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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 577 records · Page 32
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Temperature-controlled metal/ligand stoichiometric ratio in Ag-TCNE coordination networks

The deposition of tetracyanoethylene (TCNE) on Ag(111), both at Room Temperature (RT, 300 K) and low temperatures (150 K), leads to the formation of coordination networks involving silver adatoms, as revealed by Variable-Temperature Scanning Tunneling Microscopy. Our results indicate that TCNE molecules etch away material from the step edges and possibly also from the terraces, which facilitates the formation of the observed coordination networks. Moreover, such process is temperature dependent, which allows for different stoichiometric ratios between Ag and TCNE just by adjusting the deposition temperature. X-ray Photoelectron Spectroscopy and Density Functional Theory calculations reveal that charge-transfer from the surface to the molecule and the concomitant geometrical distortions at both sides of the organic/inorganic interface might facilitate the extraction of silver atoms from the step-edges and, thus, its incorporation into the observed TCNE coordination networks.

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

Galvanic Replacement-Driven Transformations of Atomically Intermixed Bimetallic Colloidal Nanocrystals: Effects of Compositional Stoichiometry and Structural Ordering

Galvanic replacement reactions dictated by deliberately designed nanoparticulate templates have emerged as a robust and versatile approach that controllably transforms solid monometallic nanocrystals into a diverse set of architecturally more sophisticated multimetallic hollow nanostructures. The galvanic atomic exchange at the nanoparticle/liquid interfaces induces a series of intriguing structure-transforming processes that interplay over multiple time and length scales. Using colloidal Au–Cu alloy and intermetallic nanoparticles as structurally and compositionally fine-tunable bimetallic sacrificial templates, we show that atomically intermixed bimetallic nanocrystals undergo galvanic replacement-driven structural transformations remarkably more complicated than those of their monometallic counterparts. Here, we interpret the versatile structure-transforming behaviors of the bimetallic nanocrystals in the context of a unified mechanistic picture that rigorously interprets the interplay of three key structure-evolutionary pathways: dealloying, Kirkendall diffusion, and Ostwald ripening. By deliberately tuning the compositional stoichiometry and atomic-level structural ordering of the Au–Cu bimetallic nanocrystals, we have been able to fine-maneuver the relative rates of dealloying and Kirkendall diffusion with respect to that of Ostwald ripening through which an entire family of architecturally distinct complex nanostructures are created in a selective and controllable manner upon galvanic replacement reactions. The insights gained from our systematic comparative studies form a central knowledge framework that allows us to fully understand how multiple classic effects and processes interplay within the confinement by a colloidal nanocrystal to synergistically guide the structural transformations of complex nanostructures at both the atomic and nanoparticulate levels.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Catastrophic degradation of the interface of epitaxial silicon carbide on silicon at high temperatures

Epitaxial cubic silicon carbide on silicon is of high potential technological relevance for the integration of a wide range of applications and materials with silicon technologies, such as micro electro mechanical systems, wide-bandgap electronics, and graphene. The hetero-epitaxial system engenders mechanical stresses at least up to a GPa, pressures making it extremely challenging to maintain the integrity of the silicon carbide/silicon interface. In this work, we investigate the stability of said interface and we find that high temperature annealing leads to a loss of integrity. High–resolution transmission electron microscopy analysis shows a morphologically degraded SiC/Si interface, while mechanical stress measurements indicate considerable relaxation of the interfacial stress. From an electrical point of view, the diode behaviour of the initial p-Si/n-SiC junction is catastrophically lost due to considerable inter-diffusion of atoms and charges across the interface upon annealing. Temperature dependent transport measurements confirm a severe electrical shorting of the epitaxial silicon carbide to the underlying substrate, indicating vast predominance of the silicon carriers in lateral transport above 25 K. This finding has crucial consequences on the integration of epitaxial silicon carbide on silicon and its potential applications.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2025

Substrate effects on phase formation and interfacial stability in superconducting vanadium silicide thin films

