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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.

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At least 415 records · Page 23
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Interplay of light transmission and catalytic exchange current in photoelectrochemical systems

We develop an analytic current-voltage expression for a variable junction photoelectrochemical (PEC) cell and use it to investigate and illustrate the influence of the optical and electrical properties of catalysts on the optoelectronic performance of PEC devices. Specifically, the model enables a simple, yet accurate accounting of nanostructured catalyst optical and electrical properties through incorporation of an optical transmission factor and active catalytic area factor. We demonstrate the utility of this model via the output power characteristics of an exemplary dual tandem solar cell with indium gallium phosphide and indium gallium arsenide absorbers with varying rhodium catalyst nanoparticle loading. The approach highlights the importance of considering interactions between independently optimized components for optimal PEC device design.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Ab initio structural and vibrational properties of GaAs diamondoids and nanocrystals

Gallium arsenide diamondoids structural and vibrational properties are investigated using density functional theory at the PBE/6-31(d) level and basis including polarization functions. Variation of energy gap as these diamondoids increase in size is seen to follow confinement theory for diamondoids having nearly equiaxed dimensions. Density of energy states transforms from nearly single levels to band structure as we reach larger diamondoids. Bonds of surface hydrogen with As atoms are relatively localized and shorter than that bonded to Ga atoms. Ga-As bonds have a distribution range of values due to surface reconstruction and effect of bonding to hydrogen atoms. Experimental bulk Ga-As bond length (2.45 Å) is within this distribution range. Tetrahedral and dihedral angles approach values of bulk as we go to higher diamondoids. Optical-phonon energy of larger diamondoids stabilizes at 0.037 eV (297 cm{sup -1}) compared to experimental 0.035 eV (285.2 cm{sup -1}). Ga-As force constant reaches 1.7 mDyne/Å which is comparable to Ga-Ge force constant (1.74 mDyne/Å). Hydrogen related vibrations are nearly constant and serve as a fingerprint of GaAs diamondoids while Ga-As vibrations vary with size of diamondoids.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Structural properties and growth mechanism of Cd{sub 3}As{sub 2} nanowires

Cadmium arsenide (Cd{sub 3}As{sub 2}) is a material well-known for its very high room-temperature carrier mobility. Recently, it has also been shown to be a three-dimensional Dirac semimetal—the three-dimensional analogue of graphene. Here, we present a detailed structural study of the self-catalyzed growth of Cd{sub 3}As{sub 2} nanowires. The crystal structure is confirmed using X-ray diffraction and transmission electron microscopy. Scanning electron microscopy and energy-dispersive X-ray spectroscopy are used to gain insight into the vapor-solid growth mechanism. The role of group II and V elements is reversed in contrast to III-V and other II-V systems, and the tips are found to be As-rich.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Plasmonic light trapping in an ultrathin photovoltaic layer with film-coupled metamaterial structures

A film-coupled metamaterial structure is numerically investigated for enhancing the light absorption in an ultrathin photovoltaic layer of crystalline gallium arsenide (GaAs). The top subwavelength concave grating and the bottom metallic film could not only effectively trap light with the help of wave interference and magnetic resonance effects excited above the bandgap, but also practically serve as electrical contacts for photon-generated charge collection. The energy absorbed by the active layer is greatly enhanced with the help of the film-coupled metamaterial structure, resulting in significant improvement on the short-circuit current density by three times over a free-standing GaAs layer at the same thickness. The performance of the proposed light trapping structure is demonstrated to be little affected by the grating ridge width considering the geometric tolerance during fabrication. The optical absorption at oblique incidences also shows direction-insensitive behavior, which is highly desired for efficiently converting off-normal sunlight to electricity. The results would facilitate the development of next-generation ultrathin solar cells with lower cost and higher efficiency.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Pressure effects on the elastic and lattice dynamics properties of AlP from first-principles calculations

