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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 1,063 records · Page 59
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Influence of oxygen pressure and aging on LaAlO{sub 3} films grown by pulsed laser deposition on SrTiO{sub 3} substrates

The crystal structures of LaAlO{sub 3} films grown by pulsed laser deposition on SrTiO{sub 3} substrates at oxygen pressure of 10{sup −3} millibars or 10{sup −5} millibars, where kinetics of ablated species hardly depend on oxygen background pressure, are compared. Our results show that the interface between LaAlO{sub 3} and SrTiO{sub 3} is sharper when the oxygen pressure is lower. Over time, the formation of various crystalline phases is observed while the crystalline thickness of the LaAlO{sub 3} layer remains unchanged. X-ray scattering as well as atomic force microscopy measurements indicate three-dimensional growth of such phases, which appear to be fed from an amorphous capping layer present in as-grown samples.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

A well-structured metastable ceria surface

By the growth of a 180 nm thick film on Si(111), we produce a metastable ceria surface with a morphology dominated by terraced pyramids with an oriented triangular base. Changes in the nanoscale surface morphology and local surface potential due to annealing at temperatures ranging from 300 K to 1150 K in the ultra-high vacuum are studied with non-contact atomic force microscopy and Kelvin probe force microscopy. As the surface is stable in the temperature range of 300 K to 850 K, it is most interesting for applications requiring regular steps with a height of one O-Ce-O triple layer.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Controlled epitaxial graphene growth within removable amorphous carbon corrals

We address the question of control of the silicon carbide (SiC) steps and terraces under epitaxial graphene on SiC and demonstrate amorphous carbon (aC) corrals as an ideal method to pin SiC surface steps. aC is compatible with graphene growth, structurally stable at high temperatures, and can be removed after graphene growth. For this, aC is first evaporated and patterned on SiC, then annealed in the graphene growth furnace. There at temperatures above 1200 °C, mobile SiC steps accumulate at the aC corral that provide effective step flow barriers. Aligned step free regions are thereby formed for subsequent graphene growth at temperatures above 1330 °C. Atomic force microscopy imaging supports the formation of step-free terraces on SiC with the step morphology aligned to the aC corrals. Raman spectroscopy indicates the presence of good graphene sheets on the step-free terraces.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

"Soiling, Cleaning, and Abrasion: The Results of the Five-Year Photovoltaic Glass Coating Field Study" [Slides]

External contamination ("soiling") of the incident surface is a major limiting factor for solar technologies. A 5-year field glass coupon study was conducted to better understand external contamination and its effects; compare cleaning methods and the use of preventative coatings; and explore the abrasion resulting from cleaning to advise on accelerated abrasion testing. Test sites included the cities of Dubai (UAE), Kuwait City (Kuwait), Mesa (AZ), Mumbai (India), and Sacramento (CA). Through the 5-year cumulative study, dry brush, water spray, and wet sponge and squeegee cleaning methods were compared to no cleaning. Optical microscopy was used to obtain images, including representative color images, grayscale images for object analysis, and oblique images for coating integrity assessment. A thresholding protocol was developed to analyze and distinguish specimens using the ImageJ software. Optical performance was quantified using a spectrophotometer, including comprehensive optical characterization (transmittance, reflectance, and absorptance in addition to forward- and back-scattering). Atomic force microscopy was used to verify the abrasion damage morphology, including the width and depth of surface scratches. Analysis of the results included correlation of optical performance and particle area coverage, rank order (by coating or location), and the acceleration factor for abrasion damage. The efficacy of external cleaning was more readily distinguished from the effectiveness of antisoiling coatings. The acceleration factor for dry brush cleaning of a porous silica coating was found to be on the order of unity.

14 SOLAR ENERGY↗
DOE OSTI2013

Effect of Ti seed layers on structure of self-organized epitaxial face-centered-cubic-Ag(001) oriented nanodots

