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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 433 records · Page 24
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Self‐Assembled NiO Nanocrystal Arrays as Memristive Elements

Abstract Memristive switching, a nonlinear current–voltage ( I – V ) characteristic, has seen a tremendous surge in interest as an approach to achieve implementation of synaptic functions. The memristive switching behavior of self‐assembled NiO nanocrystals is investigated via scanning probe microscopy, based on first‐order reversal curve current–voltage spectroscopy. Synaptic switching is clearly observed as a direct consequence of filament growth (i.e., gradually increased conductance) in the nanocrystals. A spatial dependency of the conduction in the nanocrystals suggests that there is a localization of the switching filament. The current understanding of this localization ignores features related to local lateral variation current, which can generate an excessive local heat and temperature such as electrical dissipation. The observation of low electrical dissipation at the edge of the nanocrystals shows that less energy is wasted as heat such that the bias applied can be utilized more efficiently to assist the nucleation of the filament and thus reduces the power consumption. Electrical power dissipation is also found to scale with nanocrystal height and has spatial dependence within the nanocrystals. The combination of synaptic switching and high density of the nanocrystals demonstrate that it is feasible to exploit them to create a basic architecture for neuromorphic memory devices.

Kurnia, Fran↗
DOE OSTI2014

Designed Single-Step Synthesis, Structure, and Derivative Textural Properties of Well-Ordered Layered Penta-coordinate Silicon Alcoholate Complexes

The controllable synthesis of well-ordered layered materials with specific nanoarchitecture poses a grand challenge in materials chemistry. Here the solvothermal synthesis of two structurally analogous 5-coordinate organosilicate complexes through a novel transesterification mechanism is reported. Since the polycrystalline nature of the intrinsic hypervalent Si complex thwarts the endeavor in determining its structure, a novel strategy concerning the elegant addition of a small fraction of B species as an effective crystal growth mediator and a sacrificial agent is proposed to directly prepare diffraction-quality single crystals without disrupting the intrinsic elemental type. In the determined crystal structure, two monomeric primary building units (PBUs) self-assemble into a dimeric asymmetric secondary BU via strong Na+O2- ionic bonds. The designed one-pot synthesis is straightforward, robust, and efficient, leading to a well-ordered (10)-parallel layered Si complex with its principal interlayers intercalated with extensive van der Waals gaps in spite of the presence of substantial Na+ counter-ions as a result of unique atomic arrangement in its structure. However, upon fast pyrolysis, followed by acid leaching, both complexes are converted into two SiO2 composites bearing BET surface areas of 163.3 and 254.7m(2)g(-1) for the pyrolyzed intrinsic and B-assisted Si complexes, respectively. The transesterification methodology merely involving alcoholysis but without any hydrolysis side reaction is designed to have generalized applicability for use in synthesizing new layered metal-organic compounds with tailored PBUs and corresponding metal oxide particles with hierarchical porosity.

Li, XS↗
NASA NTRS2009

Very Long Single and Few-Walled Boron Nitride Nanotubes via the Pressurized Vapor/Condenser Method

A new method for producing long, small diameter, single and few-walled, boron nitride nanotubes (BNNTs) in macroscopic quantities is reported. The pressurized vapor/condenser (PVC) method produces, without catalysts, highly crystalline, very long, small diameter, BNNTs. Palm-sized, cotton-like masses of BNNT raw material were grown by this technique and spun directly into centimeters-long yarn. Nanotube lengths were observed to be 100 times that of those grown by the most closely related method. Self-assembly and growth models for these long BNNTs are discussed.