We have performed first-principles calculations to investigate the behavior under hydrostatic pressure of the structural, elastic and lattice dynamics properties of aluminum phosphide crystal (AlP), in both zinc-blende (B3) and nickel arsenide (B8) phases. Our calculated structural and electronic properties are in good agreement with previous theoretical and experimental results. The elastic constants, bulk modulus (B), shear modulus (G), and Young's modulus (E), Born effective charge and static dielectric constant ε{sub 0}, were calculated with the generalized gradient approximations and the density functional perturbation theory (DFPT). Our results in the pressure behavior of the elastic and dielectric properties of both phases are compared and contrasted with the common III–V materials. The Born effective charge ZB decreases linearly with pressure increasing, while the static dielectric constant decreases quadratically with the increase of pressure.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

The efficiency limit of CH{sub 3}NH{sub 3}PbI{sub 3} perovskite solar cells

With the consideration of photon recycling effect, the efficiency limit of methylammonium lead iodide (CH{sub 3}NH{sub 3}PbI{sub 3}) perovskite solar cells is predicted by a detailed balance model. To obtain convincing predictions, both AM 1.5 spectrum of Sun and experimentally measured complex refractive index of perovskite material are employed in the detailed balance model. The roles of light trapping and angular restriction in improving the maximal output power of thin-film perovskite solar cells are also clarified. The efficiency limit of perovskite cells (without the angular restriction) is about 31%, which approaches to Shockley-Queisser limit (33%) achievable by gallium arsenide (GaAs) cells. Moreover, the Shockley-Queisser limit could be reached with a 200 nm-thick perovskite solar cell, through integrating a wavelength-dependent angular-restriction design with a textured light-trapping structure. Additionally, the influence of the trap-assisted nonradiative recombination on the device efficiency is investigated. The work is fundamentally important to high-performance perovskite photovoltaics.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Integrated three-dimensional photonic nanostructures for achieving near-unity solar absorption and superhydrophobicity

In this paper, we proposed and realized 3D photonic nanostructures consisting of ultra-thin graded index antireflective coatings (ARCs) and woodpile photonic crystals. The use of the integrated ARC and photonic crystal structure can achieve broadband, broad-angle near unity solar absorption. The amorphous silicon based photonic nanostructure experimentally shows an average absorption of ∼95% for λ = 400–620 nm over a wide angular acceptance of θ = 0°–60°. Theoretical studies show that a Gallium Arsenide (GaAs) based structure can achieve an average absorption of >95% for λ = 400–870 nm. Furthermore, the use of the slanted SiO{sub 2} nanorod ARC surface layer by glancing angle deposition exhibits Cassie-Baxter state wetting, and superhydrophobic surface is obtained with highest water contact angle θ{sub CB} ∼ 153°. These properties are fundamentally important for achieving maximum solar absorption and surface self-cleaning in thin film solar cell applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Transient reflectance of photoexcited Cd{sub 3}As{sub 2}

We report ultrafast transient-grating measurements of crystals of the three-dimensional Dirac semimetal cadmium arsenide, Cd{sub 3}As{sub 2}, at both room temperature and 80 K. After photoexcitation with 1.5-eV photons, charge-carriers relax by two processes, one of duration 500 fs and the other of duration 3.1 ps. By measuring the complex phase of the change in reflectance, we determine that the faster signal corresponds to a decrease in absorption, and the slower signal to a decrease in the light's phase velocity, at the probe energy. We attribute these signals to electrons' filling of phase space, first near the photon energy and later at lower energy. We attribute their decay to cooling by rapid emission of optical phonons, then slower emission of acoustic phonons. We also present evidence that both the electrons and the lattice are strongly heated.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Evaluation of DAST and zinc telluride nonlinear crystals for efficient terahertz generation

Terahertz (THz) signal is generated from 4-N, N-dimethylamino-4’-N’-methyl-stilbazolium tosylate (i.e. DAST Crystal) and Zinc telluride (ZnTe) nonlinear crystals by employing 140 fs laser pulses at 800 nm with 80 MHz repetition rate. The semi insulating gallium arsenide photoconductive stripline antennas (gap =5 µm, length = 20 µm) is used as a Terahertz detector. The detected temporal profile of Terahertz radiation generated from DAST crystal is high as compared to ZnTe crystal in terms of amplitude. THz effective bandwidths of these crystals are extended up to 1.1 THz range. The potential of THz generation of DAST and ZnTe crystals are evaluated with respect to incident laser power.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Buffer layer between a planar optical concentrator and a solar cell