Thin films of vanadium silicide (V-silicide) in the A15 cubic phase (V 3 Si) are promising for superconducting quantum devices due to their high transition temperature and potential compatibility with scalable semiconductor fabrication. However, solid-phase synthesis often yields secondary silicide phases that degrade performance. Here, in this study, we investigate the influence of substrate properties using two silicon-on-insulator architectures: one with a polycrystalline HfO 2 buried layer (group A) and the other with amorphous SiO 2 (group B). Both systems exhibit superconductivity consistent with V 3 Si formation, yet structural analysis reveals mixed-phase films in both cases. Crucially, only group A maintains an atomically sharp and chemically stable interface, a prerequisite for phase purity. Prolonged annealing in group A reduces the unwanted V 5 Si 3 phase but also leads to the emergence of Si-rich VSi 2 , likely due to localized substrate degradation. Preliminary atomic-resolution imaging suggests that HfO 2 crystallinity may promote local phase-selective nucleation. These findings highlight the importance of substrate design in promoting phase control and maintaining interfacial integrity in superconducting silicides.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Structural Rearrangement of Au–Pd Nanoparticles under Reaction Conditions: An ab Initio Molecular Dynamics Study

The structure, composition, and atomic distribution of nanoalloys under operating conditions are of significant importance for their catalytic activity. In the present work, ab initio molecular dynamics simulations are performed to understand the structural behavior of a Au-Pd nanoalloy supported on rutile TiO2 under different conditions. We find that the Au-Pd structure is strongly dependent on the redox properties of the support, originating from strong metal-support interactions. Under reducing conditions, Pd atoms are inclined to move toward the metal/oxide interface, evidenced by a significant increase of Pd-Ti bonds. This could be attributed to the charge localization at the interface that leads to Coulomb attractions to positively charged Pd atoms. In contrast, Pd atoms would rather stay inside the nanoparticle or on the alloy surface under oxidizing conditions. Moreover, Pd atoms on can be stabilized by hydrogen adsorption, forming Pd-H bonds, which are stronger than Au-H bonds. Our work provides an important insight into the structure design of Pd-Au nanoalloy catalyst and may lead to potential applications in dioxygen activation processes where Pd atoms are generally considered to be the active sites.

Xu, Cong-Qiao↗
DOE OSTI2014

Electronic properties of InP (001)/HfO{sub 2} (001) interface: Band offsets and oxygen dependence

Using ab-initio methods, atomic structures and electronic properties of InP (001)/HfO{sub 2} (001) interface are studied within the framework of density functional theory. We examine the InP/HfO{sub 2} model interface electronic structures under varying oxidation conditions. The effects of indium and phosphorous concentrations on interfacial bonding, defect states, band offsets, and the thermodynamic stability at the interface are also investigated. The origin of interfacial gap states in InP (001)/HfO{sub 2} (001) interface are proposed, mainly from the P-rich oxides, which is validated by our experimental work. This highlights the importance of surface passivation prior to high-κ deposition based on the in situ spectroscopic results of atomic layer deposition of HfO{sub 2} on InP.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

In situ atomic-scale imaging of the metal/oxide interfacial transformation

Directly probing structure dynamics at metal/oxide interfaces has been a major challenge due to their buried nature.Using environmental transmission electron microscopy,here we report observations of the in-place formation of Cu 2 O/Cu interfaces via the oxidation of Cu, and subsequently probe the atomic mechanisms by which interfacial transformation and grain rotation occur at the interfaces during reduction in an H 2 gas environment. The Cu 2 O→Cu transformation is observed to occur initially along the Cu 2 O/Cu interface in a layer-by-layer manner.The accumulation of oxygen vacancies at the Cu 2 O/Cu interface drives the collapse of the Cu 2 O lattice near the interface region, which results in a tilted Cu 2 O/Cu interface with concomitant Cu 2 O island rotation. Lastly, these results provide unprecedented microscopic detail regarding the redox reactions of supported oxides,which differs fundamentally from the reduction of bulk or isolated oxides that requires the formation of new interfaces between the parent oxide and the reduced phase.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Effect of moisture on the traction-separation behavior of cellulose nanocrystal interfaces

Interfaces and stress transfer between cellulose nanocrystals (CNCs) dictate the mechanical properties of hierarchical cellulose materials such as neat films and nanocomposites. An interesting question that remains is how the behavior of these interfaces changes due to environmental stimuli, most notably moisture. We present analyses on the traction-separation behavior between Iβ CNC elementary fibrils, providing insight into how the presence of a single atomic layer of water at these interfaces can drastically change the mechanical behavior. We find that molecular water at the interface between hydrophilic CNC surfaces has a negligible effect on the tensile separation adhesion energy. However, when water cannot hydrogen bond easily to the surface (i.e., hydrophobic surface), it tends to maintain hydrogen bonds with other water molecules across the interface and form a capillary bridge that serves to increase the energy required to separate the crystals. Under shear loading, water lowers the energy barriers to sliding by reducing the atomic friction and consequently the interlayer shear modulus between crystals. Our simulations indicate that these nanoscale interfaces and physical phenomena such as interfacial adhesion, interlayer shear properties, and stick-slip friction behavior can be drastically altered by the presence of water.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Mineral–Water Interface Structure of Xenotime (YPO 4 ) {100}

