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At least 325 records · Page 18

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NASA NTRS1976

Potential biomedical applications of ion beam technology

Electron bombardment ion thrusters used as ion sources have demonstrated a unique capability to vary the surface morphology of surgical implant materials. The microscopically rough surface texture produced by ion beam sputtering of these materials may result in improvements in the biological response and/or performance of implanted devices. Control of surface roughness may result in improved attachment of the implant to soft tissue, hard tissue, bone cement, or components deposited from blood. Potential biomedical applications of ion beam texturing discussed include: vascular prostheses, artificial heart pump diaphragms, pacemaker fixation, percutaneous connectors, orthopedic pros-thesis fixtion, and dental implants.

Banks, B. A.↗

NASA NTRS1976

Potential biomedical applications of ion beam technology

Banks, B. A.↗

NASA NTRS1998

Integrated Studies of Impact-Basin Ejecta as Probes of the Lunar Crust: Imbrium and Serenitatis

The large, late, basin impacts on the Earth side of the Moon fundamentally reshaped the structure of the crust, its surface morphology, and the composition of the megaregolith and surface soils. The latest (except for Orientale on the western limb) and largest was the Imbrium impact, which produced massive ejecta deposits over much of the Procellarum region and beyond, and ejected material that mixed with surface regolith nearly Moonwide. The basins serve as natural probes into the lunar crust; therefore, understanding the nature and composition of ejecta produced by them provides information about the crust at depth. Gravity data allow modeling of the structure of the crust beneath the basins, and from such models one can infer depths of excavation and the nature of crustal response following impact.

Jolliff, Brad L.↗

NASA NTRS2001

Workshop on Science and the Human Exploration of Mars

The exploration of Mars will be a multi-decadal activity. Currently, a scientific program is underway, sponsored by NASA's Office of Space Science in the United States, in collaboration with international partners France, Italy, and the European Space Agency. Plans exist for the continuation of this robotic program through the first automated return of Martian samples in 2014. Mars is also a prime long-term objective for human exploration, and within NASA, efforts are being made to provide the best integration of the robotic program and future human exploration missions. From the perspective of human exploration missions, it is important to understand the scientific objectives of human missions, in order to design the appropriate systems, tools, and operational capabilities to maximize science on those missions. In addition, data from the robotic missions can provide critical environmental data - surface morphology, materials composition, evaluations of potential toxicity of surface materials, radiation, electrical and other physical properties of the Martian environment, and assessments of the probability that humans would encounter Martian life forms. Understanding of the data needs can lead to the definition of experiments that can be done in the near-term that will make the design of human missions more effective. This workshop was convened to begin a dialog between the scientific community that is central to the robotic exploration mission program and a set of experts in systems and technologies that are critical to human exploration missions. The charge to the workshop was to develop an understanding of the types of scientific exploration that would be best suited to the human exploration missions and the capabilities and limitations of human explorers in undertaking science on those missions.

Duke, M. B.↗

DOE OSTI2017

Kinetic Monte Carlo simulations of GaN homoepitaxy on c- and m-plane surfaces

The surface orientation can have profound effects on the atomic-scale processes of crystal growth and is essential to such technologies as GaN-based light-emitting diodes and high-power electronics. We investigate the dependence of homoepitaxial growth mechanisms on the surface orientation of a hexagonal crystal using kinetic Monte Carlo simulations. To model GaN metal-organic vapor phase epitaxy, in which N species are supplied in excess, only Ga atoms on a hexagonal close-packed (HCP) lattice are considered. The results are thus potentially applicable to any HCP material. Growth behaviors on c-plane (0001) and m-plane (011¯0) surfaces are compared. We present a reciprocal space analysis of the surface morphology, which allows extraction of growth mode boundaries and direct comparison with surface X-ray diffraction experiments. For each orientation, we map the boundaries between 3-dimensional, layer-by-layer, and step flow growth modes as a function of temperature and growth rate. Two models for surface diffusion are used, which produce different effective Ehrlich-Schwoebel step-edge barriers and different adatom diffusion anisotropies on m-plane surfaces. Simulation results in agreement with observed GaN island morphologies and growth mode boundaries are obtained. These indicate that anisotropy of step edge energy, rather than adatom diffusion, is responsible for the elongated islands observed on m-plane surfaces. As a result, island nucleation spacing obeys a power-law dependence on growth rate, with exponents of –0.24 and –0.29 for the m- and c-plane, respectively.

