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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 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Deposition temperature-mediated growth of helically shaped polymers and chevron-type graphene nanoribbons from a fluorinated precursor

Graphene nanoribbons (GNRs) of precise size and shape, critical for controlling electronic properties and future device applications, can be realized via precision synthesis on surfaces using rationally designed molecular precursors. Fluorine-bearing precursors have the potential to form GNRs on nonmetallic substrates suitable for device fabrication. Here, we investigate the deposition temperature-mediated growth of a new fluorine-bearing precursor, 6,11-diiodo-1,4-bis(2-fluorophenyl)-2,3-diphenyltriphenylene (C 42 H 24 F 2 I 2 ), into helically shaped polymer intermediates and chevron-type GNRs on Au(111) by combining scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory simulations. The fluorinated precursors do not adsorb on the Au(111) surface at lower temperatures, necessitating an optimum substrate temperature to achieve maximum polymer and GNR lengths. We compare the adsorption behavior with that of pristine chevron precursors and discuss the effects of C-H and C-F bonds. The results elucidate the growth mechanism of GNRs with fluorine-bearing precursors and establish a foundation for future synthesis of GNRs on nonmetallic substrates.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Bandgap analysis and carrier localization in cation-disordered ZnGeN 2

The bandgap of ZnGeN 2 changes with the degree of cation site disorder and is sought in light emitting diodes for emission at green to amber wavelengths. By combining the perspectives of carrier localization and defect states, we analyze the impact of different degrees of disorder on electronic properties in ZnGeN 2 , addressing a gap in current studies, which largely focus on dilute or fully disordered systems. The present study demonstrates changes in the density of states and localization of carriers in ZnGeN 2 calculated using bandgap-corrected density functional theory and hybrid calculations on partially disordered supercells generated using the Monte Carlo method. We use localization and density of states to discuss the ill-defined nature of a bandgap in a disordered material and identify site disorder and its impact on the structure as a mechanism controlling electronic properties and potential device performance. Decreasing the order parameter results in a large reduction of the bandgap. The reduction in bandgap is due, in part, to isolated, localized states that form above the valence band continuum associated with nitrogen coordinated by more zinc than germanium. The prevalence of defect states in all but the perfectly ordered structure creates challenges for incorporating disordered ZnGeN 2 into optical devices, but the localization associated with these defects provides insight into the mechanisms of electron/hole recombination in the material.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Increasing the Hot‐Electron Driven Hydrogen Evolution Reaction Rate on a Metal‐Free Graphene Electrode

Abstract Recently, it has been shown that a semiconductor–insulator–graphene device can drive the hydrogen evolution reaction (HER) at the graphene surface with a reduced onset potential by injecting hot electrons into graphene. However, the catalytic properties of graphene are limited by the large hydrogen adsorption energy and lack of electrochemically active sites. To address these limitations, a n ‐silicon/insulator/plasma etched graphene device is investigated, where a dry etch process is used to increase the number of active sites on the graphene by creating a greater number of active edge sites, increasing hydrogen adsorption at a given potential. This has been shown to improve the properties of devices with cold electrons. However, here it is shown that this approach can improve the HER rate with hot electrons. The electrons injected into the graphene from the silicon shift the onset potential of HER by as high as ≈0.8 V reaching a current density of 90 mA cm −2 at an overpotential of ‐0.5 V versus RHE. Furthermore, the comparison between device with pristine graphene shows a ≈2X improvement in current density at high overpotentials. This result shows that hot‐electron devices can be improved by modifying the catalytically active sites without metal catalysts.

Chae, Hyun Uk↗
DOE OSTI2020

New Quantum Phenomena by Combining 2D Materials with Complex Oxides

This project focused on forming clean interfaces between exfoliated 2D materials and epitaxial oxides with the goal of determining the extent to which the electronic and magnetic properties of the former are influenced by the latter. Clear effects were observed in both device structures for which electronic transport properties were measured and unpatterned interfaces interrogated by polarized light to determine magnetic dichroic response. This work resulted in one paper already published and two others to be written, pending the outcome of ongoing experiments and theoretical calculations

36 MATERIALS SCIENCE↗
DOE OSTI2022

Building Atomic and Plasmonic Devices via Electron Beams: from Desired Structures to Desired Properties

The evolution of the aberration correction in Scanning Transmission Electron Microscopy (STEM) has enabled multiple advances including high-resolution structural imaging, pm-level position detection of atomic columns, and ~3-5 mV resolution electron energy loss spectroscopy of the chemical and physical functionalities. These developments have further enabled the precise manipulation of the atomic structure of materials, including removal and formation of atomic planes in 3D materials, controlled atomic motion, and even direct atomic assembly of homo- and heteroatomic artificial molecules. Here, we will discuss several recent advances in STEM-EELS automated experiments as applied to quantum and nanoplasmonic systems.