The influence of Ti seed layers on the structure of self-organized Ag nanodots, obtained with a Ti seed-layer-assisted thermal agglomeration method, has been investigated. The samples were grown on MgO(001) single crystal substrates by RF magnetron sputter deposition. The samples were deposited at room temperature and post-annealed at 350 °C for 4 h while maintaining the chamber vacuum conditions. The results of atomic force microscopy (AFM) observations indicated that the insertion of the Ti seed layer (0.6–5.0 nm) between the MgO substrate and Ag layer promotes the agglomeration process, forming the nanodot array. Comparisons between the AFM images revealed that the size of the Ag nanodots was increased with an increase in the Ti seed layer thickness. The atomic concentration of the film surface was confirmed by X-ray photoelectron spectroscopy (XPS). The XPS result suggested that the nanodot surface mainly consisted of Ag. Moreover, X-ray diffraction results proved that the initial deposition of the Ti seed layer (0.6–5.0 nm) onto MgO(001) prior to the Ag deposition yielded high-quality fcc-Ag(001) oriented epitaxial nanodots. The optical absorbance spectra of the fabricated Ag nanodots with various Ti seed layer thicknesses were obtained in the visible light range.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Reaction of Si(111) Surface with Saturated Hydrocarbon

Reaction of Si(111) surface with saturated hydrocarbon such as methane (CH{sub 4}) and ethane (C{sub 2}H{sub 6}) was carried out in a gas source molecular beam epitaxy (GSMBE). After carbonization, structures formed on the surface were observed by in situ reflection high-energy electron diffraction (RHEED). Structures transition formed on the surface were 7x7, {delta}-7x7, 1x1, and SiC structures. In the case of CH{sub 4}, the Si surfaces were carbonized at 800 deg. C for 120 min (7.2x10{sup 4} L) with a W-filament of 2800 deg. C, and SiC layers were obtained. In the case of C{sub 2}H{sub 6}, the mixture of 7x7 and SiC structure was observed. Decomposition of hydrocarbon was characterized in quadrupole mass spectroscopy (QMS) measurements. An atomic force microscopy (AFM) image of the mixture of 7x7 and SiC shows a wandering shape. Whereas, the SiC layer shows a regular step. This result seems to be related to the different in the amount of CH{sub 3} molecules on the surface.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Zeta-potential and particle size studies of silver sulphide nanoparticles

Silver sulfide (Ag{sub 2}S) nanoparticles (NPs) were prepared successfully for the first time using diethylene glycol (DEG) as a surfactant. X-ray diffraction (XRD) data revealed single phase nature of the compound and energy-dispersive X-ray (EDX) confirmed its nominal composition. Their sizes were 43 nm from XRD, 50 nm from atomic force microscopy (AFM) and 19 nm & 213 nm from dynamic light scattering (DLS); their differences have been discussed. Autotitration study of zeta potential of these NPs in deionized water by DLS at different pH values confirmed an isoelectric point at pH = 5.14 and their very unstable nature in deionized water.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2015

Formation of a regular domain structure in TGS–TGS + Cr crystals with a profile impurity distribution

A complex investigation of TGS–TGS + Cr crystals with a profile impurity distribution of chromium ions Cr{sup 3+} has been carried out at the macrolevel (measurement of dielectric properties by the method of nematic liquid crystals) and microlevel (domain structure according to atomic force microscopy data). It is established that periodic doped layers are formed only in individual growth pyramids in the regions where the polarization vector has a nonzero component along the normal to the growth faces rather than throughout the entire crystal volume. The domain configuration at the boundary of growth layers with different impurity compositions has been studied by piezoelectric force microscopy. The static unipolarity of layers with and without chromium impurity is approximately identical, whereas the domain-wall density in doped regions is higher than that in undoped ones by a factor of about 7.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Investigations on Ni-Co-Mn-Sn thin films: Effect of substrate temperature and Ar gas pressure on the martensitic transformations and exchange bias properties

We report the effect of substrate temperature (T{sub S}) and Ar gas pressure (P{sub D}) on the martensitic transformations, magnetic and exchange bias (EB) properties in Heusler type Ni-Co-Mn-Sn epitaxial thin films. Martensitic transformation temperatures and EB fields at 5 K were found to increase with increasing T{sub S}. The observed maximum EB value of 320 Oe after field cooling in the film deposited at 650 {sup ∘}C is high among the values reported for Ni-Mn-Sn thin films which is attributed to the coexistence of ferromagnetic (FM) and antiferromagnetic (AF) phases in the martensitic state. In the case of P{sub D} variation, with increase in P{sub D}, martensitic transformation temperatures were increased and a sharp transformation was observed in the film deposited at 0.06 mbar. Magnetization values at 5 K were higher for increasing P{sub D}. These observations are attributed to the compositional shift. EB effect is also present in these films. Microstructural features observed using atomic force microscopy (AFM) shows a fine twinning and reduced precipitation with increase in P{sub D}, which is also confirmed from the scanning electron microscopy (SEM) images. EB effects in both series were confirmed from the training effect. Target ageing effect has been observed in the films deposited before and after ninety days of time interval. This has been confirmed both on substrate temperature and Ar gas pressure variations.

