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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 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Mn(II) oxidation catalyzed by nanohematite surfaces and manganite/hausmannite core-shell nanowire formation by self-catalytic reaction

The present study investigated the solid products of heterogeneous catalytic oxidation of aqueous Mn(II) by O2 in the presence of hematite nanoparticles (NPs) using transmission electron microscopy (TEM), scanning TEM (STEM), and electron energy-loss spectroscopy (EELS). The oxidation experiments were conducted at room temperature (22±2?) in solutions containing 10-3 M Mn(II) at pH 7.5 in the dark. During 48 hours of reaction, a single-phase manganite (a Mn oxyhydroxide) nanowire was formed by way of metastable groutite and feitknechtite nanowires, both of which are polymorphs of manganite. Between 48 and 168 hours, the manganite further altered to core-shell structured nanowires with hausmannite (a mixed valent Mn oxide) forming a very thin (2 nm) outer-shell on the manganite wire core. The formation of Mn-oxyhydroxide nanowires was catalyzed by the hematite NPs surface through electrochemical pathways. This is described via the electron transfer across a redox couple from adsorbed Mn(II) to another site with adsorbed O2 via the band structure of the semiconducting hematite. On the other hand, the Ostwald step rule was operative in the sequential crystallization from metastable groutite and feitknechtite to stable manganite, which could have resulted from the differences in the surface free energies, sizes, and morphologies of product polymorphs at the nanoscale. The final product (at least in the time range of our experiments) exhibited manganite/hausmannite core-shell structures. This product is most likely to be formed by the reactions catalyzed by the manganite nanowires themselves. Microscopic investigation of interfacial changes in structure and chemistry at the nanoscale is key, and it can be essential to understand the kinetic and thermodynamic phenomena related to the redox chemistry and phase stability of nanoparticles in complex, heterogeneous systems such as natural environments.

Inoue, Sayako↗
DOE OSTI2020

Morphology-controlled transformation of Cu@Au core-shell nanowires into thermally stable Cu 3 Au intermetallic nanowires

Multimetallic nanowires with long-range atomic ordering hold the promise of unique physicochemical properties in many applications. Here we demonstrate the synthesis and study the stability of Cu 3 Au intermetallic nanowires. The synthesis is achieved by using Cu@Au core-shell nanowires as precursors. With appropriate Cu/Au stoichiometry, the Cu@Au core-shell nanowires are transformed into fully ordered Cu 3 Au nanowires under thermal annealing. Thermally-driven atom diffusion accounts for this transformation as revealed by X-ray diffraction and electron microscopy studies. The twin boundaries abundant in the Cu@Au core-shell nanowires facilitate the ordering process. The resulting Cu 3 Au intermetallic nanowires have uniform and accurate atomic positioning in the crystal lattice, which enhances the nobility of Cu. No obvious copper oxides are observed in fully ordered Cu 3 Au nanowires after annealing in air at 200 °C, a temperature that is much higher than those observed in Cu@Au core-shell and pure Cu nanowires. This work opens up an opportunity for further research into the development and applications of intermetallic nanowires.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

In Situ Transmission Electron Microscopy of Oxide Shell-Induced Pore Formation in (De)lithiated Silicon Nanowires

Silicon (Si) nanowires with a silicon oxide (SiOx) shell were observed in situ during lithiation and delithiation using transmission electron microscopy (TEM). The oxide shell constrains the volume expansion, which prevents full lithiation of the nanowire and retention of a crystalline Si core. Pores form in the amorphized silicon upon the first delithiation. We propose that this is caused by a high barrier to vacancy migration in the SiOx shell, which prevents vacancies formed during delithiation from transporting through the shell to the nanowire surface. These in situ studies demonstrate the importance of the direct observation of structural changes that occur in a material during cycling upon the addition of a coating.