Crystal truncation rod (CTR) measurements and density functional theory (DFT) calculations were performed to determine the atomic structure of the mineral–water interface of the {100} surface of xenotime (nominally YPO 4 ). This mineral is important, because it incorporates a variety of rare earth elements (REEs) in its crystal structure. REEs are critical materials necessary for a variety of renewable and energy efficient technologies. Current beneficiation techniques are not highly selective for REE ore minerals, and large amounts go to waste; this is a first step toward designing more efficient beneficiation. Evidence is found for minor relaxation of the surface within the topmost monolayer with little or no relaxation in subsurface layers. Justification for ordered water at the interface is found, where water binds to surface cations and donates hydrogen bonds to surface phosphates. The average bond lengths between cations and oxygens on water are 228 pm in the best fit to the CTR data, versus 243 and 251 pm for the DFT. No agreement on water positions bound to surface phosphates is obtained. Altogether, the findings suggest that ligands used in beneficiation with a single anionic headgroup, such as fatty acids, will have limited selectivity for xenotime relative to undesirable minerals.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Four High-Temperature Ferromagnets in the Hf–Fe–Sn System

We report the synthesis and characterization of four new ferromagnetic compounds discovered using Sn flux: Hf1.823(16)Fe5Sn3.815(14), HfFe2-xSnx, and two polymorphs of Hf1-xFe2Snx. All are closely related to HfFe2 Laves phase parent structures. HfFe2-xSnx (x approximate to 0.3-0.4) adopts the MgZn2-type (C14) crystal structure, whereas Hf1-xFe2Snx (x approximate to 0.1-0.4) adopts both the MgCu2-type (C15), and MgNi2-type (C36) structures. They crystallize in P6(3)/mmc, Fd3m, and P6(3)/mmc, respectively, with measured unit-cell parameters of a = 4.9238(7) angstrom and c = 7.9643(12) angstrom; a = 7.068(2) angstrom; and a = 4.9944(4) angstrom and c = 16.2604(15) angstrom, although phase width leads to a range of unit cell edge lengths. Hf1.823(16)Fe5Sn3.815(14) adopts a more complicated, incommensurately modulated structure in the superspace group Xmmm(00 gamma)000 with an orthorhombic subcell a = 9.7034(12) angstrom, b = 16.823(2) angstrom, and c = 8.4473(10) angstrom, three centering vectors of (1/2 0 0 1/2), (0 1/2 0 1/2), and (1/2 1/2 0 0), and a single-component modulation vector q = 0.2768(8)c*. The structure is composed of alternating slabs of the Fe-bonded Kagome nets observed in the HfFe2 parent structures alternated with Sn-rich Th2Zn17-type slabs, with Hf atoms primarily occurring at the interfaces between the slabs. All four compounds are ferromagnetic metals at room temperature, with Curie temperatures ranging from 467(2) to 658(2) K. Their coercive fields are remarkably low, between 2(1) and 15(2) Oe. Interestingly, in two of three cases the addition of nonmagnetic Sn atoms in place of magnetic Hf or Fe atoms in the HfFe2 structure seems to strengthen rather than weaken magnetic coupling and increase TC. Fits to electrical resistivity data for the compound suggest that electron scattering in the Laves phase polymorphs shows substantial contributions from electron-magnon and/or electron-electron scattering, while the electrical behavior of Hf1.823(16)Fe5Sn3.815(14) is dominated by electron-phonon scattering, as is the case in most metals.