Smith, Michael W.↗
DOE OSTI2019

Supramolecular Tessellations by a Rigid Naphthalene Diimide Triangle

Tessellation of organic polygons though [π···π] and charge-transfer (CT) interactions offers a unique opportunity to construct supramolecular organic electronic materials with 2D topologies. Our approach to exploring the 3D topology of 2D tessellations of a naphthalene diimide-based molecular triangle (NDI-Δ) reveals that the 2D molecular arrangement is sensitive to the identity of the solvent and solute concentrations. Utilization of nonhalogenated solvents, combined with careful tailoring of the concentrations, results in NDI-Δ self-assembling though [π···π] interactions into 2D honeycomb triangular and hexagonal tiling patterns. Cocrystallization of NDI-Δ with tetrathiafulvalene (TTF) leads systematically to the formation of 2D tessellations as a result of superstructure-directing CT interactions. Different solvents lead to different packing arrangements. Using MeCN, CHCl 3 , and CH 2 Cl 2 , we identified three sets of cocrystals, namely CT-A, CT-B, and CT-C, respectively. Here, solvent modulation plays a critical role in controlling not only the NDI-Δ:TTF stoichiometric ratios and the molecular arrangements in the crystal superstructures, but also prevents the inclusion of TTF guests inside the cavities of NDI-Δ. Confinement of TTF inside the NDI-Δ cavities in the CT-A superstructure enhances the CT character with the observation of a broad absorption band in the NIR region. In the CT-B superstructure, the CHCl 3 lattice molecules establish a set of [Cl···Cl] and [Cl···S] intermolecular interactions, leading to the formation of a hexagonal grid of solvent in which NDI-Δ forms a triangular grid. In the CT-C superstructure, three TTF molecules self-assemble, forming a supramolecular isosceles triangle TTF-Δ, which tiles in a plane alongside the NDI-Δ, producing a 3 + 3 honeycomb tiling pattern of the two different polygons. Solid-state spectroscopic investigations on CT-C revealed the existence of an absorption band at 2500 nm, which on the basis of TDDFT calculations, was attributed to the mixed-valence character between two TTF • + radical cations and one neutral TTF molecule.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Photocatalytic oxidation of the organic monolayers on TiO{sub 2} surface investigated by in-situ sum frequency generation spectroscopy

In-situ vibrational sum frequency generation (SFG) spectroscopy has been employed to investigate the photocatalytic oxidation of two types of well-ordered organic monolayers, namely, an arachidic acid (AA) monolayer prepared by the Langmuir-Blodgett method and an octadecyltrichlorosilane (OTS) monolayer prepared by the self-assembling method, on a TiO{sub 2} surface under ultraviolet (UV) irradiation. The extremely high sensitivity and unique selectivity of the SFG spectroscopy enabled us to directly probe the structural changes in these monolayers during the surface photocatalytic oxidation and further elucidate their reaction mechanisms at a molecular level. It was revealed that the ordering of the monolayers during the photocatalytic reaction is strongly dependent on their interaction with the substrate; the AA monolayer maintains its ordered conformation until the final oxidation stage, while the OTS monolayer shows a large increase in disordering during the initial oxidation stage, indicating a different photocatalytic reaction mechanism of the two monolayers on the TiO{sub 2} surface.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Dynamic control of polarization-inverted modes in three-dimensionally trapped multiple nanogaps

We propose a guiding principle for the dynamic control of polarization-inverted modes in multiple nanogaps for unconventional optical transitions of molecules at arbitrary three-dimensional spatial positions. Based on our developed self-consistent theory for the optical assembly of nanoparticles (NPs), we clarified that spherical silver NPs can be optically trapped and aligned in the light-propagating direction via longitudinally polarized light; they form a rod-like nano-composite with multiple nanogaps. During trapping, there is a possibility that an additional irradiation of linearly polarized far-field light may excite the bonding and anti-bonding dark plasmon modes with low radiative decay rate of several meV via cancellation of inverted polarization. Our finding reveals that not only the steep change in the enhanced intensity of light field but also the phase inversion of light field between the dynamically formed nanogaps will pave the way to the highly sensitive sensors for molecules, the unconventional chemical reactions, and so on.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Gelation of plasmonic metal oxide nanocrystals by polymer-induced depletion attractions

Significance Self-supported gelation of optically active nanocrystals offers a modular pathway to harness optoelectronic functionality in multiscale materials by directly controlling volume fraction, bonding, and structure during assembly. We combine depletion attractions that emerge from the incorporation of small polymer chains and electrostatic repulsions to induce the gelation of isotropic metal oxide nanocrystals. We develop a theoretical model to assess our experimental fluid-to-gel-phase progression observations. By preventing nanocrystal fusion during network assembly, we achieve gels with a strong near-infrared absorption, reminiscent of the inherent near-infrared localized surface plasmon resonance of the nanocrystal building blocks.