Adkins, Emily R.↗
DOE OSTI2019

In situ Characterization of Kinetics and Mass Transport of PbSe Nanowire Growth via LS and VLS Mechanisms

As a versatile approach, the vapor-liquid-solid (VLS) mechanism is widely used to grow nanowires. However, the details of the mass transit and mechanism of compound nanowire growth have been limited because of the complex growth process. In this study, we grew binary PbSe nanowires using an in situ gas-heating cell and elucidated two different growth mechanisms and mass transit pathways of different growth species, as well as their controlling factors, using in situ high-resolution transmission electron microscopy and composition analysis. Initial high supersaturation led to poly-nucleation and fast PbSe crystal growth through bulk diffusion of Pb and Se growth species from liquid to solid—a liquid-solid (LS) mechanism. In situ composition analysis showed that Pb dissolved in liquid catalyst at miscible atomic percentages as a precursor during growth. Se precursor was depleted after initial growth and was supplied from the vapor phase, leading to subsequently relatively slow VLS controlled mono-nuclear growth. We revealed the correlation between the nucleation mechanisms and the growth kinetics using a generalized Avrami model. Our study highlighted the key correlations among supersaturation, nucleation, mass transit, and growth kinetics, thereby enabling morphological and compositional control of nanowires.

Song, Miao↗
DOE OSTI2020

Effect of Anisotropic Confinement on Electronic Structure and Dynamics of Band Edge Excitons in Inorganic Perovskite Nanowires

Inorganic lead halide perovskite nanostructures show promise as the active layers in photovoltaics, light emitting diodes, and other optoelectronic devices. They are robust in the presence of oxygen and water, and the electronic structure and dynamics of these nanostructures can be tuned through quantum confinement. Here we create aligned bundles of CsPbBr3 nanowires with widths resulting in quantum confinement of the electronic wave functions and subject them to ultrafast microscopy. We directly image rapid one-dimensional exciton diffusion along the nanowires, and we measure an exciton trap density of roughly one per nanowire. Using transient absorption microscopy, we observe a polarization-dependent splitting of the band edge exciton line, and from the polarized fluorescence of nanowires in solution, we determine that the exciton transition dipole moments are anisotropic in strength. Furthermore, our observations are consistent with a model in which splitting is driven by shape anisotropy in conjunction with long-range exchange.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Textural properties determined CO 2 capture of tetraethylenepentamine loaded SiO 2 nanowires from α-sepiolite

To understand the effect of porous parameters and chemical components on CO 2 capturing performance of solid state amine absorbents and their kinetic parameters, a clay mineral-based CO 2 absorbent was prepared by impregnating five different liquid amines onto SiO 2 nanowire clusters from fibrous sepiolite (Sep). Characterization was conducted by XRD, SEM, FTIR, and N 2 adsorption–desorption isotherm, CO 2 adsorption performances were measured by TGA. After acid treatment, the raw Sep mineral changed into amorphous silica nanowires, and the largest S BET of the obtained SiO 2 nanowires reached 320 m 2 /g, which was 8 times larger than the pristine mineral. Amongst the adapted amines, tetraethylenepentamine (TEPA) showed superiority in the CO 2 adsorption capacity. The optimized adsorption capacity reached 3.7 mmol/g at 75 °C from a CO 2 + N 2 mixture on a ~50 wt% TEPA loaded SiO 2 sample, and the capacity remained 3.6 mmol/g after ten circles of adsorption/desorption tests. The pore size and pore structure of the matrix, rather than the surface area, will obviously affect the adsorption ability of the product. Kinetic parameters for the CO 2 adsorption process fitted well with a fractional-order model, confirming that the CO 2 adsorption process by the solid absorbent should be a combination of both physical and chemical reactions. Results in this article provide detailed parameters for preparing and understanding these clay mineral based solid state CO 2 absorbents.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Sub-diffractional waveguiding by mid-infrared plasmonic resonators in semiconductor nanowires

Chains of nanoscale plasmonic resonators are capable of sub-diffractional waveguiding and have applications in nanophotonics and thermal radiation transport. Practical uses have largely been limited, however, due to high optical losses or low group velocities. In this work, we predict the waveguide performance of a material structure capable of overcoming these limitations: plasmonic resonators embedded in highdielectric nanowires. Due to the enhanced near-field coupling between resonators, we find that the group velocities and propagation lengths for doped Si plasmonic resonators in intrinsic Si nanowires can be increased by up to an order of magnitude compared to the case of isotropic vacuum surroundings. We investigate the impact of resonator aspect ratio, doping, and spacing on waveguide performance, and we find that propagation lengths are maximized for large aspect ratios and high dopant concentrations at small spacings. To study these complex anisotropic systems, we develop a new analytical “absorption spectra” method to extract waveguide information from simple far-field absorption experiments (or simulations) of only two coupled resonators.