36 MATERIALS SCIENCE↗
DOE OSTI2010

Electromechanical probing of ionic currents in energy storage materials

The electrochemical processes in energy storage materials are generally linked with changes of molar volume of the host compound. Here, the frequency dependent strain response of one-dimensional electrochemically active system to periodic electric bias is analyzed. The sensitivity and resolution of electrochemical strain measurements are compared to the current-based electrochemical impedance spectroscopy. The resolution and detection limits of interferometric and atomic force microscopy based systems for probing electrochemical reactions on the nanoscale are analyzed.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

Growth mechanisms study of microcrystalline silicon deposited by SiH{sub 4}/H{sub 2} plasma using tailored voltage waveforms

The use of Tailored Voltage Waveforms is a technique wherein one uses non-sinusoidal waveforms with a period equivalent to RF frequencies to excite a plasma. It has been shown to be an effective technique to decouple maximum Ion Bombardment Energy (IBE) from the ion flux at the surface of the electrodes. In this paper, we use it for the first time as a way to scan through the IBE in order to study the growth mechanism of hydrogenated microcrystalline silicon using a SiH{sub 4}/H{sub 2} chemistry. We find that at critical energies, a stepwise increase in the amorphous to microcrystalline transition thickness is observed, as detected by Real Time Spectroscopic Ellipsometry. The same energy thresholds (30 eV and 70 eV) are found to be very influential on the final surface morphology of the samples, as observed by Atomic Force Microscopy. These thresholds correspond to SiH{sub x}{sup +} bulk displacement (30 eV) and H{sub x}{sup +} (70 eV) surface displacement energies. A model is therefore proposed to account for the impact of these ions on the morphology of μc-Si:H growth.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Comparison of trimethylgallium and triethylgallium as “Ga” source materials for the growth of ultrathin GaN films on Si (100) substrates via hollow-cathode plasma-assisted atomic layer deposition

GaN films grown by hollow cathode plasma-assisted atomic layer deposition using trimethylgallium (TMG) and triethylgallium (TEG) as gallium precursors are compared. Optimized and saturated TMG/TEG pulse widths were used in order to study the effect of group-III precursors. The films were characterized by grazing incidence x-ray diffraction, atomic force microscopy, x-ray photoelectron spectroscopy, and spectroscopic ellipsometry. Refractive index follows the same trend of crystalline quality, mean grain, and crystallite sizes. GaN layers grown using TMG precursor exhibited improved structural and optical properties when compared to GaN films grown with TEG precursor.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Optical Diagnostics of WSe{sub 2} Monolayers

WSe{sub 2} films have been studied by the method of second harmonic generation (SHG) in the 1200–1420 nm wavelength range at room temperature. Preliminary investigations of photoluminescence spectra and atomic force microscopy showed that the film consists of extended microcrystals and has a thickness of about 1 nm, which corresponds to a single monolayer. The SHG intensity spectrum showed the presence of a 1s (B)-exciton for 1.95-eV photon energy.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Epitaxial ZnO/LiNbO{sub 3}/ZnO stacked layer waveguide for application to thin-film Pockels sensors

We produced slab waveguides consisting of a LiNbO{sub 3} (LN) core layer that was sandwiched with Al-doped ZnO cladding layers. The ZnO/LN/ZnO stacked layers were grown on sapphire C-planes by electron cyclotron resonance (ECR) plasma sputtering and were subjected to structural, electrical, and optical characterizations. X-ray diffraction confirmed that the ZnO and LN layers were epitaxial without containing misoriented crystallites. The presence of 60°-rotational variants of ZnO and LN crystalline domains were identified from X-ray pole figures. Cross-sectional transmission electron microscopy images revealed a c-axis orientated columnar texture for LN crystals, which ensured operation as electro-optic sensors based on optical anisotropy along longitudinal and transversal directions. The interfacial roughness between the LN core and ZnO bottom layers as well as that between the ZnO top and the LN core layers was less than 20 nm, which agreed with surface images observed with atomic force microscopy. Outgrowth of triangular LN crystalline domains produced large roughness at the LN film surface. The RMS roughness of the LN film surface was twice that of the same structure grown on sapphire A-planes. Vertical optical transmittance of the stacked films was higher than 85% within the visible and infrared wavelength range. Following the approach adopted by Teng and Man [Appl. Phys. Lett. 56, 1734 (1990)], ac Pockels coefficients of r{sub 33} = 24-28 pm/V were derived for c-axis oriented LN films grown on low-resistive Si substrates. Light propagation within a ZnO/LN/ZnO slab waveguide as well as within a ZnO single layer waveguide was confirmed. The birefringence of these waveguides was 0.11 for the former and 0.05 for the latter.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Molecular engineering of nanoscale order in organic electro-optic glasses