36 MATERIALS SCIENCE↗

DOE OSTI2019

Growth and characterization of type-II superlattice photodiodes with cutoff wavelength of 12 μm on 4-in. wafer

High performance InAs/GaSb superlattices for long-wavelength infrared detection were grown on a 4-in. GaSb wafer using molecular beam epitaxy. The influence of the growth temperature, V/III BEP ratio on the surface morphology and microstructure was researched. The good uniformity of the surface roughness and crystallization at four location along the radial direction on the wafer was obtained by balancing the inner and outer growth temperature. The performance deviation of the four photodiodes fabricated using the superlattices from center to edge on the wafer was below 20% which revealed the uniformity of the superlattice growth and the fabrication process. The 50% cutoff wavelength was about 12 μm. The peak responsivity was 2.54 A/W corresponding to a quantum efficiency of 35.1% at 77 K. The zero bias differential resistance-area product (R{sub 0}A) was 2.1 Ω cm{sup 2} due to the barrier structure design which restrains the diffusion dark current. The calculated Johnson noise limited detectivity was 5.75 × 10{sup 10} cm √Hz/W at 10 μm and 4.13 × 10{sup 10} cm √Hz/W at 14 μm.

36 MATERIALS SCIENCE↗

DOE OSTI2024

Laterally Modulating Carrier Concentration by Ion Irradiation in CdO Thin Films for Mid‐IR Plasmonics

This report demonstrates tunable carrier densities in CdO thin films through local ion irradiation, providing lateral control of mid‐IR optical properties. Ion‐solid interactions produce donor‐like defects that boost electron concentrations from the practical minimum of 2.5 × 10 19 cm −3 to a maximum of 2.5 × 10 20 cm −3 by metered ion exposure. This range is achieved using He, N, Ar, or Au ions at 1–2.8 MeV; when normalized by displacements per atom, all ion species produce comparable results. Since CdO is well‐described by the Drude model, irradiation‐tuned carrier densities directly alter the infrared dielectric function, and in turn, mid‐infrared optical properties. Further, it is demonstrated that by combining irradiation with traditional lithography, CdO films expose to ions in the presence of 3‐µm thick, patterned photoresist exhibit lateral carrier density profiles with ≈400‐nm resolution. Scanning near‐field optical microscopy reveals sharp optical interfaces with almost no companion contrast in surface morphology, microstructure, or crystallinity. Finally, CdO lateral homostructures supporting surface plasmon polaritons (SPPs) are demonstrated whose dispersion relation can be tuned through periodic patterning in a monolithic platform by simple nanofabrication. Numerical simulations show these polaritons result from strong coupling between excitations at CdO plasma frequencies and SPPs supported by the platinum substrate.

cadmium oxide↗

DOE OSTI2018

First-principles study of stability of helium-vacancy complexes below tungsten surfaces

Density function theory calculations have been performed to study the stability of small helium-vacancy (He-V) complexes near tungsten (W) surfaces of different orientations. The results show that the stability of vacancies and He-V complexes near W surfaces depends on surface orientation. However, as the depth below the surface increased beyond about 0.65–0.8 nm, the stability of He-V complexes is similar to the bulk. The formation energies of single vacancies and di-vacancies at depths less than 0.2 nm below the W(110) surface are higher than for W(100) or W(111) surfaces, but have lower energies at depths between 0.2 and 0.65 nm. The formation energies of He-V complexes below W surfaces are sensitive to the geometric orientation of the He and vacancy, especially below the W(111) surface. Within about 0.2 nm of the top layer of the three W surfaces, neither a vacancy nor a di-vacancy can trap He. Because of the lower formation energy of He-V complexes and higher He binding energy to vacancies below the W(110) surface, the He desorption from the W(110) surface is less likely to occur than from the W(100) and W(111) surfaces. Here, our results provide fundamental insight into the differences in surface morphology changes observed in single W crystals with different surface orientations under He plasma exposure.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2026

