Search NASA⌕ Search

SEARCH · Search NASA

Results for “electron doping”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 559 records · Page 31
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Anomalous nanoclusters, anisotropy, and electronic nematicity in the doped manganite L a 1 / 3 C a 2 / 3 Mn O 3

In doped manganites, a superlattice (SL) modulation associated with charge/orbital ordering is accepted as a key component in understanding many intriguing properties. It has been reported that the SL modulation always appears on the a axis of the crystals. Here in this study, by using multiple transmission electron microscopic techniques, we observe a type of anomalous nanocluster in which the SL modulation appears on the c axis of La 1/3 Ca 2/3 MnO 3 . By correlating the thermal evolution of the anomalous nanoclusters to other property measurements, we suggest that strain is responsible for the formation of the anomalous nanoclusters. The phase separation and phase transition scenario in La 1/3 Ca 2/3 MnO 3 are also described using electronic-liquid-crystal (ELC) phases. Lastly, an ELC phase diagram in La 1/3 Ca 2/3 MnO 3 is constructed as a function of temperature based on our observations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Interplay between Composition, Electronic Structure, Disorder, and Doping due to Dual Sublattice Mixing in Nonequilibrium Synthesis of ZnSnN 2 :O

We report the opportunity for enhanced functional properties in semiconductor solid solutions has attracted vast scientific interest for a variety of novel applications. However, the functional versatility originating from the additional degrees of freedom due to atomic composition and ordering comes along with new challenges in characterization and modeling. Developing predictive synthesis-structure-property relationships is prerequisite for effective materials design strategies. Here, a first-principles based model for property prediction in such complex semiconductor materials is presented. This framework incorporates nonequilibrium synthesis, dopants and defects, and the change of the electronic structure with composition and short range order. This approach is applied to ZnSnN 2 (ZTN) which has attracted recent interest for photovoltaics. The unintentional oxygen incorporation and its correlation with the cation stoichiometry leads to the formation of a solid solution with dual sublattice mixing. A nonmonotonic doping behavior as a function of the composition is uncovered. Finally, the degenerate doping of near-stoichiometric ZTN, which is detrimental for potential applications, can be lowered into the 10 17 cm -3 range in highly off-stoichiometric material, in quantitative agreement with experiments.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Origin of resolution enhancement by co-doping of scintillators: Insight from electronic structure calculations

It was recently shown that the energy resolution of Ce-doped LaBr{sub 3} scintillator radiation detectors can be crucially improved by co-doping with Sr, Ca, or Ba. Here, we outline a mechanism for this enhancement on the basis of electronic structure calculations. We show that (i) Br vacancies are the primary electron traps during the initial stage of thermalization of hot carriers, prior to hole capture by Ce dopants; (ii) isolated Br vacancies are associated with deep levels; (iii) Sr doping increases the Br vacancy concentration by several orders of magnitude; (iv) Sr{sub La} binds to V{sub Br} resulting in a stable neutral complex; and (v) association with Sr causes the deep vacancy level to move toward the conduction band edge. The latter is essential for reducing the effective carrier density available for Auger quenching during thermalization of hot carriers. Subsequent de-trapping of electrons from Sr{sub La}–V{sub Br} complexes can activate Ce dopants that have previously captured a hole leading to luminescence. This mechanism implies an overall reduction of Auger quenching of free carriers, which is expected to improve the linearity of the photon light yield with respect to the energy of incident electron or photon.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Electronic structure evolution in doping of fullerene (C{sub 60}) by molybdenum trioxide

Molybdenum oxide doping of fullerene has been investigated using ultraviolet photoemission spectroscopy (UPS), X-ray photoemission spectroscopy (XPS), and inverse photoemission spectroscopy (IPES). The lowest unoccupied molecular orbital and the highest occupied molecular orbital (HOMO) can be observed directly with IPES and UPS. It is observed that the Fermi level position in fullerene is modified by molybdenum oxide doping, and the HOMO onset is shifted to less than 0.3 eV below the Fermi level. The energy level shift is found to saturate at doping ratio of 18%. Till this stage, the shift depends on the doping concentration in a semi-logarithmic scale, with a slope substantially higher than that of the traditional semiconductor theory. The XPS results indicate that charge transfer continues beyond the energy level shift saturation till the doping ratio reaches 66% as evidenced by the Mo{sup 5+} component. At higher doping concentration, there is more Mo{sup 6+} component, which indicates the saturation of the charge transfer between MoO{sub x} and C{sub 60}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Nonlinear electron-phonon coupling in doped manganites