42 ENGINEERING↗
NASA NTRS2003

Numerical Modeling of Nanoelectronic Devices

Nanoelectronic Modeling 3-D (NEMO 3-D) is a computer program for numerical modeling of the electronic structure properties of a semiconductor device that is embodied in a crystal containing as many as 16 million atoms in an arbitrary configuration and that has overall dimensions of the order of tens of nanometers. The underlying mathematical model represents the quantummechanical behavior of the device resolved to the atomistic level of granularity. The system of electrons in the device is represented by a sparse Hamiltonian matrix that contains hundreds of millions of terms. NEMO 3-D solves the matrix equation on a Beowulf-class cluster computer, by use of a parallel-processing matrix vector multiplication algorithm coupled to a Lanczos and/or Rayleigh-Ritz algorithm that solves for eigenvalues. In a recent update of NEMO 3-D, a new strain treatment, parameterized for bulk material properties of GaAs and InAs, was developed for two tight-binding submodels. The utility of the NEMO 3-D was demonstrated in an atomistic analysis of the effects of disorder in alloys and, in particular, in bulk In(x)Ga(l-x)As and in In0.6Ga0.4As quantum dots.

Klimeck, Gerhard↗
DOE OSTI2024

Selective modulation of electronic transport in VO 2 induced by 10 keV helium ion irradiation

Vanadium dioxide (VO 2 ) manifests an abrupt metal–insulator transition (MIT) from monoclinic to rutile phases, with potential use for tunable electronic and optical properties and spiking neuromorphic devices. Understanding pathways to modulate electronic transport in VO 2 , as well as its response to irradiation (e.g., for space applications), is critical to better enable these applications. In this work, we investigate the selective modulation of electronic transport in VO 2 films subject to different 10 keV helium ion (He + ) fluences. Under these conditions, the resistivity in the individual monoclinic and rutile phases varied by 50%–200%, while the MIT transformation temperature remains constant within 4 °C independent of irradiation fluence. Importantly, different trends in the resistivity of the monoclinic and rutile phases were observed both as a function of total He fluence as well as in films grown on different substrates (amorphous SiO 2 /Si vs single crystal Al 2 O 3 ). Through a combination of measurements including majority carrier sign via Seebeck, low frequency noise, and TEM, our investigation supports the presence of different kinds of point defects (V in; O in), which may arise due to grain boundary defect interactions. Our work suggests the utility of He irradiation for the selective modulation of VO 2 transport properties for neuromorphic, in contrast to other established but non-selective methods, like doping.

36 MATERIALS SCIENCE↗
DOE OSTI2024

The hierarchical structure of organic mixed ionic–electronic conductors and its evolution in water

Polymeric organic mixed ionic–electronic conductors underpin several technologies in which their electrochemical properties are desirable. These properties, however, depend on the microstructure that develops in their aqueous operational environment. We investigated the structure of a model organic mixed ionic–electronic conductor across multiple length scales using cryogenic four-dimensional scanning transmission electron microscopy in both its dry and hydrated states. Four-dimensional scanning transmission electron microscopy allows us to identify the prevalent defects in the polymer crystalline regions and to analyse the liquid crystalline nature of the polymer. The orientation maps of the dry and hydrated polymers show that swelling-induced disorder is mostly localized in discrete regions, thereby largely preserving the liquid crystalline order. Therefore, the liquid crystalline mesostructure makes electronic transport robust to electrolyte ingress. Furthermore, this study demonstrates that cryogenic four-dimensional scanning transmission electron microscopy provides multiscale structural insights into complex, hierarchical structures such as polymeric organic mixed ionic–electronic conductors, even in their hydrated operating state.

36 MATERIALS SCIENCE↗
NASA NTRS1998

Substrate Effects on Electronic Properties of Atomic Chains

When the device size is reduced down to 0.07 micrometers, the number of dopant atoms in the channel will no longer be macroscopic, typically less than a hundred. A spatial distribution of these dopant atoms will fluctuate statistically from device to device even in identically designed devices, and this places a serious limitation for integration. It is, however, impractical to control dopant positions within atomic dimension. One fundamental solution to this problem is to create electronics with atomically precise, but preferably simple structures. Atomic chains, precise structures of adatoms created on an atomically regulated surface, are candidates for constituent components in future electronics. All the adatoms will be placed at designated positions on the substrate, and all the device structures will be precise, free from any deviations. It was predicted using the tight-binding calculation with universal parameters that silicon chains were metallic and magnesium chains were semiconducting regardless of the lattice spacing, and a possible doping method was also proposed. In these treatments, the substrate was assumed to serve as a non-interacting template holding the adatoms without a formation of chemical bonding with substrate atoms. However, this scheme may not be easy to implement experimentally. Adatoms will have to be fixed with a van der Waals force on the substrate, but the force is generally weak and an extremely low temperature environment has to be prepared to suppress their unwanted thermal displacement. It may be logical to seek a scheme to allow the adatoms to form chemical bonding with the substrate atoms and secure their positions. The substrate effects are studied in detail.

