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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 1,801 records · Page 100
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Behavioral Features of MIS Memristors with a Si{sub 3}N{sub 4} Nanolayer Fabricated on a Conductive Si Substrate

The effect of the material of the metal plates (Au, Ta, W) and exposure to a high-power blue laser on the memristive characteristics of metal–insulator–semiconductor (MIS) capacitors with a Si{sub 3}N{sub 4} film 6 nm thick fabricated on the basis of n{sup +}-Si is studied. It is shown that bipolar switching by the current appears only in capacitors with Au. The causes of the absence of bipolar switching in capacitors with Ta and W are explained. The switching of capacitors with Ta by the current and light and the photomemory effect are detected. It is shown that, despite the high doping level of the semiconductor substrate, it decreases the MIS memristor response rate due to a high density of surface states localized at the Si{sub 3}N{sub 4}/n{sup +}-Si interface. However, illumination allows a significant increase in the response rate due to a decrease in the semiconductor resistivity. The surface state densities are determined. To improve the frequency characteristics of MIS memristors, it is necessary to achieve a low surface state density.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microlens-Enhanced Substrate Patterning and MBE Growth of GaP Nanowires

In this paper we demonstrate the results on selective area growth of GaP nanowires via self-catalyzed growth method using molecular beam epitaxy (MBE) technique on patterned Si(111) substrates. The pattern fabrication method on a base of the photolithography process over an array of microspherical lenses has been studied theoretically and then optimized in order to obtain the nanostructures with controlled morphology. It was found that the positive resist thickness corresponding to the best achievable resolution in the subwavelength region is 250 nm in case of 1.5 μm silica spheres and excitation with 365 nm LED. The silica growth mask for selective epitaxy was fabricated. The ordered array of GaP nanowires was synthesized with MBE. Large scale ordering and selectivity of the growth technique is demonstrated.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Electrical and Photoluminescence Studies of {LT-GaAs/GaAs:Si} Superlattices Grown by MBE on (100)- and (111)A-Oriented GaAs Substrates

The results of studying semiconductor structures proposed for the first time and grown, which combine the properties of LT-GaAs with p-type conductivity upon doping with Si, are presented. The structures are {LT-GaAs/GaAs:Si} superlattices, in which the LT-GaAs layers are grown at a low temperature (in the range 280–350°C) and the GaAs:Si layers at a higher temperature (470°C). The p-type conductivity upon doping with Si is provided by the use of GaAs(111)A substrates and the choice of the growth temperature and the ratio between As{sub 4} and Ga fluxes. The hole concentration steadily decreases, as the growth temperature of LT-GaAs layers is lowered from 350 to 280°C, which is attributed to an increase in the roughness of interfaces between layers and to the formation of regions depleted of charge carriers at the interfaces between the GaAs:Si and LT-GaAS layers. The evolution of the photoluminescence spectra at 77 K under variations in the growth temperature of LT-GaAs is interpreted as a result of changes in the concentration of Ga{sub As} and V{sub Ga} point defects and Si{sub Ga}–V{sub Ga}, V{sub As}–Si{sub As}, and Si{sub As}–Si{sub Ga} complexes.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Type-I Indirect-Gap Semiconductor Heterostructures on (110) Substrates

Type-I indirect-gap heterostructures are convenient objects for studying the spin dynamics of localized excitons, which are difficult to investigate in heterostructures of other types. It is shown that structures with such an energy spectrum can be formed from III–V binary compounds on substrates with the (110) orientation. The effect of the strain distribution and conduction-band structure in quasimomentum space on the energy spectrum of electronic states in the heterostructures is discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

The Structure, Dielectric, and Optical Properties of c-Oriented SBN-50 Films Grown on Pt/Al{sub 2}O{sub 3} Substrate

Studies of the structure, dielectric and optical properties of thin films of a relaxor ferroelectric Ba{sub 0.5}Sr{sub 0.5}Nb{sub 2}O{sub 6} grown on Pt(111)/Al{sub 2}O{sub 3} (c-cut) substrate by high-frequency RF-sputtering in oxygen atmosphere, were performed. X-ray diffraction studies showed that Ba{sub 0.5}Sr{sub 0.5}Nb{sub 2}O{sub 6} films are c-oriented. The unit cell parameters in the tetragonal approximation were c = 3.949(1) Å and a = 12.38(1) Å. It was established that the ferroelectric–paraelectric phase-transition temperature rises and optical anisotropy increases in the object in comparison with the bulk material. Reasons for the elucidated regularities are discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Obtaining Iron-Oxide Films on Sapphire Substrates

Iron-oxide films on sapphire substrates were obtained using laser radiation with 1064-nm wavelength and studied by methods of scanning electron microscopy, energy-dispersive X-ray spectroscopy, and transmission Mössbauer spectroscopy. The obtained Fe{sub 2}O{sub 3} films probably occur in a superparamagnetic state and may find application in gas sensors and various magnetic devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Stimulated Emission at 1.3-μm Wavelength in Metamorphic InGaAs/InGaAsP Structure with Quantum Wells Grown on Ge/Si(001) Substrate

A laser structure comprising metamorphic InGaAsP layer and InGaAs quantum wells on a non-inclined Si(001) substrate with relaxed Ge buffer layer has been grown for the first time by metal-organic vapor phase epitaxy (MOVPE). The optically pumped lasers exhibit stimulated emission at a wavelength of 1.3 μm. At liquid-nitrogen temperature, the threshold power density of pumping at 0.8 μm amounted to 250 kW/cm{sup 2}.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