Here, we employ time-resolved resonant x-ray diffraction to study the melting of charge order and the associated insulator-to-metal transition in the doped manganite Pr 0.5 Ca 0.5 MnO 3 after resonant excitation of a high-frequency infrared-active lattice mode. We find that the charge order reduces promptly and highly nonlinearly as function of excitation fluence. Density-functional theory calculations suggest that direct anharmonic coupling between the excited lattice mode and the electronic structure drives these dynamics, highlighting a new avenue of nonlinear phonon control.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
NASA NTRS1997

Low-Energy Electron Detection with Delta-Doped CCDs

In this paper, we will briefly discuss delta-doped CCDs and their application of low-energy electron detection. We show that modification of the surface this way can greatly improve sensitivity to low energy electrons.

electron↗
NASA NTRS1999

The Production of Polycyclic Aromatic Hydrocarbon Anions in Inert Gas Matrices Doped with Alkali Metals. Electronic Absorption Spectra of the Pentacene Anion (C22H14(-))

The absorption spectra of pentacene (C22H14) and its radical cation (C22H14(+)) and anion (C22H14(-)) isolated in inert-gas matrices of Ne, Ar, and Kr are reported from the ultraviolet to the near-infrared. The associated vibronic band systems and their spectroscopic assignments are discussed together with the physical and chemical conditions governing ion (and counterion) production in the solid matrix. In particular, the formation of isolated pentacene anions is found to be optimized in matrices doped with alkali metal (Na and K).

Halasinski, Thomas M.↗
DOE OSTI2023

Spectral emission properties of a nitrogen-doped diamond (001) photocathode: Hot electron transport and transverse momentum filtering

The electron emission properties of a single-crystal nitrogen-doped diamond(001) photocathode inserted in a 10 kV DC photoelectron gun are determined using a tunable (235–410 nm) ultraviolet laser radiation source for photoemission from both the back nitrogen-doped substrate face and the front homoepitaxially grown and undoped diamond crystal face. Here, the measured spectral trends of the mean transverse energy and quantum efficiency of the emitted electrons are both anomalous and nonmonotonic, but are shown to be consistent with (i) the known physics of electron photoexcitation from the nitrogen substitution states into the conduction bands of diamond, (ii) the energy position and dispersion characteristics of the conduction bands of diamond in the (001) emission direction, (iii) the effective electron affinity of the crystal faces, (iv) the strong electron-(optical)phonon coupling in diamond, and (v) the associated hot electron transport dynamics under energy equipartition with the optical phonons. Notably, the observed hot electron emission is shown to be restricted parallel to the photocathode surface by the low transverse effective masses of the emitting band states—a transverse momentum filtering effect.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Long-lived nanosecond spin relaxation and spin coherence of electrons in monolayer MoS 2 and WS 2

The recently discovered monolayer transition metal dichalcogenides (TMDCs) provide a fertile playground to explore new coupled spin–valley physics. Although robust spin and valley degrees of freedom are inferred from polarized photoluminescence (PL) experiments PL timescales are necessarily constrained by short-lived (3–100 ps) electron–hole recombination9, 10. Direct probes of spin/valley polarization dynamics of resident carriers in electron (or hole)-doped TMDCs, which may persist long after recombination ceases, are at an early stage. Here we directly measure the coupled spin–valley dynamics in electron-doped MoS 2 and WS 2 monolayers using optical Kerr spectroscopy, and reveal very long electron spin lifetimes, exceeding 3 ns at 5 K (2-3 orders of magnitude longer than typical exciton recombination times). In contrast with conventional III–V or II–VI semiconductors, spin relaxation accelerates rapidly in small transverse magnetic fields. Supported by a model of coupled spin–valley dynamics, these results indicate a novel mechanism of itinerant electron spin dephasing in the rapidly fluctuating internal spin–orbit field in TMDCs, driven by fast inter-valley scattering. Additionally, a long-lived spin coherence is observed at lower energies, commensurate with localized states. These studies provide insight into the physics underpinning spin and valley dynamics of resident electrons in atomically thin TMDCs.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Electronic transport in heavily doped Ag/n-Si composite films