Temperature-Dependent Sn Incorporation and Defect Formation in Pseudomorphic SiSn Layers on Si (001) via Molecular Beam Epitaxy

SiSn alloys have attracted growing interest for group-IV bandgap engineering, although their epitaxial growth remains challenging due to the extremely low equilibrium solubility of Sn in Si. In this work, fully strained (pseudomorphic) SiSn epitaxial layers were grown on Si (001) substrates by means of molecular beam epitaxy. A systematic investigation reveals a strong inverse correlation between growth temperature and Sn incorporation efficiency. Despite a constant Sn flux, the incorporated Sn composition decreases from 5.5% to 3.2% as the growth temperature increases, indicating a pronounced temperature dependence of Sn incorporation. Reflection high-energy electron diffraction indicates a gradual transition of the growth from two-dimensional to three-dimensional with increasing film thickness. Structural characterization by means of X-ray diffraction, atomic force microscopy, and transmission electron microscopy confirms the pseudomorphic growth and smooth surface morphology and reveals twins and stacking faults near the surface region. These results establish a quantitative reference for SiSn growth kinetics and provide guidance for future studies of SiSn and SiGeSn alloys in silicon-compatible electronic and optoelectronic applications.

SiSn alloys↗

NASA NTRS1993

Ion-Implanted Diamond Films and Their Tribological Properties

This paper reports the physical characterization and tribological evaluation of ion-implanted diamond films. Diamond films were produced by microwave plasma, chemical vapor deposition technique. Diamond films with various grain sizes (0.3 and 3 microns) and roughness (9.1 and 92.1 nm r.m.s. respectively) were implanted with C(+) (m/e = 12) at an ion energy of 160 eV and a fluence of 6.72 x 10(exp 17) ions/sq cm. Unidirectional sliding friction experiments were conducted in ultrahigh vacuum (6.6 x 10(exp -7)Pa), dry nitrogen and humid air (40% RH) environments. The effects of C(+) ion bombardment on fine and coarse-grained diamond films are as follows: the surface morphology of the diamond films did not change; the surface roughness increased (16.3 and 135.3 nm r.m.s.); the diamond structures were damaged and formed a thin layer of amorphous non-diamond carbon; the friction coefficients dramatically decreased in the ultrahigh vacuum (0.1 and 0.4); the friction coefficients decreased slightly in the dry nitrogen and humid air environments.

Wu, Richard L. C.↗

DOE OSTI2015

Development of micro-four-point probe in a scanning tunneling microscope for in situ electrical transport measurement

Electrons at surface may behave differently from those in bulk of a material. Multi-functional tools are essential in comprehensive studies on a crystal surface. Here, we developed an in situ microscopic four-point probe (4PP) transport measurement system on the basis of a scanning tunneling microscope (STM). In particular, convenient replacement between STM tips and micro-4PPs enables systematic investigations of surface morphology, electronic structure, and electrical transport property of a same sample surface. Performances of the instrument are demonstrated with high-quality STM images, tunneling spectra, and low-noise electrical I-V characteristic curves of a single-layer FeSe film grown on a conductive SrTiO{sub 3} surface.

36 MATERIALS SCIENCE↗

DOE OSTI2015

Rapid control of mold temperature during injection molding process

The control of mold surface temperature is an important factor that determines surface morphology and its dimension in thickness direction. It can also affect the frozen molecular orientation and the mold surface replicability in injection molded products. In this work, thin thermally active films were used to quickly control the mold surface temperature. In particular, an active high electrical conductivity carbon black loaded polyimide composites sandwiched between two insulating thin polymeric layers was used to condition the mold surface. By controlling the heating time, it was possible to control precisely the temporal variation of the mold temperature surface during the entire cycle. The surface heating rate was about 40°C/s and upon contact with the polymer the surface temperature decreased back to 40°C within about 5 s; the overall cycle time increased only slightly. The effect on cross section sample morphology of samples of iPP were analyzed and discussed on the basis of the recorded temperature evolution.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