The Role of the Charge State of Surface Atoms of a Metal Substrate in Doping of Quasi-Free-Standing Graphene

To determine the effect of a metal substrate on the electronic state of quasi-free-standing graphene, a simple model taking into account the presence of the double electric layer on the metal surface is proposed. The arising electrostatic field shifts the Dirac point of graphene, which leads to its doping. An analytical expression for the charge of graphene atoms is obtained. Numerical estimations are made for (111) faces of Cu, Ag, Au, and Pt. The agreement of the obtained estimations with the available results of numerical calculations is discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Features of Pulsed Laser Annealing of BC{sub 3} Films on a Sapphire Substrate

The morphology, chemical composition, microstructure, and electrical properties of BC{sub 3} thin films subjected to melting by a nanosecond laser pulse are investigated. The original films have been created by pulsed laser codeposition of B and C onto a sapphire substrate at 150 and 350°C. Morphological changes in the films depended on their initial structural state. However, a “frozen” structure of both films after irradiation corresponded to the B-saturated graphite-like phase, the local composition of which varied due to the formation of inclusions of amorphous boron carbide. Before and after irradiation, the films exhibited a slightly decreasing dependence of the surface resistance with increasing temperature from 4.2 to 330 K. After laser irradiation, the films resistance has decreased by a factor of ~2.6.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

The Influence of the Configuration and Material of Interdigital Transducers on the Excitation of Surface and Pseudosurface Acoustic Waves in Lithium Niobate Substrates

We have studied the excitation, propagation, and interaction of surface and pseudosurface acoustic waves in X-cut lithium niobate substrate. Resonance excitation frequencies, propagation velocities, and dispersion characteristics for the acoustic waves of each type are determined. It is established that the electrode material influences the efficiency of excitation and interaction between the acoustic waves of various types. Requirements for the material and configuration of electrodes in integrated acousto-optic devices are determined.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1999

Determination of human DNA polymerase utilization for the repair of a model ionizing radiation-induced DNA strand break lesion in a defined vector substrate

Human DNA polymerase and DNA ligase utilization for the repair of a major class of ionizing radiation-induced DNA lesion [DNA single-strand breaks containing 3'-phosphoglycolate (3'-PG)] was examined using a novel, chemically defined vector substrate containing a single, site-specific 3'-PG single-strand break lesion. In addition, the major human AP endonuclease, HAP1 (also known as APE1, APEX, Ref-1), was tested to determine if it was involved in initiating repair of 3'-PG-containing single-strand break lesions. DNA polymerase beta was found to be the primary polymerase responsible for nucleotide incorporation at the lesion site following excision of the 3'-PG blocking group. However, DNA polymerase delta/straightepsilon was also capable of nucleotide incorporation at the lesion site following 3'-PG excision. In addition, repair reactions catalyzed by DNA polymerase beta were found to be most effective in the presence of DNA ligase III, while those catalyzed by DNA polymerase delta/straightepsilon appeared to be more effective in the presence of DNA ligase I. Also, it was demonstrated that the repair initiating 3'-PG excision reaction was not dependent upon HAP1 activity, as judged by inhibition of HAP1 with neutralizing HAP1-specific polyclonal antibody.

Non-NASA Center↗
NASA NTRS1997

Isolation of novel ribozymes that ligate AMP-activated RNA substrates

BACKGROUND: The protein enzymes RNA ligase and DNA ligase catalyze the ligation of nucleic acids via an adenosine-5'-5'-pyrophosphate 'capped' RNA or DNA intermediate. The activation of nucleic acid substrates by adenosine 5'-monophosphate (AMP) may be a vestige of 'RNA world' catalysis. AMP-activated ligation seems ideally suited for catalysis by ribozymes (RNA enzymes), because an RNA motif capable of tightly and specifically binding AMP has previously been isolated. RESULTS: We used in vitro selection and directed evolution to explore the ability of ribozymes to catalyze the template-directed ligation of AMP-activated RNAs. We subjected a pool of 10(15) RNA molecules, each consisting of long random sequences flanking a mutagenized adenosine triphosphate (ATP) aptamer, to ten rounds of in vitro selection, including three rounds involving mutagenic polymerase chain reaction. Selection was for the ligation of an oligonucleotide to the 5'-capped active pool RNA species. Many different ligase ribozymes were isolated; these ribozymes had rates of reaction up to 0.4 ligations per hour, corresponding to rate accelerations of approximately 5 x10(5) over the templated, but otherwise uncatalyzed, background reaction rate. Three characterized ribozymes catalyzed the formation of 3'-5'-phosphodiester bonds and were highly specific for activation by AMP at the ligation site. CONCLUSIONS: The existence of a new class of ligase ribozymes is consistent with the hypothesis that the unusual mechanism of the biological ligases resulted from a conservation of mechanism during an evolutionary replacement of a primordial ribozyme ligase by a more modern protein enzyme. The newly isolated ligase ribozymes may also provide a starting point for the isolation of ribozymes that catalyze the polymerization of AMP-activated oligonucleotides or mononucleotides, which might have been the prebiotic analogs of nucleoside triphosphates.

NASA Discipline Exobiology↗

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