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At least 1,585 records · Page 88
All sourcesNASA NTRSDOE OSTI
DOE OSTI2010

Double K-shell photoionization and hypersatellite x-ray transitions of 12{<=}Z{<=}23 atoms

Single-photon double K-shell ionization of low-Z neutral atoms in the range 12{<=}Z{<=}23 is investigated. The experimental method was based on measurements of the high-resolution K{alpha}{sup h} hypersatellite x-ray spectra following the radiative decay of the K-shell double-vacancy states excited by monochromatic synchrotron radiation. The photon energy dependence of the double K-shell ionization was measured over a wide range of photon energies from threshold up to and beyond the maximum of the double-to-single photoionization cross section ratios. From the high-resolution x-ray emission spectra the energies and linewidths of the hypersatellite transitions, as well as the K{alpha}{sub 1}{sup h}:K{alpha}{sub 2}{sup h} intensity ratios, were determined. The relative importance of the initial-state and final-state electron-electron interactions to the K-shell double photoionization is addressed. Physical mechanisms and scaling laws of the K-shell double photoionization are examined. A semiempirical universal scaling of the double-photoionization cross sections with the effective nuclear charge for neutral atoms in the range 2{<=}Z{<=}47 is established.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Charge and magnetic states of Mn-, Fe-, and Co-doped monolayer MoS{sub 2}

First-principles calculations have been performed to investigate the electronic and magnetic properties of monolayer MoS{sub 2} substitutionally doped with Mn, Fe, and Co in possible charge states (q). We find that the Mn, Fe, and Co dopants substituting for a Mo atom in monolayer MoS{sub 2} (Mn@Mo, Fe@Mo, and Co@Mo) are all magnetic in their neutral and charge states except in the highest positive charge states. Mn@Mo, Fe@Mo, and Co@Mo have the same highest negative charge states of q=−2 for chemical potential of electron just below the conduction band minimum, which corresponds to the electron doping. In the q=−2 state, Mn@Mo has a much larger magnetic moment than its neutral state with the antiferromagnetic coupling between the Mn dopant and its neighboring S atoms maintained, while Fe@Mo and Co@Mo have equal or smaller magnetic moments than their neutral states. The possible charge states of Mn@Mo, Fe@Mo, and Co@Mo and the variation of the magnetic moments for different dopants and charge states are due to the change of the occupation and energy of the anti-bonding defect levels in the band gap. The rich magnetic properties of the neutral and charge states suggest possible realization of the substitutionally Mn-, Fe-, and Co-doped monolayer MoS{sub 2} as dilute magnetic semiconductors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2010

Investigation of the stability of Co-doped apatite ionic conductors in NH{sub 3}

Hydrogen powered solid oxide fuel cells (SOFCs) are of enormous interest as devices for the efficient and clean production of electrical energy. However, a number of problems linked to hydrogen production, storage and transportation are slowing down the larger scale use of SOFCs. Identifying alternative fuel sources to act as intermediate during the transition to the full use of hydrogen is, therefore, of importance. One excellent alternative is ammonia, which is produced on a large scale, is relatively cheap and has the infrastructure for storage and transportation already in place. However, considering that SOFCs operate at temperatures higher than 500 {sup o}C, a potential problem is the interaction of gaseous ammonia with the materials in the cathode, anode and solid electrolyte. In this paper, we extend earlier work on high temperature reactions of apatite electrolytes with NH{sub 3} to the transition metal (Co) doped systems, La{sub 9.67}Si{sub 5}CoO{sub 26} and La{sub 10}(Si/Ge){sub 5}CoO{sub 26.5}. A combination of PXRD, TGA and XAFS spectroscopy data showed a better structural stability for the silicate systems. Apatite silicates and germanates not containing transition metals tend to substitute nitride anions for their interstitial oxide anions, when reacted with NH{sub 3} at high temperature and, consequentially, lower the interstitial oxide content. In La{sub 9.67}Si{sub 5}CoO{sub 26} and La{sub 10}(Si/Ge){sub 5}CoO{sub 26.5} reduction of Co occurs as a competing process, favouring lower levels of nitride-oxide substitution. -- Graphical Abstract: In reactions between the apatites La{sub 9.67}Si{sub 5}CoO{sub 26} and La{sub 10}(Si/Ge){sub 5}CoO{sub 26.5} and NH{sub 3} (g) at temperatures T>500 {sup o}C, the partial substitution of the Si and Ge by Co seems to discourage O{sup 2-}/N{sup 3-} substitution in favour of the reduction of the metal. Display Omitted