The rational design of bulk nanoscale order in organic electro-optic materials, where the strong dipole-dipole interactions tend to dominate over the weaker forces exploited for self-assembly processes, remains an attractive yet elusive goal. Towards this end, a series of pseudo-discotic dipolar nonlinear optical chromophores have been synthesized and fully characterized. Theoretical guidance and an iterative molecular design process have succeeded in engineering long-range nanoscale order in organic electro-optic glasses. Small-angle thin-film X-ray diffraction experiments demonstrate a self-assembled lamellar morphology in a majority of these materials. Cryogenic crystallography, using a synchrotron X-ray source, afforded the structure of a representative system. This structure, in concert with thin-film X-ray diffraction, atomic force microscopy, UV-vis-NIR absorption spectroscopy, and refractive index experiments elucidated the nanoscale order in the films. Application of these materials in electro-optics is discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Integrating atomic layer deposition and ultra-high vacuum physical vapor deposition for in situ fabrication of tunnel junctions

Atomic Layer Deposition (ALD) is a promising technique for growing ultrathin, pristine dielectrics on metal substrates, which is essential to many electronic devices. Tunnel junctions are an excellent example which require a leak-free, ultrathin dielectric tunnel barrier of typical thickness around 1 nm between two metal electrodes. A challenge in the development of ultrathin dielectric tunnel barriers using ALD is controlling the nucleation of dielectrics on metals with minimal formation of native oxides at the metal surface for high-quality interfaces between the tunnel barrier and metal electrodes. This poses a critical need for integrating ALD with ultra-high vacuum (UHV) physical vapor deposition. In order to address these challenges, a viscous-flow ALD chamber was designed and interfaced to an UHV magnetron sputtering chamber via a load lock. A sample transportation system was implemented for in situ sample transfer between the ALD, load lock, and sputtering chambers. Using this integrated ALD-UHV sputtering system, superconductor-insulator-superconductor (SIS) Nb-Al/Al{sub 2}O{sub 2}/Nb Josephson tunnel junctions were fabricated with tunnel barriers of thickness varied from sub-nm to ∼1 nm. The suitability of using an Al wetting layer for initiation of the ALD Al{sub 2}O{sub 3} tunnel barrier was investigated with ellipsometry, atomic force microscopy, and electrical transport measurements. With optimized processing conditions, leak-free SIS tunnel junctions were obtained, demonstrating the viability of this integrated ALD-UHV sputtering system for the fabrication of tunnel junctions and devices comprised of metal-dielectric-metal multilayers.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Structural studies of zirconium doped Ba{sub 0.70}Sr{sub 0.30}TiO{sub 3} lead free ferroelectric thin films

Ba{sub 0.7}Sr{sub 0.3}(Zr{sub x}Ti{sub 1-x})O{sub 3}(BSZT, x=0,0.05,0.10,0.15,0.20) thin films were prepared by using sol gel method. Structural and microstructural properties were studied by using XRD, Raman Spectroscopy and atomic force microscopy (AFM) respectively. XRD and Raman Spectroscopy show the presence of tetragonal phase in multilayer BSZT thin film. The experimental results demonstrate that structural and microstructural properties of BSZT thin film were significantly dependent on variation of Zr content.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Structural and morphological modifications of polymer thin film in the presence of nonsolvent

Thin films of sodium poly(acrylic acid) salt (Na-PAA) have been investigated to obtain the modification of the out-of-plane structure and surface morphology in the presence of toluene which is considered as nonsolvent for Na-PAA. X-ray reflectivity analysis show that the out-of-plane thickness of the Na-PAA film increases if the film is kept for longer time inside the toluene. For the thicker film the effect of toluene is more pronounced than the thinner one. Surface morphology obtained from the atomic force microscopy shows that the top surface becomes relatively rough after the dipping of the Na-PAA film inside toluene. Although toluene is nonsolvent for Na-PAA molecules, however, the effect of restructuring of the nanometer-thick polymer film cannot be ignored. The reason for such structural modification has been proposed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