42 ENGINEERING↗
DOE OSTI2018

Isotopic Heft on the $B_{1l}$ Silent Mode in Ultra-Narrow Gallium Nitride Nanowires

Wurtzite semiconductor compounds have two silent modes, $B_{1l}$ and $B_{lh}$. A silent mode is a vibrational mode that carries neither a dipole moment nor Raman polarizability. Thus, they are forbidden in both infrared reflectivity and Raman spectroscopy. Astonishingly, we detected the $B_{1l}$ mode in high-quality, ultra-narrow GaN nanowires using resonant Raman scattering, although the $B_{lh}$ was not observed, and there is no immediate explanation for this asymmetric finding. In this work, the Raman experiments were performed using several laser lines from 647 to 325 nm; the latter is a wavelength in which Raman becomes resonant. Actually, we observed the $B_{1l}$ mode only in resonance, indicating that the appearance of this mode is related to Frohlich electron-phonon interactions; i.e., a dipole moment emerging in the $B_{1l}$ silent mode may not be present in the $B_{lh}$ mode. To shed light onto the physical origin of these observations, we performed density functional theory calculations of the lattice dynamics in GaN. We performed a careful analysis of the different physical mechanisms that allow the forbidden mode to appear to explain the physics underlying the nonzero dipole moment in the $B_{1l}$ mode, and the reason why this dipole moment is not present in the $B_{1h}$ mode.

crystalline perfection↗
DOE OSTI2018

Two-dimensional PdSe 2 -Pd 2 Se 3 junctions can serve as nanowires

While the exfoliation of almost all layered materials results in a monolayer with the same atomic geometry as its bulk counterpart, the exfoliation of PdSe 2 results in a monolayer with a different atomic geometry and a new stoichiometry, Pd 2 Se 3 , which is a fusion of two PdSe 2 monolayers mediated by Se emission. Here we first report first-principles calculations of lateral junctions between a PdSe 2 bilayer and a Pd 2 Se 3 monolayer. In this work, we find that, while several distinct junction geometries are possible, they all exhibit empty interface states below the conduction band. As a result, light n-type doping of either or both sides, e.g. by halogen atoms replacing Se atoms, leads to a remotely-doped interface, i.e. a 1D conducting nanowire that runs along the junction, in between the two semiconductors. We have fabricated such junctions inside a scanning transmission electron microscope (STEM), but doping and transport measurements are not currently practical.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Structural color from a coupled nanowire pair beyond the bonding and antibonding model

Optical resonances in nanostructures can be harnessed to produce a wide range of structural colors. Conversely, the analysis of structural colors has been used to clarify the nature of optical resonances. Here, we show that silicon nanowire (NW) pairs can display a wide range of structural colors by controlling their radiative coupling. This is accomplished by exciting a series of Fabry–Pérot-like modes where light is repeatedly scattered between two NWs. These modes are beyond the expectation from the conventional chemical bonding model under a quasi-electrostatic approximation, in which only bonding and antibonding modes can be formed in a pair system through modal hybridization. The additional eigenmodes found in a two-resonator system originate from the nonlinear, frequency-dependent coupling strength derived from the radiative nature of low- Q resonators. The Fabry–Pérot modes can be tuned across the entire visible frequency range by varying the distance between two NWs, leading to what we believe is a new type of universal building blocks that can provide structural color within a subwavelength footprint. The presented results pave the way toward the design and usage of highly tunable resonances that exploit the radiative coupling of high-index nanostructures.

Li, Qitong↗