The effect of inserting a buffer layer between a periodically multilayered isotropic dielectric (PMLID) material acting as a planar optical concentrator and a photovoltaic solar cell was theoretically investigated. The substitution of the photovoltaic material by a cheaper dielectric material in a large area of the structure could reduce the fabrication costs without significantly reducing the efficiency of the solar cell. Both crystalline silicon (c-Si) and gallium arsenide (GaAs) were considered as the photovoltaic material. We found that the buffer layer can act as an antireflection coating at the interface of the PMLID and the photovoltaic materials, and the structure increases the spectrally averaged electron-hole pair density by 36% for c-Si and 38% for GaAs compared to the structure without buffer layer. Numerical evidence indicates that the optimal structure is robust with respect to small changes in the grating profile.

14 SOLAR ENERGY↗
DOE OSTI2015

Raman spectroscopy and electrical properties of InAs nanowires with local oxidation enabled by substrate micro-trenches and laser irradiation

The thermal gradients along indium arsenide nanowires were engineered by a combination of fabricated micro-trenches in the supporting substrate and focused laser irradiation. This allowed local spatial control of thermally activated oxidation reactions of the nanowire on the scale of the diffraction limit. The locality of the oxidation was detected by micro-Raman mapping, and the results were found to be consistent with numerical simulations of the temperature profile. Applying the technique to nanowires in electrical devices the locally oxidized nanowires remained conducting with a lower conductance as expected for an effectively thinner conducting core.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Photocurrent response of B{sub 12}As{sub 2} crystals to blue light, and its temperature- dependent electrical characterizations

With the global shortage of {sup 3}He gas, researchers worldwide are looking for alternative materials for detecting neutrons. Among the candidate materials, semiconductors are attractive because of their light weight and ease in handling. Currently, we are looking into the suitability of boron arsenide (B{sub 12}As{sub 2}) for this specific application. As the first step in evaluating the material qualitatively, the photo-response of B{sub 12}As{sub 2} bulk crystals to light with different wavelengths was examined. The crystals showed photocurrent response to a band of 407- and 470- nm blue light. The maximum measured photoresponsivity and the photocurrent density at 0.7 V for 470 nm blue light at room temperature were 0.25 A ⋅ W{sup −1} and 2.47 mA ⋅ cm{sup −2}, respectively. In addition to photo current measurements, the electrical properties as a function of temperature (range: 50-320 K) were measured. Reliable data were obtained for the low-temperature I-V characteristics, the temperature dependence of dark current and its density, and the resistivity variations with temperature in B{sub 12}As{sub 2} bulk crystals. The experiments showed an exponential dependence on temperature for the dark current, current density, and resistivity; these three electrical parameters, respectively, had a variation of a few nA to μA, 1-100 μA ⋅ cm{sup −2} and 7.6x10{sup 5}-7.7x10{sup 3} Ω ⋅ cm, for temperature increasing from 50 K to 320 K. The results from this study reported the first photoresponse and demonstrated that B{sub 12}As{sub 2} is a potential candidate for thermal-neutron detectors.

36 MATERIALS SCIENCE↗
DOE OSTI2016

The LDA+U calculation of electronic band structure of GaAs

We present the electronic band structure of bulk gallium arsenide (GaAs) using first principle approach. A series of calculations has been performed by applying norm-conserving pseudopotentials and ultrasoft non-norm-conserving pseudopotentials within the density functional theory. These calculations yield too small band gap as compare to experiment. Thus, we use semiemperical approach called local density approximation plus the multi-orbital mean-field Hubbard model (LDA+U), which is quite effective in order to describe the band gap of GaAs.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Gallium arsenide/gold nanostructures deposited using plasma method