Hall measurements characterized Ag/n-Si composite films 1 micron thick produced by magnetron co-sputtering onto high resistivity Si (111) substrates at 550°C. The targets were Ag and n-type Si doped with 3 × 10{sup 19}/cm{sup 3} of antimony. Films were prepared with 13, 16 and 22 at. % Ag and measured over a temperature range 77–500°K. Conduction takes place at low temperatures by variable rang hopping in localized states at the Fermi level and by thermal activation over grain boundaries at higher temperatures. The Log Resistivity vs 1/kT curves for the three Ag concentrations vary in a similar manner, but decrease in magnitude with increasing Ag due to the smaller number of grain boundaries between Ag nanoparticles occurring with increasing Ag concentration. At low temperatures Hall mobilities are essentially independent of temperature as the carrier densities for the three Ag concentrations are constant from 77 to slightly under 300°K with resistivities varying by small amounts. The mobilities at all Ag concentrations increase with temperature and approach each other as the effects of grain boundaries become less important. This work presents for the first time the effects of metal particles embedded in a semiconductor on the transport properties of carriers in the semiconductor. Though these effects are for a given average particle size most of the results are expected to hold over a range of particle sizes. Free electrons produced in films containing 13 and 16 at. % Ag result in concentrations of 1.5 × 10{sup 19}/cm{sup 3}, one half the antimony doping, while those with 22 at. % Ag, the carrier concentrations are three orders of magnitude higher. These constant carrier concentrations are due to the metal-insulator transition that occurs in doped crystalline and polycrystalline silicon for carrier densities n{sub c} >3.9 × 10{sup 18}/cm{sup 3}. The three orders of magnitude higher carrier concentration produced in films with 22 at. % Ag is argued to be due to doping of the Si matrix by the Ag nanoparticles at this concentration, a doping effect previously observed in ErAs in InGaAs. A discussion of this doping effect is presented that is in keeping with the experimental observations though other factors such as particle size effects need further experimental confirmation. Thermal activation energies and grain boundary barrier heights were obtained from resistivity and Hall mobility data. The barrier heights of 0.360, 0.390 and 0.470 eV measured for films with Ag concentrations of 13, 16 and 22 at. % Ag respectively, are quite high compared to those obtained by other methods used for producing polycrystalline Si (10-100 mev) and need to be considerably reduced in order to increase the efficiency of the detectors for which these Ag/n-Si composite films will be used. Thermal annealing of as-deposited films is suggested as one means to accomplish this result.

36 MATERIALS SCIENCE↗
DOE OSTI2014

MO-G-BRF-07: Optical Characterization of Novel Terbium-Doped Nanophosphors Excited by Clinical Electron and Photon Beams for Potential Use in Molecular Imaging Or Photodynamic Therapy

Purpose: Optical properties of terbium (Tb3+)-doped gadolinium trifluoride (GdF3) nanoplates irradiated by electron and photon beams were investigated for their potential as optical probes. The contribution of induced Cerenkov radiation in exciting the nanophosphors was investigated as well. Methods: The emission spectra of Terbium-doped GdF3 dispersed in hexane, embedded in tissue mimicking phantoms were collected by an optical fiber connected to a CCD-coupled spectrograph, while the samples were irradiated by a medical linear accelerator with electron beams of energies 6, 9, 12, 16, and 20 MeV or X-ray beams of energies of 6, and 15 MV. The contribution of induced Cerenkov radiation in exciting the nanophosphores was investigated in a dedicated experimental apparatus through optical isolation of the samples and also by using 125 kVp X-ray beams whose energy is below the threshold for generating Cerenkov radiation in that medium. Results: Terbium-doped GdF3 nanoplates show characteristic cathodoluminescence emission peaks at 488, 543, 586, and 619 nm, which are responsible for the characteristic f-f transition of terbium ion. In a series of experiments, the contribution of Cerenkov radiation in the luminescence of such nanophosphors was ruled out. Conclusion: We have characterized the optical properties of Terbium-doped GdF3 nanoplates. Such nanocrystals with emission tunability and high surface area that facilitates attachment with targeting reagents are promising in situ light source candidates for molecular imaging or exciting a photosensitizer for ultralow fluence photodynamic therapy. This work is supported by the Department of Radiation Oncology at the University of Pennsylvania, the American Cancer Society through IRG-78-002-28, and the University of Pennsylvania's Nano/Bio Interface Center through NSEC DMR08-32802.