Saez Cabezas, Camila A.↗
DOE OSTI2013

A molecular design principle of lyotropic liquid-crystalline conjugated polymers with directed alignment capability for plastic electronics

Conjugated polymers with a one-dimensional p-orbital overlap exhibit optoelectronic anisotropy. Their unique anisotropic properties can be fully realized in device applications only when the conjugated chains are aligned. Here, we report a molecular design principle of conjugated polymers to achieve concentration-regulated chain planarization, self-assembly, liquid-crystal-like good mobility and non-interdigitated side chains. As a consequence of these intra- and intermolecular attributes, chain alignment along an applied flow field occurs. This liquid-crystalline conjugated polymer was realized by incorporating intramolecular sulphur–fluorine interactions and bulky side chains linked to a tetrahedral carbon having a large form factor. By optimizing the polymer concentration and the flow field, we could achieve a high dichroic ratio of 16.67 in emission from conducting conjugated polymer films. Two-dimensional grazing-incidence X-ray diffraction was performed to analyse a well-defined conjugated polymer alignment. Thin-film transistors built on highly aligned conjugated polymer films showed more than three orders of magnitude faster carrier mobility along the conjugated polymer alignment direction than the perpendicular direction.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Natural selection in the colloid world: active chiral spirals

We present a model system in which to study natural selection in the colloid world. In the assembly of active Janus particles into rotating pinwheels when mixed with trace amounts of homogeneous colloids in the presence of an AC electric field, broken symmetry in the rotation direction produces spiral, chiral shapes. Locked into a central rotation point by the centre particle, the spiral arms are found to trail rotation of the overall cluster. To achieve a steady state, the spiral arms undergo an evolutionary process to coordinate their motion. Because all the particles as segments of the pinwheel arms are self-propelled, asymmetric arm lengths are tolerated. Reconfiguration of these structures can happen in various ways and various mechanisms of this directed structural change are analyzed in detail. Here, we introduce the concept of VIP (very important particles) to express that sustainability of active structures is most sensitive to only a few particles at strategic locations in the moving self-assembled structures.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Self-similar mesocrystals form via interface-driven nucleation and assembly

Crystallization by particle attachment (CPA) is a frequently occurring mechanism of colloidal crystallization that results in hierarchical morphologies. CPA has been exploited to create nanomaterials with unusual properties and is implicated in the development of complex mineral textures. Oriented attachment–a form of CPA in which particles align along specific crystallographic directions–produces mesocrystals that diffract as single crystals do, although the constituent particles are still discernible. The conventional view of CPA is that nucleation provides a supply of particles that aggregate via Brownian motion biased by attractive interparticle potentials. However, mesocrystals often exhibit regular morphologies and uniform sizes. Although many crystal systems form mesocrystals and individual attachment events have been directly visualized10, how random attachment events lead to well defined, self-similar morphologies remains unknown, as does the role of surface-bound ligands, which are ubiquitous in nanoparticle systems. Attempts to understand mesocrystal formation are further complicated in many systems by the presence of precursor nanoparticles with a phase distinct from that of the bulk. Some studies propose that such particles convert before attachment, whereas others attribute conversion to the attachment process itself and yet others conclude that transformation occurs after the mesocrystals exceed a characteristic size. Here we investigate mesocrystal formation by iron oxides, which are important colloidal phases in natural environments and classic examples of systems forming ubiquitous precursor phases and undergoing CPA accompanied by phase transformations. Combining in situ transmission electron microscopy (TEM) at 80 degrees Celsius with ‘freeze-and-look’ TEM, we tracked the formation of haematite (Hm) mesocrystals in the presence of oxalate (Ox), which is abundant in soils, where iron oxides are common. In this work, we find that isolated Hm particles rarely appear, but once formed, interfacial gradients at the Ox-covered surfaces drive Hm particles to nucleate repeatedly about two nanometres from the surfaces, to which they then attach, thereby generating mesocrystals. Comparison to natural and synthetic systems suggests that interface-driven pathways are widespread.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Cellular Delivery of Nanoparticles Revealed with Combined Optical and Isotopic Nanoscopy

Synthetic drug-carrying nanomaterials offer great potential as targeted cellular delivery vehicles. Typically, their size, morphology, surface chemistry and stability are optimized in order to control their effect on drug release kinetics, cellular uptake pathways, efficiency and site of action. However, methods to track the carriers and their cargo independently at the micro- and nanoscale have been severely underutilized preventing the correlation between structure and function. Here we show that by using combined optical and isotopic nanoscopy we can track the uptake in cancer cells and subsequent drug release of a Pt(II)-loaded anticancer nanoparticle (NP) system. We found that by directly polymerizing an oxaliplatin analogue containing a norbornyl moiety amenable to polymerization via ring opening metathesis polymerization (ROMP) we could generate amphiphiles in one pot. Spontaneous self-assembly of the drug-containing polymers in aqueous solution led to well-defined NPs in a reproducible manner. Our results demonstrate that the covalently loaded NPs are equipotent with free oxaliplatin and are taken up intact via endocytic pathways before release of the cytotoxic cargo. This was confirmed by super resolution fluorescence structured illumination microscopy (SIM) and nanoscale secondary ion mass spectrometry (NanoSIMS). We anticipate that this type of multimodal cellular tracking of NP and drug will bridge the knowledge gap between particle structure and performance for the vast array of currently generalizable systems in the literature. Furthermore, the use of covalently loaded NP drug systems should allow development of more stable, reproducible and site specific nanodelivery agents.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2020