36 MATERIALS SCIENCE↗
DOE OSTI2017

Effect of the energy of bombarding electrons on the conductivity of n-4H-SiC (CVD) epitaxial layers

The electrical characteristics of epitaxial layers of n-4H-SiC (CVD) irradiated with 0.9 and 3.5MeV electrons are studied. It is shown that the donor removal rate becomes nearly four times higher as the energy of impinging electrons increases by a factor of 4, although the formation cross section of primary radiation defects (Frenkel pairs in the carbon sublattice) responsible for conductivity compensation of the material is almost energy independent in this range. It is assumed that the reason for the observed differences is the influence exerted by primary knocked-out atoms. First, cascade processes start to manifest themselves with increasing energy of primary knocked-out atoms. Second, the average distance between genetically related Frenkel pairs grows, and, as a consequence, the fraction of defects that do not recombine under irradiation becomes larger. The recombination radius of Frenkel pairs in the carbon sublattice is estimated and the possible charge state of the recombining components is assessed.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Heavy doping of H{sup -} ion in 12CaO.7Al{sub 2}O{sub 3}

12CaO.7Al{sub 2}O{sub 3} (C12A7, mayenite), which has a nanoscale porous structure that can accommodate extraframework species such as hydride (H{sup -}), oxide (O{sup 2-}), hydroxide (OH{sup -}) ions, and electrons, has been doped with H{sup -} ions to investigate its effects as dominant extraframework species. Chemical doping with CaH{sub 2} enables the concentration of H{sup -} ions to reach almost the theoretical maximum. The concentration of H{sup -} ions is characterized by optical absorption intensity ascribed to photoionization of H{sup -} ions, and {sup 1}H magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy. Persistent electron generation, which is accompanied by the formation of an F{sup +} absorption band and electrical conductivity, by irradiation with ultraviolet light at room temperature increases as the H{sup -} ion doping increases until it reaches half the theoretical maximum and then decreases as the H{sup -} ion concentration increases further. This dependence indicates that both H{sup -} and O{sup 2-} ions are necessary for the generation of persistent electrons. - Graphical abstract: CaH{sub 2} reduction markedly enhances H{sup -} ion doping in 12CaO.7Al{sub 2}O{sub 3}. The H{sup -} ion incorporation is evaluated using optical absorption and {sup 1}H-MAS-NMR spectra. Both H{sup -} and O{sup 2-} ions are responsible for photogeneration of electrons. Highlights: > CaH{sub 2} reduction markedly enhances H{sup -} ion doping in 12CaO.7Al{sub 2}O{sub 3}. {yields}The H{sup -} ion incorporation is evaluated using optical absorption and {sup 1}H-MAS-NMR spectra. > Both H{sup -} and O{sup 2-} ions are responsible for photogeneration of electrons.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2011

Oxygen-Atom Defects In 6H Silicon Carbide Implanted Using 24- MeV O{sup 3+} Ions Measured Using Three-Dimensional Positron Annihilation Spectroscopy System (3DPASS)

