal to defect production and hole trapping rates in p-type InP after irradiation by 1-MeV electrons. Specific contributions to carrier removal from defect levels H3, H4, and H5 were determined from combined deep-level transient spectroscopy (DLTS) and measured carrier concentrations. An additional contribution was attributed to one or more defects not observed by the present DLTS measurements. The high trapping rate observed for H5 suggests that this defect, if present in relatively high concentration, could be dominant in p-type InP.

Estimated Maternal Pesticide Exposure from Drinking Water and Heart Defects in Offspring

PubMed Central

Kim, Jihye; Swartz, Michael D.; Langlois, Peter H.; Romitti, Paul A.; Weyer, Peter; Mitchell, Laura E.; Ramakrishnan, Anushuya; Malik, Sadia; Lupo, Philip J.; Feldkamp, Marcia L.; Meyer, Robert E.; Winston, Jennifer J.; Reefhuis, Jennita; Blossom, Sarah J.; Bell, Erin; Agopian, A. J.

2017-01-01

Our objective was to examine the relationship between estimated maternal exposure to pesticides in public drinking water and the risk of congenital heart defects (CHD). We used mixed-effects logistic regression to analyze data from 18,291 nonsyndromic cases with heart defects from the Texas Birth Defects Registry and 4414 randomly-selected controls delivered in Texas from 1999 through 2005. Water district-level pesticide exposure was estimated by linking each maternal residential address to the corresponding public water supply district’s measured atrazine levels. We repeated analyses among independent subjects from the National Birth Defects Prevention Study (NBDPS) (1620 nonsyndromic cases with heart defects and 1335 controls delivered from 1999 through 2005). No positive associations were observed between high versus low atrazine level and eight CHD subtypes or all included heart defects combined. These findings should be interpreted with caution, in light of potential misclassification and relatively large proportions of subjects with missing atrazine data. Thus, more consistent and complete monitoring and reporting of drinking water contaminants will aid in better understanding the relationships between pesticide water contaminants and birth defects. PMID:28786932

Estimated Maternal Pesticide Exposure from Drinking Water and Heart Defects in Offspring.

PubMed

Kim, Jihye; Swartz, Michael D; Langlois, Peter H; Romitti, Paul A; Weyer, Peter; Mitchell, Laura E; Luben, Thomas J; Ramakrishnan, Anushuya; Malik, Sadia; Lupo, Philip J; Feldkamp, Marcia L; Meyer, Robert E; Winston, Jennifer J; Reefhuis, Jennita; Blossom, Sarah J; Bell, Erin; Agopian, A J

2017-08-08

Our objective was to examine the relationship between estimated maternal exposure to pesticides in public drinking water and the risk of congenital heart defects (CHD). We used mixed-effects logistic regression to analyze data from 18,291 nonsyndromic cases with heart defects from the Texas Birth Defects Registry and 4414 randomly-selected controls delivered in Texas from 1999 through 2005. Water district-level pesticide exposure was estimated by linking each maternal residential address to the corresponding public water supply district's measured atrazine levels. We repeated analyses among independent subjects from the National Birth Defects Prevention Study (NBDPS) (1620 nonsyndromic cases with heart defects and 1335 controls delivered from 1999 through 2005). No positive associations were observed between high versus low atrazine level and eight CHD subtypes or all included heart defects combined. These findings should be interpreted with caution, in light of potential misclassification and relatively large proportions of subjects with missing atrazine data. Thus, more consistent and complete monitoring and reporting of drinking water contaminants will aid in better understanding the relationships between pesticide water contaminants and birth defects.

Cobalt related defect levels in silicon analyzed by temperature- and injection-dependent lifetime spectroscopy

NASA Astrophysics Data System (ADS)

Diez, S.; Rein, S.; Roth, T.; Glunz, S. W.

2007-02-01

Temperature- and injection-dependent lifetime spectroscopy (TIDLS) as a method to characterize point defects in silicon with several energy levels is demonstrated. An intentionally cobalt-contaminated p-type wafer was investigated by means of lifetime measurements performed at different temperatures up to 151°C. Two defect energy levels were required to model the lifetime curves on basis of the Shockley-Read-Hall statistics. The detailed analysis is based on the determination of the recently introduced defect parameter solution surface (DPSS) in order to extract the underlying defect parameters. A unique solution has been found for a deep defect level located in the upper band gap half with an energy depth of EC-Et=0.38±0.01eV, with a corresponding ratio of capture cross sections k =σn/σp=0.16 within the interval of uncertainty of 0.06-0.69. Additionally, a deep donor level in the lower band gap half known from the literature could be assigned to a second energy level within the DPSS analysis at Et-EV=0.41±0.02eV with a corresponding ratio of capture cross sections k =σn/σp=16±3. An investigation of the temperature dependence of the capture cross section for electrons suggests that the underlying recombination process of the defect in the lower band gap half is driven by a two stage cascade capture with an activation energy of ΔE =52±2meV. These results show that TIDLS in combination with DPSS analysis is a powerful method to characterize even multiple defect levels that are affecting carrier recombination lifetime in parallel.

Optical Metrology for Directed Self-assembly Patterning Using Mueller Matrix Spectroscopic Ellipsometry Based Scatterometry

NASA Astrophysics Data System (ADS)

Dixit, Dhairya J.

The semiconductor industry continues to drive patterning solutions that enable devices with higher memory storage capacity, faster computing performance, lower cost per transistors, and higher transistor density. These developments in the field of semiconductor manufacturing along with the overall minimization of the size of transistors require cutting-edge metrology tools for characterization. Directed self-assembly (DSA) patterning process can be used to fabricate nanoscale line-space patterns and contact holes via thermodynamically driven micro-phase separation of block copolymer (BCP) films with boundary constraints from guiding templates. Its main advantages are high pattern resolution (~10 nm), high throughput, no requirement of a high-resolution mask, and compatibility with standard fab-equipment and processes. Although research into DSA patterning has demonstrated a high potential as a nanoscale patterning process, there are critical challenges that must be overcome before transferring DSA into high volume manufacturing, including achievement of low defect density and high process stability. For this, advances in critical dimension (CD) and overlay measurement as well as rapid defect characterization are required. Both scatterometry and critical dimension-scanning electron microscopy (CD-SEM) are routinely used for inline dimensional metrology. CD-SEM inspection is limited, as it does not easily provide detailed line-shape information, whereas scatterometry has the capability of measuring important feature dimensions including: line-width, line-shape, sidewall-angle, and thickness of the patterned samples quickly and non-destructively. The present work describes the application of Mueller matrix spectroscopic ellipsometry (MMSE) based scatterometry to optically characterize DSA patterned line- space grating and contact hole structures fabricated with phase-separated polystyrene-b-polymethylmethacrylate (PS-b-PMMA) at various integration steps of BCP DSA based patterning process. This work focuses on understanding the efficacy of MMSE base scatterometry for characterizing complex DSA structures. For example, the use of symmetry-antisymmetry properties associated with Mueller matrix (MM) elements to understand the topography of the periodic nanostructures and measure defectivity. Simulations (the forward problem approach of scatterometry) are used to investigate MM elements' sensitivity to changes in DSA structure such as one vs. two contact hole patterns and predict sensitivity to dimensional changes. A regression-based approach is used to extract feature shape parameters of the DSA structures by fitting simulated optical spectra to experimental optical spectra. Detection of the DSA defects is a key to reducing defect density for eventual manufacturability and production use of DSA process. Simulations of optical models of structures containing defects are used to evaluate the sensitivity of MM elements to DSA defects. This study describes the application of MMSE to determine the DSA pattern defectivity via spectral comparisons based on optical anisotropy and depolarization. The use of depolarization and optical anisotropy for characterization of experimental MMSE data is a very recent development in scatterometry. In addition, reconstructed scatterometry models are used to calculate line edge roughness in 28 nm pitch Si fins fabricated using DSA patterning process.

ZnO twin-cones: synthesis, photoluminescence, and catalytic decomposition of ammonium perchlorate.

PubMed

Sun, Xuefei; Qiu, Xiaoqing; Li, Liping; Li, Guangshe

2008-05-19

ZnO twin-cones, a new member to the ZnO family, were prepared directly by a solvothermal method using a mixed solution of zinc nitrate and ethanol. The reaction and growth mechanisms of ZnO twin-cones were investigated by X-ray diffraction, UV-visible spectra, infrared and ion trap mass spectra, and transmission electron microscopy. All as-prepared ZnO cones consisted of tiny single crystals with lengths of several micrometers. With prolonging of the reaction time from 1.5 h to 7 days, the twin-cone shape did not change at all, while the lattice parameters increased slightly and the emission peak of photoluminescence shifted from the green region to the near orange region. ZnO twin-cones are also explored as an additive to promote the thermal decomposition of ammonium perchlorate. The variations of photoluminescence spectra and catalytic roles in ammonium perchlorate decomposition were discussed in terms of the defect structure of ZnO twin-cones.

Single-photon cesium Rydberg excitation spectroscopy using 318.6-nm UV laser and room-temperature vapor cell.

PubMed

Wang, Jieying; Bai, Jiandong; He, Jun; Wang, Junmin

2017-09-18

We demonstrate a single-photon Rydberg excitation spectroscopy of cesium (Cs) atoms in a room-temperature vapor cell. Cs atoms are excited directly from 6S 1/2 ground state to nP 3/2 (n = 70 - 100) Rydberg states with a 318.6 nm ultraviolet (UV) laser, and Rydberg excitation spectra are obtained by transmission enhancement of a probe beam resonant to Cs 6S 1/2 , F = 4 - 6P 3/2 , F' = 5 transition as partial population on F = 4 ground state are transferred to Rydberg state. Analysis reveals that the observed spectra are velocity-selective spectroscopy of Rydberg state, from which the amplitude and linewidth influenced by lasers' Rabi frequency have been investigated. Fitting to energies of Cs nP 3/2 (n = 70 -100) states, the determined quantum defect is 3.56671(42). The demodulated spectra can also be employed as frequency references to stabilize the UV laser frequency to specific Cs Rydberg transition.

Magnetic states of linear defects in graphene monolayers: Effects of strain and interaction

NASA Astrophysics Data System (ADS)

Alexandre, Simone S.; Nunes, R. W.

2017-08-01

The combined effects of defect-defect interaction and strains of up to 10% on the onset of magnetic states in the quasi-one-dimensional electronic states generated by the so-called 558 linear defect in graphene monolayers are investigated by means of ab initio calculations. Results are analyzed on the basis of the heuristics of the Stoner criterion. We find that conditions for the emergence of magnetic states on the 558 defect can be tuned by uniaxial tensile parallel strains (along the defect direction) as well as by uniaxial compressive perpendicular strains, at both limits of isolated and interacting 558 defects. Parallel tensile strains and perpendicular compressive strains are shown to give rise to two cooperative effects that favor the emergence of itinerant magnetism on the 558 defect in graphene: enhancement of the density of states (DOS) of the resonant defect states in the region of the Fermi level and tuning of the Fermi level to the maximum of the related DOS peak. On the other hand, parallel compressive strains and perpendicular tensile strains are shown to be detrimental to the development of magnetic states in the 558 defect, because in these cases the Fermi level is found to shift away from the maximum of the DOS of the defect states. Effects of isotropic and unisotropic biaxial strains are also analyzed in terms of the conditions encoded in the Stoner criterion.

The determination of complete human mitochondrial DNA sequences in single cells: implications for the study of somatic mitochondrial DNA point mutations

PubMed Central

Taylor, Robert W.; Taylor, Geoffrey A.; Durham, Steve E.; Turnbull, Douglass M.

2001-01-01

Studies of single cells have previously shown intracellular clonal expansion of mitochondrial DNA (mtDNA) mutations to levels that can cause a focal cytochrome c oxidase (COX) defect. Whilst techniques are available to study mtDNA rearrangements at the level of the single cell, recent interest has focused on the possible role of somatic mtDNA point mutations in ageing, neurodegenerative disease and cancer. We have therefore developed a method that permits the reliable determination of the entire mtDNA sequence from single cells without amplifying contaminating, nuclear-embedded pseudogenes. Sequencing and PCR–RFLP analyses of individual COX-negative muscle fibres from a patient with a previously described heteroplasmic COX II (T7587C) mutation indicate that mutant loads as low as 30% can be reliably detected by sequencing. This technique will be particularly useful in identifying the mtDNA mutational spectra in age-related COX-negative cells and will increase our understanding of the pathogenetic mechanisms by which they occur. PMID:11470889

Magnetic characteristics of a new cubic defect spinel Li0.5Mg0.5MnO3

NASA Astrophysics Data System (ADS)

Singh, V.; Seehra, M. S.; Manivannan, A.; Kumta, P. N.

2012-04-01

Magnetic properties of Li0.5Mg0.5MnO3-δ nanoparticles (size ≃ 20 nm) synthesized by the Pechini method are investigated using temperature dependence of its magnetization (M) and electron magnetic resonance (EMR) spectra at 9.286 GHz. Analysis of the x-ray diffraction spectra yields its structure to be a cubic defect spinel with the formula 4(Li0.5Mg0.5MnO2.75) = 3{[Li2/3Mg1/3][Mn4/3Mg1/3□1/3]O11/3]} so that Mn occupies the octahedral B-sites only. The data of M versus T yields a blocking temperature TB ≃ 9 K above which the Curie-Weiss law variation with θ = 13 K and μ = 3.96μB characteristic of Mn4+ ions is established. For T < 9 K, temperature dependent coercivity and remanence are observed. The observed temperature dependence of the EMR parameters (linewidth ΔH, resonance field Hr, and intensity Io) for T < 30 K is interpreted in terms of TB (EMR) ≃ 30 K. Formation of ferromagnetic Mn4+ clusters, resulting from the co-presence of non-magnetic Mg2+ and vacancies on the B-sites, is inferred.

Effect of annealing temperature on the structural and optical properties of CeO{sub 2}:Ni thin films

DOE Office of Scientific and Technical Information (OSTI.GOV)

Murugan, R.; Vijayaprasath, G.; Sakthivel, P.

2016-05-06

High quality Ni-doped CeO{sub 2} (CeO{sub 2}:Ni) thin films were deposited on glass substrates at room temperature by using radio frequency magnetron sputtering. The effect of annealing temperature on structural and optical properties of the CeO{sub 2}:Ni films was investigated. The structural, optical and vibrational properties of the films were determined using X-ray diffraction (XRD), photoluminescence spectrometer (PL) and Raman spectrometer. It was found that the as-deposited film has a fluorite cubic structure. By increasing annealing temperature from 100°C to 300°C, the crystalline quality of the thin films could be improved. The UV and visible band emissions were observed inmore » the photoluminescence spectra, due to exciton, defect related emissions respectively. The micro-Raman results show the characteristic peak of CeO{sub 2} F{sub 2g} at 465 cm{sup −1} and 2L0 at 1142 cm{sup −1}. Defect peaks like D and 0 bands were observed at 641 cm{sup −1} and 548 cm{sup −1} respectively. It is found from the spectra that the peak intensity of the films increased with increase of annealing temperature.« less

Research on the optical spectra, g factors and defect structures for two tetragonal Y²+ centers in the irradiated CaF₂: Y crystal.

PubMed

Zheng, Wen-Chen; Mei, Yang; Yang, Yu-Guang; Liu, Hong-Gang

2012-11-01

Based on the defect models that the tetragonal Y(2+) (1) center in the irradiated CaF(2): Y crystal is due to Y(2+) at Ca(2+) site associated with a nearest interstitial F(-) ion along C(4) axis and the tetragonal Y(2+) (2) center is Y(2+) at Ca(2+) site where the tetragonal distortion is caused by the static Jahn-Teller effect, the two optical spectral bands and anisotropic g factors for both tetragonal Y(2+) centers are calculated. The calculations are made by using two methods based on the cluster approach, one is the complete diagonalization (of energy matrix) method (CDM) and another is the perturbation theory method (PTM). The calculated results for each Y(2+) center from CDM and PTM coincide and show reasonable agreement with the experimental values. The calculated isotropic g factor for Y(2+) (2) center at higher temperature owing to the dynamical Jahn-Teller effect is also consistent with the observed value. The defect structures (i.e., tetragonal distortion) of the two Y(2+) centers are obtained from the calculation. It appears that both theoretical methods can be applied to explain the optical and EPR data, to study the defect model and to determine the defect structures for d(1) ions in crystals. Copyright © 2012 Elsevier B.V. All rights reserved.

Concentration of point defects in 4H-SiC characterized by a magnetic measurement

DOE Office of Scientific and Technical Information (OSTI.GOV)

Peng, B.; Jia, R. X., E-mail: rxjia@mail.xidian.edu.cn; Wang, Y. T.

A magnetic method is presented to characterize the concentration of point defects in silicon carbide. In this method, the concentration of common charged point defects, which is related to the density of paramagnetic centers, is determined by fitting the paramagnetic component of the specimen to the Brillouin function. Several parameters in the Brillouin function can be measured such as: the g-factor can be obtained from electron spin resonance spectroscopy, and the magnetic moment of paramagnetic centers can be obtained from positron lifetime spectroscopy combined with a first-principles calculation. To evaluate the characterization method, silicon carbide specimens with different concentrations ofmore » point defects are prepared with aluminum ion implantation. The fitting results of the densities of paramagnetic centers for the implanted doses of 1 × 10{sup 14} cm{sup −2}, 1 × 10{sup 15} cm{sup −2} and 1 × 10{sup 16} cm{sup −2} are 6.52 × 10{sup 14}/g, 1.14 × 10{sup 15}/g and 9.45 × 10{sup 14}/g, respectively. The same trends are also observed for the S-parameters in the Doppler broadening spectra. It is shown that this method is an accurate and convenient way to obtain the concentration of point defects in 4H-SiC.« less

Native defects in Tl 6SI 4: Density functional calculations

DOE PAGES

Shi, Hongliang; Du, Mao -Hua

2015-05-05

In this study, Tl 6SI 4 is a promising room-temperature semiconductor radiation detection material. Here, we report density functional calculations of native defects and dielectric properties of Tl 6SI 4. Formation energies and defect levels of native point defects and defect complexes are calculated. Donor-acceptor defect complexes are shown to be abundant in Tl 6SI 4. High resistivity can be obtained by Fermi level pinning by native donor and acceptor defects. Deep donors that are detrimental to electron transport are identified and methods to mitigate such problem are discussed. Furthermore, we show that mixed ionic-covalent character of Tl 6SI 4more » gives rise to enhanced Born effective charges and large static dielectric constant, which provides effective screening of charged defects and impurities.« less

Detection of Quadrupole Interactions by Muon Level Crossing Resonance

NASA Astrophysics Data System (ADS)

Cox, S. F. J.

1992-02-01

The positive muon proves to be a very versatile and sensitive magnetic resonance probe: implanted in virtually any material its polarisation may be monitored via the asymmetry in its radioactive decay, giving information on the sites occupied by the muon in lattices or molecules, and the local fields experienced at these sites. The scope of these experiments has been greatly extended by the development of a technique of cross relaxation or level crossing resonance which allows quadrupole splittings on nuclei adjacent to the muon to be measured. The principles of the technique and the conditions necessary for detection of the spectra are described, together with a number of applications. Of especial interest is the manner in which muons mimic the behaviour of protons in matter. In metal lattices, for instance, muons invariably adopt the same interstitial sites as do protons in the dilute hydride phases, so that they can be used to study problems of localisation and diffusion common to those of hydrogen in metals. Studies of the muon level crossing resonance in copper have given valuable information on the crystallographic site, electronic structure and low temperature mobility of the interstitial defect. In semiconductors, muons are expected to trap at other impurities - notably acceptors - in processes analogous to the passivation of dopants by hydrogen. Muon resonance offers the exciting prospect of spectroscopic study of these passivation complexes. In molecular materials, substitution of protons by muons can be thought of rather like deuteration. Muons implanted in ice produce a significant change in the quadrupole coupling constant of adjacent 17O nuclei which may be traced to the effects of the large muon zero point energy; the resonance spectrum also exhibits temperature dependent features which may be informative on the nature and lifetime of defects in the ice structure. Muon level crossing resonance has already been studied in an oxide superconductor and this relatively young field is now wide open for quadrupole interaction studies in other materials, using a variety of nuclei.

Cell performance and defect behavior in proton-irradiated lithium-counterdoped n(+)p silicon solar cells

NASA Technical Reports Server (NTRS)

Weinberg, I.; Stupica, J. W.; Swartz, C. K.; Goradia, C.

1986-01-01

Lithium-counterdoped n(+)p silicon solar cells were irradiated by 10-MeV protons, and their performance was determined as a function of fluence. It was found that the cell with the highest lithium concentration exhibited the higher radiation resistance. Deep-level transient spectroscopy studies of deep-level defects were used to identify two lithium-related defects. Defect energy levels obtained after the present 10-MeV irradiations were found to be markedly different than those observed after previous 1-MeV electron irradiations. However, the present DLTS data are consistent with previous suggestion by Weinberg et al. (1984) of a lithium-oxygen interaction which tends to inhibit formation of an interstitial boron-oxygen defect.

Optical properties of ion-beam-synthesized Au nanoparticles in SiO2 matrix

NASA Astrophysics Data System (ADS)

Hsieh, Chang-Lin; Oyoshi, Keiji; Chao, Der-Sheng; Tsai, Hsu-Sheng; Hong, Wei-Lun; Takeda, Yoshihiko; Liang, Jenq-Horng

2016-05-01

In recent years, gold (Au) nanoparticles have been synthesized via various methods and used in optical and biomedical detection. Au nanoparticles contain some remarkable dimension-dependent optical properties due to surface plasmon resonance (SPR) in Au nanoparticles which causes high absorption in visible light regions. Since SPR in well-crystallized Au nanoparticles can enhance the local electromagnetic field, it is thus expected that greater efficiency in the photoluminescence (PL) originating from oxygen deficiency centers (ODC) can be achieved in Au-implanted SiO2 matrix. In order to demonstrate the enhancement of PL, Au nanoparticles were formed in SiO2 film using ion beam synthesis and their optical and microstructural properties were also investigated in this study. The results revealed that a clear absorption peak at approximately 530 nm was identified in the UV-Vis spectra and was attributed to SPR induced by Au nanoparticles in SiO2. The SPR of Au nanoparticles is also dependent on thermal treatment conditions, such as post-annealing temperature and ambient. The Au nanoparticle-containing SiO2 film also displayed several distinctive peaks at approximately 320, 360, 460, and 600 nm in the PL spectra and were found to be associated with ODC-related defects and non-bridging oxygen hole centers (NBOHC) in SiO2. In addition, the PL peak intensities increased as post-annealing temperature increased, a finding contradictory to the defect recovery but highly consistent with the SPR tendency. A maximum PL emission was achieved when the Au-implanted SiO2 film was annealed at 1100 °C for 1 h under N2. Therefore, the existence of Au nanoparticles in SiO2 film can induce SPR effects as well as enhance PL emission resulting from defect-related luminescence centers.

Electrically active defects in p-type silicon after alpha-particle irradiation

NASA Astrophysics Data System (ADS)

Danga, Helga T.; Auret, F. Danie; Tunhuma, Shandirai M.; Omotoso, Ezekiel; Igumbor, Emmanuel; Meyer, Walter E.

2018-04-01

In this work, we investigated the defects introduced when boron (B) doped silicon (Si) was irradiated by making use of a 5.4 MeV americium (Am) 241 foil radioactive source with a fluence rate of 7×106 cm-2 s-1 at room temperature. Deep level transient spectroscopy (DLTS) and Laplace-DLTS measurements were used to investigate the electronic properties of the introduced defects. After exposure at a fluence of 5.1×1010 cm-2, the energy levels of the hole traps measured were: H(0.10), H(0.16), H(0.33) and H(0.52) The defect level H(0.10) was tri-vacancy related. H(0.33) was identified as the interstitial carbon (Ci) related defect which was a result of radiation induced damage. H(0.52) was a B-related defect. Explicit deductions about the origin of H(0.16) have not yet been achieved.