The fabrication of gallium arsenide (GaAs) nanostructures on gold coated glass, quartz and silicon substrates using the high fluence and highly energetic ions has been reported. The high fluence and highly energetic ions are produced by the hot, dense and extremely non-equilibrium plasma in a modified dense plasma focus device. The nanostructures having mean size about 14 nm, 13 nm and 18 nm are deposited on gold coated glass, quartz and silicon substrates, respectively. The optical properties of nanostructures studied using absorption spectra show surface plasmon resonance peak of gold nanoparticles. In addition, the band-gap of GaAs nanoparticles is more than that of bulk GaAs suggesting potential applications in the field of optoelectronic and sensor systems.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2011

Symmetry-Breaking Orbital Anisotropy Observed for Detwinned Ba(Fe1-xCox)2As2 above the Spin Density Wave Transition

Nematicity, defined as broken rotational symmetry, has recently been observed in competing phases proximate to the superconducting phase in the cuprate high temperature superconductors. Similarly, the new iron-based high temperature superconductors exhibit a tetragonal to orthorhombic structural transition (i.e. a broken C{sub 4} symmetry) that either precedes or is coincident with a collinear spin density wave (SDW) transition in undoped parent compounds, and superconductivity arises when both transitions are suppressed via doping. Evidence for strong in-plane anisotropy in the SDW state in this family of compounds has been reported by neutron scattering, scanning tunneling microscopy, and transport measurements. Here we present an angle resolved photoemission spectroscopy study of detwinned single crystals of a representative family of electron-doped iron-arsenide superconductors, Ba(Fe{sub 1-x}Co{sub x}){sub 2}As{sub 2} in the underdoped region. The crystals were detwinned via application of in-plane uniaxial stress, enabling measurements of single domain electronic structure in the orthorhombic state. At low temperatures, our results clearly demonstrate an in-plane electronic anisotropy characterized by a large energy splitting of two orthogonal bands with dominant d{sub xz} and d{sub yz} character, which is consistent with anisotropy observed by other probes. For compositions x > 0, for which the structural transition (T{sub S}) precedes the magnetic transition (T{sub SDW}), an anisotropic splitting is observed to develop above T{sub SDW}, indicating that it is specifically associated with T{sub S}. For unstressed crystals, the band splitting is observed close to T{sub S}, whereas for stressed crystals the splitting is observed to considerably higher temperatures, revealing the presence of a surprisingly large in-plane nematic susceptibility in the electronic structure.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2012

Effect of tensile stress on the in-plane resistivity anisotropy in BaFe[subscript 2]As[subscript 2]

The effect of uniaxial tensile stress and the resultant strain on the structural/magnetic transition in the parent compound of the iron arsenide superconductor BaFe{sub 2}As{sub 2} is characterized by temperature-dependent electrical resistivity, x-ray diffraction, and quantitative polarized light imaging. We show that strain induces a measurable uniaxial structural distortion above the first-order magnetic transition and significantly smears the structural transition. This response is different from that found in another parent compound, SrFe{sub 2}As{sub 2}, where the coupled structural and magnetic transitions are strongly first order. This difference in the structural responses explains the in-plane resistivity anisotropy above the transition in BaFe{sub 2}As{sub 2}. This conclusion is supported by the Ginzburg-Landau-type phenomenological model for the effect of the uniaxial strain on the resistivity anisotropy.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Neutron scattering studies of spin excitations in superconducting Rb0.82Fe1.68Se2

We use inelastic neutron scattering to show that superconducting (SC) rubidium iron selenide Rb0.82Fe1.68Se2 exhibits antiferromagnetic (AF) spin excitations near the in-plane wave vector Q = ( ,0) identical to that for iron arsenide superconductors. Moreover, we find that these excitations change from incommensurate to commensurate with increasing energy and occur at the expense of spin waves associated with the coexisting 5 5 block AF phase. Since these spin excitations cannot come from Fermi surface nesting based on angle resolved photoemission experiments, our results indicate the presence of local moments in SC Rb0.82Fe1.68Se2 that may have a similar origin as the hourglass-like spin excitations in copper oxide superconductors.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