Three dimensional electron-positron (e{sup -}-e{sup +}) momentum distributions were measured for single crystal 6H silicon carbide (SiC); both virgin and having implanted oxygen-atom defects. 6H SiC samples were irradiated by 24- MeV O{sup 3+} ions at 20 particle-nanoamps at the Sandia National Laboratory's Ion Beam Facility. O{sup 3+} ions were implanted 10.8 {mu}m deep normal to the (0001) face of one side of the SiC samples. During positron annihilation measurements, the opposite face of the 254.0-{mu}m thick SiC samples was exposed to positrons from a {sup 22}Na source. This technique reduced the influence on the momentum measurements of vacancy-type defects resulting from knock-on damage by the O{sup 3+} ions. A three-dimensional positron annihilation spectroscopy system (3DPASS) was used to measure e{sup -}-e{sup +} momentum distributions for virgin and irradiated 6H SiC crystal both before and following annealing. 3DPASS simultaneously measures coincident Doppler-broadening (DBAR) and angular correlation of annihilation radiation (ACAR) spectra. DBAR ratio plots and 2D ACAR spectra are presented. Changes in the momentum anisotropies relative to crystal orientation observed in 2D ACAR spectra for annealed O-implanted SiC agree with the local structure of defect distortion predicted using Surface Integrated Molecular Orbital/Molecular Mechanics (SIMOMM). Oxygen atoms insert between Si and C atoms increasing their separation by 0.9 A forming a Si-O-C bond angle of {approx}150 deg.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Geometrical defect detection for additive manufacturing with machine learning models

This study proposed a scheme based on Machine Learning (ML) models to detect geometric defects of additively manufactured objects. The ML models are trained with synthetic 3D point clouds with defects and then applied to detect defects in actual production. Using synthetic 3D point clouds rather than experimental data could save a huge amount of training time and costs associated with many prints for each design. Besides distance differences of individual points between source and target point clouds, this scheme uses a new concept called “patch” to capture macro-level information about nearby points for ML training and implementation. Numerical comparisons of prediction results on experimental data with different shapes showed that the proposed scheme outperformed the existing Z-difference method in the literature. Five ML methods (Bagging of Trees, Gradient Boosting, Random Forest, K-nearest Neighbors and Linear Supported Vector Machine) were compared under various conditions, such as different point cloud densities and defect sizes. Bagging and Random Forest were found the two best models regarding predictability; and the right patch size was found to be at 20. The proposed ML-based scheme is applicable to in-situ defect detection during additive manufacturing with the aid of a proper 3D data acquisition system.

Additive manufacturing↗
DOE OSTI2020

Enhancing Photocarrier Bulk Lifetime with Defect Engineering of Polycrystalline Passivated-Contact n-Cz Photovoltaic Devices

We study the photocarrier lifetime evolution of n-Cz Si material throughout the processing sequence for polycrystalline passivated contact devices. We show that a high temperature annealing pretreatment (known as Tabula Rasa) has a clear effect on enhancing bulk lifetimes of n-Cz Si. We further this development by integrating such defect engineering into the lower-temperature annealing of passivated contact. By applying oxidizing ambient gases during these anneals we report a photocarrier lifetime enhancement over an N 2 environment. This enhancement is exhibited in a 1-sun iV OC of 735 mV when annealed in O 2 rather than 708 mV in N 2.

14 SOLAR ENERGY↗
DOE OSTI2018

Effect of Tuning the Structure on the Optical and Magnetic Properties by Various Transition Metal Doping in ZnO/TM (TM = Fe, FeCo, Cr, and Mn) Thin Films

We have investigated in this work the physical, optical, electronic, and the magnetic behavior (Curie temperature, magnetic moment) of Zn.1-x.MxO (M = Fe 5%, Co 1%, Cr 5%, and Mn 5%), diluted magnetic semiconductors (DMSs). The samples were deposited on glass substrate by the spray pyrolysis technique, and the results of the x-ray diffraction (XRD) of the prepared substrates was used to prove the incorporation of the dopants into the ZnO lattice host; the ferromagnetic and the antiferromagnetic state competitions and their effects on the physical, magnetic, and optical properties, were investigated. The electronic structure and magnetic properties of transition metal (TM) defects, were investigated in detail, by using the Korringa–Kohn–Rostoker (KKR) method combined with the coherent potential approximation (CPA). As a result, doping by TM impurities induce the ferromagnetism with different competitions between the ferromagnetic and antiferromagnetic states, which affects the physical properties of the TM (Fe, Fe/Co, Cr, and Mn)-doped ZnO, with a Curie temperature closer to room-temperature ferromagnetic.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Radiative capture rates at deep defects from electronic structure calculations