RESIDENTIAL RADON AND BIRTH DEFECTS: A POPULATION-BASED ASSESSMENT

PubMed Central

Langlois, Peter H; Lee, MinJae; Lupo, Philip J; Rahbar, Mohammad H; Cortez, Ruben K

2015-01-01

BACKGROUND Associations have been reported between maternal radiation exposure and birth defects. No such studies were found on radon. Our objective was to determine if there is an association between living in areas with higher radon levels and birth defects. METHODS The Texas Birth Defects Registry provided data on all birth defects from 1999–2009 from the entire state. Mean radon levels by geologic region came from the Texas Indoor Radon Survey. The association between radon and birth defects was estimated using multilevel mixed effect Poisson regression. RESULTS Birth defects overall were not associated with residential radon levels. Of the 100 other birth defect groups with at least 500 cases, 14 were significantly elevated in areas with high mean radon level in crude analyses, and 9 after adjustment for confounders. Cleft lip with/without cleft palate had an adjusted prevalence ratio (aPR) of 1.16 per 1 picoCurie/liter (pCi/l) increase in exposure to region mean radon, 95% confidence interval (CI) 1.08, 1.26. Cystic hygroma / lymphangioma had an aPR of 1.22 per 1 pCi/l increase, 95% CI 1.02, 1.46. Other associations were suggested but not as consistent: three skeletal defects, Down syndrome, other specified anomalies of the brain, and other specified anomalies of the bladder and urethra. CONCLUSIONS In the first study of residential radon and birth defects, we found associations with cleft lip w/wo cleft palate and cystic hygroma / lymphangioma. Other associations were suggested. The ecological nature of this study and multiple comparisons suggest that our results be interpreted with caution. PMID:25846606

Size-dependent structural transformations of hematite nanoparticles. 1. Phase transition.

PubMed

Chernyshova, I V; Hochella, M F; Madden, A S

2007-04-14

Using Fourier Transform InfraRed (FTIR) spectroscopy, Raman spectroscopy, X-ray diffraction (XRD), and Transmission Electron Microscopy (TEM), we characterize the structure and/or morphology of hematite (alpha-Fe(2)O(3)) particles with sizes of 7, 18, 39 and 120 nm. It is found that these nanoparticles possess maghemite (gamma-Fe(2)O(3))-like defects in the near surface regions, to which a vibrational mode at 690 cm(-1), active both in FTIR and Raman spectra, is assigned. The fraction of the maghemite-like defects and the net lattice disorder are inversely related to the particle size. However, the effect is opposite for nanoparticles grown by sintering of smaller hematite precursors under conditions when the formation of a uniform hematite-like structure throughout the aggregate is restricted by kinetic issues. This means that not only particle size but also the growth kinetics determines the structure of the nanoparticles. The observed structural changes are interpreted as size-induced alpha-Fe(2)O(3)<-->gamma-Fe(2)O(3) phase transitions. We develop a general model that considers spinel defects and absorbed/adsorbed species (in our case, hydroxyls) as dominant controls on structural changes with particle size in hematite nanoparticles, including solid-state phase transitions. These changes are represented by trajectories in a phase diagram built in three phase coordinates-concentrations of spinel defects, absorbed impurities, and adsorbed species. The critical size for the onset of the alpha-->gamma phase transition depends on the particle environment, and for the dry particles used in this study is about 40 nm. The model supports the existence of intermediate phases (protohematite and hydrohematite) during dehydration of goethite. We also demonstrate that the hematite structure is significantly less defective when the nanoparticles are immersed in water or KBr matrix, which is explained by the effects of the electrochemical double layer and increased rigidity of the particle environment. Finally, we revise the problem of applicability of IR spectroscopy to the lattice vibrations of hematite nanoparticles, demonstrating that structural comparison of different samples is much more reliable if it is based on the E(u) band at about 460 cm(-1) and the spinel band at 690 cm(-1), instead of the A(2u)/E(u) band at about 550 cm(-1) used in previous work. The new methodology is applied to analysis of the reported IR spectra of Martian hematite.

Leveling coatings for reducing the atomic oxygen defect density in protected graphite fiber epoxy composites

NASA Astrophysics Data System (ADS)

Jaworske, D. A.; Degroh, Kim K.; Podojil, G.; McCollum, T.; Anzic, J.

1992-11-01

Pinholes or other defect sites in a protective oxide coating provide pathways for atomic oxygen in low Earth orbit to reach underlying material. One concept of enhancing the lifetime of materials in low Earth orbit is to apply a leveling coating to the material prior to applying any reflective and protective coatings. Using a surface tension leveling coating concept, a low viscosity epoxy was applied to the surface of several composite coupons. A protective layer of 1000 A of SiO2 was deposited on top of the leveling coating, and the coupons were exposed to an atomic oxygen environment in a plasma asher. Pinhole populations per unit area were estimated by counting the number of undercut sites observed by scanning electron microscopy. Defect density values of 180,000 defects/sq cm were reduced to about 1000 defects/sq cm as a result of the applied leveling coating. These improvements occur at a mass penalty of about 2.5 mg/sq cm.

Leveling coatings for reducing the atomic oxygen defect density in protected graphite fiber epoxy composites

NASA Technical Reports Server (NTRS)

Jaworske, D. A.; Degroh, K. K.; Podojil, G.; Mccollum, T.; Anzic, J.

1992-01-01

Pinholes or other defect sites in a protective oxide coating provide pathways for atomic oxygen in low Earth orbit to reach underlying material. One concept for enhancing the lifetime of materials in low Earth orbits is to apply a leveling coating to the material prior to applying any reflective and protective coatings. Using a surface tension leveling coating concept, a low viscosity epoxy was applied to the surface of several composite coupons. A protective layer of 1000 A of SiO2 was deposited on top of the leveling coating, and the coupons were exposed to an atomic oxygen environment in a plasma asher. Pinhole populations per unit area were estimated by counting the number of undercut sites observed by scanning electron microscopy. Defect density values of 180,000 defects/sq cm were reduced to about 1000 defects/sq cm as a result of the applied leveling coating. These improvements occur at a mass penalty of about 2.5 mg/sq cm.

Leveling coatings for reducing the atomic oxygen defect density in protected graphite fiber epoxy composites

NASA Technical Reports Server (NTRS)

Jaworske, D. A.; Degroh, Kim K.; Podojil, G.; Mccollum, T.; Anzic, J.

1992-01-01

Pinholes or other defect sites in a protective oxide coating provide pathways for atomic oxygen in low Earth orbit to reach underlying material. One concept of enhancing the lifetime of materials in low Earth orbit is to apply a leveling coating to the material prior to applying any reflective and protective coatings. Using a surface tension leveling coating concept, a low viscosity epoxy was applied to the surface of several composite coupons. A protective layer of 1000 A of SiO2 was deposited on top of the leveling coating, and the coupons were exposed to an atomic oxygen environment in a plasma asher. Pinhole populations per unit area were estimated by counting the number of undercut sites observed by scanning electron microscopy. Defect density values of 180,000 defects/sq cm were reduced to about 1000 defects/sq cm as a result of the applied leveling coating. These improvements occur at a mass penalty of about 2.5 mg/sq cm.

Non-radiative carrier recombination enhanced by two-level process: A first-principles study

DOE PAGES

Yang, Ji -Hui; Shi, Lin; Wang, Lin -Wang; ...

2016-02-16

In this study, non-radiative recombination plays an important role in the performance of optoelectronic semiconductor devices such as solar cells and light-emitting diodes. Most textbook examples assume that the recombination process occurs through a single defect level, where one electron and one hole are captured and recombined. Based on this simple picture, conventional wisdom is that only defect levels near the center of the bandgap can be effective recombination centers. Here, we present a new two-level recombination mechanism: first, one type of carrier is captured through a defect level forming a metastable state; then the local defect configuration rapidly changesmore » to a stable state, where the other type of carrier is captured and recombined through another defect level. This novel mechanism is applied to the recombination center Te 2+ cd in CdTe. We show that this two-level process can significantly increase the recombination rate (by three orders of magnitude) in agreement with experiments. We expect that this two-level recombination process can exist in a wide range of semiconductors, so its effect should be carefully examined in characterizing optoelectronic materials.« less

Defect Detection in Textures through the Use of Entropy as a Means for Automatically Selecting the Wavelet Decomposition Level.

PubMed

Navarro, Pedro J; Fernández-Isla, Carlos; Alcover, Pedro María; Suardíaz, Juan

2016-07-27

This paper presents a robust method for defect detection in textures, entropy-based automatic selection of the wavelet decomposition level (EADL), based on a wavelet reconstruction scheme, for detecting defects in a wide variety of structural and statistical textures. Two main features are presented. One of the new features is an original use of the normalized absolute function value (NABS) calculated from the wavelet coefficients derived at various different decomposition levels in order to identify textures where the defect can be isolated by eliminating the texture pattern in the first decomposition level. The second is the use of Shannon's entropy, calculated over detail subimages, for automatic selection of the band for image reconstruction, which, unlike other techniques, such as those based on the co-occurrence matrix or on energy calculation, provides a lower decomposition level, thus avoiding excessive degradation of the image, allowing a more accurate defect segmentation. A metric analysis of the results of the proposed method with nine different thresholding algorithms determined that selecting the appropriate thresholding method is important to achieve optimum performance in defect detection. As a consequence, several different thresholding algorithms depending on the type of texture are proposed.

Circuit analysis method for thin-film solar cell modules

NASA Technical Reports Server (NTRS)

Burger, D. R.

1985-01-01

The design of a thin-film solar cell module is dependent on the probability of occurrence of pinhole shunt defects. Using known or assumed defect density data, dichotomous population statistics can be used to calculate the number of defects expected in a module. Probability theory is then used to assign the defective cells to individual strings in a selected series-parallel circuit design. Iterative numerical calculation is used to calcuate I-V curves using cell test values or assumed defective cell values as inputs. Good and shunted cell I-V curves are added to determine the module output power and I-V curve. Different levels of shunt resistance can be selected to model different defect levels.

Condition Assessment of Kevlar Composite Materials Using Raman Spectroscopy

NASA Technical Reports Server (NTRS)

Washer, Glenn; Brooks, Thomas; Saulsberry, Regor

2007-01-01

This viewgraph presentation includes the following main concepts. Goal: To evaluate Raman spectroscopy as a potential NDE tool for the detection of stress rupture in Kevlar. Objective: Test a series of strand samples that have been aged under various conditions and evaluate differences and trends in the Raman response. Hypothesis: Reduction in strength associated with stress rupture may manifest from changes in the polymer at a molecular level. If so, than these changes may effect the vibrational characteristics of the material, and consequently the Raman spectra produced from the material. Problem Statement: Kevlar composite over-wrapped pressure vessels (COPVs) on the space shuttles are greater than 25 years old. Stress rupture phenomena is not well understood for COPVs. Other COPVs are planned for hydrogen-fueled vehicles using Carbon composite material. Raman spectroscopy is being explored as an non-destructive evaluation (NDE) technique to predict the onset of stress rupture in Kevlar composite materials. Test aged Kevlar strands to discover trends in the Raman response. Strength reduction in Kevlar polymer will manifest itself on the Raman spectra. Conclusions: Raman spectroscopy has shown relative changes in the intensity and FWHM of the 1613 cm(exp -1) peak. Reduction in relative intensity for creep, fleet leader, and SIM specimens compared to the virgin strands. Increase in FWHM has been observed for the creep and fleet leader specimens compared to the virgin strands. Changes in the Raman spectra may result from redistributing loads within the material due to the disruption of hydrogen bonding between crystallites or defects in the crystallites from aging the Kevlar strands. Peak shifting has not been observed to date. Analysis is ongoing. Stress measurements may provide a tool in the short term.

Electricity generation from defective tomatoes.

PubMed

Shrestha, Namita; Fogg, Alex; Wilder, Joseph; Franco, Daniel; Komisar, Simeon; Gadhamshetty, Venkataramana

2016-12-01

The United States faces a significant burden in treating 0.61billionkg of defective tomatoes (culls) every year. We present a proof-of-concept for generating electricity from culled tomatoes in microbial-electrochemical systems (MESs). This study delineates impedance behavior of the culled tomatoes in MESs and compares its impedance spectra with that of soluble substrates (dextrose, acetate, and wastewater). A series of AC and DC diagnostic tests have revealed the superior performance of the culled tomatoes compared to the pure substrates. Cyclic voltammetry results have indicated the active role of indigenous, diffusible redox-active pigments in the culled tomatoes on overall electricity production. Electrochemical impedance spectroscopy results have elucidated the role of peel and seed on the oxidation behavior of the culled tomatoes. Copyright © 2016 Elsevier B.V. All rights reserved.

Fabrication of Worm-Like Nanorods and Ultrafine Nanospheres of Silver Via Solid-State Photochemical Decomposition

PubMed Central

2009-01-01

Worm-like nanorods and nanospheres of silver have been synthesized by photochemical decomposition of silver oxalate in water by UV irradiation in the presence of CTAB and PVP, respectively. No external seeds have been employed for the synthesis of Ag nanorods. The synthesized Ag colloids have been characterized by UV-visible spectra, powder XRD, HRTEM, and selected area electron diffraction (SAED). Ag nanospheres of average size around 2 nm have been obtained in the presence of PVP. XRD and TEM analyses revealed that top and basal planes of nanorods are bound with {111} facets. Williamson–Hall plot has revealed the presence of defects in the Ag nanospheres and nanorods. Formation of defective Ag nanocrystals is attributed to the heating effect of UV-visible irradiation. PMID:20596513

The Effect of Low Energy Nitrogen Ion Implantation on Graphene Nanosheets

NASA Astrophysics Data System (ADS)

Mishra, Mukesh; Alwarappan, Subbiah; Kanjilal, Dinakar; Mohanty, Tanuja

2018-03-01

Herein, we report the effect 50 keV nitrogen ion implantation at varying fluence on the optical properties of graphene nanosheets (number of layers < 5). Initially, graphene nanosheets synthesized by the direct liquid exfoliation of graphite layers were deposited on a cleaned Si-substrate by drop cast method. These graphene nanosheets are implanted with 50 keV nitrogen-ion beam at six different fluences. Raman spectroscopic results show that the D, D' and G peak get broadened up to the nitrogen ion fluence of 1 × 1015 ions/cm2, while 2D peak of graphene nanosheets disappeared for nitrogen-ions have fluence more than 1014 ions/cm2. However, further increase of fluence causes the indistinguishable superimposition of D, D' and G peaks. Surface contact potential value analysis for ion implanted graphene nanosheets shows the increase in defect concentration from 1.15 × 1012 to 1.98 × 1014 defects/cm2 with increasing the nitrogen ion fluence, which resembles the Fermi level shift towards conduction band. XRD spectra confirmed that the crystallinity of graphene nanosheets was found to tamper with increasing fluence. These results revealed that the limit of nitrogen ion implantation resistant on the vibrational behaviors for graphene nanosheets was 1015 ions/cm2 that opens up the scope of application of graphene nanosheets in device fabrication for ion-active environment and space applications.

Effects of oxygen vacancies on the structural and optical properties of β-Ga2O3

PubMed Central

Dong, Linpeng; Jia, Renxu; Xin, Bin; Peng, Bo; Zhang, Yuming

2017-01-01

The structural, electronic, and optical properties of β-Ga2O3 with oxygen vacancies are studied by employing first-principles calculations based on density function theory. Based on the defects formation energies, we conclude the oxygen vacancies are most stable in their fully charge states. The electronic structures and optical properties of β-Ga2O3 are calculated by Generalized Gradient Approximation + U formalisms with the Hubbard U parameters set 7.0 eV and 8.5 eV for Ga and O ions, respectively. The calculated bandgap is 4.92 eV, which is consistent with the experimental value. The static real dielectric constants of the defective structures are increased compared with the intrinsic one, which is attributed to the level caused by the Ga-4s states in the bandgap. Extra peaks are introduced in the absorption spectra, which are related to Ga-4s and O-2p states. Experimentally, β-Ga2O3 films are deposited under different O2 volume percentage with ratio-frequency magnetron sputtering method. The measured results indicate that oxygen vacancies can induce extra emission peaks in the photoluminescence spectrum, the location of these peaks are close to the calculated results. Extra O2 can increase the formation energies of oxygen vacancies and thus reduce oxygen vacancies in β-Ga2O3. PMID:28065936

Effects of oxygen vacancies on the structural and optical properties of β-Ga2O3.

PubMed

Dong, Linpeng; Jia, Renxu; Xin, Bin; Peng, Bo; Zhang, Yuming

2017-01-09

The structural, electronic, and optical properties of β-Ga 2 O 3 with oxygen vacancies are studied by employing first-principles calculations based on density function theory. Based on the defects formation energies, we conclude the oxygen vacancies are most stable in their fully charge states. The electronic structures and optical properties of β-Ga 2 O 3 are calculated by Generalized Gradient Approximation + U formalisms with the Hubbard U parameters set 7.0 eV and 8.5 eV for Ga and O ions, respectively. The calculated bandgap is 4.92 eV, which is consistent with the experimental value. The static real dielectric constants of the defective structures are increased compared with the intrinsic one, which is attributed to the level caused by the Ga-4s states in the bandgap. Extra peaks are introduced in the absorption spectra, which are related to Ga-4s and O-2p states. Experimentally, β-Ga 2 O 3 films are deposited under different O 2 volume percentage with ratio-frequency magnetron sputtering method. The measured results indicate that oxygen vacancies can induce extra emission peaks in the photoluminescence spectrum, the location of these peaks are close to the calculated results. Extra O 2 can increase the formation energies of oxygen vacancies and thus reduce oxygen vacancies in β-Ga 2 O 3 .

Exciton Level Structure and Dynamics in Tubular Porphyrin Aggregates

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wan, Yan; Stradomska, Anna; Fong, Sarah

2014-10-30

We present an account of the optical properties of the Frenkel excitons in self-assembled porphyrin tubular aggregates that represent an analog to natural photosynthetic antennae. Using a combination of ultrafast optical spectroscopy and stochastic exciton modeling, we address both linear and nonlinear exciton absorption, relaxation pathways, and the role of disorder. The static disorder-dominated absorption and fluorescence line widths show little temperature dependence for the lowest excitons (Q band), which we successfully simulate using a model of exciton scattering on acoustic phonons in the host matrix. Temperature-dependent transient absorption of and fluorescence from the excitons in the tubular aggregates aremore » marked by nonexponential decays with time scales ranging from a few picoseconds to a few nanoseconds, reflecting complex relaxation mechanisms. Combined experimental and theoretical investigations indicate that nonradiative pathways induced by traps and defects dominate the relaxation of excitons in the tubular aggregates. We model the pumpprobe spectra and ascribe the excited-state absorption to transitions from one-exciton states to a manifold of mixed one- and two-exciton states. Our results demonstrate that while the delocalized Frenkel excitons (over 208 (1036) molecules for the optically dominant excitons in the Q (B) band) resulting from strong intermolecular coupling in these aggregates could potentially facilitate efficient energy transfer, fast relaxation due to defects and disorder probably present a major limitation for exciton transport over large distances.« less

Exciton polariton spectra and limiting factors for the room-temperature photoluminescence efficiency in ZnO

NASA Astrophysics Data System (ADS)

Chichibu, S. F.; Uedono, A.; Tsukazaki, A.; Onuma, T.; Zamfirescu, M.; Ohtomo, A.; Kavokin, A.; Cantwell, G.; Litton, C. W.; Sota, T.; Kawasaki, M.

2005-04-01

Static and dynamic responses of excitons in state-of-the-art bulk and epitaxial ZnO are reviewed to support the possible realization of polariton lasers, which are coherent and monochromatic light sources due to Bose condensation of exciton-polaritons in semiconductor microcavities (MCs). To grasp the current problems and to pave the way for obtaining ZnO epilayers of improved quality, the following four principal subjects are treated: (i) polarized optical reflectance (OR), photoreflectance (PR) and photoluminescence (PL) spectra of the bulk and epitaxial ZnO were recorded at 8 K. Energies of PR resonances corresponded to those of upper and lower exciton-polariton branches, where A-, B- and C-excitons couple simultaneously to an electromagnetic wave. PL peaks due to the corresponding polariton branches were observed. Longitudinal-transverse splittings (ωLT) of the corresponding excitons were 1.5, 11.1 and 13.1 meV, respectively. The latter two values are more than two orders of magnitude greater than that of GaAs being 0.08 meV. (ii) Using these values and material parameters, corresponding vacuum-field Rabi splitting of exciton-polaritons coupled to a model MC mode was calculated to be 191 meV, which is the highest value ever reported for semiconductor MCs and satisfies the requirements to observe the strong exciton-light coupling regime necessary for polariton lasing above room temperature. (iii) Polarized OR and PR spectra of an out-plane nonpolar (1\\,1\\,\\bar{2}\\,0) ZnO epilayer grown by laser-assisted molecular beam epitaxy (L-MBE) were measured, since ZnO quantum wells (QWs) grown in nonpolar orientations are expected to show higher emission efficiencies due to the elimination of spontaneous and piezoelectric polarization fields normal to the QW plane. They exhibited in-plane anisotropic exciton resonances according to the polarization selection rules for anisotropically-strained wurzite material. (iv) Impacts of point defects on the nonradiative processes in L-MBE ZnO were studied using time-resolved PL making a connection with the results of positron annihilation measurement. Free excitonic PL intensity at room temperature naturally increased with the increase in nonradiative lifetime (τnr). The value of τnr increased and density or size of Zn vacancies (VZn) decreased with increasing growth temperature (Tg) in heteroepitaxial films grown on a ScAlMgO4 substrate, and the use of homoepitaxial substrates further reduced VZn density. The value of τnr was shown to increase with the decrease in gross density of positively and negatively charged and neutral point defects including complexes rather than with the decrease in VZn density. The results indicate that the nonradiative recombination process is governed not by single point defects, but by certain defects introduced with the incorporation of VZn, such as VZn-defect complexes. As a result of defect elimination by growing the films at high Tg followed by subsequent post-growth in situ annealing, combined with the use of high-temperature-annealed ZnO self-buffer layer, a record long τnr for spontaneous emission of 3.8 ns was obtained at room temperature. By using progressively improving epitaxial growth methods, the polariton laser effect is expected to be observed at room temperature in the near future.

First Principles Study of Chemically Functionalized Graphene

NASA Astrophysics Data System (ADS)

Jha, Sanjiv; Vasiliev, Igor

2015-03-01

The electronic, structural and vibrational properties of carbon nanomaterials can be affected by chemical functionalization. We applied ab initio computational methods based on density functional theory to study the covalent functionalization of graphene with benzyne, carboxyl groups and tetracyanoethylene oxide (TCNEO). Our calculations were carried out using the SIESTA and Quantum-ESPRESSO electronic structure codes combined with the local density and generalized gradient approximations for the exchange correlation functional and norm-conserving Troullier-Martins pseudopotentials. The simulated Raman and infrared spectra of graphene functionalized with carboxyl groups and TCNEO were consistent with the available experimental results. The computed vibrational spectra of graphene functionalized with carboxyl groups showed that the presence of point defects near the functionalization site affects the Raman and infrared spectroscopic signatures of functionalized graphene. Supported by NSF CHE-1112388.

Hyperfine Interactions in the Electron Paramagnetic Resonance Spectra of Point Defects in Wide-Band-Gap Semiconductors

DTIC Science & Technology

2014-09-18

compensation) during growth due to their preferred trivalent charge states. The electron paramagnetic resonance spectrum of the singly ionized chromium ...neutral nitrogen acceptor in ZnO . . . . . . . . . . . . . . . . . . 45 16 Spectrum of the singly ionized chromium acceptor in TiO2 . . . . . . . . . 49...is a single crystal of magnesium oxide that has been doped with chromium . Chromium Cr3+ substitutes for magnesium Mg2+ and creates a paramagnetic

Spin noise spectroscopy of ZnO

NASA Astrophysics Data System (ADS)

Horn, H.; Berski, F.; Balocchi, A.; Marie, X.; Mansur-Al-Suleiman, M.; Bakin, A.; Waag, A.; Hübner, J.; Oestreich, M.