Rate Enhancement of Acid-Catalyzed Alcohol Dehydration by Supramolecular Organic Capsules

The presence of a (sub)nanometric environment has been demonstrated to enhance the rate of homogeneous acid-catalyzed reactions. Conventionally, such catalysts are constructed by directly embedding a catalytically active acid site into the structure of a porous material, linking the acid properties with the properties of the embedding solid. Here, a holoenzyme mimicking approach was used, in which a self-assembled hexameric resorcinarene capsule (1.4 nm 3 cavity) acts like an apoenzyme that transforms into a catalytically active site by hosting hydrated HCl as a cofactor. This capsule binds the hydronium ion together with cyclohexanol (CyOH) and catalyzes the dehydration to cyclohexene with enhanced rates that are 2 orders of magnitude higher than those in the unconstrained environment. Kinetic analysis shows that cyclohexanol dehydration proceeds via an E2 mechanism on hydrated HCl in the unconstrained environment, while the constrained environment of the active capsule induces the stabilization of an ionic intermediate, allowing an E1 mechanism to dominate. By comparison with the reaction in zeolite environments, we show that the direct chemical environment of the capsule or a zeolite wall exerts only a minor direct influence. The volume constraint increases the reaction rate by inducing a larger activation entropy, indicating the stabilization of a later transition state in the E1 mechanism, as well as a higher reaction space of the carbenium ion constituting the transition state.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Colloidal self assembly of non-magnetic particles in magnetic nanofluid

Here we present a technique using magnetic nanofluid to induce bidispersed suspension of nonmagnetic particles to assemble into colloidal chain, triangle, rectangle, ring-flower configurations. By changing the amplitude and direction of the magnetic field, we could tune the structure of nonmagnetic particles in magnetic nanofluid. The structures are assembled using magneto static interactions between effectively nonmagnetic particles dispersed in magnetizable magnetic nanofluid. The assembly of complex structures out of simple colloidal building blocks is of practical interest in photonic crystals and DNA biosensors.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Modeling the self-assembly of ordered nanoporous materials

This report describes progress on a collaborative project on the multiscale modeling of the assembly processes in the synthesis of nanoporous materials. Such materials are of enormous importance in modern technology with application in the chemical process industries, biomedicine and biotechnology as well as microelectronics. The project focuses on two important classes of materials: i) microporous crystalline materials, such as zeolites, and ii) ordered mesoporous materials. In the first case the pores are part of the crystalline structure, while in the second the structures are amorphous on the atomistic length scale but where surfactant templating gives rise to order on the length scale of 2 - 20 nm. We have developed a modeling framework that encompasses both these kinds of materials. Our models focus on the assembly of corner sharing silica tetrahedra in the presence of structure directing agents. We emphasize a balance between sufficient realism in the models and computational tractibility given the complex many-body phenomena. We use both on-lattice and off-lattice models and the primary computational tools are Monte Carlo simulations with sampling techniques and ensembles appropriate to specific situations. Our modeling approach is the first to capture silica polymerization, nanopore crystallization, and mesopore formation through computer-simulated self assembly.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Phase segregation of a composite air electrode unlocks the high performance of reversible protonic ceramic electrochemical cells

One breakthrough in developing highly efficient air electrodes for reversible protonic ceramic electrochemical cells (R-PCECs) is optimizing the sluggish oxygen reduction and water oxidation reactions. Here, we present a novel composite material with a nominal formula of high-entropy Ce 0.2 Ba 0.2 Sr 0.2 La 0.2 Ca 0.2 CoO 3–δ (CBSLCC) that spontaneously self-assembles to three-phase electrocatalysts composed of deficient Ce 0.2–y Ba 0.2 Sr 0.2–x La 0.2–x Ca 0.2 CoO 3–δ (CD-CBSLCC), CeO 2 , and La 0.5 Sr 0.5 CoO 3–δ (LSC). Mechanistic studies corroborate that oxygen reduction may occur on entire air electrode surfaces, followed by water formation preferentially at or near CD-CBSLCC. The CeO 2 phase could provide or consume protons to facilitate the oxygen evolution/reduction kinetics in R-PCECs. The developed electrodes demonstrate a record-high electrochemical performance in dual modes of fuel cells and electrolysis cells, delivering a peak power density of 1.66 W cm –2 at 600 °C and a current density of –1.76 A cm –2 at 1.3 V and 600 °C. Finally, excellent operational stabilities of the fuel cell (200 h at 600 °C), electrolysis cell (200 h at 600 °C), and reversible cycling (548 h at 550 °C) provide a promising and reliable step towards realizing the commercialization of R-PCECs.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2018