Calta, Nicholas P.↗
DOE OSTI2015

Glide dislocation nucleation from dislocation nodes at semi-coherent {111} Cu–Ni interfaces

Using atomistic simulations and dislocation theory on a model system of semi-coherent {1 1 1} interfaces, we show that misfit dislocation nodes adopt multiple atomic arrangements corresponding to the creation and redistribution of excess volume at the nodes. We identified four distinctive node structures: volume-smeared nodes with (i) spiral or (ii) straight dislocation patterns, and volume-condensed nodes with (iii) triangular or (iv) hexagonal dislocation patterns. Volume-smeared nodes contain interfacial dislocations lying in the Cu–Ni interface but volume-condensed nodes contain two sets of interfacial dislocations in the two adjacent interfaces and jogs across the atomic layer between the two adjacent interfaces. Finally, under biaxial tension/compression applied parallel to the interface, we show that the nucleation of lattice dislocations is preferred at the nodes and is correlated with the reduction of excess volume at the nodes.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Delocalized electrons in atomic and molecular nanoclusters

The aim of the award (Program director: Dr. Mark Pederson) was to facilitate the attendance of researchers, students, and postdocs from the U.S. at the international workshop co-organized by the applicant. The award succeeded in making it possible for a number of US attendees to present their work and participate in the meeting, which was a significant event in the research community at the interdisciplinary interface of physical chemistry, nanoscience, atomic and molecular physics, condensed matter physics, and spectroscopy. The workshop did not issue proceedings, but the present report includes present the schedule, the abstracts, and the attendance list of the July 2016 Workshop. DOE sponsorship is gratefully acknowledged in the program.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2015

An affinity-structure database of helix-turn-helix: DNA complexes with a universal coordinate system

Molecular interactions between proteins and DNA molecules underlie many cellular processes, including transcriptional regulation, chromosome replication, and nucleosome positioning. Computational analyses of protein-DNA interactions rely on experimental data characterizing known protein-DNA interactions structurally and biochemically. While many databases exist that contain either structural or biochemical data, few integrate these two data sources in a unified fashion. Such integration is becoming increasingly critical with the rapid growth of structural and biochemical data, and the emergence of algorithms that rely on the synthesis of multiple data types to derive computational models of molecular interactions. We have developed an integrated affinity-structure database in which the experimental and quantitative DNA binding affinities of helix-turn-helix proteins are mapped onto the crystal structures of the corresponding protein-DNA complexes. This database provides access to: (i) protein-DNA structures, (ii) quantitative summaries of protein-DNA binding affinities using position weight matrices, and (iii) raw experimental data of protein-DNA binding instances. Critically, this database establishes a correspondence between experimental structural data and quantitative binding affinity data at the single basepair level. Furthermore, we present a novel alignment algorithm that structurally aligns the protein-DNA complexes in the database and creates a unified residue-level coordinate system for comparing the physico-chemical environments at the interface between complexes. Using this unified coordinate system, we compute the statistics of atomic interactions at the protein-DNA interface of helix-turn-helix proteins. We provide an interactive website for visualization, querying, and analyzing this database, and a downloadable version to facilitate programmatic analysis. Lastly, this database will facilitate the analysis of protein-DNA interactions and the development of programmatic computational methods that capitalize on integration of structural and biochemical datasets. The database can be accessed at http://ProteinDNA.hms.harvard.edu.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2018

Influence of Chemistry and Misfit Dislocation Structure on Dopant Segregation at Complex Oxide Heterointerfaces

Complex oxide heterostructures and thin films have emerged as promising candidates for diverse applications. Lattice mismatch between the two oxides lead to the formation of misfit dislocations, which influence vital material features. Trivalent dopant segregation to misfit dislocations at semi-coherent oxide heterointerfaces, while not well understood, is anticipated to impact interface-governed properties. Atomistic simulations elucidating the influence of misfit dislocations on dopant segregation at SrTiO 3 /MgO heterointerfaces are reported. SrO- and TiO 2 -terminated interfaces that have differing misfit dislocation structure were considered for trivalent dopants segregation. At SrO-terminated interface, dopants tend to segregate toward but not precisely to the heterointerface, whereas at TiO 2 -terminated interface, dopants exhibit a thermodynamic preference to accumulate at the heterointerface. Most favorable segregation sites at SrO-terminated interface are located within the coherent terrace, whereas those at TiO 2 -terminated interface are at misfit dislocation intersections. Atomic layer chemistry and the resulting misfit dislocation structure at the heterointerface, along with concomitant strain at the heterointerface due to mismatched dopants, play a critical role in influencing the observed trends for dopant segregation. Overall, the present results offer a fundamental atomic scale perspective of dopant behavior at semi-coherent complex oxide heterointerfaces and the interplay between dopant chemistry, interface chemistry, and misfit dislocation structure.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Thin sensing layer based on semi-conducting β-cyclodextrin rotaxane for toxic metals detection