2013-12-01

We investigate the thermal equilibrium dynamics of electron spins bound to donors in nanoporous ZnO by optical spin noise spectroscopy. The spin noise spectra reveal two noise contributions: A weak spin noise signal from undisturbed localized donor electrons with a dephasing time of 24 ns due to hyperfine interaction and a strong spin noise signal with a spin dephasing time of 5 ns which we attribute to localized donor electrons which interact with lattice defects.

Effect of varying Ga content in ZnO:GaN solid solution synthesized by solution combustion technique for photocatalytic applications

NASA Astrophysics Data System (ADS)

Menon, Sumithra Sivadas; Janani, R.; Baskar, K.; Gupta, Bhavana; Singh, Shubra

2017-05-01

ZnO:GaN (oxy)nitride solid solution has been established as the most efficient non-oxide photocatalyst for water splitting under visible irradiation with one step photoexcitation and also boasts a band gap tunability from 2.8 eV to 2.5 eV[1]. The solid solution of GaN in ZnO is formed by the intersubstitution of few of Zn/O ions by Ga/N ions, and this results in the introduction of new defect levels above the valence band which narrows the effective band gap enabling activity under visible region of spectra. In this work, we report the synthesis of ZnO:GaN solid solution by a solution combustion technique where metal nitrates and urea are used as precursors. The Zn/Ga ratio was varied from 16 to 1 in the precursors. The as synthesized samples were characterized as phase pure by X-ray diffraction, where the wurtzite structure was retained up to Zn/Ga ratio of 5. The Diffuse reflectance spectroscopy studies revealed that as the Ga content in the solid solution increases there is a reduction in band gap, from 2.9 eV to 2.4 eV. The reduced band gap of the samples facilitates its photocatalytic activity under visible region of the spectra as evaluated by photoelectrochemical measurements.

Molecular beam epitaxial growth, transmittance and photoluminescence spectra of zinc-blende CdTe thin films with high-quality on perovskite SrTiO3 (1 1 1) substrates

NASA Astrophysics Data System (ADS)

Song, Kun; Zhu, Xuanting; Tang, Kai; Bai, W.; Zhu, Liangqing; Yang, Jing; Zhang, Yuanyuan; Tang, Xiaodong; Chu, Junhao

2018-03-01

High-crystalline quality CdTe thin films are grown on the largely lattice-mismatched SrTiO3 (STO) (1 1 1) substrates by molecular beam epitaxy. A transformation from a three dimensional regime to a two dimensional one is observed by the reflection high energy electron diffraction (RHEED) and atomic force microscopy (AFM). The formation of an elastic deformation CdTe layer on STO (1 1 1), namely a pseudomorphic growth mode with a critical thickness of ∼40 nm, is supported by the RHEED, AFM and X-ray diffraction. Crystal structures and epitaxial relationships of CdTe epitaxial films on STO (1 1 1) are characterized by 2θ-ω scans and reciprocal space mapping. Two strong absorption peaks at the energies of ∼1.621 eV and ∼1.597 eV at 5 K are clearly observed for a ∼120 nm thick CdTe epitaxial film, which are proposed to be ascribed to the strained and unstrained epitaxial CdTe layers, respectively. Moreover, the presence of the exciton band while the absence of deep level defect states for the ∼120 nm thick CdTe film characterized by the temperature dependent photoluminescence spectra further supports the high-crystalline quality.

Toward Intelligent Software Defect Detection

NASA Technical Reports Server (NTRS)

Benson, Markland J.

2011-01-01

Source code level software defect detection has gone from state of the art to a software engineering best practice. Automated code analysis tools streamline many of the aspects of formal code inspections but have the drawback of being difficult to construct and either prone to false positives or severely limited in the set of defects that can be detected. Machine learning technology provides the promise of learning software defects by example, easing construction of detectors and broadening the range of defects that can be found. Pinpointing software defects with the same level of granularity as prominent source code analysis tools distinguishes this research from past efforts, which focused on analyzing software engineering metrics data with granularity limited to that of a particular function rather than a line of code.

Relating the defect band gap and the density functional band gap

NASA Astrophysics Data System (ADS)

Schultz, Peter; Edwards, Arthur

2014-03-01

Density functional theory (DFT) is an important tool to probe the physics of materials. The Kohn-Sham (KS) gap in DFT is typically (much) smaller than the observed band gap for materials in nature, the infamous ``band gap problem.'' Accurate prediction of defect energy levels is often claimed to be a casualty--the band gap defines the energy scale for defect levels. By applying rigorous control of boundary conditions in size-converged supercell calculations, however, we compute defect levels in Si and GaAs with accuracies of ~0.1 eV, across the full gap, unhampered by a band gap problem. Using GaAs as a theoretical laboratory, we show that the defect band gap--the span of computed defect levels--is insensitive to variations in the KS gap (with functional and pseudopotential), these KS gaps ranging from 0.1 to 1.1 eV. The defect gap matches the experimental 1.52 eV gap. The computed defect gaps for several other III-V, II-VI, I-VII, and other compounds also agree with the experimental gap, and show no correlation with the KS gap. Where, then, is the band gap problem? This talk presents these results, discusses why the defect gap and the KS gap are distinct, implying that current understanding of what the ``band gap problem'' means--and how to ``fix'' it--need to be rethought. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Company, for the U.S. Department of Energy's NNSA under contract DE-AC04-94AL85000.

Lattice defects of ZnO and hybrids with GO: Characterization, EPR and optoelectronic properties

NASA Astrophysics Data System (ADS)

Ahmed, Gulzar; Hanif, Muddasir; Mahmood, Khalid; Yao, Rihui; Ning, Honglong; jiao, Dongling; Wu, Mingmei; Khan, Javid; Liu, Zhongwu

2018-02-01

We have prepared and combined ZnO nanoparticles (ZnO-NPs) with different graphene oxide (GO) contents (10%, 20% and 30%) via microwave processing. The procedure provided well-dispersed ZnO-NPs between and onto the rGO layers (GZCs). The annealing temperature and graphene oxide contents affected the UV-Vis absorption, PL emission, defect-states of the ZnO, EPR signals, photo-electrochemical response and charge transfer properties. The HRTEM microscopy images of the GZCs showed interpenetrating structures and clearly visible vacancy defects. The results indicated that the defect sites (Zn interstitials, oxygen vacancy, ionized zinc vacancy and oxygen interstitials) significantly decreased after hybridization with GO. The photo-conversion efficiency of the GZC-10% (η = 13.1 x 10-3%), is 13 times higher than the ZnO-NPs (η = 1.02 x 10-3%) illustrating higher exciton production and separation efficiency of the GZCs under photo-excitation. The GZC-10% has lower (8-15 Ω) charge transfer resistance (Rct) compared to all the GZCs under same experimental conditions, therefore an important reason of better performance of the GZC 10%. The EPR spectra showed presence of radicals in all the samples with GZC 10% most intense signal among the different GZCs.

Characterization and manipulation of individual defects in insulating hexagonal boron nitride using scanning tunnelling microscopy.

PubMed

Wong, Dillon; Velasco, Jairo; Ju, Long; Lee, Juwon; Kahn, Salman; Tsai, Hsin-Zon; Germany, Chad; Taniguchi, Takashi; Watanabe, Kenji; Zettl, Alex; Wang, Feng; Crommie, Michael F

2015-11-01

Defects play a key role in determining the properties and technological applications of nanoscale materials and, because they tend to be highly localized, characterizing them at the single-defect level is of particular importance. Scanning tunnelling microscopy has long been used to image the electronic structure of individual point defects in conductors, semiconductors and ultrathin films, but such single-defect electronic characterization remains an elusive goal for intrinsic bulk insulators. Here, we show that individual native defects in an intrinsic bulk hexagonal boron nitride insulator can be characterized and manipulated using a scanning tunnelling microscope. This would typically be impossible due to the lack of a conducting drain path for electrical current. We overcome this problem by using a graphene/boron nitride heterostructure, which exploits the atomically thin nature of graphene to allow the visualization of defect phenomena in the underlying bulk boron nitride. We observe three different defect structures that we attribute to defects within the bulk insulating boron nitride. Using scanning tunnelling spectroscopy we obtain charge and energy-level information for these boron nitride defect structures. We also show that it is possible to manipulate the defects through voltage pulses applied to the scanning tunnelling microscope tip.

Clustered vacancies in ZnO: chemical aspects and consequences on physical properties

NASA Astrophysics Data System (ADS)

Pal, S.; Gogurla, N.; Das, Avishek; Singha, S. S.; Kumar, Pravin; Kanjilal, D.; Singha, A.; Chattopadhyay, S.; Jana, D.; Sarkar, A.

2018-03-01

The chemical nature of point defects, their segregation, cluster or complex formation in ZnO is an important area of investigation. The evolution of a defective state with MeV Ar ion irradiation fluence 1  ×  1014 and 1  ×  1016 ions cm-2 has been monitored here using x-ray photoelectron spectroscopy (XPS), photoluminescence (PL) and Raman spectroscopy. The XPS study shows the presence of oxygen vacancies (V O) in Ar irradiated ZnO. Zn(LMM) Auger spectra clearly identifies a transition involving metallic zinc in the irradiated samples. An intense PL emission from interstitial Zn (I Zn)-related shallow donor bound excitons (DBX) is visible in the 10 K spectra for all samples. Although overall PL is largely reduced with irradiation disorder, DBX intensity is increased for the highest fluence irradiated sample. The Raman study indicates damage in both the zinc and oxygen sub-lattice by an energetic ion beam. Representative Raman modes from defect complexes involving V O, I Zn and I O are visible after irradiation with intermediate fluence. A further increase of fluence shows, to some extent, a homogenization of disorder. A huge reduction of resistance is also noted for this sample. Certainly, high irradiation fluence induces a qualitative modification of the conventional (and highly resistive) grain boundary (GB) structure of granular ZnO. A low resistive path, involving I Zn related shallow donors, across the GB can be presumed to explain resistance reduction. Open volumes (V Zn and V O) agglomerate more and more with increasing irradiation fluence and are finally transformed to voids. The results as a whole have been elucidated with a model which emphasizes the possible evolution of a new defect microstructure that is distinctively different from the GB-related disorder. Based on the model, qualitative explanations of commonly observed radiation hardness, colouration and ferromagnetism in disordered ZnO have been put forward. A coherent scenario on disorder accumulation in ZnO has been presented, which we believe will guide further discussion on this topic.

Investigation of defect properties in Cu(In,Ga)Se 2 solar cells by deep-level transient spectroscopy

NASA Astrophysics Data System (ADS)

Kerr, L. L.; Li, Sheng S.; Johnston, S. W.; Anderson, T. J.; Crisalle, O. D.; Kim, W. K.; Abushama, J.; Noufi, R. N.

2004-09-01

The performance of the chalcopyrite material Cu(In,Ga)Se 2 (CIGS) used as an absorber layer in thin-film photovoltaic devices is significantly affected by the presence of native defects. The deep-level transient spectroscopy (DLTS) technique is used in this work to characterize the defect properties, yielding relevant information about the defect types, their capture cross-sections, and energy levels and densities in the CIGS cells. Three solar cells developed using different absorber growth technologies were analyzed using DLTS, capacitance-voltage ( C- V), and capacitance-temperature ( C- T) techniques. It was found that CIS cells grown at the University of Florida exhibits a middle-gap defect level that may relate to the cell's low fill factor and open-circuit voltage values observed. A high efficiency ( ηc>18%) CIGS cell produced by the National Renewable Energy Laboratory (NREL) was found to contain three minority-carrier (electron) traps and a 13% CIGS cell produced by the Energy Photovoltaics Inc. (EPV) exhibited one majority (hole) trap. The approach followed using the DLTS technique serves as a paradigm for revealing the presence of significant defect levels in absorber materials, and may be used to support the identification of remedial processing operations.

Adjunctive Effect of Autologus Platelet-Rich Fibrin to Barrier Membrane in the Treatment of Periodontal Intrabony Defects.

PubMed

Panda, Saurav; Sankari, Malaiappan; Satpathy, Anurag; Jayakumar, Doraiswamy; Mozzati, Marco; Mortellaro, Carmen; Gallesio, Giorgia; Taschieri, Silvio; Del Fabbro, Massimo

2016-05-01

Autologous platelet-rich fibrin (PRF) and barrier membranes in the treatment of intrabony defects in chronic periodontitis patients have shown significant clinical benefits. This study evaluates the additive effect of autologous PRF in combination with a barrier membrane versus the use of barrier membrane alone for the treatment of intrabony defects in chronic periodontitis patients. A randomized split-mouth design was used. Sixteen patients with 32 paired intrabony defects were included. In each patient 1 defect was treated using a resorbable collagen membrane along with PRF (test group) and the other defect by guided tissue regeneration alone (control group). The following clinical parameters were measured at baseline and after 9 months: plaque index, modified sulcus bleeding index, probing pocket depth, clinical attachment level, and gingival marginal level. The radiographic defect depth was also assessed at baseline and after 9 months. Test group showed a statistically significant improvement for probing depth (P = 0.002), clinical attachment level (P = 0.001), and radiographic defect depth (P < 0.001) after 9 months as compared with the control sites. Radiographic defect depth reduction was 58.19 ± 13.24% in the test group as compared with 24.86 ± 9.94% reduction in the control group. The adjunctive use of PRF in combination with barrier membrane is more effective in the treatment of intrabony defects in chronic periodontitis as compared with barrier membrane alone.

Thermal degradation of InP in open tube processing: deep-level photoluminescence

NASA Astrophysics Data System (ADS)

Banerjee, S.; Srivastava, A. K.; Arora, B. M.

1990-09-01

Thermal processing of InP at temperatures above 500 °C is indispensable in the growth and device fabrication of InGaAsP alloy semiconductors for optoelectronic and microwave applications. Incongruous loss of P at these temperatures creates native defects and their complexes. The presence of such defects modifies the electrical and optical properties of the material resulting in poor device performance. In addition, native defects play a significant role in dopant diffusion which is a topic of current interest. We have measured deep-level photoluminescence (PL) on undoped InP after heat treatments at 500 and 550 °C in an open-tube processing system in different protective environments of powder InP, and Sn-InP melt together with an InP cover. In this paper we shall present the PL results which have bearing on the question of defects. We find that (1) the Sn-InP melt provides better protection in preserving the overall luminescence in InP; (2) the deep-level PL related to defects has at least two components in the virgin samples, viz., MnIn, and band C, which is a native defect complex related to VP; (3) a new defect appears in samples heated in a P-deficient environment; and (4) the enhancement in the deep-level luminescence intensity after heat treatment can be attributed to the excess defect concentrations existing under nonequilibrium conditions of an open-tube processing environment.

Excitation mechanisms of Er optical centers in GaN epilayers

DOE Office of Scientific and Technical Information (OSTI.GOV)

George, D. K.; Hawkins, M. D.; McLaren, M.

2015-10-26

We report direct evidence of two mechanisms responsible for the excitation of optically active Er{sup 3+} ions in GaN epilayers grown by metal-organic chemical vapor deposition. These mechanisms, resonant excitation via the higher-lying inner 4f shell transitions and band-to-band excitation of the semiconductor host, lead to narrow emission lines from isolated and the defect-related Er optical centers. However, these centers have different photoluminescence spectra, local defect environments, decay dynamics, and excitation cross sections. The photoluminescence at 1.54 μm from the isolated Er optical center which can be excited by either mechanism has the same decay dynamics, but possesses a much highermore » excitation cross-section under band-to-band excitation. In contrast, the photoluminescence at 1.54 μm from the defect-related Er optical center can only be observed through band-to-band excitation but has the largest excitation cross-section. These results explain the difficulty in achieving gain in Er doped GaN and indicate approaches for realization of optical amplification, and possibly lasing, at room temperature.« less

High-frequency EPR of surface impurities on nanodiamond

NASA Astrophysics Data System (ADS)

Peng, Zaili; Stepanov, Viktor; Takahashi, Susumu

Diamond is a fascinating material, hosting nitrogen-vacancy (NV) defect centers with unique magnetic and optical properties. There have been many reports that suggest the existence of paramagnetic impurities near surface of various kinds of diamonds. Electron paramagnetic resonance (EPR) investigation of mechanically crushed nanodiamonds (NDs) as well as detonation NDs revealed g 2 like signals that are attributed to structural defects and dangling bonds near the diamond surface. In this presentation, we investigate paramagnetic impurities in various sizes of NDs using high-frequency (HF) continuous wave (cw) and pulsed EPR spectroscopy. Strong size dependence on the linewidth of HF cw EPR spectra reveals the existence of paramagnetic impurities in the vicinity of the diamond surface. We also study the size dependence of the spin-lattice and spin-spin relaxation times (T1 and T2) of single substitutional nitrogen defects in NDs Significant deviations from the temperature dependence of the phonon-assisted T1 process were observed in the ND samples, and were attributed to the contribution from the surface impurities. This work was supported by the Searle Scholars Program and the National Science Foundation (DMR-1508661 and CHE-1611134).

Finite-volume spectra of the Lee-Yang model

NASA Astrophysics Data System (ADS)

Bajnok, Zoltan; el Deeb, Omar; Pearce, Paul A.

2015-04-01

We consider the non-unitary Lee-Yang minimal model in three different finite geometries: (i) on the interval with integrable boundary conditions labelled by the Kac labels ( r, s) = (1 , 1) , (1 , 2), (ii) on the circle with periodic boundary conditions and (iii) on the periodic circle including an integrable purely transmitting defect. We apply φ 1,3 integrable perturbations on the boundary and on the defect and describe the flow of the spectrum. Adding a Φ1,3 integrable perturbation to move off-criticality in the bulk, we determine the finite size spectrum of the massive scattering theory in the three geometries via Thermodynamic Bethe Ansatz (TBA) equations. We derive these integral equations for all excitations by solving, in the continuum scaling limit, the TBA functional equations satisfied by the transfer matrices of the associated A 4 RSOS lattice model of Forrester and Baxter in Regime III. The excitations are classified in terms of ( m, n) systems. The excited state TBA equations agree with the previously conjectured equations in the boundary and periodic cases. In the defect case, new TBA equations confirm previously conjectured transmission factors.

Diamond structure recovery during ion irradiation at elevated temperatures

NASA Astrophysics Data System (ADS)

Deslandes, Alec; Guenette, Mathew C.; Belay, Kidane; Elliman, Robert G.; Karatchevtseva, Inna; Thomsen, Lars; Riley, Daniel P.; Lumpkin, Gregory R.

2015-12-01

CVD diamond is irradiated by 5 MeV carbon ions, with each sample held at a different temperature (300-873 K) during irradiations. The defect structures resulting from the irradiations are evident as vacancy, interstitial and amorphous carbon signals in Raman spectra. The observed variation of the full width at half maximum (FWHM) and peak position of the diamond peak suggests that disorder in the diamond lattice is reduced for high temperature irradiations. The dumbbell interstitial signal is reduced for irradiations at 873 K, which suggests this defect is unstable at these temperatures and that interstitials have migrated to crystal surfaces. Near edge X-ray absorption fine structure (NEXAFS) spectroscopy results indicate that damage to the diamond structure at the surface has occurred for room temperature irradiations, however, this structure is at least partially recovered for irradiations performed at 473 K and above. The results suggest that, in a high temperature irradiation environment such as a nuclear fusion device, in situ annealing of radiation-created defects can maintain the diamond structure and prolong the lifetime of diamond components.

Self-regulation of charged defect compensation and formation energy pinning in semiconductors

PubMed Central

Yang, Ji-Hui; Yin, Wan-Jian; Park, Ji-Sang; Wei, Su-Huai

2015-01-01

Current theoretical analyses of defect properties without solving the detailed balance equations often estimate Fermi-level pinning position by omitting free carriers and assume defect concentrations can be always tuned by atomic chemical potentials. This could be misleading in some circumstance. Here we clarify that: (1) Because the Fermi-level pinning is determined not only by defect states but also by free carriers from band-edge states, band-edge states should be treated explicitly in the same footing as the defect states in practice; (2) defect formation energy, thus defect density, could be pinned and independent on atomic chemical potentials due to the entanglement of atomic chemical potentials and Fermi energy, in contrast to the usual expectation that defect formation energy can always be tuned by varying the atomic chemical potentials; and (3) the charged defect compensation behavior, i.e., most of donors are compensated by acceptors or vice versa, is self-regulated when defect formation energies are pinned. The last two phenomena are more dominant in wide-gap semiconductors or when the defect formation energies are small. Using NaCl and CH3NH3PbI3 as examples, we illustrate these unexpected behaviors. Our analysis thus provides new insights that enrich the understanding of the defect physics in semiconductors and insulators. PMID:26584670

Characterization of Thallium Bromide (TlBr) for Room Temperature Radiation Detectors

NASA Astrophysics Data System (ADS)

Smith, Holland McTyeire

Thallium bromide (TlBr) has emerged as a remarkably well-suited material for room temperature radiation detection. The unique combination of high-Z elements, high density, suitable band gap, and excellent electrical transport properties present in TlBr have brought device performance up to par with CdZnTe (CZT), the current market-leading room temperature radiation detector material. TlBr research is at an earlier stage than that of CZT, giving hope that the material will see even further improvement in electronic properties. Improving a resistive semiconductor material requires knowledge of deep levels present in the material and the effects of these deep levels on transport properties. Very few deep level studies have been conducted on TlBr, and none with the depth required to generate useful growth suggestions. In this dissertation, deep levels in nominally undoped and doped TlBr samples are studied with electrical and optical methods. Photo-Induced Conductivity Transient Spectroscopy (PICTS) is used to discover many deep levels in TlBr electrically. These levels are compared to sub-band gap optical transitions originating from defects observed in emission spectra. The results of this research indicate that the origin of resistivity in TlBr is likely due to deep level defects pinning the Fermi level at least ˜0.7 eV from either the conduction or valence band edge. The effect of dopants and deep levels on transport in TlBr is assessed with microwave photoconductivity decay analysis. It is found that Pb-, Se-, and O-doping decreases carrier lifetime in TlBr, whereas C-doping does not. TlBr exhibits weak ionic conductivity at room temperature, which both negatively affects the leakage current of detectors and leads to device degradation over time. Researchers are actively looking for ways to reduce or eliminate the ionic conductivity, but are faced with an intriguing challenge of materials engineering: is it possible to mitigate the ionic conduction of TlBr without harming the excellent electronic transport properties? Doping TlBr in order to control the ionic conductivity has been proposed and shown to be effective in reducing dark ionic current, but the electronic effects of the dopants has not been previously studied in detail. In this dissertation, the electronic effects of dopants introduced for ionic reasons are evaluated.

Characterization of dipole defects in MgAl2O4 spinel

NASA Astrophysics Data System (ADS)

Carvalhaes, R. P. M.; da Rocha, M. S. F.; de Souza, S. S.; Blak, A. R.

2005-01-01

Dipole defects in gamma-irradiated and thermal treated MgAl2O4 samples have been studied through thermally stimulated depolarisation currents(TSDC) technique and computer modelling methods. The presence of TSDC bands varies from sample to sample and some crystals do not present any band. The origin of these bands has been investigated in several different samples. In the spectra of spinels showing TSDC peaks, three bands at 130K, 160K and 320K are observed. The peaks at 130K and 160K have been attributed to dipole defects. After 1200kGy of gamma irradiation the broad band at 320K dislocates to 290K and increases ten times. Pulsed thermal treatments between 350K and 470K produce a progressive reduction of the peak area and a shift in the peak position back to 320K. A detailed analysis of the curve indicates the possibility of a superposition of peaks. Gamma irradiation restores the 320K TSDC peak. Taking into account optical absorption(OA) and electron paramagnetic resonance(EPR) results, the thermal reduction of the 320K TSDC band was attributed to V-type centres as a result of hole trapping at tetrahedral and octahedral cation vacancies. Computer modelling methods, based on lattice energy and defect minimisation, were applied to identify dipole defects that occur in these crystals. The calculations were made in normal and inverse spinel structures, doped with Cr, Co, Mn and Fe in order to justify the presence of dipole bands.