We present a methodology to calculate radiative carrier capture coefficients at deep defects in semiconductors and insulators from first principles. Electronic structure and lattice relaxations are accurately described with hybrid density functional theory. Calculations of capture coefficients provide an additional validation of the accuracy of these functionals in dealing with localized defect states. We also discuss the validity of the Condon approximation, showing that even in the event of large lattice relaxations the approximation is accurate. We test the method on GaAs:VGa-TeAs and GaN:CN, for which reliable experiments are available, and demonstrate very good agreement with measured capture coefficients.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Intrinsic Defect Properties in Halide Double Perovskites for Optoelectronic Applications

Lead-free halide double perovskites with the formula of quaternary A 2 + B + B ' 3 + X 6 - have recently attracted intense interest as alternatives to lead-halide-perovskite-based optoelectronic materials for their nontoxicity and enhanced chemical and thermodynamic stability. However, the understanding of intrinsic defect properties and their effects on carrier transport and Fermi level tuning is still limited. In this paper, we show that, by exploring the phase diagram of a halide double perovskite, one can control the effects of intrinsic defects on carrier trapping and Fermi-level pinning. Furthermore, we reveal the ideal growth conditions to grow p type Cs 2 Ag In Cl 6 and Cs 2 Ag Bi Cl 6 as well as semi-insulating Cs 2 Ag Bi Br 6 with low trap density for targeted photovoltaic or visible light/radiation detection application.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Intrinsic defect in BiNbO{sub 4}: A density functional theory study

Formation energies, transition energy levels, and electronic properties of various intrinsic defects in BiNbO{sub 4} systems are studied based on the first-principles density-functional theory. Our results indicate that the acceptor defects form easier than donor defects under O rich condition, while it is opposite under Bi rich condition. Under O-rich condition, Bi vacancies (Bi{sub vac}) leading to p-type conductivity are the dominant intrinsic defects, whereas O vacancies (O{sub vac}) inducing moderate n-type conductivity are the dominant intrinsic defects under Bi-rich condition. Among these intrinsic defects, O{sub vac} is a deep donor; to the contrary, Bi{sub vac} is found to be a shallow acceptor which is benefit to the separation and migration of the photogenerated carriers. Consequently, the BiNbO{sub 4} with Bi{sub vac} under O-rich growth condition should be of better photocatalytic performance.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Structural defect generation and band-structure features in the HfNi{sub 1−x}Co{sub x}Sn semiconductor

The crystal and electronic structure and magnetic, energy, and kinetic properties of the n-HfNiSn semiconductor heavily doped with the Co acceptor impurity (HfNi{sub 1−x}Co{sub x}Sn) are investigated in the temperature and Co concentration ranges T = 80–400 K and N{sub A}{sup Co} ≈ 9.5 × 10{sup 19}-5.7 × 10{sup 21} cm{sup −3} (x = 0.005–0.30), respectively, and under magnetic field H ≤ 10 kOe. It is established that the degree of compensation of the semiconductor changes due to transformation of the crystal structure upon doping, which leads to the generation of acceptor and donor structural defects. The calculated electronic structure is consistent with the experiment; the HfNi{sub 1−x}Co{sub x}Sn semiconductor is shown to be a promising thermoelectric material. The results obtained are discussed within the Shklovsky-Efros model for a heavily doped and compensated semiconductor.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Vacancy-type defects in In{sub x}Ga{sub 1-x}N alloys probed using a monoenergetic positron beam

Native defects in In{sub x}Ga{sub 1-x}N grown by plasma-assisted molecular beam epitaxy were probed by a monoenergetic positron beam. Doppler broadening spectra of the annihilation radiation were measured, and these were compared with results obtained using first-principles calculation. The defect concentration increased with increasing In composition x and reached the maximum at x = 0.44{approx}0.56. A clear correlation between the line-width of photoluminescence and the defect concentration was obtained. The major defect species detected by positron annihilation was identified as cation vacancies coupled with multiple nitrogen vacancies (V{sub N}s), and their introduction mechanism is discussed in terms of the strain energy due to bond-length/angle distortions and the suppression of the V{sub N} formation energy by neighboring In atoms.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Magnetic defects in chemically converted graphene nanoribbons: electron spin resonance investigation

