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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 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Material properties and device evaluations of ECR-deposited a-Si:H and a-SiC:H films

Device-quality a-Si:H and a-SiC:H films have been deposited using electron cyclotron resonance (ECR) microwave plasmas of SiH4, CH4, and H2 mixtures. Typical material properties of ECR-deposited, photosensitive a-Si:H films are: (1) high photosensitivity up to 2 x 106 with a photoconductivity of 10 exp -5 to 10 exp -4/(Ohm-cm), (2) a Tauc gap of 1.75 to 1.85 eV, (3) an Urbach slope of 50-60 meV determined by the constant photocurrent method, and (4) an integrated defect density of 1-2 x 10 exp 16/cu cm determined by junction capacitance measurements. Highly conductive, p-type a-SiC:H films have been produced by ECR plasmas with a conductivity of 0.2/(Ohm-cm).

Shing, Y. H.↗
DOE OSTI2023

Direct Observation of Magnetic Bubble Lattices and Magnetoelastic Effects in van der Waals Cr 2 Ge 2 Te 6

Abstract Ferromagnetic van der Waals (vdW) materials are of large current interest for the fundamental study of low‐dimensional magnetism and for potential applications in multilayer heterostructures. Cr 2 Ge 2 Te 6 (CGT) is particularly exciting because it is a ferromagnetic semiconductor with tunable electronic and magnetic properties. Controlling the magnetic domain structure of CGT is a requirement for understanding its novel interface physics and for tuning behavior for potential devices. Herein, cryo‐Lorentz transmission electron microscopy is performed in the temperature range of 12–50K to directly image the magnetic domain structures in CGT. A rich phase diagram of domain structures including stripe domains, magnetic bubble lattices of mixed‐chirality, and topologically‐protected lattices of homochiral magnetic bubbles is observed. The types and chiralities of the bubbles can be controlled by topographical changes in the CGT flakes. Additionally, it is observed that in‐plane strain and magnetoelastic coupling can align and organize both bubble lattices and stripe domains. This study provides insights into creating and controlling complex magnetic domain structures for integration into multilayer heterostructures and for future studies of 2D magnetism.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Pulsed studies of intervalley transfer in Al 0.35 In 0.65 As: A paradigm for valley photovoltaics

Al 0.35 In 0.65 As is a direct semiconductor whose multivalley conduction-band structure has been proposed for use in so-called valley photovoltaics. In such hot-carrier solar cells, energetic (hot) photocarriers are stored in satellite valleys away from the Γ point, allowing them to be extracted prior to thermalization and to thereby increase power-conversion efficiency. While prior theoretical work has highlighted the potential of Al 0.35 In 0.65 As—a widely used barrier material in electronic and optoelectronic devices, for use in valley photovoltaics—surprisingly little is known about its electrical properties, especially how these are impacted by the application of high fields. In this work, we therefore undertake a detailed characterization of the electrical properties of Te-doped ($\textit{n}$-type) Al 0.35 In 0.65 As, over wide ranges of temperature (3–400 K) and electric field (<50 kV/cm). Using pulsed measurements to suppress the influence of Joule heating, we reveal the presence of clear negative-differential conductance in the current-voltage characteristics of the films, suggestive of the intervalley transfer of hot electrons. This conclusion is supported by the results of ensemble Monte Carlo simulations of the hot-carrier action, which confirm the connection of the observed negative-differential conductance to hot-electron transfer from the conduction-band (Γ) minimum, to the side valleys at the $\textit{L}$ point. Further, the quantitative features of the experimentally determined velocity-field curves are found to be in good agreement with the results of these calculations, providing further confidence in the role of the implied intervalley transfer mechanism. Overall, these results confirm the excellent potential of Al 0.35 In 0.65 As for use as the absorber material in hot-carrier solar cell technology.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Exciton Transport in Perovskite Materials

Halide perovskites have emerged as promising materials for a wide variety of optoelectronic applications, including solar cells, light‐emitting devices, photodetectors, and quantum information applications. In addition to their desirable optical and electronic properties, halide perovskites provide tremendous synthetic flexibility through variation of not only their chemical composition but also their structure and morphology. At the heart of their use in optoelectronic technologies is the interaction of light with electronic excitations in the form of excitons. This review discusses the properties and behavior of excitons in halide perovskite materials, with a particular emphasis on low‐dimensional perovskites and the effects of nanoscale morphology on excitonic behavior. The basic theory of excitonic energy migration in semiconductor nanomaterials is introduced, and novel observations in halide perovskite nanomaterials that have evolved our current understanding are explored. Lastly, many important questions that remain unanswered are presented and exciting emerging directions in low‐dimensional perovskite exciton physics are discussed.