Defect characterization in Mg-doped GaN studied using a monoenergetic positron beam

NASA Astrophysics Data System (ADS)

Uedono, A.; Ishibashi, S.; Tenjinbayashi, K.; Tsutsui, T.; Nakahara, K.; Takamizu, D.; Chichibu, S. F.

2012-01-01

Vacancy-type defects in Mg-doped GaN grown by metalorganic vapor phase epitaxy were probed using a monoenergetic positron beam. For a sample fabricated with a high H2-flow rate, before post-growth annealing the major defect species detected by positrons was identified as vacancy-clusters. Evidence suggested that other donor-type defects such as nitrogen vacancies also existed. The defects increased the Fermi level position, and enhanced the diffusion of positrons toward the surface. The annihilation of positrons at the top surface was suppressed by Mg-doping. This was attributed to the introduction of a subsurface layer (<6 nm) with a low defect concentration, where the Fermi level position was considered to decrease due to partial activation of Mg. For samples after annealing, the trapping of positrons by residual vacancy-type defects was observed, and the sample crystal quality was found to depend on that before annealing.

Objectively assessed physical activity and sedentary behaviour does not differ between children and adolescents with and without a congenital heart defect: a pilot examination.

PubMed

Ewalt, Lauren A; Danduran, Michael J; Strath, Scott J; Moerchen, Victoria; Swartz, Ann M

2012-02-01

To objectively evaluate and describe physical activity levels in children with a stable congenital heart defect and compare those levels with children who do not have a congenital heart defect. We matched 21 pairs of children for gender and grade in school and gave them an accelerometer-based motion sensor to wear for 7 consecutive days. Physical activity levels did not differ between children with and without a congenital heart defect. During the 7 days of monitoring, children in this study spent most of their time in sedentary behaviours, that is, 6.7 hours of the 13 monitored hours, 54 minutes in moderate-intensity physical activity, and 12 minutes in vigorous-intensity physical activity. Less than one-fifth of all participants, with or without a congenital heart defect, accumulated sufficient physical activity to meet current physical activity recommendations for children and adolescents. Children with a stable congenital heart defect have activity behaviours that are similar to children without a congenital heart defect. Habitual physical activity in children with a congenital heart defect should be encouraged early on in life to develop strong physical activity habits that will hopefully follow them across their lifespan.

Native point defects in MoS2 and their influences on optical properties by first principles calculations

NASA Astrophysics Data System (ADS)

Saha, Ashim Kumar; Yoshiya, Masato

2018-03-01

Stability of native point defect species and optical properties are quantitatively examined through first principles calculations in order to identify possible native point defect species in MoS2 and its influences on electronic structures and resultant optical properties. Possible native point defect species are identified as functions of thermodynamic environment and location of Fermi-level in MoS2. It is found that sulphur vacancies can be introduced more easily than other point defect species which will create impurity levels both in bandgap and in valence band. Additionally, antisite Mo and/or Mo vacancies can be created depending on chemical potential of sulphur, both of which will create impurity levels in bandgap and in valence band. Those impurity levels result in pronounced photon absorption in visible light region, though each of these point defects alone has limited impact on the optical properties unless their concentration remained low. Thus, attention must be paid when intentional impurity doping is made to MoS2 to avoid unwanted modification of optical properties of MoS2. Those impurity may enable further exploitation of photovoltaic energy conversion at longer wavelength.

Influence of a Levelness Defect in a Thrust Bearing on the Dynamic Behaviour of AN Elastic Shaft

NASA Astrophysics Data System (ADS)

BERGER, S.; BONNEAU, O.; FRÊNE, J.

2002-01-01

This paper examines the non-linear dynamic behaviour of a flexible shaft. The shaft is mounted on two journal bearings and the axial load is supported by a defective hydrodynamic thrust bearing at one end. The defect is a levelness defect of the rotor. The thrust bearing behaviour must be considered to be non-linear because of the effects of the defect. The shaft is modelled with typical beam finite elements including effects such as the gyroscopic effects. A modal technique is used to reduce the number of degrees of freedom. Results show that the thrust bearing defects introduce supplementary critical speeds. The linear approach is unable to show the supplementary critical speeds which are obtained only by using non-linear analysis.

Observations of the high vibrational levels of the B ' ' B ¯ 1 Σu + state of H2

NASA Astrophysics Data System (ADS)

Chartrand, A. M.; Duan, W.; Ekey, R. C.; McCormack, E. F.

2016-01-01

Double-resonance laser spectroscopy via the E F 1 Σg + , v ' = 6 , J ' = 0 -2 state was used to probe the high vibrational levels of the B ' ' B ¯ 1 Σu + state of molecular hydrogen. Resonantly enhanced multiphoton ionization spectra were recorded by detecting ion production as a function of energy using a time of flight mass spectrometer. New measurements of energies for the v = 51-66 levels for the B ' ' B ¯ state of H2 are reported, which, taken with previous results, span the v = 46-69 vibrational levels. Results for energy levels are compared to theoretical close-coupled calculations [L. Wolniewicz, T. Orlikowski, and G. Staszewska, J. Mol. Spectrosc. 238, 118-126 (2006)]. The average difference between the 84 measured energies and calculated energies is -3.8 cm-1 with a standard deviation of 5.3 cm-1. This level of agreement showcases the success of the theoretical calculations in accounting for the strong rovibronic mixing of the 1 Σu + and 1 Πu + states. Due to the ion-pair character of the outer well, the observed energies of the vibrational levels below the third dissociation limit smoothly connect with previously observed energies of ion-pair states above this limit. The results provide an opportunity for testing a heavy Rydberg multi-channel quantum defect analysis of the high vibrational states below the third dissociation limit.

Origin of reverse annealing in radiation-damaged silicon solar cells

NASA Technical Reports Server (NTRS)

Weinberg, I.; Swartz, C. K.

1980-01-01

The paper employs relative defect concentrations, energy levels, capture cross sections, and minority carrier diffusion lengths in order to identify the defect responsible for the reverse annealing observed in a radiation damaged n(+)/p silicon solar cell. It is reported that the responsible defect, with the energy level at +0.30 eV, has been tentatively identified as boron-oxygen-vacancy complex. In conclusion, it is shown that removal of this defect could result in significant cell recovery when annealing at temperatures well below the currently required 400 C.

Chornobyl, radiation, neural tube defects, and microcephaly.

PubMed

Wertelecki, Wladimir; Yevtushok, Lyubov; Kuznietsov, Illia; Komov, Oleksandr; Lapchenko, Serhii; Akhmedzanova, Diana; Ostapchuk, Lyubov

2018-06-13

Pregnant women residing in areas impacted by the Chornobyl ionizing radiation of the Rivne Province in Ukraine have persistent higher levels of incorporated cesium-137. In these areas the neural tube defects and microcephaly rates are significantly higher than in areas with lower maternal cesium-137 incorporated levels. In two Rivne counties with populations proximal to nuclear power plants the rates of neural tube defects and microcephaly are the highest in the province. The neural tube defects rates in Rivne are persistently among the highest in Europe. Copyright © 2018 Elsevier Masson SAS. All rights reserved.

DOE Office of Scientific and Technical Information (OSTI.GOV)

S. Dong; N. Paterson; S.G. Kazarian

A suite of tuyere-level coke samples have been withdrawn from a working blast furnace during coal injection, using the core-drilling technique. The samples have been characterized by size exclusion chromatography (SEC), Fourier transform Raman spectroscopy (FT-RS), and X-ray powder diffraction (XRD) spectroscopy. The 1-methyl-2-pyrrolidinone (NMP) extracts of the cokes sampled from the 'bosh', the rear of the 'bird's nest', and the 'dead man' zones were found by SEC to contain heavy soot-like materials (ca. 10{sup 7}-10{sup 8} apparent mass units). In contrast, NMP extracts of cokes taken from the raceway and the front of the 'bird's nest' only contained amore » small amount of material of relatively lower apparent molecular mass (up to ca. 10{sup 5} u). Since the feed coke contained no materials extractable by the present method, the soot-like materials are thought to have formed during the reactions of volatile matter released from the injectant coal, probably via dehydrogenation and repolymerization of the tars. The Raman spectra of the NMP-extracted core-drilled coke samples showed variations reflecting their temperature histories. Area ratios of D-band to G-band decreased as the exposure temperature increased, while intensity ratios of D to G band and those of 2D to G bands increased with temperature. The graphitic (G), defect (D), and random (R) fractions of the carbon structure of the cokes were also derived from the Raman spectra. The R fractions decreased with increasing temperature, whereas G fractions increased, while the D fractions showed a more complex variation with temperature. These data appear to give clues regarding the graphitization mechanism of tuyere-level cokes in the blast furnace. 41 refs., 9 figs., 6 tabs.« less

Crystal growth and structure, electrical, and optical characterization of the semiconductor Cu2SnSe3

NASA Astrophysics Data System (ADS)

Marcano, G.; Rincón, C.; de Chalbaud, L. M.; Bracho, D. B.; Pérez, G. Sánchez

2001-08-01

X-ray powder diffraction by p-type Cu2SnSe3, prepared by the vertical Bridgman-Stockbarger technique, shows that this material crystallizes in a monoclinic structure, space group Cc, with unit cell parameters a=6.5936(1) Å, b=12.1593(4) Å, c=6.6084(3) Å, and β=108.56(2)°. The temperature variation of the hole concentration p obtained from the Hall effect and electrical resistivity measurements from about 160 to 300 K, is explained as due to the thermal activation of an acceptor level with an ionization energy of 0.067 eV, whereas below 100 K, the conduction in the impurity band dominates the electrical transport process. From the analysis of the p vs T data, the density-of-states effective mass of the holes is estimated to be nearly of the same magnitude as the free electron mass. In the valence band, the temperature variation of the hole mobility is analyzed by taking into account the scattering of charge carriers by ionized and neutral impurities, and acoustic phonons. In the impurity band, the mobility is explained as due to the thermally activated hopping transport. From the analysis of the optical absorption spectra at room temperature, the fundamental energy gap was determined to be 0.843 eV. The photoconductivity spectra show the presence of a narrow band gap whose main peak is observed at 0.771 eV. This band is attributed to a free-to-bound transition from the defect acceptor level to the conduction band. The origin of this acceptor state, consistent with the chemical composition of the samples and screening effects, is tentatively attributed to selenium interstitials.

TOPICAL REVIEW: The doping process and dopant characteristics of GaN

NASA Astrophysics Data System (ADS)

Sheu, J. K.; Chi, G. C.

2002-06-01

The characteristic effects of doping with impurities such as Si, Ge, Se, O, Mg, Be, and Zn on the electrical and optical properties of GaN-based materials are reviewed. In addition, the roles of unintentionally introduced impurities, such as C, H, and O, and grown-in defects, such as vacancy and antisite point defects, are also discussed. The doping process during epitaxial growth of GaN, AlGaN, InGaN, and their superlattice structures is described. Doping using the diffusion process and ion implantation techniques is also discussed. A p-n junction formed by Si implantation into p-type GaN is successfully fabricated. The results on crystal structure, electrical resistivity, carrier mobility, and optical spectra obtained by means of x-rays, low-temperature Hall measurements, and photoluminescence are also discussed.

RBS/C, HRTEM and HRXRD study of damage accumulation in irradiated SrTiO3

DOE Office of Scientific and Technical Information (OSTI.GOV)

Jagielski, Jacek; Jozwik, Przemyslaw A.; Jozwik Biala, Iwona

2013-05-14

Damage accumulation in argon-irradiated SrTiO3 single crystals has been studied by using combination of Rutherford Backscattering/Channeling (RBS/C), High Resolution Transmission Electron Microscopy (HRTEM) and High Resolution X-Ray Diffraction (HRXRD) techniques. The RBS/C spectra were fitted using McChasy, a Monte Carlo simulation code allowing the quantitative analysis of amorphous-like and dislocation-like types of defects. The results were interpreted by using a Multi-Step Damage Accumulation model which assumes, that the damage accumulation occurs in a series of structural transformations, the defect transformations are triggered by a stress caused by formation of a free volume in the irradiated crystal. This assumption has beenmore » confirmed by High Resolution Transmission Electron Microscopy and High Resolution X-Ray Diffraction analysis.« less

Low temperature detection of phase transitions and relaxation processes in strontium titanate by means of cathodoluminescence

NASA Astrophysics Data System (ADS)

Yang, B.; Townsend, P. D.; Fromknecht, R.

2004-11-01

Cathodoluminescence is an effective tool for investigating phase changes and relaxation processes in insulators and data are presented for strontium titanate. The results demonstrate considerable sensitivity to the origin of the samples as the detailed spectra and intensity changes with temperature are strongly dependent on the growth conditions, trace impurities and radiation induced defects. It is of particular note that in the defective surface layer the normal second-order phase transition cited near 105 K transforms into a sharply defined first-order transition because of the relaxation of the near surface layer in doped crystals. Detection of the other main relaxation stages is also straightforward via intensity and spectral changes. Secondary effects of phase changes incorporated within the surface layers are clearly evident, particularly for the 197 K sublimation of CO2 nanoparticle inclusions.

Identification of a limiting mechanism in GaSb-rich superlattice midwave infrared detector

DOE Office of Scientific and Technical Information (OSTI.GOV)

Delmas, Marie; Rodriguez, Jean-Baptiste; Rossignol, Rémi

2016-05-07

GaSb-rich superlattice (SL) p-i-n photodiodes grown by molecular beam epitaxy were studied theoretically and experimentally in order to understand the poor dark current characteristics typically obtained. This behavior, independent of the SL-grown material quality, is usually attributed to the presence of defects due to Ga-related bonds, limiting the SL carrier lifetime. By analyzing the photoresponse spectra of reverse-biased photodiodes at 80 K, we have highlighted the presence of an electric field, breaking the minibands into localized Wannier-Stark states. Besides the influence of defects in such GaSb-rich SL structures, this electric field induces a strong tunneling current at low bias which canmore » be the main limiting mechanism explaining the high dark current density of the GaSb-rich SL diode.« less

Impact of iron-site defects on superconductivity in LiFeAs

DOE PAGES

Chi, Shun; Aluru, Ramakrishna; Singh, Udai Raj; ...

2016-10-19

In conventional s -wave superconductors, only magnetic impurities exhibit impurity bound states, whereas for an s ± order parameter they can occur for both magnetic and nonmagnetic impurities. Impurity bound states in superconductors can thus provide important insight into the order parameter. We present a combined experimental and theoretical study of native and engineered iron-site defects in LiFeAs. A detailed comparison of tunneling spectra measured on impurities with spin-fluctuation theory reveals a continuous evolution from negligible impurity-bound-state features for weaker scattering potential to clearly detectable states for somewhat stronger scattering potentials. Furthermore, all bound states for these intermediate strengthmore » potentials are pinned at or close to the gap edge of the smaller gap, a phenomenon that we explain and ascribe to multiorbital physics.« less

Temperature, stress, and annealing effects on the luminescence from electron-irradiated silicon

NASA Technical Reports Server (NTRS)

Jones, C. E.; Johnson, E. S.; Compton, W. D.; Noonan, J. R.; Streetman, B. G.

1973-01-01

Low-temperature photoluminescence spectra are presented for Si crystals which have been irradiated with high-energy electrons. Studies of isochronal annealing, stress effects, and the temperature dependences of the luminescence are used to discuss the nature of the luminescent transitions and the properties of defects. Two dominant bands present after room-temperature anneal of irradiated material are discussed, and correlations of the properties of these bands are made with known Si defects. A band between 0.8 and 1.0 eV has properties which are related to those of the divacancy, and a band between 0.6 and 0.8 eV has properties related to those of the Si-G15(K) center. Additional peaks appear in the luminescence after high-temperature anneal; the influence of impurities and the effects of annealing of these lines are discussed.

Evaluating and enhancing quantum capacitance in graphene-based electrodes from first principles

NASA Astrophysics Data System (ADS)

Ogitsu, Tadashi; Otani, Minoru; Lee, Jonathan; Bagge-Hansen, Michael; Biener, Juergen; Wood, Brandon

2013-03-01

Graphene derivatives are attractive as supercapacitor electrodes because they are lightweight, chemically inert, have high surface area and conductivity, and are stable in electrolyte solutions. Nevertheless, devising reliable strategies for improving energy density relies on an understanding of the specific factors that control electrode performance. We use density-functional theory calculations of pristine and defective graphene to extract quantum capacitance, as well as to identify specific limiting factors. The effect of structural point defects and strain-related morphological changes on the density of states is also evaluated. The results are combined with predicted and measured in situ X-ray absorption spectra in order to give insight into the structural and chemical features present in synthesized carbon aerogel samples. Performed under the auspices of the U.S. DOE by LLNL under Contract DE-AC52-07NA27344.

Practical ranges of loudness levels of various types of environmental noise, including traffic noise, aircraft noise, and industrial noise.

PubMed

Salomons, Erik M; Janssen, Sabine A

2011-06-01

In environmental noise control one commonly employs the A-weighted sound level as an approximate measure of the effect of noise on people. A measure that is more closely related to direct human perception of noise is the loudness level. At constant A-weighted sound level, the loudness level of a noise signal varies considerably with the shape of the frequency spectrum of the noise signal. In particular the bandwidth of the spectrum has a large effect on the loudness level, due to the effect of critical bands in the human hearing system. The low-frequency content of the spectrum also has an effect on the loudness level. In this note the relation between loudness level and A-weighted sound level is analyzed for various environmental noise spectra, including spectra of traffic noise, aircraft noise, and industrial noise. From loudness levels calculated for these environmental noise spectra, diagrams are constructed that show the relation between loudness level, A-weighted sound level, and shape of the spectrum. The diagrams show that the upper limits of the loudness level for broadband environmental noise spectra are about 20 to 40 phon higher than the lower limits for narrowband spectra, which correspond to the loudness levels of pure tones. The diagrams are useful for assessing limitations and potential improvements of environmental noise control methods and policy based on A-weighted sound levels.

Hexagonal ice in pure water and biological NMR samples.

PubMed

Bauer, Thomas; Gath, Julia; Hunkeler, Andreas; Ernst, Matthias; Böckmann, Anja; Meier, Beat H

2017-01-01

Ice, in addition to "liquid" water and protein, is an important component of protein samples for NMR spectroscopy at subfreezing temperatures but it has rarely been observed spectroscopically in this context. We characterize its spectroscopic behavior in the temperature range from 100 to 273 K, and find that it behaves like pure water ice. The interference of magic-angle spinning (MAS) as well as rf multiple-pulse sequences with Bjerrum-defect motion greatly influences the ice spectra.

High-spin europium and gadolinium centers in yttrium-aluminum garnet

NASA Astrophysics Data System (ADS)

Vazhenin, V. A.; Potapov, A. P.; Asatryan, G. R.; Uspenskaya, Yu. A.; Petrosyan, A. G.; Fokin, A. V.

2016-08-01

Electron-spin resonance spectra of Eu2+ and Gd3+ centers substituting Y3+ ions in single-crystal yttrium-aluminum garnet have been studied and the parameters of their rhombic spin Hamiltonian have been determined. The fine-structure parameters of the above ions have been calculated in the superposition model disregarding changes in the angular coordinates of the ligand environment of the impurity defect thus demonstrating the necessity of taking these changes into account.

Simple intrinsic defects in GaP and InP

NASA Astrophysics Data System (ADS)

Schultz, Peter A.

2012-02-01

To faithfully simulate evolution of defect chemistry and electrical response in irradiated semiconductor devices requires accurate defect reaction energies and energy levels. In III-Vs, good data is scarce, theory hampered by band gap and supercell problems. I apply density functional theory (DFT) to intrinsic defects in GaP and InP, predicting stable charge states, ground state configurations, defect energy levels, and identifying mobile species. The SeqQuest calculations incorporate rigorous charge boundary conditions removing supercell artifacts, demonstrated converged to the infinite limit. Computed defect levels are not limited by a band gap problem, despite Kohn-Sham gaps much smaller than the experimental gap. As in GaAs, [P.A. Schultz and O.A. von Lilienfeld, Modeling Simul. Mater. Sci. Eng. 17, 084007 (2009)], defects in GaP and InP exhibit great complexity---multitudes of charge states, bistabilities, and negative U systems---but show similarities to each other (and to GaAs). Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Company, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

An improved AE detection method of rail defect based on multi-level ANC with VSS-LMS

NASA Astrophysics Data System (ADS)

Zhang, Xin; Cui, Yiming; Wang, Yan; Sun, Mingjian; Hu, Hengshan

2018-01-01

In order to ensure the safety and reliability of railway system, Acoustic Emission (AE) method is employed to investigate rail defect detection. However, little attention has been paid to the defect detection at high speed, especially for noise interference suppression. Based on AE technology, this paper presents an improved rail defect detection method by multi-level ANC with VSS-LMS. Multi-level noise cancellation based on SANC and ANC is utilized to eliminate complex noises at high speed, and tongue-shaped curve with index adjustment factor is proposed to enhance the performance of variable step-size algorithm. Defect signals and reference signals are acquired by the rail-wheel test rig. The features of noise signals and defect signals are analyzed for effective detection. The effectiveness of the proposed method is demonstrated by comparing with the previous study, and different filter lengths are investigated to obtain a better noise suppression performance. Meanwhile, the detection ability of the proposed method is verified at the top speed of the test rig. The results clearly illustrate that the proposed method is effective in detecting rail defects at high speed, especially for noise interference suppression.

Defect phase diagram for doping of Ga2O3

NASA Astrophysics Data System (ADS)

Lany, Stephan

2018-04-01

For the case of n-type doping of β-Ga2O3 by group 14 dopants (C, Si, Ge, Sn), a defect phase diagram is constructed from defect equilibria calculated over a range of temperatures (T), O partial pressures (pO2), and dopant concentrations. The underlying defect levels and formation energies are determined from first-principles supercell calculations with GW bandgap corrections. Only Si is found to be a truly shallow donor, C is a deep DX-like (lattice relaxed donor) center, and Ge and Sn have defect levels close to the conduction band minimum. The thermodynamic modeling includes the effect of association of dopant-defect pairs and complexes, which causes the net doping to decline when exceeding a certain optimal dopant concentration. The optimal doping levels are surprisingly low, between about 0.01% and 1% of cation substitution, depending on the (T, pO2) conditions. Considering further the stability constraints due to sublimation of molecular Ga2O, specific predictions of optimized pO2 and Si dopant concentrations are given. The incomplete passivation of dopant-defect complexes in β-Ga2O3 suggests a design rule for metastable doping above the solubility limit.

Reduced graphene oxide-germanium quantum dot nanocomposite: electronic, optical and magnetic properties

NASA Astrophysics Data System (ADS)

Amollo, Tabitha A.; Mola, Genene T.; Nyamori, Vincent O.

2017-12-01

Graphene provides numerous possibilities for structural modification and functionalization of its carbon backbone. Localized magnetic moments can, as well, be induced in graphene by the formation of structural defects which include vacancies, edges, and adatoms. In this work, graphene was functionalized using germanium atoms, we report the effect of the Ge ad atoms on the structural, electrical, optical and magnetic properties of graphene. Reduced graphene oxide (rGO)-germanium quantum dot nanocomposites of high crystalline quality were synthesized by the microwave-assisted solvothermal reaction. Highly crystalline spherical shaped germanium quantum dots, of diameter ranging between 1.6-9.0 nm, are anchored on the basal planes of rGO. The nanocomposites exhibit high electrical conductivity with a sheet resistance of up to 16 Ω sq-1. The electrical conductivity is observed to increase with the increase in Ge content in the nanocomposites. High defect-induced magnetization is attained in the composites via germanium adatoms. The evolution of the magnetic moments in the nanocomposites and the coercivity showed marked dependence on the Ge quantum dots size and concentration. Quantum confinement effects is evidenced in the UV-vis absorbance spectra and photoluminescence emission spectra of the nanocomposites which show marked size-dependence. The composites manifest strong absorption in the UV region, strong luminescence in the near UV region, and a moderate luminescence in the visible region.

Unusual surface and edge morphologies, sp2 to sp3 hybridized transformation and electronic damage after Ar+ ion irradiation of few-layer graphene surfaces.

