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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 775 records · Page 43
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Surface Conversion of Single-Crystal Bi 2 Se 3 to β-In 2 Se 3

In this work, we demonstrate that the surface layers of single-crystal layered-2D Bi 2 Se 3 can be converted to layered-2D rhombohedral ß-In 2 Se 3 by annealing under a trimethylindium (TMIn) flux. Samples were prepared in a metalorganic chemical vapor deposition (MOCVD) chamber, then transferred under vacuum to a surface analysis chamber for analysis with low-energy electron diffraction (LEED) and Auger electron spectroscopy. Additional ex situ characterization included x-ray diffraction, transmission-electron microscopy (TEM), energy dispersive x-ray spectroscopy (EDS) elemental mapping, and Raman spectroscopy. The resulting single-crystal ß-In 2 Se 3 adopts the rhombohedral crystal structure (space group R-3m) and orientation of the underlying Bi 2 Se 3 , and the excess Bi atoms generated by this process create an underlying region of Bi-rich Bi x Se y . Due to the difference in bandgap between Bi 2 Se 3 and In 2 Se 3 , this conversion reaction presents a pathway to lateral heterojunctions if only selected regions are converted by masking the surface to spatially define the TMIn exposure. The conversion may also have implications for heteroepitaxy, because the in-plane lattice constants of Bi 2 Se 3 and In 2 Se 3 (0001) surfaces match those of InP and GaAs (111), respectively, and the natural cleavage planes of a layered-2D crystal facilitate substrate removal and reuse.

14 SOLAR ENERGY↗
DOE OSTI2014

Improvement of band gap profile in Cu(InGa)Se{sub 2} solar cells through rapid thermal annealing

Highlights: • Proper RTA treatment can effectively optimize band gap profile to more expected level. • Inter-diffusion of atoms account for the improvement of the graded band gap profile. • The variation of the band gap profile created an absolute gain in the efficiency by 1.22%. - Abstract: In the paper, the effect of rapid thermal annealing on non-optimal double-graded band gap profiles was investigated by using X-ray photoelectron spectroscopy and capacitance–voltage measurement techniques. Experimental results revealed that proper rapid thermal annealing treatment can effectively improve band gap profile to more optimal level. The annealing treatment could not only reduce the values of front band gap and minimum band gap, but also shift the position of the minimum band gap toward front electrode and enter into space charge region. In addition, the thickness of Cu(InGa)Se{sub 2} thin film decreased by 25 nm after rapid thermal annealing treatment. All of these modifications were attributed to the inter-diffusion of atoms during thermal treatment process. Simultaneously, the variation of the band gap profile created an absolute gain in the efficiency by 1.22%, short-circuit current density by 2.16 mA/cm{sup 2} and filled factor by 3.57%.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Elucidating the microscopic origin of electrochemical corrosion and electrical conductivity by lattice response to severe plastic deformation in Al-Mn-Si alloy

Highlights: • Al-Mn-Si specimens were underwent severe plastic deformation by CGP and cold-rolling and their combined routes. • Microscopical observations depicted particles' fragmentation and non-preferential distribution under heavy straining. • XRD analysis revealed dislocations annihilation and lattice swelling (d-spacing increase) due to dynamic recrystallization. • Polarization outputs disclosed direct correlation of corrosion behavior with dislocations density. • Four-point probe test proved that electrical conductivity was a function of lattice d-spacing. - Abstract: Al-Mn-Si specimens were underwent SPD by CGP, cold-rolling and their combined paths. Acquired OM and SEM micrographs revealed that random distribution of intermetallic precipitates beside their fragmentation under heavy imposed strains deteriorated particles’ influence on the electrochemical corrosion and inhibited pitting. Analysis of XRD patterns pointed out that post-rolling of CGPed sheets resulted in dynamic annihilation of dislocations and accordingly crystal lattice swelling (d-spacing increase) due to dynamic recrystallization. From polarization results, crystallites’ refinement along with density increment of sub-grain boundaries within structure as a consequence of SPD beside dislocation density enhancement accelerated corrosion. Outputs of four-point probe electrical conductivity examination indicated that apart from the employed processing, lattice compaction and atomic layers approach would limit the electrons’ mobility space and hence reduce the conductivity of material. In this regard, combined effects of CGP and rolling as employed dual-straining route on Al-Mn-Si alloy enhanced the electrical conductivity by d-spacing growth.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Mechanistically Informed Strategies for Site-Selective Atomic Layer Deposition