Yamada, Toshishige↗
DOE OSTI2020

Numerical simulation of high-efficiency, scalable, all-back-contact Cd(Se,Te) solar cells

In this work, all-back-contact thin film photovoltaic devices in lattice back contact (LBC) configuration are studied by device simulation to set the requirements for achieving efficiency > 20% with lateral contact spacing > 50 µm. The numerical device model is first validated against published data for an LBC perovskite device and extended to predict the performance of Cd(Se,Te)-based polycrystalline devices. Recent advances in the electronic properties of Cd(Se,Te) films have made high efficiency LBC devices feasible with industrially-scalable contact dimensions. Constraints on bulk, interface, and contact properties are quantified and discussed

14 SOLAR ENERGY↗
DOE OSTI2024

Synthesis of 2D layered transition metal (Ni, Co) hydroxides via edge-on condensation

Abstract Layered transition metal hydroxides (LTMHs) with transition metal centers sandwiched between layers of coordinating hydroxide anions have attracted considerable interest for their potential in developing clean energy sources and storage technologies. However, two-dimensional (2D) LTMHs remain largely understudied in terms of physical properties and applications in electronic devices. Here, for the first time we report > 20 μm α-Ni(OH) 2 2D crystals, synthesized from hydrothermal reaction. And an edge-on condensation mechanism assisted with the crystal field geometry is proposed to understand the 2D intra-planar growth of the crystals, which is also testified through series of systematic comparative studies. We also report the successful synthesis of 2D Co(OH) 2 crystals (> 40 μm) with more irregular shape due to the slightly distorted octahedral geometry of the crystal field. Moreover, the detailed structural characterization of synthesized α-Ni(OH) 2 are performed. The optical band gap energy is extrapolated as 2.54 eV from optical absorption measurements and the electronic bandgap is measured as 2.52 eV from reflected electrons energy loss spectroscopy (REELS). We further demonstrate its potential as a wide bandgap (WBG) semiconductor for high voltage operation in 2D electronics with a high breakdown strength, 4.77 MV/cm with 4.9 nm thickness. The successful realization of the 2D LTMHs opens the door for future exploration of more fundamental physical properties and device applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1983

Recovery of damage in rad-hard MOS devices during and after irradiation by electrons, protons, alphas, and gamma rays

This paper reports on the recovery properties of rad-hard MOS devices during and after irradiation by electrons, protons, alphas, and gamma rays. The results indicated that complex recovery properties controlled the damage sensitivities of the tested parts. The results also indicated that damage sensitivities depended on dose rate, total dose, supply bias, gate bias, transistor type, radiation source, and particle energy. The complex nature of these dependencies make interpretation of LSI device performance in space (exposure to entire electron and proton spectra) difficult, if not impossible, without respective ground tests and analyses. Complete recovery of n-channel shifts was observed, in some cases within hours after irradiation, with equilibrium values of threshold voltages greater than their pre-irradiation values. This effect depended on total dose, radiation source, and gate bias during exposure. In contrast, the p-channel shifts recovered only 20 percent within 30 days after irradiation.

Brucker, G. J.↗
NASA NTRS1985

Crystal Growth of Device Quality Gaas in Space

The GaAs research evolves about these key thrust areas. The overall program combines: (1) studies of crystal growth on novel approaches to engineering of semiconductor material (i.e., GaAs and related compounds); (2) investigation and correlation of materials properties and electronic characteristics on a macro- and microscale; and (3) investigation of electronic properties and phenomena controlling device applications and device performance. This effort is aimed at the essential ground-based program which would insure successful experimentation with and eventually processing of GaAs in near zero gravity environment. It is believed that this program addresses in a unique way materials engineering aspects which bear directly on the future exploitation of the potential of GaAs and related materials in device and systems applications.

Gatos, H. C.↗
DOE OSTI2022

Correlation-driven electron-hole asymmetry in graphene field effect devices

Electron-hole asymmetry is a fundamental property in solids that can determine the nature of quantum phase transitions and the regime of operation for devices. The observation of electron-hole asymmetry in graphene and recently in twisted graphene and moiré heterostructures has spurred interest into whether it stems from single-particle effects or from correlations, which are core to the emergence of intriguing phases in moiré systems. Here, we report an effective way to access electron-hole asymmetry in 2D materials by directly measuring the quasiparticle self-energy in graphene/Boron Nitride field-effect devices. As the chemical potential moves from the hole to the electron-doped side, we see an increased strength of electronic correlations manifested by an increase in the band velocity and inverse quasiparticle lifetime. These results suggest that electronic correlations intrinsically drive the electron-hole asymmetry in graphene and by leveraging this asymmetry can provide alternative avenues to generate exotic phases in twisted moiré heterostructures.