NASA NTRS1993

Visible Imaging on the Pluto Fast Flyby Mission

Objectives for visible imaging of the Pluto-Charon system, as prescribed by the Outer Planets Science Working Group, are to acquire (1) global observations (FOV of approximately 5000 IFOV's) at 1 km/line-pair for the purpose of characterizing surface morphology and geology, (2) global observations in 3-5 broadband colors at 5-10 km/line-pair for studies of surface properties and composition as it relates to morphology, and (3) selected observations at higher spatial resolution for study of surface processes. Several factors of the Pluto Fast Flyby mission make these difficult to achieve. These factors along with other aspects of the mission are discussed.

Malin, M. C.↗

DOE OSTI2015

Suppression of surface charge accumulation on Al{sub 2}O{sub 3}-filled epoxy resin insulator under dc voltage by direct fluorination

Surface charge accumulation on insulators under high dc voltage is a major factor that may lead to the reduction of insulation levels in gas insulated devices. In this paper, disc insulators made of Al{sub 2}O{sub 3}-filled epoxy resin were surface fluorinated using a F{sub 2}/N{sub 2} mixture (12.5% F{sub 2}) at 50 °C and 0.1 MPa for different durations of 15 min, 30 min and 60 min. A dc voltage was applied to the insulator for 30 min and the charge density on its surface was measured by an electrostatic probe. The results revealed significant lower surface charge densities on the fluorinated insulators in comparison with the original one. Surface conductivity measurements indicated a higher surface conductivity by over three orders of magnitude after fluorination, which would allow the charges to transfer along the surface and thus may suppress their accumulation. Further, attenuated total reflection infrared analysis and surface morphology observations of the samples revealed that the introduction of fluoride groups altered the surface physicochemical properties. These structure changes, especially the physical defects reduced the depth of charge traps in the surface layer, which was verified by the measurement of energy distributions of the electron and hole traps based on the isothermal current theory. The results in this paper demonstrate that fluorination can be a promising and effective method to suppress surface charge accumulation on epoxy insulators in gas insulated devices.

36 MATERIALS SCIENCE↗

DOE OSTI2016

Fast and low-cost method to fabricate large-area superhydrophobic surface on steel substrate with anticorrosion and anti-icing properties

A facile and quick fabrication method was proposed to prepare superhydrophobic surfaces on iron substrate by chemical immersion and subsequent stearic acid modification. The association between wettability and surface morphology was studied through altering the copper ion concentration and immersion time. Surface tension instrument, scanning electron microscopy, x-ray diffraction, x-ray photoelectron spectroscopy, and electrochemical workstation were used to characterize the wettability, physical morphology, chemical composition, and corrosion resistance ability of the prepared film. Results showed that both the rough micro/nanostructures and low surface energy material play critical roles in surface wettability. The superhydrophobic film achieved a better anticorrosion property compared to barrier iron by analysis of open circuit potential, potentiodynamic polarization curves, and Nyquist plots. In addition, the superhydrophobic surface showed excellent performance of acid and alkali resistance, anti-icing, and self-cleaning through a series of environmental tests. This study provides a valid method for quick-preparation of the stable superhydrophobic surfaces, which has a promising application in steel buildings and facilities.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2022

Pit rim decomposition into multiple quantum dots on surfaces of epitaxial thin films grown on pit-patterned substrates