PubMed

Al-Harthi, Salim Hamood; Elzain, Mohammed; Al-Barwani, Muataz; Kora'a, Amal; Hysen, Thomas; Myint, Myo Tay Zar; Anantharaman, Maliemadom Ramaswamy

2012-08-19

Roughness and defects induced on few-layer graphene (FLG) irradiated by Ar+ ions at different energies were investigated using X-ray photoemission spectroscopy (XPS) and atomic force microscopy techniques. The results provide direct experimental evidence of ripple formation, sp2 to sp3 hybridized carbon transformation, electronic damage, Ar+ implantation, unusual defects and edge reconstructions in FLG, which depend on the irradiation energy. In addition, shadowing effects similar to those found in oblique-angle growth of thin films were seen. Reliable quantification of the transition from the sp2-bonding to sp3-hybridized state as a result of Ar+ ion irradiation is achieved from the deconvolution of the XPS C (1s) peak. Although the ion irradiation effect is demonstrated through the shape of the derivative of the Auger transition C KVV spectra, we show that the D parameter values obtained from these spectra which are normally used in the literature fail to account for the sp2 to sp3 hybridization transition. In contrast to what is known, it is revealed that using ion irradiation at large FLG sample tilt angles can lead to edge reconstructions. Furthermore, FLG irradiation by low energy of 0.25 keV can be a plausible way of peeling graphene layers without the need of Joule heating reported previously.

Radiation coloring of nonstoichiometric M(1-x)R(x)F(2+x) single crystals with a fluorite defect structure

NASA Astrophysics Data System (ADS)

Rustamov, Ia.; Tavshunskii, G. A.; Khabibullaev, P. K.; Bessonova, T. S.; Sobolev, B. P.

1985-06-01

Experimental results are reported concerning the radiation coloring of nonstoichiometric crystals of the M(1-x)R(x)F(2+x) type in the presence of fluorite defects. Samples of the crystals are cut using the Stockbarger technique in a chemically active fluoridating atmosphere generated by pyrolysis of tetrafluoroethylene. The samples were irradiated at 77 and 300 K using a Co-60 gamma-ray source and the total doses were in the range 10 to the 6th to 10 to the 7th roentgen. Absorption spectra of the crystals were analogous spectra for MF2-RF3 single crystals with RF 3 contents of less than 1 mole percent. It is shown that the properties of radiation coloring of the two types of crystal are very different: F-centers formed at 300 K in Ca(1-x)R(x) F(2+x), but not at 77 K. Complex color centers were observed at 77 K in Ca(1-x)R(x)F(2+x) single crystals and the intensity of the centers was determined by the competition among the electron trapping processes involving the r3(+) ions. It is concluded that the coloring characteristics of the M(1-x)R(x)F(2+x) crystals are related to their structural characteristics as compared with the MF2-RF3 crystals.

Superstructure Ta2O5 mesocrystals derived from (NH4)2Ta2O3F6 mesocrystals with efficient photocatalytic activity.

PubMed

Yu, Xin; Li, Wei; Huang, Jian; Li, Zhonghua; Liu, Jiawen; Hu, PingAn

2018-02-06

Superstructured mesocrystalline Ta 2 O 5 nanosheets were successfully prepared from mesocrystalline (NH 4 ) 2 Ta 2 O 3 F 6 nanorods by the annealing method for the first time. The as-prepared mesocrystalline Ta 2 O 5 nanosheets in this work showed remarkable visible light absorption, mainly due to the formation of oxygen vacancy defects in the mesocrystalline Ta 2 O 5 nanosheets, which was also confirmed by XPS spectra, Raman spectra and EPR spectra. Besides, the mesocrystalline Ta 2 O 5 nanosheets showed a highly enhanced photocatalytic activity of 11 268.24 μmol g -1 h -1 , about 3.95 times that of commercial Ta 2 O 5 . Moreover, the specific surface area of the mesocrystalline Ta 2 O 5 -800 nanosheets was 16.34 m 2 g -1 , about 5.32 times that of the commercial Ta 2 O 5 (3.072 m 2 g -1 ). The valence band XPS spectra indicated a strong oxidizing ability of the mesocrystalline Ta 2 O 5 nanosheets in comparison to that of commercial Ta 2 O 5 . The formation of superstructured Ta 2 O 5 mesocrystals generated long lifetime carriers and effective conduction pathways, which greatly enhanced the photocatalytic activity for hydrogen production.

Electrical level of defects in single-layer two-dimensional TiO2

NASA Astrophysics Data System (ADS)

Song, X. F.; Hu, L. F.; Li, D. H.; Chen, L.; Sun, Q. Q.; Zhou, P.; Zhang, D. W.

2015-11-01

The remarkable properties of graphene and transition metal dichalcogenides (TMDCs) have attracted increasing attention on two-dimensional materials, but the gate oxide, one of the key components of two-dimensional electronic devices, has rarely reported. We found the single-layer oxide can be used as the two dimensional gate oxide in 2D electronic structure, such as TiO2. However, the electrical performance is seriously influenced by the defects existing in the single-layer oxide. In this paper, a nondestructive and noncontact solution based on spectroscopic ellipsometry has been used to detect the defect states and energy level of single-layer TiO2 films. By fitting the Lorentz oscillator model, the results indicate the exact position of defect energy levels depends on the estimated band gap and the charge state of the point defects of TiO2.

Sum frequency generation (SFG) vibrational spectroscopy of planar phosphatidylethanolamine hybrid bilayer membranes under water.

PubMed

Kett, Peter J N; Casford, Michael T L; Davies, Paul B

2010-06-15

Sum frequency generation (SFG) spectroscopy has been used to study the structure of phosphatidylethanolamine hybrid bilayer membranes (HBMs) under water at ambient temperatures. The HBMs were formed using a modified Langmuir-Schaefer technique and consisted of a layer of dipalmitoyl phosphatidylethanolamine (DPPE) physisorbed onto an octadecanethiol (ODT) self-assembled monolayer (SAM) at a series of surface pressures from 1 to 40 mN m(-1). The DPPE and ODT were selectively deuterated so that the contributions to the SFG spectra from the two layers could be determined separately. SFG spectra in both the C-H and C-D stretching regions confirmed that a monolayer of DPPE had been adsorbed to the ODT SAM and that there were gauche defects within the alkyl chains of the phospholipid. On adsorption of a layer of DPPE, methylene modes from the ODT SAM were detected, indicating that the phospholipid had partially disordered the alkanethiol monolayer. SFG spectra recorded in air indicated that removal of water from the surface of the HBM resulted in disruption of the DPPE layer and the formation of phospholipid bilayers.

Electroluminescence and electrical degradation of insulating polymers at electrode interfaces under divergent fields

NASA Astrophysics Data System (ADS)

Zhang, Shuai; Li, Qi; Hu, Jun; Zhang, Bo; He, Jinliang

2018-04-01

Electrical degradation of insulating polymers at electrode interfaces is an essential factor in determining long-term reliability. A critical challenge is that the exact mechanism of degradation is not fully understood, either experimentally or theoretically, due to the inherent complex processes. Consequently, in this study, we investigate electroluminescence (EL) at the interface of an electrode and insulator, and determine the relationship between EL and electrical degradation. Using a tip-plate electrode structure, the unique features of EL under a highly divergent field are investigated. The voltage type (alternating or direct current), the polymer matrix, and the time of pressing are also investigated separately. A study of EL from insulators under a divergent field is provided, and the relationship between EL spectra and degradation is discussed. It is shown that EL spectra under a divergent field have unique characteristics compared with EL spectra from polymer films under a uniform field and the most obvious one is the UV emission. The results obtained in the current investigation bring us a step closer to understanding the process of electrical degradation and provide a potential way to diagnose insulator defects.

Combining UV photodissociation action spectroscopy with electron transfer dissociation for structure analysis of gas-phase peptide cation-radicals.

PubMed

Shaffer, Christopher J; Pepin, Robert; Tureček, František

2015-12-01

We report the first example of using ultraviolet (UV) photodissociation action spectroscopy for the investigation of gas-phase peptide cation-radicals produced by electron transfer dissociation. z-Type fragment ions (●) Gly-Gly-Lys(+), coordinated to 18-crown-6-ether (CE), are generated, selected by mass and photodissociated in the 200-400 nm region. The UVPD action spectra indicate the presence of valence-bond isomers differing in the position of the Cα radical defect, (α-Gly)-Gly-Lys(+) (CE), Gly-(α-Gly)-Lys(+) (CE) and Gly-Gly-(α-Lys(+))(CE). The isomers are readily distinguishable by UV absorption spectra obtained by time-dependent density functional theory (TD-DFT) calculations. In contrast, conformational isomers of these radical types are calculated to have similar UV spectra. UV photodissociation action spectroscopy represents a new tool for the investigation of transient intermediates of ion-electron reactions. Specifically, z-type cation radicals are shown to undergo spontaneous hydrogen atom migrations upon electron transfer dissociation. Copyright © 2015 John Wiley & Sons, Ltd.

Picolinic and isonicotinic acids: a Fourier transform microwave spectroscopy study.

PubMed

Peña, Isabel; Varela, Marcelino; Franco, Vanina G; López, Juan C; Cabezas, Carlos; Alonso, José L

2014-12-04

The rotational spectra of laser ablated picolinic and isonicotinic acids have been studied using broadband chirped pulse (CP-FTMW) and narrowband molecular beam (MB-FTMW) Fourier transform microwave spectroscopies. Two conformers of picolinic acid, s-cis-I and s-cis-II, and one conformer of isonicotinic acid have been identified through the analysis of their rotational spectra. The values of the inertial defect and the quadrupole coupling constants obtained for the most stable s-cis-I conformer of picolinic acid, evidence the formation of an O-H···N hydrogen bond between the acid group and the endocyclic N atom. The stabilization provided by this hydrogen bond compensates the destabilization energy due to the adoption of a -COOH trans configuration in this conformer. Its rs structure has been derived from the rotational spectra of several (13)C, (15)N, and (18)O species observed in their natural abundances. Mesomeric effects have been revealed by comparing the experimental values of the (14)N nuclear quadrupole coupling constants in the isomeric series of picolinic, isonicotinic, and nicotinic acids.

Multiscale simulations of defect dipole-enhanced electromechanical coupling at dilute defect concentrations

NASA Astrophysics Data System (ADS)

Liu, Shi; Cohen, R. E.

2017-08-01

The role of defects in solids of mixed ionic-covalent bonds such as ferroelectric oxides is complex. Current understanding of defects on ferroelectric properties at the single-defect level remains mostly at the empirical level, and the detailed atomistic mechanisms for many defect-mediated polarization-switching processes have not been convincingly revealed quantum mechanically. We simulate the polarization-electric field (P-E) and strain-electric field (ɛ-E) hysteresis loops for BaTiO3 in the presence of generic defect dipoles with large-scale molecular dynamics and provide a detailed atomistic picture of the defect dipole-enhanced electromechanical coupling. We develop a general first-principles-based atomistic model, enabling a quantitative understanding of the relationship between macroscopic ferroelectric properties and dipolar impurities of different orientations, concentrations, and dipole moments. We find that the collective orientation of dipolar defects relative to the external field is the key microscopic structure feature that strongly affects materials hardening/softening and electromechanical coupling. We show that a small concentration (≈0.1 at. %) of defect dipoles dramatically improves electromechanical responses. This offers the opportunity to improve the performance of inexpensive polycrystalline ferroelectric ceramics through defect dipole engineering for a range of applications including piezoelectric sensors, actuators, and transducers.

Automated defect spatial signature analysis for semiconductor manufacturing process

DOEpatents

Tobin, Jr., Kenneth W.; Gleason, Shaun S.; Karnowski, Thomas P.; Sari-Sarraf, Hamed

1999-01-01

An apparatus and method for performing automated defect spatial signature alysis on a data set representing defect coordinates and wafer processing information includes categorizing data from the data set into a plurality of high level categories, classifying the categorized data contained in each high level category into user-labeled signature events, and correlating the categorized, classified signature events to a present or incipient anomalous process condition.

Laterality Defects Other Than Situs Inversus Totalis in Primary Ciliary Dyskinesia

PubMed Central

Davis, Stephanie D.; Ferkol, Thomas; Dell, Sharon D.; Rosenfeld, Margaret; Olivier, Kenneth N.; Sagel, Scott D.; Milla, Carlos; Zariwala, Maimoona A.; Wolf, Whitney; Carson, Johnny L.; Hazucha, Milan J.; Burns, Kimberlie; Robinson, Blair; Knowles, Michael R.; Leigh, Margaret W.

2014-01-01

BACKGROUND: Motile cilia dysfunction causes primary ciliary dyskinesia (PCD), situs inversus totalis (SI), and a spectrum of laterality defects, yet the prevalence of laterality defects other than SI in PCD has not been prospectively studied. METHODS: In this prospective study, participants with suspected PCD were referred to our multisite consortium. We measured nasal nitric oxide (nNO) level, examined cilia with electron microscopy, and analyzed PCD-causing gene mutations. Situs was classified as (1) situs solitus (SS), (2) SI, or (3) situs ambiguus (SA), including heterotaxy. Participants with hallmark electron microscopic defects, biallelic gene mutations, or both were considered to have classic PCD. RESULTS: Of 767 participants (median age, 8.1 years, range, 0.1-58 years), classic PCD was defined in 305, including 143 (46.9%), 125 (41.0%), and 37 (12.1%) with SS, SI, and SA, respectively. A spectrum of laterality defects was identified with classic PCD, including 2.6% and 2.3% with SA plus complex or simple cardiac defects, respectively; 4.6% with SA but no cardiac defect; and 2.6% with an isolated possible laterality defect. Participants with SA and classic PCD had a higher prevalence of PCD-associated respiratory symptoms vs SA control participants (year-round wet cough, P < .001; year-round nasal congestion, P = .015; neonatal respiratory distress, P = .009; digital clubbing, P = .021) and lower nNO levels (median, 12 nL/min vs 252 nL/min; P < .001). CONCLUSIONS: At least 12.1% of patients with classic PCD have SA and laterality defects ranging from classic heterotaxy to subtle laterality defects. Specific clinical features of PCD and low nNO levels help to identify PCD in patients with laterality defects. TRIAL REGISTRY: ClinicalTrials.gov; No.: NCT00323167; URL: www.clinicaltrials.gov PMID:24577564

Defects level evaluation of LiTiZn ferrite ceramics using temperature dependence of initial permeability

NASA Astrophysics Data System (ADS)

Malyshev, A. V.; Petrova, A. B.; Sokolovskiy, A. N.; Surzhikov, A. P.

2018-06-01

The method for evaluating the integral defects level and chemical homogeneity of ferrite ceramics based on temperature dependence analysis of initial permeability is suggested. A phenomenological expression for the description of such dependence was suggested and an interpretation of its main parameters was given. It was shown, that the main criterion of the integral defects level of ferrite ceramics is relation of two parameters correlating with elastic stress value in a material. An indicator of structural perfection can be a maximum value of initial permeability close to Curie point as well. The temperature dependences of initial permeability have analyzed for samples sintered in laboratory conditions and for the ferrite industrial product. The proposed method allows controlling integral defects level of the soft ferrite products and has high sensitivity compare to typical X-ray methods.

Energy conversion modeling of the intrinsic persistent luminescence of solids via energy transfer paths between transition levels.

PubMed

Huang, Bolong; Sun, Mingzi

2017-04-05

An energy conversion model has been established for the intrinsic persistent luminescence in solids related to the native point defect levels, formations, and transitions. In this study, we showed how the recombination of charge carriers between different defect levels along the zero phonon line (ZPL) can lead to energy conversions supporting the intrinsic persistent phosphorescence in solids. This suggests that the key driving force for this optical phenomenon is the pair of electrons hopping between different charged defects with negative-U eff . Such a negative correlation energy will provide a sustainable energy source for electron-holes to further recombine in a new cycle with a specific quantum yield. This will help us to understand the intrinsic persistent luminescence with respect to native point defect levels as well as the correlations of electronics and energetics.

Generation of narrowband elastic waves with a fiber laser and its application to the imaging of defects in a plate.

PubMed

Hayashi, Takahiro; Ishihara, Ken

2017-05-01

Pulsed laser equipment can be used to generate elastic waves through the instantaneous reaction of thermal expansion or ablation of the material; however, we cannot control the waveform generated by the laser in the same manner that we can when piezoelectric transducers are used as exciters. This study investigates the generation of narrowband tone-burst waves using a fiber laser of the type that is widely used in laser beam machining. Fiber lasers can emit laser pulses with a high repetition rate on the order of MHz, and the laser pulses can be modulated to a burst train by external signals. As a consequence of the burst laser emission, a narrowband tone-burst elastic wave is generated. We experimentally confirmed that the elastic waves agreed well with the modulation signals in time domain waveforms and their frequency spectra, and that waveforms can be controlled by the generation technique. We also apply the generation technique to defect imaging with a scanning laser source. In the experiments, with small laser emission energy, we were not able to obtain defect images from the signal amplitude due to low signal-to-noise ratio, whereas using frequency spectrum peaks of the tone-burst signals gave clear defect images, which indicates that the signal-to-noise ratio is improved in the frequency domain by using this technique for the generation of narrowband elastic waves. Moreover, even for defect imaging at a single receiving point, defect images were enhanced by taking an average of distributions of frequency spectrum peaks at different frequencies. Copyright © 2017 Elsevier B.V. All rights reserved.

Self-regulation of charged defect compensation and formation energy pinning in semiconductors

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yang, Ji -Hui; Yin, Wan -Jian; Park, Ji -Sang

2015-11-20

Current theoretical analyses of defect properties without solving the detailed balance equations often estimate Fermi-level pinning position by omitting free carriers and assume defect concentrations can be always tuned by atomic chemical potentials. This could be misleading in some circumstance. Here we clarify that: (1) Because the Fermi-level pinning is determined not only by defect states but also by free carriers from band-edge states, band-edge states should be treated explicitly in the same footing as the defect states in practice; (2) defect formation energy, thus defect density, could be pinned and independent on atomic chemical potentials due to the entanglementmore » of atomic chemical potentials and Fermi energy, in contrast to the usual expectation that defect formation energy can always be tuned by varying the atomic chemical potentials; and (3) the charged defect compensation behavior, i.e., most of donors are compensated by acceptors or vice versa, is self-regulated when defect formation energies are pinned. The last two phenomena are more dominant in wide-gap semiconductors or when the defect formation energies are small. Using NaCl and CH 3NH 3PbI 3 as examples, we illustrate these unexpected behaviors. Furthermore, our analysis thus provides new insights that enrich the understanding of the defect physics in semiconductors and insulators.« less

Capacitance Techniques | Photovoltaic Research | NREL

Science.gov Websites

transient spectroscopy generated graph showing six defect levels; DLTS signal (Y-axis) versus Temperature (X -axis). DLTS characterizes defect levels to assist in identification of impurities and potential levels of interface states (or both) that often exist between the surfaces of dissimilar materials. Deep

Effect of intermixing at CdS/CdTe interface on defect properties

NASA Astrophysics Data System (ADS)

Park, Ji-Sang; Yang, Ji-Hui; Barnes, Teresa; Wei, Su-Huai

2016-07-01

We investigated the stability and electronic properties of defects in CdTe1-xSx that can be formed at the CdS/CdTe interface. As the anions mix at the interface, the defect properties are significantly affected, especially those defects centered at cation sites like Cd vacancy, VCd, and Te on Cd antisite, TeCd, because the environment surrounding the defect sites can have different configurations. We show that at a given composition, the transition energy levels of VCd and TeCd become close to the valence band maximum when the defect has more S atoms in their local environment, thus improving the device performance. Such beneficial role is also found at the grain boundaries when the Te atom is replaced by S in the Te-Te wrong bonds, reducing the energy of the grain boundary level. On the other hand, the transition levels with respect to the valence band edge of CdTe1-xSx increases with the S concentration as the valence band edge decreases with the S concentration, resulting in the reduced p-type doping efficiency.

Outstanding features of Cu-doped ZnS nanoclusters

NASA Astrophysics Data System (ADS)

Tawfik, Wael Z.; Farghali, A. A.; Moneim, Ahmed; Imam, N. G.; El-Dek, S. I.

2018-05-01

ZnS and their Cu-doped nanoclusters (NCs) were synthesized successfully using the wet chemical route with different Cu content. The crystalline structure was investigated using x-ray powder diffraction which assured the single-phase formation in cubic symmetry. High-resolution transmission electron microscope indicated the microstructure of NCs with a size ranging from 2–4 nm. A butterfly hysteresis (M-H) loop was observed at room temperature with large values of coercivity for the Cu content of x = 0.05. Photoluminescence emission spectra were recorded from 500–615 nm for pure and Cu-doped ZnS NCs at a 350 nm excitation wavelength. The sample exhibited green fluorescence bands peaking at 535, 544, 552.5, 558.2, and 560.6 nm, which confirmed the characteristic feature of Zn2+ as luminescent centers in the lattice. The additional yellow and orange emissions are due to defect levels or/and impurity centers. The dielectric constant as well as the conductivity values increased with increasing Cu content.

Photoelectrochemical properties of highly mobilized Li-doped ZnO thin films.

PubMed

Shinde, S S; Bhosale, C H; Rajpure, K Y

2013-03-05

Li-doped ZnO thin films with preferred (002) orientation have been prepared by spray pyrolysis technique in aqueous medium on to the corning glass substrates. The effect of Li-doping on to the photoelectrochemical, structural, morphological, optical, luminescence, electrical and thermal properties has been investigated. XRD and Raman study indicates that the films have hexagonal crystal structure. The transmittance, reflectance, refractive index, extinction coefficient and bandgap have been analyzed by optical study. PL spectra consist of a near band edge and visible emission due to the electronic defects, which are related to deep level emissions, such as oxide antisite (OZn), interstitial zinc (Zni), interstitial oxygen (Oi) and zinc vacancy (VZn). The Li-doped ZnO films prepared for 1at% doping possesses the highest electron mobility of 102cm(2)/Vs and carrier concentration of 3.62×10(19)cm(-3). Finally, degradation of 2,4,6-Trinitrotoluene using Li-doped ZnO thin films has been reported. Copyright © 2013 Elsevier B.V. All rights reserved.

Effect of Eu3+ doping on the structural, morphological and luminescence properties ZnO nanostructures

NASA Astrophysics Data System (ADS)

Vinoditha, U.; Balakrishna, K. M.; Sarojini, B. K.; Narayana, B.; Kumara, K.

2018-05-01

Pure and Eu3+ ions (1, 3, 5 atomic wt%) doped ZnO nanostructures are synthesized by a surfactant assisted hydrothermal method. The effect of doping concentrations on structural, morphological and optical properties of ZnO nanostructures is studied. The XRD analysis shows good crystallinity and the phase purity of the ZnO nanostructures. A shift in the standard Zn-O stretching mode after Eu3+ doping is observed in the FTIR spectra. The images of FESEM demonstrate the morphological variations from hexagonal nanorods to nanoflowers on varying the dopant concentrations. Substitution of Eu3+ ions into Zn2+ sites is confirmed by EDX analysis. The dominance of particle shape over the UV-Visible absorption properties of the prepared samples is noticed. The photoluminescence (PL) emission of undoped and doped ZnO nanostructures show dominant near band edge emission (NBE) in the UV region and minor defect induced deep level emissions in the visible region.