36 MATERIALS SCIENCE↗
DOE OSTI2021

In Operando Angle‐Resolved Photoemission Spectroscopy with Nanoscale Spatial Resolution: Spatial Mapping of the Electronic Structure of Twisted Bilayer Graphene

To pinpoint the electronic and structural mechanisms that affect intrinsic and extrinsic performance limits of 2D material devices, it is of critical importance to resolve the electronic properties on the mesoscopic length scale of such devices under operating conditions. Herein, angle‐resolved photoemission spectroscopy with nanoscale spatial resolution (nanoARPES) is used to map the quasiparticle electronic structure of a twisted bilayer graphene device. The dispersion and linewidth of the Dirac cones associated with top and bottom graphene layers are determined as a function of spatial position on the device under both static and operating conditions. The analysis reveals that microscopic rotational domains in the two graphene layers establish a range of twist angles from 9.8° to 12.7°. Application of current and electrostatic gating lead to strong electric fields with peak strengths of 0.75 V/μm at the rotational domain boundaries in the device. These proof‐of‐principle results demonstrate the potential of nanoARPES to link mesoscale structural variations with electronic states in operating device conditions and to disentangle such extrinsic factors from the intrinsic quasiparticle dispersion.

Majchrzak, Paulina↗
NASA NTRS1993

Micro-opto-mechanical devices and systems using epitaxial lift off

The integration of high quality, single crystal thin film gallium arsenide (GaAs) and indium phosphide (InP) based photonic and electronic materials and devices with host microstructures fabricated from materials such as silicon (Si), glass, and polymers will enable the fabrication of the next generation of micro-opto-mechanical systems (MOMS) and optoelectronic integrated circuits. Thin film semiconductor devices deposited onto arbitrary host substrates and structures create hybrid (more than one material) near-monolithic integrated systems which can be interconnected electrically using standard inexpensive microfabrication techniques such as vacuum metallization and photolithography. These integrated systems take advantage of the optical and electronic properties of compound semiconductor devices while still using host substrate materials such as silicon, polysilicon, glass and polymers in the microstructures. This type of materials optimization for specific tasks creates higher performance systems than those systems which must use trade-offs in device performance to integrate all of the function in a single material system. The low weight of these thin film devices also makes them attractive for integration with micromechanical devices which may have difficulty supporting and translating the full weight of a standard device. These thin film devices and integrated systems will be attractive for applications, however, only when the development of low cost, high yield fabrication and integration techniques makes their use economically feasible. In this paper, we discuss methods for alignment, selective deposition, and interconnection of thin film epitaxial GaAs and InP based devices onto host substrates and host microstructures.

Camperi-Ginestet, C.↗
NASA NTRS1984

Crystal growth of device quality GaAs in space

The crystal growth, device processing and device related properties and phenomena of GaAs are investigated. Our GaAs research evolves about these key thrust areas. The overall program combines: (1) studies of crystal growth on novel approaches to engineering of semiconductor materials (i.e., GaAs and related compounds); (2) investigation and correlation of materials properties and electronic characteristics on a macro- and microscale; (3) investigation of electronic properties and phenomena controlling device applications and device performance. The ground based program is developed which would insure successful experimentation with and eventually processing of GaAs in a near zero gravity environment.

Gatos, H. C.↗
DOE OSTI2021

Spin splitting of dopant edge state in magnetic zigzag graphene nanoribbons

Spin-ordered electronic states in hydrogen-terminated zigzag nanographene give rise to magnetic quantum phenomena that have sparked renewed interest in carbon-based spintronics. Zigzag graphene nanoribbons (ZGNRs)-quasi one-dimensional semiconducting strips of graphene bounded by parallel zigzag edges-host intrinsic electronic edge states that are ferromagnetically ordered along the edges of the ribbon and antiferromagnetically coupled across its width. Despite recent advances in the bottom-up synthesis of GNRs featuring symmetry protected topological phases and even metallic zero mode bands, the unique magnetic edge structure of ZGNRs has long been obscured from direct observation by a strong hybridization of the zigzag edge states with the surface states of the underlying support. Here, we present a general technique to thermodynamically stabilize and electronically decouple the highly reactive spin-polarized edge states by introducing a superlattice of substitutional N-atom dopants along the edges of a ZGNR. First-principles GW calculations and scanning tunnelling spectroscopy reveal a giant spin splitting of low-lying nitrogen lone-pair flat bands by an exchange field (~850 tesla) induced by the ferromagnetically ordered edge states of ZGNRs. Finally, our findings directly corroborate the nature of the predicted emergent magnetic order in ZGNRs and provide a robust platform for their exploration and functional integration into nanoscale sensing and logic devices.

36 MATERIALS SCIENCE↗