While atomic layer deposition (ALD) processes from across the periodic table have been designed to deposit conformal thin films, an atomistic view of disparate substrate sites reveals the possibility of even greater synthetic control and precision. An understanding of the mechanism by which a particular ALD precursor reacts (or does not react) at myriad surface sites remains in its infancy. Here, in this Perspective, we summarize site-specific chemical reaction strategies that utilize ALD precursors and tailored surface chemistry to discriminate among potential deposition sites as well as describe techniques and tools that can be used to investigate site-selective ALD (SS-ALD). The Perspective is focused on the science of site-selective vapor-phase surface reactions but inevitably reveals the potential utility of such surface synthetic precision.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Toward the Development of a Fundamentally Based Chemical Model for Cyclopentanone: High-Pressure-Limit Rate Constants for H Atom Abstraction and Fuel Radical Decomposition

Theoretical aspects of the development of a chemical kinetic model for the pyrolysis and combustion of a cyclic ketone, cyclopentanone, are considered. We present calculated thermodynamic and kinetic data for the first time for the principal species including 2- and 3-oxo-cyclopentyl radicals, which are in reasonable agreement with the literature. Furthermore, these radicals can be formed via H atom abstraction reactions by H and Ö atoms and OH, HO 2 , and CH 3 radicals, the rate constants of which have been calculated. Abstraction from the β-hydrogen atom is the dominant process when OH is involved, but the reverse holds true for HO 2 radicals. We also determined the subsequent β-scission of the radicals formed, and it is shown that recent tunable VUV photoionization mass spectrometry experiments can be interpreted in this light. The bulk of the calculations used the composite model chemistry G4, which was benchmarked in the simplest case with a coupled cluster treatment, CCSD(T), in the complete basis set limit.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
DOE OSTI2015

Dynamics and diffusion mechanism of low-density liquid silicon

A first-order phase transition from a high-density liquid to a low-density liquid has been proposed to explain the various thermodynamic anomies of water. It also has been proposed that such liquid–liquid phase transition would exist in supercooled silicon. Computer simulation studies show that, across the transition, the diffusivity drops roughly 2 orders of magnitude, and the structures exhibit considerable tetrahedral ordering. The resulting phase is a highly viscous, low-density liquid silicon. Investigations on the atomic diffusion of such a novel form of liquid silicon are of high interest. Here we report such diffusion results from molecular dynamics simulations using the classical Stillinger–Weber (SW) potential of silicon. We show that the atomic diffusion of the low-density liquid is highly correlated with local tetrahedral geometries. We also show that atoms diffuse through hopping processes within short ranges, which gradually accumulate to an overall random motion for long ranges as in normal liquids. There is a close relationship between dynamical heterogeneity and hopping process. We point out that the above diffusion mechanism is closely related to the strong directional bonding nature of the distorted tetrahedral network. Here, our work offers new insights into the complex behavior of the highly viscous low density liquid silicon, suggesting similar diffusion behaviors in other tetrahedral coordinated liquids that exhibit liquid–liquid phase transition such as carbon and germanium.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Selective Hydroxylation of In 2 O 3 as A Route to Site-Selective Atomic Layer Deposition