2D materials↗
DOE OSTI2023

Permeability of Single–Layer–Free–Standing Meshes at Varying Capillary Pressure via a Novel Method

The permeability of mesh wicks is important for various applications, including two–phase heat transfer. However, the understanding of the permeability of single–layer, free–standing mesh wicks, with liquid–gas interfaces on both sides, is limited. A novel and simpler method is presented to determine the permeability of a free–standing wick and apply it to a representative mesh. This method involves modifying the capillary pressure via elevation and simultaneously measuring the permeability to determine the permeability–capillary pressure relationship. When applied to a copper mesh with plain weave having undergone surface cleaning, the permeability is found to decrease as capillary pressure for deionized water increases. A dimensional analysis is presented to generalize this data for other mesh sizes with similar weaves and fluids. The behavior of mesh in application is modeled, based on the integration of Darcy's law with an analytic function fit to measured data, and parametric studies are conducted to investigate the superficial velocity of liquids through the mesh under varying driving pressures, transport lengths, and liquid viscosity, based on the obtained capillary pressure–permeability relationship. This study provides valuable insights into the transport properties of mesh wicks, with potential applications in fields such as electronics cooling, electrochemical devices, and fluid purification technologies.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Needle-Like Ferroelastic Domains in Individual Ferroelectric Nanoparticles

Superior structural, physical and electronic properties make ferroelectric nanocrystals essential in enabling a range of next-generation devices. Ferroelectric responses are determined by crystal structure and domain morphology. The ability to reversibly displace, create and annihilate elastic domains is critical to device applications. Whereas electric-field control has been demonstrated for ferroelectric 180° surface domain walls and vortices, similar control of ferroelastic domains and domain boundaries within individual nanocrystals remains challenging. Using controlled external compressive and tensile axial stress, deterministic and reversible control of highly mobile ferroelastic domains and axial polarization in three dimensions is demonstrated in this study. While many studies exist on ferroelastic domains in thin films and bulk, little is known about ferroelastic interactions at the single nanoparticle level, especially involving domain boundaries. Through combining Bragg coherent X-ray diffractive imaging and Landau theory, strain gradients in individual BaTiO3 nanocrystals are shown to stabilize needle-like ferroelastic twin domains. These domains are highly labile under applied axial stress, producing a locally enhanced electric polarization mediated by a ferroelectric phase transition. The efficacy of Bragg coherent X-ray diffractive imaging in studying in operando domains in three-dimensions is demonstrated, while synergy with theory provides a paradigm for domain boundary engineering and potential for nanoscale functional devices.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Substrate-Directed Dimensional and Phase Control of Peptide Assemblies on Two-Dimensional van der Waals Materials

Understanding and controlling biomolecular self-assembly on van der Waals (vdW) materials has the potential to advance hybrid bioelectronic devices by enabling precise tuning of the interface and modulation of the resulting electronic properties of the biomolecule-vdW heterostructure. However, how surface properties of vdW materials direct biomolecule assembly remains poorly understood. To fill this knowledge gap, we investigated the assembly of a peptide known to assemble into two-dimensional (2D) crystalline films on MoS 2 on three representative vdW surfaces: WS 2 , MoS 2 , and highly oriented pyrolytic graphite (HOPG). Using in situ atomic force microscopy (AFM), we find that assembly is substrate-dependent, resulting in multilayers on WS 2 , monolayers on MoS 2 , and multiple coexisting phases on HOPG. WS 2 exhibits a higher negative charge, strong long-range electrostatic interactions, and extensive hydration layering that may promote multilayer stacking. In contrast, MoS 2 has stronger short-range interactions with the peptides but much weaker long-range interactions and hydration structure, which may favor monolayer formation. Molecular dynamics simulations predict a corresponding switch from monolayer to multilayer aggregates of the adsorbed monomers, reflected in their relative mobilities. On hydrophobic HOPG, the peptides bind most strongly and remain as monomers with high surface mobility. The peptide dimers comprising the basic unit of the crystals are more compact on HOPG, which has a smaller lattice constant than WS 2 or MoS 2 , suggesting strain contributes to stabilizing multiple phases. Our results provide mechanistic insights into how surface charge and hydration structure, and the lattice structure of the substrates governs peptide assembly on vdW materials, offering a framework to rationally control the 2D peptide-vdW heterostructures.