07 ISOTOPE AND RADIATION SOURCES↗
DOE OSTI2015

Intrinsic evolutions of dielectric function and electronic transition in tungsten doping Ge{sub 2}Sb{sub 2}Te{sub 5} phase change films discovered by ellipsometry at elevated temperatures

Tungsten (W) doping effects on Ge{sub 2}Sb{sub 2}Te{sub 5} (GSTW) phase change films with different concentrations (3.2, 7.1, and 10.8%) have been investigated by variable-temperature spectroscopic ellipsometry. The dielectric functions from 210 K to 660 K have been evaluated with the aid of Tauc-Lorentz and Drude dispersion models. The analysis of Tauc gap energy (E{sub g}) and partial spectral weight integral reveal the correlation between optical properties and local structural change. The order degree increment and chemical bond change from covalent to resonant should be responsible for band gap narrowing and electronic transition enhancement during the phase change process. It is found that the elevated crystalline temperature for GSTW can be related to improved disorder degree. Furthermore, the shrinkage of E{sub g} for GSTW should be attributed to the enhanced metallicity compared with undoped GST.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Enhancing the Electron Mobility via Delta-Doping in SrTiO3

We fabricated high-mobility {delta}-doped structures in SrTiO{sub 3} thin films in order to investigate the low temperature electronic transport properties of confined carriers in this system. An enhancement of the electron mobility above the bulk value was observed as the doped layer thickness decreased. High-field Hall measurements revealed that this mobility enhancement originates from higher-mobility electrons in the undoped clean regions, which have quantum-mechanically broadened from the doped layer. Because of the absence of apparent lattice misfit between the layers, this structure is highly suitable for investigating two-dimensional electron gases in SrTiO{sub 3}

36 MATERIALS SCIENCE↗
DOE OSTI2018

Increasing Saturated Electron-Drift Velocity in Donor–Acceptor Doped pHEMT Heterostructures

Field dependences of the electron-drift velocity in typical pseudomorphic high-electron-mobility transistor (pHEMT) heteroepitaxial structures (HESs) and in those with donor–acceptor doped (DApHEMT) heterostructures with quantum-well (QW) depth increased by 0.8–0.9 eV with the aid of acceptor layers have been studied by a pulsed technique. It is established that the saturated electron-drift velocity in DA-pHEMT-HESs is 1.2–1.3 times greater than that in the usual pHEMT-HESs. The electroluminescence (EL) spectra of DA-pHEMT-HESs do not contain emission bands related to the recombination in widebandgap layers (QW barriers). The EL intensity in these HESs is not saturated with increasing electric field. This is indicative of a suppressed real-space transfer of hot electrons from QW to barrier layers, which accounts for the observed increase in the saturated electron-drift velocity.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2012

Obtaining Mixed Ionic/Electronic Conductivity in Perovskite Oxides in a Reducing Environment: A Computational Prediction for Doped SrTiO3

The electronic conductivity and thermodynamic stability of mixed p- and n-doped SrTiO3 perovskites have been investigated under anodic solid oxide fuel cell conditions using density functional theory (DFT). In particular, constrained ab initio thermodynamic calculations have been performed to evaluate the phase stability of various Ga- and La-doped SrTiO3 at synthesized and anodic SOFC conditions. The density of states (DOS) of these materials was analyzed to determine the number of charge carriers and the degree of electronic conductivity. We find that a mixed ionic/electronic conductor can be obtained when doping SrTiO3 perovskite oxide with both p-type and n-type dopants. Calculations show that 10% Ga- and 20% La-doped SrTiO3 exhibit mixed ionic/electronic conductivity at high temperature and low oxygen partial pressure whereas doping with higher concentrations of Ga, e.g., 20%, diminishes the electronic conductivity of the material. Furthermore, changing the n-dopant from La (A-site) to Nb (B-site) does not significantly affect the reducibility and number of charge carriers in p- and n-doped SrTiO3. However, a higher degree of oxygen vacancy clustering is observed for the La-doped material which reduces the oxygen ion diffusion rate and traps electrons. Nevertheless, our findings suggest that independent of doping site, mixed ionic/ electronic conductivity can be obtained in SrTiO3 perovskite oxides under reducing conditions and high temperatures when using a mixed p- and n-type doping strategy that uses a p-dopant concentration smaller than the n-dopant concentration.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2016