Here, we report results of dynamical simulations according to an experimentally validated surface morphological evolution model on the formation of regular arrays of quantum dot molecules (QDMs) consisting of 1D arrays of smaller interacting quantum dots (QDs). These QD arrays form along the sides of each pit rim on the surface of a coherently strained thin film epitaxially deposited on a semiconductor substrate, the surface of which consists of a periodic pattern of inverted prismatic pits with rectangular pit openings. We find that this complex QDM pattern results from the decomposition of the pit rim from a “quantum fortress” with four elongated QDs into four 1D arrays of multiple smaller QDs arranged along each side of the pit rim. Systematic parametric analysis indicates that varying the pit opening dimensions and the pit wall inclination directly impacts the number of QDs in the resulting QDM pattern, while varying the pit depth only affects the dimensions of the QDs in the nanostructure pattern. Therefore, the number, arrangement, and sizes of QDs in the resulting pattern of QDMs on the film surface can be engineered precisely by proper tuning of the pit design parameters. Our simulation results are supported by predictions of morphological stability analysis, which explains the pit rim decomposition into multiple QDs as the outcome of a tip-splitting instability and provides a fundamental characterization of the post-instability nanostructure pattern. Our theoretical findings can play a vital role in designing optimal semiconductor surface patterns toward enabling future nanofabrication technologies.

36 MATERIALS SCIENCE↗

DOE OSTI2025

Control over Banded Morphologies and Circular Dichroism in Chiral Halide Perovskites

Chiral halide perovskites (c-HPs) merge the chirality of organic cations with the semiconducting properties of metal halide frameworks, creating a family of chiral semiconductors with tunable chiroptoelectronic behavior. Here, we describe the impact of periodic banded morphologies of textured c-HP ( R/S -NEA) 2 PbBr 4 films (NEA = 1- (1-naphthyl)ethylammonium) on their chiroptical behavior. Due to the interplay between the crystalline and glassy phases, the c-HP film growth is driven by rhythmic precipitation, producing a distinctive controllable radial banded pattern with the ( R/S -NEA) 2 PbBr 4 inorganic planes oriented parallel to the substrate. The banded morphology can be controlled, as evidenced by the growth temperature dictating the ridge-to-ridge spacing as well as the density of banded regions. The resulting circular dichroism (CD) spectral shape, intensity, and polarity vary in a seemingly random manner across processing conditions. However, these spectral features can be explained by considering key features of the banded morphology, such as refraction of the incident light due to surface morphology, birefringence, and stacked, rotated crystallites. These effects cannot be canceled by averaging front and back CD spectra of c-HP films, and our model incorporating these effects reproduces all observed CD spectra remarkably well. The control over the c-HP morphology and prediction capabilities of our CD modeling leads to further understanding of this class of semiconductors and the possibility of exploiting structural features for light polarization control akin to enhanced metamaterials.

14 SOLAR ENERGY↗

DOE OSTI2015

Fabrication of anti-adhesion surfaces on aluminium substrates of rubber plastic moulds using electrolysis plasma treatment

An anti-adhesion surface with a water contact angle of 167° was fabricated on aluminium samples of rubber plastic moulds by electrolysis plasma treatment using mixed electrolytes of C{sub 6}H{sub 5}O{sub 7}(NH{sub 4}){sub 3} and Na{sub 2}SO{sub 4}, followed by fluorination. To optimise the fabrication conditions, several important processing parameters such as the discharge voltage, discharge time, concentrations of supporting electrolyte and stearic acid ethanol solution were examined systematically. Using scanning electron microscopy (SEM) to analyse surfaces morphology, micrometer scale pits, and protrusions were found on the surface, with numerous nanometer mastoids contained in the protrusions. These binary micro/nano-scale structures, which are similar to the micro-structures of soil-burrowing animals, play a critical role in achieving low adhesion properties. Otherwise, the anti-adhesion behaviours of the resulting samples were analysed by the atomic force microscope (AFM), Fourier-transform infrared spectrophotometer (FTIR), electrons probe micro-analyzer (EPMA), optical contact angle meter, digital Vickers microhardness (Hv) tester, and electronic universal testing. The results show that the electrolysis plasma treatment does not require complex processing parameters, using a simple device, and is an environment-friendly and effective method. Under the optimised conditions, the contact angle (CA) for the modified anti-adhesion surface is up to 167°, the sliding angle (SA) is less than 2°, roughness of the sample surface is only 0.409μm. Moreover, the adhesion force and H{sub v} are 0. 9KN and 385, respectively.

36 MATERIALS SCIENCE↗