Systemic Treatment with Strontium Ranelate Accelerates the Filling of a Bone Defect and Improves the Material Level Properties of the Healing Bone

PubMed Central

Zacchetti, Giovanna; Rizzoli, René

2014-01-01

Rapid bone defect filling with normal bone is a challenge in orthopaedics and dentistry. Strontium ranelate (SrRan) has been shown to in vitro decrease bone resorption and increase bone formation, and represents a potential agent with the capacity to accelerate bone defect filling. In this study, bone tibial defects of 2.5 mm in diameter were created in 6-month-old female rats orally fed SrRan (625 mg/kg/d; 5/7 days) or vehicle for 4, 8, or 12 weeks (10 rats per group per time point) from the time of surgery. Tibias were removed. Micro-architecture was determined by micro-computed tomography (µCT) and material level properties by nanoindentation analysis. µCT analysis showed that SrRan administration significantly improved microarchitecture of trabecular bone growing into the defect after 8 and 12 weeks of treatment compared to vehicle. SrRan treatment also accelerated the growth of cortical bone over the defect, but with different kinetics compared to trabecular bone, as the effects were already significant after 4 weeks. Nanoindentation analysis demonstrated that SrRan treatment significantly increased material level properties of both trabecular bone and cortical bone filling the defect compared to vehicle. SrRan accelerates the filling of bone defect by improving cortical and trabecular bone microarchitecture both quantitatively and qualitatively. PMID:25243150

Self-passivation Rule and the Effect of Post-treatment in GBs of Solar Cell Materials

NASA Astrophysics Data System (ADS)

Liu, Chengyan; Chen, Shiyou; Xiang, Hongjun; Gong, Xingao

Grain boundaries (GBs) existing in polycrystalline semiconductors alloys inducing a great deal of deep defect levels are usually harmful to cells' photovoltaic performance. Experimental and theoretical investigations verified that these defect levels come from the GBs' dangling bonds. We find that, the defect levels in anion core of GB can be passivated by its cations, called by self-passivation. For instance, the post-treated by CdCl2, Cd can eliminate the defect levels by saturating Te dangling bonds in the grain boundary of CdTe. We verify that the idea of self-passivation rule can perfectly explain the benign GBs of CISe and CZTS by sodium treatment. The present work reveals a general mechanism about how dopants in GBs eliminate the defect states through passivating the dangling bonds in covalent polycrystalline semiconductors, and sheds light on how to passivate dangling bonds in GBs with alterative processes. National Science Foundation of China, international collaboration project of MOST, Pujiang plan, Program for Professor of Special Appointment (Eastern Scholar), and Shanghai Rising-star program.

Phase time delay and Hartman effect in a one-dimensional photonic crystal with four-level atomic defect layer

NASA Astrophysics Data System (ADS)

Jamil, Rabia; Ali, Abu Bakar; Abbas, Muqaddar; Badshah, Fazal; Qamar, Sajid

2017-08-01

The Hartman effect is revisited using a Gaussian beam incident on a one-dimensional photonic crystal (1DPC) having a defect layer doped with four-level atoms. It is considered that each atom of the defect layer interacts with three driving fields, whereas a Gaussian beam of width w is used as a probe light to study Hartman effect. The atom-field interaction inside the defect layer exhibits electromagnetically induced transparency (EIT). The 1DPC acts as positive index material (PIM) and negative index material (NIM) corresponding to the normal and anomalous dispersion of the defect layer, respectively, via control of the phase associated with the driving fields and probe detuning. The positive and negative Hartman effects are noticed for PIM and NIM, respectively, via control of the relative phase corresponding to the driving fields and probe detuning. The advantage of using four-level EIT system is that a much smaller absorption of the transmitted beam occurs as compared to three-level EIT system corresponding to the anomalous dispersion, leading to negative Hartman effect.

Electrical conductivity of In2O3 and Ga2O3 after low temperature ion irradiation; implications for instrinsic defect formation and charge neutrality level.

PubMed

Vines, L; Bhoodoo, C; von Wenckstern, H; Grundmann, M

2017-12-13

The evolution of sheet resistance of n-type In 2 O 3 and Ga 2 O 3 exposed to bombardment with MeV 12 C and 28 Si ions at 35 K is studied in situ. While the sheet resistance of Ga 2 O 3 increased by more than eight orders of magnitude as a result of ion irradiation, In 2 O 3 showed a more complex defect evolution and became more conductive when irradiated at the highest doses. Heating up to room temperature reduced the sheet resistivity somewhat, but Ga 2 O 3 remained highly resistive, while In 2 O 3 showed a lower resistance than as deposited samples. Thermal admittance spectroscopy and deep level transient spectroscopy did not reveal new defect levels for irradiation up to [Formula: see text] cm -2 . A model where larger defect complexes preferentially produce donor like defects in In 2 O 3 is proposed, and may reveal a microscopic view of a charge neutrality level within the conduction band, as previously proposed.

Electrical conductivity of In2O3 and Ga2O3 after low temperature ion irradiation; implications for instrinsic defect formation and charge neutrality level

NASA Astrophysics Data System (ADS)

Vines, L.; Bhoodoo, C.; von Wenckstern, H.; Grundmann, M.

2018-01-01

The evolution of sheet resistance of n-type In2O3 and Ga2O3 exposed to bombardment with MeV 12C and 28Si ions at 35 K is studied in situ. While the sheet resistance of Ga2O3 increased by more than eight orders of magnitude as a result of ion irradiation, In2O3 showed a more complex defect evolution and became more conductive when irradiated at the highest doses. Heating up to room temperature reduced the sheet resistivity somewhat, but Ga2O3 remained highly resistive, while In2O3 showed a lower resistance than as deposited samples. Thermal admittance spectroscopy and deep level transient spectroscopy did not reveal new defect levels for irradiation up to 2 × 1012 cm-2. A model where larger defect complexes preferentially produce donor like defects in In2O3 is proposed, and may reveal a microscopic view of a charge neutrality level within the conduction band, as previously proposed.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Upadhyaya, Mihir; Jindal, Vibhu; Basavalingappa, Adarsh

The availability of defect-free masks is considered to be a critical issue for enabling extreme ultraviolet lithography (EUVL) as the next generation technology. Since completely defect-free masks will be hard to achieve, it is essential to have a good understanding of the printability of the native EUV mask defects. In this work, we performed a systematic study of native mask defects to understand the defect printability caused by them. The multilayer growth over native substrate mask blank defects was correlated to the multilayer growth over regular-shaped defects having similar profiles in terms of their width and height. To model themore » multilayer growth over the defects, a novel level-set multilayer growth model was used that took into account the tool deposition conditions of the Veeco Nexus ion beam deposition tool. The same tool was used for performing the actual deposition of the multilayer stack over the characterized native defects, thus ensuring a fair comparison between the actual multilayer growth over native defects, and modeled multilayer growth over regular-shaped defects. Further, the printability of the characterized native defects was studied with the SEMATECH-Berkeley Actinic Inspection Tool (AIT), an EUV mask-imaging microscope at Lawrence Berkeley National Laboratory (LBNL). Printability of the modeled regular-shaped defects, which were propagated up the multilayer stack using level-set growth model was studied using defect printability simulations implementing the waveguide algorithm. Good comparison was observed between AIT and the simulation results, thus demonstrating that multilayer growth over a defect is primarily a function of a defect’s width and height, irrespective of its shape. This would allow us to predict printability of the arbitrarily-shaped native EUV mask defects in a systematic and robust manner.« less

System-size convergence of point defect properties: The case of the silicon vacancy

NASA Astrophysics Data System (ADS)

Corsetti, Fabiano; Mostofi, Arash A.

2011-07-01

We present a comprehensive study of the vacancy in bulk silicon in all its charge states from 2+ to 2-, using a supercell approach within plane-wave density-functional theory, and systematically quantify the various contributions to the well-known finite size errors associated with calculating formation energies and stable charge state transition levels of isolated defects with periodic boundary conditions. Furthermore, we find that transition levels converge faster with respect to supercell size when only the Γ-point is sampled in the Brillouin zone, as opposed to a dense k-point sampling. This arises from the fact that defect level at the Γ-point quickly converges to a fixed value which correctly describes the bonding at the defect center. Our calculated transition levels with 1000-atom supercells and Γ-point only sampling are in good agreement with available experimental results. We also demonstrate two simple and accurate approaches for calculating the valence band offsets that are required for computing formation energies of charged defects, one based on a potential averaging scheme and the other using maximally-localized Wannier functions (MLWFs). Finally, we show that MLWFs provide a clear description of the nature of the electronic bonding at the defect center that verifies the canonical Watkins model.

21 CFR 101.79 - Health claims: Folate and neural tube defects.

Code of Federal Regulations, 2013 CFR

2013-04-01

... result in infant mortality or serious disability. The birth defects anencephaly and spina bifida are the... development. Because the neural tube forms and closes during early pregnancy, the defect may occur before a... pregnancy had a reduced risk of having a child with a neural tube defect. (Products containing this level of...

21 CFR 101.79 - Health claims: Folate and neural tube defects.

Code of Federal Regulations, 2014 CFR

2014-04-01

... result in infant mortality or serious disability. The birth defects anencephaly and spina bifida are the... development. Because the neural tube forms and closes during early pregnancy, the defect may occur before a... pregnancy had a reduced risk of having a child with a neural tube defect. (Products containing this level of...

21 CFR 101.79 - Health claims: Folate and neural tube defects.

Code of Federal Regulations, 2012 CFR

2012-04-01

... result in infant mortality or serious disability. The birth defects anencephaly and spina bifida are the... development. Because the neural tube forms and closes during early pregnancy, the defect may occur before a... pregnancy had a reduced risk of having a child with a neural tube defect. (Products containing this level of...

Reduction of Fermi level pinning and recombination at polycrystalline CdTe surfaces by laser irradiation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Simonds, Brian J.; Kheraj, Vipul; Department of Applied Physics, S. V. National Institute of Technology, Surat 395 007

2015-06-14

Laser processing of polycrystalline CdTe is a promising approach that could potentially increase module manufacturing throughput while reducing capital expenditure costs. For these benefits to be realized, the basic effects of laser irradiation on CdTe must be ascertained. In this study, we utilize surface photovoltage spectroscopy (SPS) to investigate the changes to the electronic properties of the surface of polycrystalline CdTe solar cell stacks induced by continuous-wave laser annealing. The experimental data explained within a model consisting of two space charge regions, one at the CdTe/air interface and one at the CdTe/CdS junction, are used to interpret our SPS results.more » The frequency dependence and phase spectra of the SPS signal are also discussed. To support the SPS findings, low-temperature spectrally-resolved photoluminescence and time-resolved photoluminescence were also measured. The data show that a modest laser treatment of 250 W/cm{sup 2} with a dwell time of 20 s is sufficient to reduce the effects of Fermi level pinning at the surface due to surface defects.« less

Annealing effect on the photoluminescence properties of ZnO nanorod array prepared by a PLD-assistant wet chemical method

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wei Sufeng; Lian Jianshe; Wu Hua, E-mail: weisufeng@gmail.com

2010-11-15

Well-aligned ZnO nanorod arrays were synthesized by a wet chemical method on the glass substrate with ZnO thin film as seed layer prepared by pulsed laser deposition. The effect of annealing temperature on the luminescence characteristics was investigated. As the annealing temperature increased, the photoluminescence properties show a general enhancing tendency. The nanorod array with high ultraviolet emission and negligible visible light emission (designated by the photoluminescence intensity ratio of ultraviolet to visible emission of 66.4) is obtained by annealing the sample at 700 deg. C for 1 h. Based on the results of X-ray photoelectron spectroscopy and photoluminescence spectra,more » the mechanisms of visible emission were discussed. - Research Highlights: {yields} ZnO nanorod array with good crystallography, low defects concentration and good optical property was obtained after annealed at 700 deg. C for 1 h. {yields} The transition from the conduction band to the O{sub i} level may be responsible for the yellow-green emission. {yields} The yellow emission may originate from the presence of Zn(OH){sub 2} on the surface or the band transition from conduction band to V{sub o}Zn{sub i} level. {yields} The transition from the Zn{sub i} level to the level should produce an orange emission or an orange-red emission.« less

Effective scheme to determine accurate defect formation energies and charge transition levels of point defects in semiconductors

NASA Astrophysics Data System (ADS)

Yao, Cang Lang; Li, Jian Chen; Gao, Wang; Tkatchenko, Alexandre; Jiang, Qing

2017-12-01

We propose an effective method to accurately determine the defect formation energy Ef and charge transition level ɛ of the point defects using exclusively cohesive energy Ecoh and the fundamental band gap Eg of pristine host materials. We find that Ef of the point defects can be effectively separated into geometric and electronic contributions with a functional form: Ef=χ Ecoh+λ Eg , where χ and λ are dictated by the geometric and electronic factors of the point defects (χ and λ are defect dependent). Such a linear combination of Ecoh and Eg reproduces Ef with an accuracy better than 5% for electronic structure methods ranging from hybrid density-functional theory (DFT) to many-body random-phase approximation (RPA) and experiments. Accordingly, ɛ is also determined by Ecoh/Eg and the defect geometric/electronic factors. The identified correlation is rather general for monovacancies and interstitials, which holds in a wide variety of semiconductors covering Si, Ge, phosphorenes, ZnO, GaAs, and InP, and enables one to obtain reliable values of Ef and ɛ of the point defects for RPA and experiments based on semilocal DFT calculations.

Identification of the primary compensating defect level responsible for determining blocking voltage of vertical GaN power diodes

DOE PAGES

King, M. P.; Kaplar, R. J.; Dickerson, J. R.; ...

2016-10-31

Electrical performance and characterization of deep levels in vertical GaN P-i-N diodes grown on low threading dislocation density (~10 4 –10 6 cm –2) bulk GaN substrates are investigated. The lightly doped n drift region of these devices is observed to be highly compensated by several prominent deep levels detected using deep level optical spectroscopy at E c-2.13, 2.92, and 3.2 eV. A combination of steady-state photocapacitance and lighted capacitance-voltage profiling indicates the concentrations of these deep levels to be N t = 3 × 10 12, 2 × 10 15, and 5 × 10 14 cm –3, respectively. Themore » E c-2.92 eV level is observed to be the primary compensating defect in as-grown n-type metal-organic chemical vapor deposition GaN, indicating this level acts as a limiting factor for achieving controllably low doping. The device blocking voltage should increase if compensating defects reduce the free carrier concentration of the n drift region. Understanding the incorporation of as-grown and native defects in thick n-GaN is essential for enabling large V BD in the next-generation wide-bandgap power semiconductor devices. Furthermore, controlling the as-grown defects induced by epitaxial growth conditions is critical to achieve blocking voltage capability above 5 kV.« less

Defect reduction for semiconductor memory applications using jet and flash imprint lithography

NASA Astrophysics Data System (ADS)

Ye, Zhengmao; Luo, Kang; Irving, J. W.; Lu, Xiaoming; Zhang, Wei; Fletcher, Brian; Liu, Weijun; Xu, Frank; LaBrake, Dwayne; Resnick, Douglas; Sreenivasan, S. V.

2013-03-01

Imprint lithography has been shown to be an effective technique for replication of nano-scale features. Jet and Flash Imprint Lithography (J-FIL) involves the field-by-field deposition and exposure of a low viscosity resist deposited by jetting technology onto the substrate. The patterned mask is lowered into the fluid which then quickly flows into the relief patterns in the mask by capillary action. Following this filling step, the resist is crosslinked under UV radiation, and then the mask is removed leaving a patterned resist on the substrate. Acceptance of imprint lithography for manufacturing will require demonstration that it can attain defect levels commensurate with the defect specifications of high end memory devices. Typical defectivity targets are on the order of 0.10/cm2. In previous studies, we have focused on defects such as random non-fill defects occurring during the resist filling process and repeater defects caused by interactions with particles on the substrate. In this work, we attempted to identify the critical imprint defect types using a mask with NAND Flash-like patterns at dimensions as small as 26nm. The two key defect types identified were line break defects induced by small particulates and airborne contaminants which result in local adhesion failure. After identification, the root cause of the defect was determined, and corrective measures were taken to either eliminate or reduce the defect source. As a result, we have been able to reduce defectivity levels by more than three orders of magnitude in only 12 months and are now achieving defectivity adders as small as 2 adders per lot of wafers.

The Effect of Thermal Annealing on Charge Transport in Organolead Halide Perovskite Microplate Field-Effect Transistors.

PubMed

Li, Dehui; Cheng, Hung-Chieh; Wang, Yiliu; Zhao, Zipeng; Wang, Gongming; Wu, Hao; He, Qiyuan; Huang, Yu; Duan, Xiangfeng

2017-01-01

Transformation of unipolar n-type semiconductor behavior to ambipolar and finally to unipolar p-type behavior in CH 3 NH 3 PbI 3 microplate field-effect transistors by thermal annealing is reported. The photoluminescence spectra essentially maintain the same features before and after the thermal annealing process, demonstrating that the charge transport measurement provides a sensitive way to probe low-concentration defects in perovskite materials. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Core structures of haemosiderins deposited in various organs in β-thalassaemia/haemoglobin e disease

NASA Astrophysics Data System (ADS)

St. Pierre, T. G.; Tran, K. C.; Webb, J.; Macey, D. J.; Pootrakul, P.; Dickson, D. P. E.

1992-04-01

Mössbauer spectra were recorded of tissue from β-thalassaemia/haemoglobin E spleen, liver, pancreas and heart and of crude haemosiderins (insoluble iron fractions) isolated from the organs. Iron in the crude haemosiderins from the spleen and heart remains paramagnetic below 4.2K indicating that the iron is in a non-crystalline form. Superparamagnetic behaviour of the crude haemosiderins from the pancreas and liver indicate the presence of ferrihydrite cores with some cores with a structure based on defect-goethite.

Test simulation of neutron damage to electronic components using accelerator facilities

NASA Astrophysics Data System (ADS)

King, D. B.; Fleming, R. M.; Bielejec, E. S.; McDonald, J. K.; Vizkelethy, G.

2015-12-01

The purpose of this work is to demonstrate equivalent bipolar transistor damage response to neutrons and silicon ions. We report on irradiation tests performed at the White Sands Missile Range Fast Burst Reactor, the Sandia National Laboratories (SNL) Annular Core Research Reactor, the SNL SPHINX accelerator, and the SNL Ion Beam Laboratory using commercial silicon npn bipolar junction transistors (BJTs) and III-V Npn heterojunction bipolar transistors (HBTs). Late time and early time gain metrics as well as defect spectra measurements are reported.

Forbidden 2P–nP and 2P–nF transitions in the energy spectrum of ultracold Rydberg lithium-7 atoms

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zelener, B. B., E-mail: bobozel@mail.ru; Saakyan, S. A.; Sautenkov, V. A.

2016-04-15

Forbidden 2P–nP and 2P–nF transitions in the ranges of the principal quantum number n = 42–114 and n = 38–48 have been detected in the optical spectra of ultracold highly excited lithium-7 atoms. The presence of forbidden transitions is due to induced external electric fields. The quantum defects and ionization energy obtained in various experiments and predicted theoretically have been discussed.

Intrinsic and extrinsic defects in a family of coal-derived graphene quantum dots

DOE Office of Scientific and Technical Information (OSTI.GOV)

Singamaneni, Srinivasa Rao, E-mail: ssingam@ncsu.edu, E-mail: tour@rice.edu; Department of Material Science and Engineering, North Carolina State University, Raleigh, North Carolina 27695; Tol, Johan van

In this letter, we report on the high frequency (239.2 and 336 GHz) electron spin resonance (ESR) studies performed on graphene quantum dots (GQDs), prepared through a wet chemistry route from three types of coal: (a) bituminous, (b) anthracite, and (c) coke; and from non-coal derived GQDs. The microwave frequency-, power-, and temperature-dependent ESR spectra coupled with computer-aided simulations reveal four distinct magnetic defect centers. In bituminous- and anthracite-derived GQDs, we have identified two of them as intrinsic carbon-centered magnetic defect centers (a broad signal of peak to peak width = 697 (10{sup −4} T), g = 2.0023; and a narrow signal of peak tomore » peak width = 60 (10{sup −4} T), g = 2.003). The third defect center is Mn{sup 2+} ({sup 6}S{sub 5/2}, 3d{sup 5}) (signal width = 61 (10{sup −4} T), g = 2.0023, A{sub iso} = 93(10{sup −4} T)), and the fourth defect is identified as Cu{sup 2+} ({sup 2}D{sub 5/2}, 3d{sup 9}) (g{sub ⊥} = 2.048 and g{sub ‖} = 2.279), previously undetected. Coke-derived and non-coal derived GQDs show Mn{sup 2+} and two-carbon related signals, and no Cu{sup 2+} signal. The extrinsic impurities most likely originate from the starting coal. Furthermore, Raman, photoluminescence, and ESR measurements detected no noticeable changes in the properties of the bituminous GQDs after one year. This study highlights the importance of employing high frequency ESR spectroscopy in identifying the (magnetic) defects, which are roadblocks for spin relaxation times of graphene-based materials. These defects would not have been possible to probe by other spin transport measurements.« less

Defect phase diagram for doping of Ga 2O 3

DOE PAGES

Lany, Stephan

2018-04-01

For the case of n-type doping of β-Ga 2O 3 by group 14 dopants (C, Si, Ge, Sn), a defect phase diagram is constructed from defect equilibria calculated over a range of temperatures (T), O partial pressures (pO 2), and dopant concentrations. The underlying defect levels and formation energies are determined from first-principles supercell calculations with GW bandgap corrections. Only Si is found to be a truly shallow donor, C is a deep DX-like (lattice relaxed donor) center, and Ge and Sn have defect levels close to the conduction band minimum. The thermodynamic modeling includes the effect of association ofmore » dopant-defect pairs and complexes, which causes the net doping to decline when exceeding a certain optimal dopant concentration. The optimal doping levels are surprisingly low, between about 0.01% and 1% of cation substitution, depending on the (T, pO 2) conditions. Considering further the stability constraints due to sublimation of molecular Ga 2O, specific predictions of optimized pO 2 and Si dopant concentrations are given. To conclude, the incomplete passivation of dopant-defect complexes in β-Ga 2O 3 suggests a design rule for metastable doping above the solubility limit.« less

Defect phase diagram for doping of Ga 2O 3

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lany, Stephan

For the case of n-type doping of β-Ga 2O 3 by group 14 dopants (C, Si, Ge, Sn), a defect phase diagram is constructed from defect equilibria calculated over a range of temperatures (T), O partial pressures (pO 2), and dopant concentrations. The underlying defect levels and formation energies are determined from first-principles supercell calculations with GW bandgap corrections. Only Si is found to be a truly shallow donor, C is a deep DX-like (lattice relaxed donor) center, and Ge and Sn have defect levels close to the conduction band minimum. The thermodynamic modeling includes the effect of association ofmore » dopant-defect pairs and complexes, which causes the net doping to decline when exceeding a certain optimal dopant concentration. The optimal doping levels are surprisingly low, between about 0.01% and 1% of cation substitution, depending on the (T, pO 2) conditions. Considering further the stability constraints due to sublimation of molecular Ga 2O, specific predictions of optimized pO 2 and Si dopant concentrations are given. To conclude, the incomplete passivation of dopant-defect complexes in β-Ga 2O 3 suggests a design rule for metastable doping above the solubility limit.« less

Significant Enhancement of Photocatalytic Reduction of CO2 with H2O over ZnO by the Formation of Basic Zinc Carbonate.

PubMed

Xin, Chunyu; Hu, Maocong; Wang, Kang; Wang, Xitao

2017-07-11

Electron-hole pair separation efficiency and adsorption performance of photocatalysts to CO 2 are the two key factors affecting the performance of photocatalytic CO 2 reduction with H 2 O. Distinct from conventional promoter addition, this study proposed a novel approach to address these two issues by tuning the own surface features of semiconductor photocatalyst. Three ZnO samples with different morphologies, surface area, and defect content were fabricated by varying preparation methods, characterized by XRD, TEM, and room-temperature PL spectra, and tested in photoreduction of CO 2 with H 2 O. The results show that the as-prepared porous ZnO nanosheets exhibit a much higher activity for photoreduction of CO 2 with H 2 O when compared to ZnO nanoparticles and nanorods attributed to the existence of more defect sites, that is, zinc and oxygen vacancies. These defects would lower the combination rate of electron-hole pair as well as promote the formation of basic zinc carbonate by Lewis acid-base interaction, which is the active intermediate species for photoreduction of CO 2 . ZnO nanoparticles and ZnO nanorods with few defects show weak adsorption for CO 2 leading to the inferior photocatalytic activities. This work provides new insight on the CO 2 activation under light irradiation.