Analysis of the structural, electronic and optic properties of Ni doped MgSiP{sub 2} semiconductor chalcopyrite compound

The structural, electronic band structure and optic properties of the Ni doped MgSiP{sub 2} chalcopyrite compound have been performed by using first-principles method in the density functional theory (DFT) as implemented in Vienna Ab-initio Simulation Package (VASP). The generalized gradient approximation (GGA) in the scheme of Perdew, Burke and Ernzerhof (PBE) is used for the exchange and correlation functional. The present lattice constant (a) follows generally the Vegard’s law. The electronic band structure, total and partial density of states (DOS and PDOS) are calculated. We present data for the frequency dependence of imaginary and real parts of dielectric functions of Ni doped MgSiP{sub 2}. For further investigation of the optical properties the reflectivity, refractive index, extinction coefficient and electron energy loss function are also predicted. Our obtained results indicate that the lattice constants, electronic band structure and optical properties of this compound are dependent on the substitution concentration of Ni.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2025

Doping Effects on Multivalence States, Electronic Structure, and Optical Band Gap in LaCrO 3 under Varied Atmospheres: An Integrated Experimental and Density Functional Theory Study

Doping effects on the valence state, electronic structure, and optical band and the effects on electrical conductivity were studied on the doped lanthanum chromite (LaCrO 3 ) system. The specific compositions studied were La 1–x Ca x CrO 3 (LCCx), La 1–x Sr x CrO 3 (LSCx), and La 0.8 Sr 0.2 Cr 1–x Mn x O 3 (LSCMx) (0.1 ≤ x ≤ 0.4). The powders were synthesized using a modified Pechini sol–gel method, and the ceramic samples were densified using a reactive sintering method resulting in densities >96% theoretical. X-ray photoelectron spectroscopy (XPS) was completed to characterize the defect states and cationic valence compensation as a result of divalent (Ca 2+ or Sr 2+ ) and trivalent (Mn 3+ ) substitutions. XPS was completed for samples tested in oxidizing and reducing atmospheres (up to 1500 °C), which provided insights into the oxidation state transitions induced by the Ca 2+ and Sr 2+ dopants. The work notably demonstrated, for the first time, the oxidation/reduction transitions of Cr 4+ to Cr 3+ in Sr 2+ /Mn 3+ co-doped samples under reducing atmospheres. Reflectance UV–vis spectrophotometry optical band gap measurements were also completed for the same materials; a decrease in the optical band gap (2.81–3.12 eV) was shown with increased substitution, suggesting electronic structure modifications in the LaCrO 3 perovskite. Density functional theory calculations validated experimental trends, predicting a diminishing band gap with a rising dopant concentration. The transition in Cr oxidation states was attributed to the presence of divalent/trivalent cations. These findings contribute some insights into methods to tune the LaCrO 3 electrical properties for various low- and high-temperature applications.

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

Electronic structure studies of Fe doped CeO{sub 2} thin films by resonance photoemission spectroscopy

We have studied the modification in the electronic properties of pulsed laser deposited CeO{sub 2} thin films due to Fe doping (2 and 6 at %), with the help of x-ray photoemission spectroscopy (XPS) and resonance photoemission spectroscopy (RPES) measurements. XPS results indicate the ionic state of Fe in the Fe doped films, ruling out the possibility of Fe metallic clusters. Valence band spectra of CeO{sub 2} show an additional feature after Fe doping, suggesting its incorporation in the CeO{sub 2} matrix. RPES studies on these films reveal the hybridization between oxygen vacancy induced Ce localized states and Fe derived states.

36 MATERIALS SCIENCE↗