Supramolecular Assemblies on Surfaces: Nanopatterning, Functionality, and Reactivity

Understanding how molecules interact to form large-scale hierarchical structures on surfaces holds promise for building designer nanoscale constructs with defined chemical and physical properties. Here, we describe early advances in this field and highlight upcoming opportunities and challenges. Both direct intermolecular interactions and those that are mediated by coordinated metal centers or substrates are discussed. These interactions can be additive, but they can also interfere with each other, leading to new assemblies in which electrical potentials vary at distances much larger than those of typical chemical interactions. Earlier spectroscopic and surface measurements have provided partial information on such interfacial effects. In the interim, scanning probe microscopies have assumed defining roles in the field of molecular organization on surfaces, delivering deeper understanding of interactions, structures, and local potentials. Furthermore, self-assembly is a key strategy to form extended structures on surfaces, advancing nanolithography into the chemical dimension and providing simultaneous control at multiple scales. In parallel, the emergence of graphene and the resulting impetus to explore 2D materials have broadened the field, as surface-confined reactions of molecular building blocks provide access to such materials as 2D polymers and graphene nanoribbons. In this Review, we describe recent advances and point out promising directions that will lead to even greater and more robust capabilities to exploit designer surfaces.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Super-Joule heating in graphene and silver nanowire network

Transistors, sensors, and transparent conductors based on randomly assembled nanowire networks lean on multi-component percolation for unique and distinctive applications in flexible electronics, biochemical sensing, and solar cells. Although conduction models for 1-D and 1-D/2-D networks have been developed, typically assuming linear electronic transport and self-heating, the model has not been validated by direct high-resolution characterization of coupled electronic pathways and thermal response. In this letter, we show the occurrence of nonlinear “super-Joule” self-heating at the transport bottlenecks in networks of silver nanowires and silver nanowire/single layer graphene hybrid using high resolution thermoreflectance (TR) imaging. TR images at the microscopic self-heating hotspots within nanowire network and nanowire/graphene hybrid network devices with submicron spatial resolution are used to infer electrical current pathways. The findings encourage a fundamental reevaluation of transport models for network-based percolating conductors.

42 ENGINEERING↗
DOE OSTI2021

Hydrogen Bonding Stiffens Peptide Amphiphile Supramolecular Filaments by Aza-Glycine Residues

Peptide amphiphiles (PAs) are a class of molecules comprised of short amino acid sequences conjugated to hydrophobic moieties that may exhibit self-assembly in water into supramolecular structures. Here, we investigate here how mechanical properties of hydrogels formed by PA supramolecular nanofibers are affected by hydrogen bond densities within their internal structure by substituting glycine for aza-glycine (azaG) residues. We found that increasing the number of PA molecules that contain azaG up to 5 mol% in PA supramolecular nanofibers increases their persistence length fivefold and decreases their diffusion coefficients as measured by fluorescence recovery after photobleaching. When these PAs are used to create hydrogels, their bulk storage modulus (G') was found to increase as azaG PA content in the supramolecular assemblies increases up to a value of 10 mol% and beyond this value a decrease was observed, likely due to diminished levels of nanofiber entanglement in the hydrogels as a direct result of increased supramolecular rigidity. Interestingly, we found that the bioactivity of the scaffolds toward dopaminergic neurons derived from induced pluripotent stem cells can be enhanced directly by persistence length independently of storage modulus. We hypothesize that this is due to interactions between the cells and the extracellular environment across different size scales: from filopodia adhering to individual nanofiber bundles to cell adhesion sites that interact with the hydrogel as a bulk substrate. Fine tuning of hydrogen bond density in self-assembling peptide biomaterials such as PAs provides an approach to control nanoscale stiffness as part of an overall strategy to optimize bioactivity in these supramolecular systems. supramolecular biomaterials.

36 MATERIALS SCIENCE↗