Highlights: • Microwave-assisted synthesis of rotaxane based on anthracene and β-cyclodextrin. • Morphological and optical characterization of thin solid film. • Elaboration of impedimetric gold/rotaxane sensor. • Investigation of the membrane sensitivity towards Hg{sup 2+}, Cu{sup 2+} and Pb{sup 2+} cations. - Abstract: An impedimetric sensor based on a new semi-conducting rotaxane has been described for detection of toxic cations. The rotaxane, consists on a π-conjugated material encapsulated into β-cyclodextrin (β-CD); it has been synthesized via the Williamson reaction under microwaves irradiation. The supramolecular structure of the compound was confirmed by NMR and FT-IR spectroscopies. A thin solid film of the rotaxane was deposited by spin-coating to develop a new electrochemical sensor. The morphological properties of the organic membrane were evaluated using contact angle measurements and atomic force microscopy. The gold/rotaxane/solution interfaces were investigated by electrochemical impedance spectroscopy and the obtained data were fitted using an equivalent electrical circuit. The response of the gold/rotaxane membrane towards Hg{sup 2+}, Cu{sup 2+} and Pb{sup 2+} cations was studied and the results showed a good sensitivity to the mercury cations.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Microstructure evolution of a dissimilar junction interface between an Al sheet and a Ni-coated Cu sheet joined by magnetic pulse welding

An Al sheet and a Ni-coated Cu sheet were lap joined by using magnetic pulse welding (MPW). Tensile tests were performed on the joined sheets, and a good lap joint was achieved at a discharge energy of > 0.9 kJ. The weld interface exhibited a wavy morphology and an intermediate layer along the weld interface. Microstructure observations of the intermediate layer revealed that the Ni coating region consisted of a Ni–Al binary amorphous alloy and that the Al sheet region contained very fine Al nanograins. Ni fragments indicative of unmelted residual Ni from the coating were also observed in parts of the intermediate layer. Formation of these features can be attributed to localize melting and a subsequent high rate cooling of molten Al and Ni confined to the interface during the MPW process. In the absence of an oxide film, atomic-scale bonding was also achieved between the intermediate layer and the sheet surfaces after the collision. MPW utilises impact energy, which affects the sheet surfaces. From the obtained results, good lap joint is attributed to an increased contact area, the anchor effect, work hardening, the absence of an oxide film, and suppressed formation of intermetallic compounds at the interface. - Highlights: •Good lap joint of an Al sheet and a Ni-coated Cu sheet was achieved by using magnetic pulse welding. •A Ni–Al binary amorphous alloy was formed as an intermediate layer at weld interface. •Atomic-scale bonding was achieved between the intermediate layer and the sheet surfaces.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Probing electronic and dielectric properties of ultrathin Ga 2 O 3 /Al 2 O 3 atomic layer stacks made with in vacuo atomic layer deposition

Ultrathin (1–4 nm) films of wide-bandgap semiconductors are important to many applications in microelectronics, and the film properties can be sensitively affected by defects especially at the substrate/film interface. Motivated by this, an in vacuo atomic layer deposition (ALD) was developed for the synthesis of ultrathin films of Ga 2 O 3 /Al 2 O 3 atomic layer stacks (ALSs) on Al electrodes. It is found that the Ga 2 O 3 /Al 2 O 3 ALS can form an interface with the Al electrode with negligible interfacial defects under the optimal ALD condition whether the starting atomic layer is Ga 2 O 3 or Al 2 O 3 . Such an interface is the key to achieving an optimal and tunable electronic structure and dielectric properties in Ga 2 O 3 /Al 2 O 3 ALS ultrathin films. In situ scanning tunneling spectroscopy confirms that the electronic structure of Ga 2 O 3 /Al 2 O 3 ALS can have tunable bandgaps (E g ) between ~2.0 eV for 100% Ga 2 O 3 and ~3.4 eV for 100% Al 2 O 3 . With variable ratios of Ga:Al, the measured E g exhibits significant non-linearity, agreeing with the density functional theory simulation, and tunable carrier concentration. Furthermore, the dielectric constant of ultrathin Ga 2 O 3 /Al 2 O 3 ALS capacitors is tunable through the variation in the ratio of the constituent Ga 2 O 3 and Al 2 O 3 atomic layer numbers from 9.83 for 100% Ga 2 O 3 to 8.28 for 100% Al 2 O 3 . The high ε leads to excellent effective oxide thickness ~1.7–2.1 nm for the ultrathin Ga 2 O 3 /Al 2 O 3 ALS, which is comparable to that of high-K dielectric materials.

36 MATERIALS SCIENCE↗