Molecular dynamics simulations↗
DOE OSTI2020

The Computational Capacity of Mem-LRC Reservoirs

Reservoir computing has a emerged as a powerful tool in data-driven time series analysis. The possibility of utilizing hardware reservoirs as specialized co-processors has generated interest in the properties of electronic reservoirs, especially those based on memristors as the nonlinearity of these devices should translate to an improved nonlinear computational capacity of the reservoir. However, designing these reservoirs requires a detailed understanding of how memristive networks process information which has thus far been lacking. In this work, we derive an equation for general memristor-inductor-resistor-capacitor (MEM-LRC) reservoirs that includes all network and dynamical constraints explicitly. Utilizing this we undertake a study of the computational capacity of these reservoirs. We demonstrate that hardware reservoirs may be constructed with extensive memory capacity and that the presence of memristors enacts a tradeoff between memory capacity and nonlinear computational capacity. Here, using these principles, we design reservoirs to tackle problems in signal processing, paving the way for applying hardware reservoirs to high-dimensional spatiotemporal systems.

Circuits with memory↗
DOE OSTI2022

Multiplication of Freestanding Compound Semiconductor Membranes from a Single Wafer by Alternating Growth with 2D Materials

Single-crystal compound semiconductors, such as III-N and III-V, are important building blocks for functional devices due to their high electron mobilities, wide range of bandgaps, and excellent optoelectronic properties. However, current methods to produce their freestanding membranes for heterointegration suffer from slow processes or poor material quality. Here, we demonstrate an approach to grow and harvest multiple wafer-scale single-crystal membranes with high throughput by introducing weak van der Waals interfaces between epitaxial layers. This is achieved by directly growing two-dimensional (2D) materials on compound semiconductors in their native deposition systems, which enables alternating growth of 2D material and compound semiconductor epitaxial layers in a single run. Each epilayer in the multi-stack structure is then sequentially harvested by mechanical exfoliation, producing multiple freestanding single-crystal membranes with extremely high throughput from a single wafer, primed for integration in mixed-dimensional heterostructures.

Han, Ne Myo↗
DOE OSTI2022

Multiplication of Freestanding Compound Semiconductor Membranes from a Single Wafer by Alternating Growth with 2D Materials

Single-crystal compound semiconductors, such as III-N and III-V, are important building blocks for functional devices due to their high electron mobilities, wide range of bandgaps, and excellent optoelectronic properties. However, current methods to produce their freestanding membranes for heterointegration suffer from slow processes or poor material quality. Here, we demonstrate an approach to grow and harvest multiple wafer-scale single-crystal membranes with high throughput by introducing weak van der Waals interfaces between epitaxial layers. This is achieved by directly growing two-dimensional (2D) materials on compound semiconductors in their native deposition systems, which enables alternating growth of 2D material and compound semiconductor epitaxial layers in a single run. Each epilayer in the multi-stack structure is then sequentially harvested by mechanical exfoliation, producing multiple freestanding single-crystal membranes with extremely high throughput from a single wafer, primed for integration in mixed-dimensional heterostructures.

Han, Ne Myo↗
NASA NTRS1975

Preliminary design report, Large Space Telescope OTA/SI Phase B study: High speed area photometer

A photometer is examined which combines several features from separate instruments into a single package. The design presented has both point and area photometry capability with provision for inserting filters to provide spectral discrimination. The electronics provide for photon counting mode for the point detectors and both photon counting and analog modes for the area detector. The area detector also serves as a target locating device for the point detectors. Topics discussed include: (1) electronic equipment requirements, (2) optical properties, (3) structural housing for the instrument, (4) motors and other mechanical components, (5) ground support equipment, and (6) environment control for the instrument. Engineering drawings and block diagrams are shown.

Source record↗
NASA NTRS2001

Characterization of Carbon Nanotubes Grown by Chemical Vapor Deposition

Since the superior properties of multi-wall carbon nanotubes (MWCNT) could improve numerous devices such as electronics and sensors, many efforts have been made in investigating the growth mechanism of MWCNT to synthesize high quality MWCNT. Chemical vapor deposition (CVD) is widely used for MWCNT synthesis, and scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS) are useful methods for analyzing the structure, morphology and composition of MWCNT. Temperature and pressure are two important growth parameters for fabricating carbon nanotubes. In MWCNT growth by CVD, the plasma assisted method is normally used for low temperature growth. However a high temperature environment is required for thermal CVD. A systematic study of temperature and pressure-dependence is very helpful to understanding MWCNT growth. Transition metal particles are commonly used as catalysis in carbon nanotube growth. It is also interesting to know how temperature and pressure affect the interface of carbon species and catalyst particles