Quasi-continuum photoluminescence: Unusual broad spectral and temporal characteristics found in defective surfaces of silica and other materials

DOE Office of Scientific and Technical Information (OSTI.GOV)

Laurence, Ted A., E-mail: laurence2@llnl.gov; Bude, Jeff D.; Shen, Nan

2014-02-28

We previously reported a novel photoluminescence (PL) with a distribution of fast decay times in fused silica surface flaws that is correlated with damage propensity by high fluence lasers. The source of the PL was not attributable to any known silica point defect. Due to its broad spectral and temporal features, we here give this PL the name quasi-continuum PL (QC-PL) and describe the features of QC-PL in more detail. The primary features of QC-PL include broad excitation and emission spectra, a broad distribution of PL lifetimes from 20 ps to 5 ns, continuous shifts in PL lifetime distributions with respectmore » to emission wavelength, and a propensity to photo-bleach and photo-brighten. We found similar PL characteristics in surface flaws of other optical materials, including CaF{sub 2}, DKDP, and quartz. Based on the commonality of the features in different optical materials and the proximity of QC-PL to surfaces, we suggest that these properties arise from interactions associated with high densities of defects, rather than a distribution over a large number of types of defects and is likely found in a wide variety of structures from nano-scale composites to bulk structures as well as in both broad and narrow band materials from dielectrics to semiconductors.« less

Tunable optical properties of ZnCdTe2-xSex(x =0.625) chalcopyrite for photovoltaics; a mBJLDA approach

NASA Astrophysics Data System (ADS)

Kashyap, Manish K.; Paudyal, D.; Harmon, B. N.

In the present study, we have performed ab-initio simulations of sp-element defect in ZnCdTe2-xSex (x =0.625) chalcopyrite to check the tuning of band gap as compared to the pristine case. The exchange and correlation (XC) effects are taken into account by an orbital independent modified Becke-Johnson (mBJ) potential as coupled with Local Density Approximation (LDA) for these calculations. The calculated energy band structures show a direct band gap at the point in the brillouin zone for the pristine as well as the defected case and the band gap decreases with inclusion of sp-disorder. The imaginary dielectric function predicts the optical band gap of pristine ZnCdTe2 very close to the experimental value and the results are in reasonable agreement without applying any scissor operator. With inclusion of sp-element defect, the optical spectra is tuned to optimal region, suitable for photovoltaics. It is apparent that mBJ functional is well suited for calculating electronic structure of pristine as well as defected ZnCdTe2chalcopyrite. MKK acknowledges financial support from UGC, India in the form of RAMAN Post-doctoral fellowship. This work at Ames Laboratory was supported by the DOE, Office of Basic Energy Sciences, Materials Sciences Division under contract No. DE-AC02-07CH11358.

Optical Dependence of Electrically Detected Magnetic Resonance in Lightly Doped Si:P Devices

NASA Astrophysics Data System (ADS)

Zhu, Lihuang; van Schooten, Kipp J.; Guy, Mallory L.; Ramanathan, Chandrasekhar

2017-06-01

Using frequency-modulated electrically detected magnetic resonance (EDMR), we show that signals measured from lightly doped (1.2 - 5 ×1 015 cm-3 ) silicon devices vary significantly with the wavelength of the optical excitation used to generate the mobile carriers. We measure EDMR spectra at 4.2 K as a function of modulation frequency and applied microwave power using a 980-nm laser, a 405-nm laser, and a broadband white-light source. EDMR signals are observed from the phosphorus donor and two distinct defect species in all of the experiments. With near-infrared irradiation, we find that the EDMR signal primarily arises from donor-defect pairs, while, at higher photon energies, there are significant additional contributions from defect-defect pairs. The contribution of spins from different spatial regions to the EDMR signal is seen to vary as the optical penetration depth changes from about 120 nm at 405-nm illumination to 100 μ m at 980-nm illumination. The modulation frequency dependence of the EDMR signal shows that the energy of the optical excitation strongly modulates the kinetics of the underlying spin-dependent recombination (SDR) process. Careful tuning of the optical photon energy could therefore be used to control both the subset of spin pairs contributing to the EDMR signal and the dynamics of the SDR process.

The defect level and ideal thermal conductivity of graphene uncovered by residual thermal reffusivity at the 0 K limit.

PubMed

Xie, Yangsu; Xu, Zaoli; Xu, Shen; Cheng, Zhe; Hashemi, Nastaran; Deng, Cheng; Wang, Xinwei

2015-06-14

Due to its intriguing thermal and electrical properties, graphene has been widely studied for potential applications in sensor and energy devices. However, the reported value for its thermal conductivity spans from dozens to thousands of W m(-1) K(-1) due to different levels of alternations and defects in graphene samples. In this work, the thermal diffusivity of suspended four-layered graphene foam (GF) is characterized from room temperature (RT) down to 17 K. For the first time, we identify the defect level in graphene by evaluating the inverse of thermal diffusivity (termed "thermal reffusivity": Θ) at the 0 K limit. By using the Debye model of Θ = Θ0 + C× e(-θ/2T) and fitting the Θ-T curve to the point of T = 0 K, we identify the defect level (Θ0) and determine the Debye temperature of graphene. Θ0 is found to be 1878 s m(-2) for the studied GF and 43-112 s m(-2) for three highly crystalline graphite materials. This uncovers a 16-43-fold higher defect level in GF than that in pyrolytic graphite. In GF, the phonon mean free path solely induced by defects and boundary scattering is determined as 166 nm. The Debye temperature of graphene is determined to be 1813 K, which is very close to the average theoretical Debye temperature (1911 K) of the three acoustic phonon modes in graphene. By subtracting the defect effect, we report the ideal thermal diffusivity and conductivity (κideal) of graphene presented in the 3D foam structure in the range of 33-299 K. Detailed physics based on chemical composition and structure analysis are given to explain the κideal-T profile by comparing with those reported for suspended graphene.

Steady state and time resolved optical characterization studies of Zn 2SnO 4 nanowires for solar cell applications

DOE PAGES

Yakami, Baichhabi R.; Poudyal, Uma; Nandyala, Shashank R.; ...

2016-10-25

Nanowires are a promising option for sensitized solar cells, sensors, and display technology. Most of the work thus far has focused on binary oxides for these nanowires, but ternary oxides have advantages in additional control of optical and electronic properties. Here, we report on the diffuse reflectance, Low Temperature and Room Temperature Photoluminescence (PL), PL excitation spectrum, and Time Resolved PL (TRPL) of Zinc Tin Oxide (ZTO) nanowires grown by Chemical Vapor Deposition. The PL from the ZTO nanowires does not exhibit any band gap or near gap emission, and the diffuse reflectance measurement confirms that these ZTO nanowires havemore » a direct forbidden transition. The broad PL spectrum reveals two Gaussian peaks centered at 1.86 eV (red) and 2.81 eV (blue), representing two distinct defect states or complexes. The PL spectra were further studied by the Time Resolved Emission Spectrum and intensity dependent PL and TRPL. The time resolved measurements show complex non-exponential decays at all wavelengths, indicative of defect to defect transitions, and the red emissive states decay much slower than the blue emissive states. The effects of annealing in air and vacuum are studied to investigate the origin of the defect states in the nanowires, showing that the blue states are related to oxygen vacancies. We propose an energy band model for the nanowires containing defect states within the band gap and the associated transitions between these states that are consistent with our measurements.« less

Steady state and time resolved optical characterization studies of Zn 2SnO 4 nanowires for solar cell applications

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yakami, Baichhabi R.; Poudyal, Uma; Nandyala, Shashank R.

Nanowires are a promising option for sensitized solar cells, sensors, and display technology. Most of the work thus far has focused on binary oxides for these nanowires, but ternary oxides have advantages in additional control of optical and electronic properties. Here, we report on the diffuse reflectance, Low Temperature and Room Temperature Photoluminescence (PL), PL excitation spectrum, and Time Resolved PL (TRPL) of Zinc Tin Oxide (ZTO) nanowires grown by Chemical Vapor Deposition. The PL from the ZTO nanowires does not exhibit any band gap or near gap emission, and the diffuse reflectance measurement confirms that these ZTO nanowires havemore » a direct forbidden transition. The broad PL spectrum reveals two Gaussian peaks centered at 1.86 eV (red) and 2.81 eV (blue), representing two distinct defect states or complexes. The PL spectra were further studied by the Time Resolved Emission Spectrum and intensity dependent PL and TRPL. The time resolved measurements show complex non-exponential decays at all wavelengths, indicative of defect to defect transitions, and the red emissive states decay much slower than the blue emissive states. The effects of annealing in air and vacuum are studied to investigate the origin of the defect states in the nanowires, showing that the blue states are related to oxygen vacancies. We propose an energy band model for the nanowires containing defect states within the band gap and the associated transitions between these states that are consistent with our measurements.« less

Effect of antimony on the deep-level traps in GaInNAsSb thin films

DOE Office of Scientific and Technical Information (OSTI.GOV)

Islam, Muhammad Monirul, E-mail: islam.monir.ke@u.tsukuba.ac.jp; Miyashita, Naoya; Ahsan, Nazmul

2014-09-15

Admittance spectroscopy has been performed to investigate the effect of antimony (Sb) on GaInNAs material in relation to the deep-level defects in this material. Two electron traps, E1 and E2 at an energy level 0.12 and 0.41 eV below the conduction band (E{sub C}), respectively, were found in undoped GaInNAs. Bias-voltage dependent admittance confirmed that E1 is an interface-type defect being spatially localized at the GaInNAs/GaAs interface, while E2 is a bulk-type defect located around mid-gap of GaInNAs layer. Introduction of Sb improved the material quality which was evident from the reduction of both the interface and bulk-type defects.

Defects in a New Class of Sulfate/Anion Transporter Link Sulfur Acclimation Responses to Intracellular Glutathione Levels and Cell Cycle Control1[W][OPEN

PubMed Central

Fang, Su-Chiung; Chung, Chin-Lin; Chen, Chun-Han; Lopez-Paz, Cristina; Umen, James G.

2014-01-01

We previously identified a mutation, suppressor of mating type locus3 15-1 (smt15-1), that partially suppresses the cell cycle defects caused by loss of the retinoblastoma tumor suppressor-related protein encoded by the MAT3 gene in Chlamydomonas reinhardtii. smt15-1 single mutants were also found to have a cell cycle defect leading to a small-cell phenotype. SMT15 belongs to a previously uncharacterized subfamily of putative membrane-localized sulfate/anion transporters that contain a sulfate transporter domain and are found in a widely distributed subset of eukaryotes and bacteria. Although we observed that smt15-1 has a defect in acclimation to sulfur-limited growth conditions, sulfur acclimation (sac) mutants, which are more severely defective for acclimation to sulfur limitation, do not have cell cycle defects and cannot suppress mat3. Moreover, we found that smt15-1, but not sac mutants, overaccumulates glutathione. In wild-type cells, glutathione fluctuated during the cell cycle, with highest levels in mid G1 phase and lower levels during S and M phases, while in smt15-1, glutathione levels remained elevated during S and M. In addition to increased total glutathione levels, smt15-1 cells had an increased reduced-to-oxidized glutathione redox ratio throughout the cell cycle. These data suggest a role for SMT15 in maintaining glutathione homeostasis that impacts the cell cycle and sulfur acclimation responses. PMID:25361960

Origin of subgap states in amorphous In-Ga-Zn-O

NASA Astrophysics Data System (ADS)

Körner, Wolfgang; Urban, Daniel F.; Elsässer, Christian

2013-10-01

We present a density functional theory analysis of stoichiometric and nonstoichiometric, crystalline and amorphous In-Ga-Zn-O (c-IGZO, a-IGZO), which connects the recently experimentally discovered electronic subgap states to structural features of a-IGZO. In particular, we show that undercoordinated oxygen atoms create electronic defect levels in the lower half of the band gap up to about 1.5 eV above the valence band edge. As a second class of fundamental defects that appear in a-IGZO, we identify mainly pairs of metal atoms which are not separated by oxygen atoms in between. These defects cause electronic defect levels in the upper part of the band gap. Furthermore, we show that hydrogen doping can suppress the deep levels due to undercoordinated oxygen atoms while those of metal defects just undergo a shift within the band gap. Altogether our results provide an explanation for the experimentally observed effect that hydrogen doping increases the transparency and improves the conductivity of a-IGZO.

Promising features of low-temperature grown Ge nanostructures on Si(001) substrates

NASA Astrophysics Data System (ADS)

Wang, Ze; Wang, Shuguang; Yin, Yefei; Liu, Tao; Lin, Dongdong; Li, De-hui; Yang, Xinju; Jiang, Zuimin; Zhong, Zhenyang

2017-03-01

High-quality Ge nanostructures are obtained by molecular beam epitaxy of Ge on Si(001) substrates at 200 °C and ex situ annealing at 400 °C. Their structural properties are comprehensively characterized by atomic force microscopy, transmission electron microscopy and Raman spectroscopy. It is disclosed that they are almost defect free except for some defects at the Ge/Si interface and in the subsequent Si capping layer. The misfit strain in the nanostructure is substantially relaxed. Dramatically strong photoluminescence (PL) from the Ge nanostructures is observed. Detailed analyses on the power- and temperature-dependent PL spectra, together with a self-consistent calculation, indicate the confinement and the high quantum efficiency of excitons within the Ge nanostructures. Our results demonstrate that the Ge nanostructures obtained via the present feasible route may have great potential in optoelectronic devices for monolithic optical-electronic integration circuits.

Spatially resolved positron annihilation spectroscopy on friction stir weld induced defects.

PubMed

Hain, Karin; Hugenschmidt, Christoph; Pikart, Philip; Böni, Peter

2010-04-01

A friction stir welded (FSW) Al alloy sample was investigated by Doppler broadening spectroscopy (DBS) of the positron annihilation line. The spatially resolved defect distribution showed that the material in the joint zone becomes completely annealed during the welding process at the shoulder of the FSW tool, whereas at the tip, annealing is prevailed by the deterioration of the material due to the tool movement. This might be responsible for the increased probability of cracking in the heat affected zone of friction stir welds. Examination of a material pairing of steel S235 and the Al alloy Silafont36 by coincident Doppler broadening spectroscopy (CDBS) indicates the formation of annealed steel clusters in the Al alloy component of the sample. The clear visibility of Fe in the CDB spectra is explained by the very efficient trapping at the interface between steel cluster and bulk.

Modification of the G-phonon mode of graphene by nitrogen doping

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lukashev, Pavel V., E-mail: pavel.lukashev@uni.edu; Hurley, Noah; Zhao, Liuyan

2016-01-25

The effect of nitrogen doping on the phonon spectra of graphene is analyzed. In particular, we employ first-principles calculations and scanning Raman analysis to investigate the dependence of phonon frequencies in graphene on the concentration of nitrogen dopants. We demonstrate that the G phonon frequency shows oscillatory behavior as a function of nitrogen concentration. We analyze different mechanisms which could potentially be responsible for this behavior, such as Friedel charge oscillations around the localized nitrogen impurity atom, the bond length change between nitrogen impurity and its nearest neighbor carbon atoms, and the long-range interactions of the nitrogen point defects. Wemore » show that the bond length change and the long range interaction of point defects are possible mechanisms responsible for the oscillatory behavior of the G frequency as a function of nitrogen concentration. At the same time, Friedel charge oscillations are unlikely to contribute to this behavior.« less

Evolution of native point defects in ZnO bulk probed by positron annihilation spectroscopy

NASA Astrophysics Data System (ADS)

Peng, Cheng-Xiao; Wang, Ke-Fan; Zhang, Yang; Guo, Feng-Li; Weng, Hui-Min; Ye, Bang-Jiao

2009-05-01

This paper studies the evolution of native point defects with temperature in ZnO single crystals by positron lifetime and coincidence Doppler broadening (CDB) spectroscopy, combined with the calculated results of positron lifetime and electron momentum distribution. The calculated and experimental results of the positron lifetime in ZnO bulk ensure the presence of zinc monovacancy, and zinc monovacancy concentration begins to decrease above 600 °C annealing treatment. CDB is an effective method to distinguish the elemental species, here we combine this technique with calculated electron momentum distribution to determine the oxygen vacancies, which do not trap positrons due to their positive charge. The CDB spectra show that oxygen vacancies do not appear until 600 °C annealing treatment, and increase with the increase of annealing temperature. This study supports the idea that green luminescence has a close relation with oxygen vacancies.

Modeling of the gate-controlled Kondo effect at carbon point defects in graphene

NASA Astrophysics Data System (ADS)

May, Daniel; Lo, Po-Wei; Deltenre, Kira; Henke, Anika; Mao, Jinhai; Jiang, Yuhang; Li, Guohong; Andrei, Eva Y.; Guo, Guang-Yu; Anders, Frithjof B.

2018-04-01

We study the magnetic properties in the vicinity of a single carbon defect in a monolayer of graphene. We include the unbound σ orbital and the vacancy-induced bound π state in an effective two-orbital single-impurity model. The local magnetic moments are stabilized by the Coulomb interaction as well as a significant ferromagnetic Hund's rule coupling between the orbitals predicted by a density functional theory calculation. A hybridization between the orbitals and the Dirac fermions is generated by the curvature of the graphene sheet in the vicinity of the vacancy. We present results for the local spectral function calculated using Wilson's numerical renormalization group approach for a realistic graphene band structure and find three different regimes depending on the filling, the controlling chemical potential, and the hybridization strength. These different regions are characterized by different magnetic properties. The calculated spectral functions qualitatively agree with recent scanning tunneling spectra on graphene vacancies.

Enhancing the luminescence efficiency of silicon-nanocrystals by interaction with H+ ions.

PubMed

Cannas, Marco; Camarda, Pietro; Vaccaro, Lavinia; Amato, Francesco; Messina, Fabrizio; Fiore, Tiziana; Li Vigni, Maria

2018-04-18

The emission of silicon nanocrystals (Si-NCs), synthesized by pulsed laser ablation in water, was investigated on varying the pH of the solution. These samples emit μs decaying orange photoluminescence (PL) associated with radiative recombination of quantum-confined excitons. Time-resolved spectra reveal that both the PL intensity and the lifetime increase by a factor of ∼20 when the pH decreases from 10 to 1 thus indicating that the emission quantum efficiency increases by inhibiting nonradiative decay rates. Infrared (IR) absorption and electron paramagnetic resonance (EPR) experiments allow addressing the origin of defects on which the excitons nonradiatively recombine. The linear correlation between the PL and the growth of SiH groups demonstrates that H+ ions passivate the nonradiative defects that are located in the interlayer between the Si-NC core and the amorphous SiO2 shell.

Stability of boron-doped graphene/copper interface: DFT, XPS and OSEE studies

NASA Astrophysics Data System (ADS)

Boukhvalov, D. W.; Zhidkov, I. S.; Kukharenko, A. I.; Slesarev, A. I.; Zatsepin, A. F.; Cholakh, S. O.; Kurmaev, E. Z.

2018-05-01

Two different types of boron-doped graphene/copper interfaces synthesized using two different flow rates of Ar through the bubbler containing the boron source were studied. X-ray photoelectron spectra (XPS) and optically stimulated electron emission (OSEE) measurements have demonstrated that boron-doped graphene coating provides a high corrosion resistivity of Cu-substrate with the light traces of the oxidation of carbon cover. The density functional theory calculations suggest that for the case of substitutional (graphitic) boron-defect only the oxidation near boron impurity is energetically favorable and creation of the vacancies that can induce the oxidation of copper substrate is energetically unfavorable. In the case of non-graphitic boron defects oxidation of the area, a nearby impurity is metastable that not only prevent oxidation but makes boron-doped graphene. Modeling of oxygen reduction reaction demonstrates high catalytic performance of these materials.

Simultaneous sensing of light and sound velocities of fluids in a two-dimensional phoXonic crystal with defects

DOE Office of Scientific and Technical Information (OSTI.GOV)

Amoudache, Samira; Laboratoire de Physique et Chimie Quantique, Université Mouloud Mammeri, B.P. 17 RP, 15000 Tizi-Ouzou; Pennec, Yan, E-mail: yan.pennec@univ-lille1.fr

2014-04-07

We theoretically investigate the potentiality of dual phononic-photonic (the so-called phoxonic) crystals for liquid sensing applications. We study the transmission through a two-dimensional (2D) crystal made of infinite cylindrical holes in a silicon substrate, where one row of holes oriented perpendicular to the propagation direction is filled with a liquid. The infiltrated holes may have a different radius than the regular holes. We show, in the defect structure, the existence of well-defined features (peaks or dips) in the transmission spectra of acoustic and optical waves and estimate their sensitivity to the sound and light velocity of the analyte. Some ofmore » the geometrical requirements behave in opposite directions when searching for an efficient sensing of either sound or light velocities. Hence, a compromise in the choice of the parameters may become necessary in making the phoxonic sensor.« less

Photoluminescence of amorphous and crystalline silicon nanoclusters in silicon nitride and oxide superlattices

DOE Office of Scientific and Technical Information (OSTI.GOV)

Shuleiko, D. V., E-mail: shuleyko.dmitriy@physics.msu.ru; Zabotnov, S. V.; Zhigunov, D. M.

2017-02-15

The photoluminescence properties of silicon nitride and oxide superlattices fabricated by plasmaenhanced chemical vapor deposition are studied. In the structures annealed at a temperature of 1150°C, photoluminescence peaks at about 1.45 eV are recorded. The peaks are defined by exciton recombination in silicon nanocrystals formed upon annealing. Along with the 1.45-eV peaks, a number of peaks defined by recombination at defects at the interface between the nanocrystals and silicon-nitride matrix are detected. The structures annealed at 900°C exhibit a number of photoluminescence peaks in the range 1.3–2.0 eV. These peaks are defined by both the recombination at defects and excitonmore » recombination in amorphous silicon nanoclusters formed at an annealing temperature of 900°C. The observed features of all of the photoluminescence spectra are confirmed by the nature of the photoluminescence kinetics.« less

PAL spectroscopy of rare-earth doped Ga-Ge-Te/Se glasses

NASA Astrophysics Data System (ADS)

Shpotyuk, Ya.; Ingram, A.; Shpotyuk, O.

2016-04-01

Positron annihilation lifetime (PAL) spectroscopy was applied for the first time to study free-volume void evolution in chalcogenide glasses of Ga-Ge-Te/Se cut-section exemplified by glassy Ga10Ge15Te75 and Ga10Ge15Te72Se3 doped with 500 ppm of Tb3+ or Pr3+. The collected PAL spectra reconstructed within two-state trapping model reveal decaying tendency in positron trapping efficiency in these glasses under rare-earth doping. This effect results in unchanged or slightly increased defect-related lifetimes τ2 at the cost of more strong decrease in I2 intensities, as well as reduced positron trapping rate in defects and fraction of trapped positrons. Observed changes are ascribed to rare-earth activated elimination of intrinsic free volumes associated mainly with negatively-charged states of chalcogen atoms especially those neighboring with Ga-based polyhedrons.

Optical spectroscopy and photo modification of individual single-photon emitters in hexagonal boron nitride

NASA Astrophysics Data System (ADS)

Jayakumar, Harishankar; Shotan, Zav; Considine, Christopher; Mazkoit, Mažena; Fedder, Helmut; Wrachtrup, Joerg; Alkauskas, Audrius; Doherty, Marcus; Menon, Vinod; Meriles, Carlos

Fluorescent defects recently observed under ambient conditions in hexagonal boron nitride (h-BN) promise to open novel opportunities for the implementation of on-chip photonic devices that rely on identical photons from single emitters. Here we report on the room temperature photo-luminescence dynamics of individual emitters in multilayer h-BN flakes exposed to blue laser light. Comparison of optical spectra recorded at successive times reveals considerable spectral diffusion, possibly the result of slowly fluctuating, trapped-carrier-induced stark shifts. Large spectral jumps - reaching up to 100 nm - followed by bleaching are observed in most cases upon prolonged exposure to blue light, an indication of one-directional, photo-chemical changes likely taking place on the flake surface. Remarkably, only a fraction of the observed emitters also fluoresces on green illumination suggesting a more complex optical excitation dynamics than previously anticipated and raising questions on the physical nature of the atomic defect at play.