Electronic spin transport properties of graphene nanoribbons (GNRs) are influenced by the presence of adatoms, adsorbates and edge functionalization. To improve the understanding of the factors that influence the spin properties of GNRs, local (element) spin-sensitive techniques such as electron spin resonance (ESR) spectroscopy are important for spintronics applications. Here, we present results of multi-frequency continuous wave (CW), pulse and hyperfine sublevel correlation (HYSCORE) ESR spectroscopy measurements performed on oxidatively unzipped graphene nanoribbons (GNRs), which were subsequently chemically converted (CCGNRs) with hydrazine. ESR spectra at 336 GHz reveal an isotropic ESR signal from the CCGNRs, of which the temperature dependence of its line width indicates the presence of localized unpaired electronic states. Upon functionalization of CCGNRs with 4-nitrobenzene diazonium tetrafluoroborate, the ESR signal is found to be 2 times narrower than that of pristine ribbons. NH{sub 3} adsorption/desorption on CCGNRs is shown to narrow the signal, while retaining the signal intensity and g value. The electron spin-spin relaxation process at 10 K is found to be characterized by slow (163 ns) and fast (39 ns) components. HYSCORE ESR data demonstrate the explicit presence of protons and {sup 13}C atoms. With the provided identification of intrinsic point magnetic defects such as proton and {sup 13}C has been reported, which are roadblocks to spin travel in graphene-based materials, this work could help in advancing the present fundamental understanding on the edge-spin (or magnetic)-based transport properties of CCGNRs.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Exchange-correlation functional challenges in modeling quaternary chalcogenides

The development of next-generation quaternary chalcogenides, such as Cu2ZnSnS4 (CZTS) and Cu2ZnGeS4 (CZGS), for solar energy and thermoelectric applications hinges upon both careful experimentation and accurate quantum mechanical modeling. To address the latter, many have turned to density functional theory (DFT), which offers several choices for the approximate treatment of electron exchange and correlation (XC). Popular XC functionals include the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) and the recently developed strongly constrained and appropriately normed (SCAN) meta-GGA. Extensions of DFT functionals, such as adding a Hubbard U correction and introducing a fraction of the Fock exchange (hybrid functionals), have been used widely to model systems containing 3d metal ions. However, no studies yet have compared comprehensively PBE(+U) and SCAN(+U) in the quality of their predictions of the bulk and defect thermodynamics of quaternary chalcogenides, which play a critical role in device fabrication and performance. Hence, here we calculate the (i) 0 K formation energies of bulk Ge compounds and (ii) neutral defect formation energies including charge-balanced (e.g., CuZn + ZnCu) and charge-imbalanced (e.g., CuSn) combinations of antisites and vacancies in CZTS and CZGS using the PBE, PBE +U, SCAN, SCAN +U, and the hybrid Heyd-Scuseria-Ernzerhof XC frameworks. We find that the formation energies of charge-imbalanced defects are more sensitive to the choice of the XC functional than those of charge-balanced defects, which can be explained by the differences in the extent of penalization of defect-generated delocalized electrons/holes by PBE, PBE +U, SCAN, and SCAN +U. Additionally, our results show that SCAN systematically underbinds Ge-containing compounds, thus highlighting the need for even further improvement of XC functionals. Based on our findings, we recommend the use of SCAN for modeling quaternary chalcogenides because its errors are systematic, and it has the firmest theoretical underpinning. Our work provides guidance for future modeling of quaternary chalcogenides.

14 SOLAR ENERGY↗
DOE OSTI2018

Electronic fingerprints of Cr and V dopants in the topological insulator Sb 2 Te 3

By combining scanning tunneling microscopy/spectroscopy and first-principles calculations, we systematically study the local electronic states of magnetic dopants V and Cr in the topological insulator (TI) Sb 2 Te 3 . Spectroscopic imaging shows diverse local defect states between Cr and V, which agree with our first-principle calculations. The unique spectroscopic features of V and Cr dopants provide electronic fingerprints for the codoped magnetic TI samples with the enhanced quantum anomalous Hall effect. Here our results also facilitate the exploration of the underlying mechanism of the enhanced quantum anomalous Hall temperature in Cr/V codoped TIs.

36 MATERIALS SCIENCE↗