Cochrane, J. C.↗
NASA NTRS2001

Orientational Growth of Carbon Nanotube for Applications

Since the superior properties of multi-wall carbon nanotubes (MWCNT) could improve numerous devices such as electronics and sensors, many efforts have been made in investigating the growth mechanism of MWCNT to synthesize high quality MWCNT. Most applications require uniform aligned CNT. In this presentation, a directional growth of CNT will be reported. Carbon nanotubes are synthesized using thermal chemical vapor deposition. Temperature and pressure are two important growth parameters for fabricating carbon nanotubes. It is found that the nanotube diameter distribution mainly depends on the growth-temperature. With the substrate surface normal either along or against the gravity vector, different growth orientations of MWCNT are observed by scanning electron microscopy although the Raman spectra are similar for samples synthesized at different locations. The sizes of these carbon nanotubes in each sample are quite uniform and the length of the tube is up to several tens of micrometers. These results suggest the gravitation effects in the growth of long and small diameter CNT.

Zhu, Shen↗
DOE OSTI2021

A perspective on the electro-thermal co-design of ultra-wide bandgap lateral devices

Fundamental research and development of ultra-wide bandgap (UWBG) semiconductor devices are under way to realize next-generation power conversion and wireless communication systems. Devices based on aluminum gallium nitride (AlxGa1-xN, x is the Al composition), ..beta..-phase gallium oxide (..beta..-Ga2O3), and diamond give promise to the development of power switching devices and radio frequency power amplifiers with higher performance and efficiency than commercial wide bandgap semiconductor devices based on gallium nitride (GaN) and silicon carbide (SiC). However, one of the most critical challenges for the successful deployment of UWBG device technologies is to overcome adverse thermal effects that impact the device performance and reliability. Overheating of UWBG devices originates from the projected high power density operation and poor intrinsic thermal properties of AlxGa1-xN and ..beta..-Ga2O3. This Perspective delineates the need and process for the "electro-thermal co-design" of laterally configured UWBG electronic devices and provides a comprehensive review of current state-of-the-art thermal characterization methods, device thermal modeling practices, and both device- and package-level thermal management solutions.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Semi-transparent p-type barium copper sulfide as a back contact interface layer for cadmium telluride solar cells

Optically transparent p-type materials play a critical role in transparent electronics including photovoltaic (PV) devices. P-type sulfide materials offer an alternative to oxides for PV application due to improved hole transport properties. Here, we report the solution-based synthesis of earth-abundant p-type transparent conducting barium copper sulfide (α-BaCu 4 S 3 , BCS) thin films. These films were characterized using scanning electron microscopy, X-ray diffraction, UV–Vis–NIR spectrophotometry, Raman spectroscopy, and spectroscopic ellipsometry. BCS films of ~100 nm thickness transmit >70% of visible light. We report on tests of the hole transport properties of these BCS films for cadmium telluride (CdTe) photovoltaics, finding that the BCS deposition process forms a beneficial tellurium (Te) rich surface on CdTe by selectively removing Cd from the surface. Based on our study, the BCS interface layer plays dual functions for CdTe PV devices as a hole transport material and as an etchant, enhancing the resulting device performance. We observed a significant increase in open-circuit voltage of CdTe solar cells with the BCS buffer layer. Furthermore, we discuss semitransparent CdTe solar cells with BCS as a hole transport layer and indium tin oxide as a finishing electrode. Semitransparent CdTe solar cells shows 13.3% conversion efficiency for the front side illumination and 1.2% efficiency for back side illumination, indicating high recombination of charge carriers generated close to the rear CdTe/BCS/ITO contact.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Carbon Nanotube Devices for Quantum Technology

Carbon nanotubes, quintessentially one-dimensional quantum objects, possess a variety of electrical, optical, and mechanical properties that are suited for developing devices that operate on quantum mechanical principles. The states of one-dimensional electrons, excitons, and phonons in carbon nanotubes with exceptionally large quantization energies are promising for high-operating-temperature quantum devices. Here, we discuss recent progress in the development of carbon-nanotube-based devices for quantum technology, i.e., quantum mechanical strategies for revolutionizing computation, sensing, and communication. We cover fundamental properties of carbon nanotubes, their growth and purification methods, and methodologies for assembling them into architectures of ordered nanotubes that manifest macroscopic quantum properties. Most importantly, recent developments and proposals for quantum information processing devices based on individual and assembled nanotubes are reviewed.

36 MATERIALS SCIENCE↗