Surface to bulk Fermi arcs via Weyl nodes as topological defects

PubMed Central

Kim, Kun Woo; Lee, Woo-Ram; Kim, Yong Baek; Park, Kwon

2016-01-01

A hallmark of Weyl semimetal is the existence of surface Fermi arcs. An intriguing question is what determines the connectivity of surface Fermi arcs, when multiple pairs of Weyl nodes are present. To answer this question, we show that the locations of surface Fermi arcs are predominantly determined by the condition that the Zak phase integrated along the normal-to-surface direction is . The Zak phase can reveal the peculiar topological structure of Weyl semimetal directly in the bulk. Here, we show that the winding of the Zak phase around each projected Weyl node manifests itself as a topological defect of the Wannier–Stark ladder, energy eigenstates under an electric field. Remarkably, this leads to bulk Fermi arcs, open-line segments in the bulk spectra. Bulk Fermi arcs should exist in conjunction with surface counterparts to conserve the Weyl fermion number under an electric field, which is supported by explicit numerical evidence. PMID:27845342

Optical spectra and band structure of Ag(x)Ga(x)Ge(1-x)Se2 (x = 0.333, 0.250, 0.200, 0.167) single crystals: experiment and theory.

PubMed

Reshak, A H; Parasyuk, O V; Fedorchuk, A O; Kamarudin, H; Auluck, S; Chyský, J

2013-12-05

Theoretical and experimental studies of the Ag(x)Ga(x)Ge(1-x)Se2 (x = 0.333, 0.250, 0.200, 0.167) single crystals are performed. These crystals possess a lot of intrinsic defects which are responsible for their optoelectronic features. The theoretical investigations were performed by means of DFT calculations using different exchange-correlation potentials. The experimental studies were carried out using the modulated VUV ellipsometry for dielectric constants and birefringence studies. The comparison of the structure obtained from X-ray with the theoretically optimized structure is presented. The crucial role of the intrinsic defect states is manifested in the choice of the exchange correlation potential used. The data may be applicable for a large number of the ternary chalcogenides which are sensitive to the presence of the local disordered states near the band edges.

Diagnosing and controlling mix in National Ignition Facility implosion experiments a)

NASA Astrophysics Data System (ADS)

Hammel, B. A.; Scott, H. A.; Regan, S. P.; Cerjan, C.; Clark, D. S.; Edwards, M. J.; Epstein, R.; Glenzer, S. H.; Haan, S. W.; Izumi, N.; Koch, J. A.; Kyrala, G. A.; Landen, O. L.; Langer, S. H.; Peterson, K.; Smalyuk, V. A.; Suter, L. J.; Wilson, D. C.

2011-05-01

High mode number instability growth of "isolated defects" on the surfaces of National Ignition Facility [Moses et al., Phys. Plasmas 16, 041006 (2009)] capsules can be large enough for the perturbation to penetrate the imploding shell, and produce a jet of ablator material that enters the hot-spot. Since internal regions of the CH ablator are doped with Ge, mixing of this material into the hot-spot results in a clear signature of Ge K-shell emission. Evidence of jets entering the hot-spot has been recorded in x-ray images and spectra, consistent with simulation predictions [Hammel et al., High Energy Density Phys. 6, 171 (2010)]. Ignition targets have been designed to minimize instability growth, and capsule fabrication improvements are underway to reduce "isolated defects." An experimental strategy has been developed where the final requirements for ignition targets can be adjusted through direct measurements of mix and experimental tuning.

Experimental chlorine partitioning between forsterite, enstatite and aqueous fluid at upper mantle conditions☆

PubMed Central

Fabbrizio, Alessandro; Stalder, Roland; Hametner, Kathrin; Günther, Detlef

2013-01-01

Cl partition coefficients between forsterite, enstatite and coexisting Cl-bearing aqueous fluids were determined in a series of high pressure and temperature piston cylinder experiments at 2 GPa between 900 and 1300 °C in the system MgO–SiO2–H2O–NaCl–BaO–C±CaCl2±TiO2±Al2O3±F. Diamond aggregates were added to the experimental capsule set-up in order to separate the fluid from the solid residue and enable in situ analysis of the quenched solute by LA-ICP-MS. The chlorine content of forsterite and enstatite was measured by electron microprobe, and the nature of hydrous defects was investigated by infrared spectroscopy. Partition coefficients show similar incompatibility for Cl in forsterite and enstatite, with DClfo/fl = 0.0012 ± 0.0006, DClen/fl = 0.0018 ± 0.0008 and DClfo/en = 1.43 ± 0.71. The values determined for mineral/fluid partitioning are very similar to previously determined values for mineral/melt. Applying the new mineral/fluid partition coefficients to fluids in subduction zones, a contribution between 0.15% and 20% of the total chlorine from the nominally anhydrous minerals is estimated. Infrared spectra of experimental forsterite show absorption bands at 3525 and 3572 cm−1 that are characteristic for hydroxyl point defects associated with trace Ti substitutions, and strongly suggest that the TiO2 content of the system can influence the chlorine and OH incorporation via the stabilization of Ti-clinohumite-like point defects. The water contents for coexisting forsterite and enstatite in some runs were determined using unpolarized IR spectra and calculated water partition coefficients DH2Ofo/en are between 0.01 and 0.5. PMID:25843971

Physical-level synthesis for digital lab-on-a-chip considering variation, contamination, and defect.

PubMed

Liao, Chen; Hu, Shiyan

2014-03-01

Microfluidic lab-on-a-chips have been widely utilized in biochemical analysis and human health studies due to high detection accuracy, high timing efficiency, and low cost. The increasing design complexity of lab-on-a-chips necessitates the computer-aided design (CAD) methodology in contrast to the classical manual design methodology. A key part in lab-on-a-chip CAD is physical-level synthesis. It includes the lab-on-a-chip placement and routing, where placement is to determine the physical location and the starting time of each operation and routing is to transport each droplet from the source to the destination. In the lab-on-a-chip design, variation, contamination, and defect need to be considered. This work designs a physical-level synthesis flow which simultaneously considers variation, contamination, and defect of the lab-on-a-chip design. It proposes a maze routing based, variation, contamination, and defect aware droplet routing technique, which is seamlessly integrated into an existing placement technique. The proposed technique improves the placement solution for routing and achieves the placement and routing co-optimization to handle variation, contamination, and defect. The simulation results demonstrate that our technique does not use any defective/contaminated grids, while the technique without considering contamination and defect uses 17.0% of the defective/contaminated grids on average. In addition, our routing variation aware technique significantly improves the average routing yield by 51.2% with only 3.5% increase in completion time compared to a routing variation unaware technique.

Electronic characterization of defects in narrow gap semiconductors

NASA Technical Reports Server (NTRS)

Patterson, James D.

1994-01-01

We use a Green's function technique to calculate the position of deep defects in narrow gap semiconductors. We consider substitutional (including antisite), vacancy, and interstitial (self and foreign) deep defects. We also use perturbation theory to look at the effect of nonparabolic bands on shallow defect energies and find nonparabolicity can increase the binding by 10 percent or so. We consider mercury cadmium telluride (MCT), mercury zinc telluride (MZT), and mercury zinc selenide (MZS). For substitutional and interstitial defects we look at the situation with and without relaxation. For substitutional impurities in MCT, MZT, and MZS, we consider x (the concentration of Cd or Zn) in the range 0.1 less than x less than 0.3 and also consider appropriate x so E(sub g) = 0.1 eV for each of the three compounds. We consider several cation site s-like deep levels and anion site p-like levels. For E(sub g) = 0.1 eV, we also consider the effects of relaxation. Similar comments apply to the interstitial deep levels whereas no relaxation is considered for the ideal vacancy model. Relaxation effects can be greater for the interstitial than the substitutional cases. Specific results are given in figures and tables and comparison to experiment is made in a limited number of cases. We find, for example, that I, Se, S, Rn, and N are possible cation site, s-like deep levels in MCT and Zn and Mg are for anion site, p-like levels (both levels for substitutional cases). The corresponding cation and anion site levels for interstitial deep defects are (Au, Ag, Hg, Cd, Cu, Zn) and (N, Ar, O, F). For the substitutional cases we have some examples of relaxation moving the levels into the band gap, whereas for the interstitial case we have examples where relaxation moves it out of the band gap. Future work involves calculating the effects of charge state interaction and seeing the effect of relaxation on vacancy levels.

Density Functional Theory Calculations of Activation Energies for Carrier Capture by Defects in Semiconductors

NASA Astrophysics Data System (ADS)

Modine, Normand; Wright, Alan; Lee, Stephen

2015-03-01

Carrier recombination due to defects can have a major impact on device performance. The rate of defect-induced recombination is determined by both defect levels and carrier capture cross-sections. Density functional theory (DFT) has been widely and successfully used to predict defect levels, but only recently has work begun to focus on using DFT to determine carrier capture cross-sections. Lang and Henry worked out the fundamental theory of carrier-capture by multiphonon emission in the 1970s and showed that, above the Debye temperature, carrier-capture cross-sections differ between defects primarily due to differences in their carrier capture activation energies. We present an approach to using DFT to calculate carrier capture activation energies that does not depend on an assumed configuration coordinate and that fully accounts for anharmonic effects, which can substantially modify carrier activation energies. We demonstrate our approach for the -3/-2 level of the Ga vacancy in wurtzite GaN. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Signatures of Phonon and Defect-Assisted Tunneling in Planar Metal-Hexagonal Boron Nitride-Graphene Junctions.

PubMed

Chandni, U; Watanabe, K; Taniguchi, T; Eisenstein, J P

2016-12-14

Electron tunneling spectroscopy measurements on van der Waals heterostructures consisting of metal and graphene (or graphite) electrodes separated by atomically thin hexagonal boron nitride tunnel barriers are reported. The tunneling conductance, dI/dV, at low voltages is relatively weak, with a strong enhancement reproducibly observed to occur at around |V| ≈ 50 mV. While the weak tunneling at low energies is attributed to the absence of substantial overlap, in momentum space, of the metal and graphene Fermi surfaces, the enhancement at higher energies signals the onset of inelastic processes in which phonons in the heterostructure provide the momentum necessary to link the Fermi surfaces. Pronounced peaks in the second derivative of the tunnel current, d 2 I/dV 2 , are observed at voltages where known phonon modes in the tunnel junction have a high density of states. In addition, features in the tunneling conductance attributed to single electron charging of nanometer-scale defects in the boron nitride are also observed in these devices. The small electronic density of states of graphene allows the charging spectra of these defect states to be electrostatically tuned, leading to "Coulomb diamonds" in the tunneling conductance.

Using the methods of radiospectroscopy (EPR, NMR) to study the nature of the defect structure of solid solutions based on lead zirconate titanate (PZT).

PubMed

Bykov, Igor; Zagorodniy, Yuriy; Yurchenko, Lesya; Korduban, Alexander; Nejezchleb, Karel; Trachevsky, Vladimir; Dimza, Vilnis; Jastrabik, Lubomir; Dejneka, Alexander

2014-08-01

The nature of intrinsic and impurity point defects in lead zirconate titanate (PZT) ceramics has been explored. Using electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR), and X-ray photoelectron spectroscopy (XPS) methods, several impurity sites have been identified in the materials, including the Fe(3+)-oxygen vacancy (VO) complex and Pb ions. Both of these centers are incorporated into the PZT lattice. The Fe(3+) –VО paramagnetic complex serves as a sensitive probe of the local crystal field in the ceramic; the symmetry of this defect roughly correlates with PZT phase diagram as the composition is varied from PbTiO3 to PbZrO3. NMR spectra (207)Pb in PbTiO3, PbZrO3, and PZT with iron content from 0 to 0.4 wt% showed that increasing the iron concentration leads to a distortion of the crystal structure and to improvement of the electrophysical parameters of the piezoceramics. This is due to the formation of a phase which has a higher symmetry, but at high concentrations of iron (>0.4 wt%), it leads to sharp degradation of electrophysical parameters.

Effect of a Nitrogen Impurity on the Fundamental Raman Band of Diamond Single Crystals

NASA Astrophysics Data System (ADS)

Gusakov, G. A.; Samtsov, M. P.; Voropay, E. S.

2018-05-01

The effect of nitrogen defects in natural and synthetic diamond single crystals on the position and half-width of the fundamental Raman band was investigated. Samples containing the main types of nitrogen lattice defects at impurity contents of 1-1500 ppm were studied. The parameters of the Stokes and anti-Stokes components in Raman spectra of crystals situated in a cell with distilled water to minimize the influence of heating by the exciting laser radiation were analyzed to determine the effect of a nitrogen impurity in the diamond crystal lattice. It was shown that an increase of impurity atoms in the crystals in the studied concentration range resulted in broadening of the Raman band from 1.61 to 2.85 cm-1 and shifting of the maximum to lower frequency from 1332.65 to 1332.3 cm-1. The observed effect was directly proportional to the impurity concentration and depended on the form of the impurity incorporated into the diamond lattice. It was found that the changes in the position and half-width of the fundamental Raman band for diamond were consistent with the magnitude of crystal lattice distortions due to the presence of impurity defects and obeyed the Gruneisen law.

A correlation between the defect states and yellow luminescence in AlGaN/GaN heterostructures

NASA Astrophysics Data System (ADS)

Jana, Dipankar; Sharma, T. K.

2017-07-01

AlGaN/GaN heterostructures are investigated by performing complementary spectroscopic measurements under novel experimental configurations. Distinct features related to the band edge of AlGaN and GaN layers are clearly observed in surface photovoltage spectroscopy (SPS) spectra. A few more SPS features, which are associated with defects in GaN, are also identified by performing the pump-probe SPS measurements. SPS results are strongly corroborated by the complementary photoluminescence and photoluminescence excitation (PLE) measurements. A correlation between the defect assisted SPS features and yellow luminescence (YL) peak is established by performing pump-probe SPS and PLE measurements. It is found that CN-ON donor complex is responsible for the generation of YL peak in our sample. Further, the deep trap states are found to be present throughout the entire GaN epilayer. It is also noticed that the deep trap states lying at the GaN/Fe-GaN interface make a strong contribution to the YL feature. A phenomenological model is proposed to explain the intensity dependence of the YL feature and the corresponding SPS features in a pump-probe configuration, where a reasonable agreement between the numerical simulations and experimental results is achieved.

Using thermally stimulated current (TSC) to investigate disorder in micronized drug substance produced at different milling energies.

PubMed

Forcino, Rachel; Brum, Jeffrey; Galop, Marc; Sun, Yan

2010-10-01

To investigate the use of thermally stimulated current (TSC) to characterize disorder resulting from micronization of a crystalline drug substance. Samples processed at different milling energies are characterized, and annealing studied. Molecular mobility in micronized drug substance was studied using TSC and compared to results from differential scanning calorimetry (DSC). The micronized drug substance TSC spectra are compared to crystalline and amorphous references. TSC shows distinct relaxation modes for micronized material in comparison to a single weak exotherm observed with DSC. Molecular mobility modes are unique for micronized material compared to the amorphous reference indicating physically distinct disorder compared to phase-separated amorphous material. Signals are ascribed as arising from crystal defects. TSC differentiates material processed at different milling energies showing reasonable correlation between the AUC of the α-relaxation and micronization energy. The annealing process of crystal defects in micronized drug appears to proceed differently for α and β relaxations. TSC proves sensitive to the crystal defects in the micronized drug substance studied here. The technique is able to differentiate distinct types of disorder and can be used to characterize noncrystalline regions arising from milling processes which are physically distinct from amorphous material.

Synthesis and electrochemical sodium-storage of few-layered MoS2/nitrogen, phosphorus-codoped graphene

NASA Astrophysics Data System (ADS)

Xu, Limei; Ma, Lin; Li, Wenyan; Yang, Xinxin; Ling, Yan

2018-07-01

Few-layered molybdenum disulfide/nitrogen, phosphorus co-doped graphene composites are synthesized by a quaternary phosphonium salt-assisted hydrothermal and annealing procedure. The prepared composites are analyzed by x-ray powder diffraction, x-ray photoelectron spectra, scanning electronic microscopy, transmission electronic microscopy, Raman spectra and nitrogen adsorption and desorption. Experimental results indicate that the MoS2 nanosheets are of few-layered and defective structures and are well anchored on flexible conductive nitrogen, phosphorus co-doped graphene to constitute mesoporous composites with increased surface areas. Benefiting from the structural merits as well as surface-dominated pseudocapacitive contribution, the composite electrode presents a high electrochemical sodium storage capacity that arrives at 542 mAh g‑1 at a current density of 100 mA g‑1 with an excellent cyclability. Moreover, a superior high-rate capability can also be achieved.

He ion irradiation effects on multiwalled carbon nanotubes structure

NASA Astrophysics Data System (ADS)

Elsehly, Emad M.; Chechenin, Nikolay G.; Makunin, Alexey V.; Shemukhin, Andrey A.; Motaweh, Hussien A.

2017-03-01

Samples of multi-walled carbon nanotubes (MWNTs) were irradiated with 80 keV He ions. Scanning electron microscopy (SEM) inspection showed that the average outer diameters of the tube decreased as a result of ion irradiation. The samples were also characterized using Raman spectrometry by analysis of the intensity of main bands in the spectra of virgin and irradiated MWNT samples. Modifications of the disorder mode (D-band) and the tangential mode (G-band) were studied as a function of irradiation fluences. Raman spectra showed that as the fluence increases, the MWNTs first show disorder due to the produced defects, and then amorphization under still higher fluence of ion irradiation. Thermal and athermal mechanisms of the radiation induced MWNTs modifications are discussed. Contribution to the Topical Issue "Many Particle Spectroscopy of Atoms, Molecules, Clusters and Surfaces", edited by A.N. Grum-Grzhimailo, E.V. Gryzlova, Yu V. Popov, and A.V. Solov'yov.

Photoluminescence properties of thallium-containing GeSe{sub 2} and GeSe{sub 3} vitreous semiconductors

DOE Office of Scientific and Technical Information (OSTI.GOV)

Babaev, A. A., E-mail: babaev-arif@mail.ru

2016-02-15

The photoluminescence properties of thallium-containing vitreous semiconductor systems with stoichiometric and nonstoichiometric compositions (GeSe{sub 2}){sub 1–x}Tl{sub x} and (GeSe{sub 3}){sub 1–x}Tl{sub x} (0 ⩽ x ⩽ 0.1) are studied at a temperature of T = 77 K. Intrinsic defects with negative correlation energy are responsible for the Gaussian shape of the photoluminescence spectra. It is established that an increase in x in the systems does not affect the shape of the spectrum, does not generate new emission bands, shifts the photoluminescence spectra to the region of low energies, reduces the intensity of radiation, and increases its half-width. Kinetics of themore » fatigue of photoluminescence is different for both systems and is characterized by one curve irrespective of Tl content in the systems.« less

Synthesis and electrochemical sodium-storage of few-layered MoS2/nitrogen, phosphorus-codoped graphene.

PubMed

Xu, Limei; Ma, Lin; Li, Wenyan; Yang, Xinxin; Ling, Yan

2018-07-27

Few-layered molybdenum disulfide/nitrogen, phosphorus co-doped graphene composites are synthesized by a quaternary phosphonium salt-assisted hydrothermal and annealing procedure. The prepared composites are analyzed by x-ray powder diffraction, x-ray photoelectron spectra, scanning electronic microscopy, transmission electronic microscopy, Raman spectra and nitrogen adsorption and desorption. Experimental results indicate that the MoS 2 nanosheets are of few-layered and defective structures and are well anchored on flexible conductive nitrogen, phosphorus co-doped graphene to constitute mesoporous composites with increased surface areas. Benefiting from the structural merits as well as surface-dominated pseudocapacitive contribution, the composite electrode presents a high electrochemical sodium storage capacity that arrives at 542 mAh g -1 at a current density of 100 mA g -1 with an excellent cyclability. Moreover, a superior high-rate capability can also be achieved.

Influence of twin boundaries on superconducting gap nodes in FeSe single crystal studied by STM/STS

NASA Astrophysics Data System (ADS)

Watashige, T.; Hanaguri, T.; Kohsaka, Y.; Iwaya, K.; Fu, Y.; Kasahara, S.; Watanabe, D.; Mizukami, Y.; Mikami, T.; Kawamoto, Y.; Kurata, S.; Shibauchi, T.; Matsuda, Y.; Böhmer, A. E.; Wolf, T.; Meingast, C.; Löhneysen, H. V.

2014-03-01

We performed scanning tunneling microscopy (STM) and spectroscopy (STS) measurements on high-quality FeSe single crystals grown by vapor transport technique to examine the superconducting-gap structure. In MBE-grown FeSe thin films, based on the V-shaped tunneling spectra, nodal superconductivity is suggested. It is interesting to investigate how the nodes are affected by various kinds of defects. We found that twin boundaries bring about drastic effects on the gap nodes. With approaching to the twin boundary, V-shaped spectra gradually change to U-shaped ones. Interestingly, in the area between the twin boundaries separated by about 30 nm, the gap node is completely lifted and there appears a finite gap over +/-0.4 meV. This unusual twin-boundary effect will give us a hint to elucidate the superconducting-gap structure.

Maternal vitamin B12 status and risk of neural tube defects in a population with high neural tube defect prevalence and no folic Acid fortification.

PubMed

Molloy, Anne M; Kirke, Peadar N; Troendle, James F; Burke, Helen; Sutton, Marie; Brody, Lawrence C; Scott, John M; Mills, James L

2009-03-01

Folic acid fortification has reduced neural tube defect prevalence by 50% to 70%. It is unlikely that fortification levels will be increased to reduce neural tube defect prevalence further. Therefore, it is important to identify other modifiable risk factors. Vitamin B(12) is metabolically related to folate; moreover, previous studies have found low B(12) status in mothers of children affected by neural tube defect. Our objective was to quantify the effect of low B(12) status on neural tube defect risk in a high-prevalence, unfortified population. We assessed pregnancy vitamin B(12) status concentrations in blood samples taken at an average of 15 weeks' gestation from 3 independent nested case-control groups of Irish women within population-based cohorts, at a time when vitamin supplementation or food fortification was rare. Group 1 blood samples were from 95 women during a neural tube defect-affected pregnancy and 265 control subjects. Group 2 included blood samples from 107 women who had a previous neural tube defect birth but whose current pregnancy was not affected and 414 control subjects. Group 3 samples were from 76 women during an affected pregnancy and 222 control subjects. Mothers of children affected by neural tube defect had significantly lower B(12) status. In all 3 groups those in the lowest B(12) quartiles, compared with the highest, had between two and threefold higher adjusted odds ratios for being the mother of a child affected by neural tube defect. Pregnancy blood B(12) concentrations of <250 ng/L were associated with the highest risks. Deficient or inadequate maternal vitamin B(12) status is associated with a significantly increased risk for neural tube defects. We suggest that women have vitamin B(12) levels of >300 ng/L (221 pmol/L) before becoming pregnant. Improving B(12) status beyond this level may afford a further reduction in risk, but this is uncertain.

Interface effects on calculated defect levels for oxide defects

NASA Astrophysics Data System (ADS)

Edwards, Arthur; Barnaby, Hugh; Schultz, Peter; Pineda, Andrew

2014-03-01

Density functional theory (DFT) has had impressive recent success predicting defect levels in insulators and semiconductors [Schultz and von Lillienfeld, 2009]. Such success requires care in accounting for long-range electrostatic effects. Recently, Komsa and Pasquarello have started to address this problem in systems with interfaces. We report a multiscale technique for calculating electrostatic energies for charged defects in oxide of the metal-oxide-silicon (MOS) system, but where account is taken of substrate doping density, oxide thickness, and gate bias. We use device modeling to calculate electric fields for a point charge a fixed distance from the interface, and used the field to numerically calculate the long-range electrostatic interactions. We find, for example, that defect levels in the oxide do depend on both the magnitude and the polarity the substrate doping density. Furthermore, below 20 Å, oxide thickness also has significant effects. So, transferring results directly from bulk calculations leads to inaccuracies up to 0.5 eV- half of the silicon band gap. We will present trends in defect levels as a function of device parameters. We show that these results explain previous experimental results, and we comment on their potential impact on models for NBTI. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the United States Department of Energy's National Nuclear Security Administration under co.