We report that several atomic layer deposition (ALD) processes are now known that leverage the distinct surface chemistry of two disparate substrates (e.g., metal vs oxide) to realize markedly different ALD nucleation rates in an approach referred to as area-selective ALD. In contrast, few ALD processes have been identified that allow selective reaction at distinct surface sites of a single material surface, in a process that might be called site-selective ALD (SS-ALD). We describe one potential strategy to discriminate among several distinct surface sites on bixbyite In 2 O 3 to achieve site selectivity. Using density functional theory, we predict the discriminant hydration and hydroxylation of In 2 O 3 terrace and step-edge sites that depend strongly on the substrate temperature at low water coverage. Infrared measurement of surface hydroxyls on In 2 O 3 nanoparticles supports the predicted temperature dependence. The in situ examination of MgO ALD nucleation also shows results consistent with the predicted temperature dependence of In 2 O 3 hydroxylation. Together, these findings suggest that a selective hydroxylation approach may be a viable route to SS-ALD on In 2 O 3 .

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

Coadsorption of Formic Acid and Hydrazine on Cu(110) Single-Crystal Surfaces

The chemistry of coadsorbed formic acid and hydrazine on Cu(110) surfaces was characterized both experimentally, by temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS), and theoretically, via density functional theory (DFT) calculations. It was found that the two reactants interact with each other via hydrogen bonds, and that this modifies their individual thermal chemistry on the surface in two main ways: by stabilizing a HCOOH:N 2 H 4 adduct, which desorbs molecularly at around 240 K, and by slightly delaying the decomposition of the hydrazine to higher temperatures and shifting the selectivity of that step from dehydrogenation and formation of N 2 H x (ads) species to scission of the N–N bond and ammonia production. The coadsorbed formic acid was determined to react at higher temperatures than hydrazine, in chemistry not affected by the latter, which is no longer present on the surface at that stage. One interesting aspect of this chemistry revealed by the DFT calculations is that formic acid may preferentially H-bond on top of adsorbed hydrazine rather than directly attach to the copper surface. The implications of these results to atomic layer deposition (ALD) processes are discussed.

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

Significantly Enhanced Emission Stability of CsPbBr 3 Nanocrystals via Chemically Induced Fusion Growth for Optoelectronic Devices

Lead–halide perovskite nanocrystals (PNCs) have attracted much attention in recent years due to their outstanding optical properties. We report here a new chemical route for triggering postsynthetic growth of CsPbBr 3 PNCs at room temperature via intentional depletion of stabilizing ligands, resulting in an immediate fusion growth of the as-synthesized PNCs. Upon fusion, the CsPbBr 3 PNCs can grow from ca. 8 nm to ca. 60 nm in lateral dimensions in 48 h, reaching about 14 nm in thickness. More importantly, it was found that the fused PNCs have significantly enhanced optical properties. They showed an exceptionally higher stability to photodegradation. They also displayed sharper emission lines and a higher quantum yield, contrary to the fact that the nanocrystals are much larger. The much-improved optical properties are attributed to the Ruddlesden–Popper (RP) planar faults formed during the fusion process and observed using atomic resolution scanning transmission electron microscopy, which are predicted to result in quantum confinement based on density-functional theory calculations. The newly grown nanocrystals with RP defects are expected to significantly improve light emission properties of the PNCs and find applications in light-emitting diodes and other optoelectronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Atomic Scale Picture of the Ion Conduction Mechanism in Tetrahedral Network of Lanthanum Barium Gallate

Combined experimental study of impedance spectroscopy, neutron powder diffraction and quasielastic neutron scattering was performed to shed light into the atomic scale ion migration processes in proton and oxide ion conductor; La0.8Ba1.2GaO3.9 . This material consist of tetrahedral GaO4 units, which are rather flexible and rocking motion of these units promotes the ionic migration process. The oxide ion (vacancy) conduction takes place on channels along c axis, involving a single elementary step, which occurs between adjacent tetrahedron (inter-tetrahedron jump). The proton conduction mechanism consists of intra-tetrahedron and inter-tetrahedron elementary processes. The intra-tetrahedron proton transport is the rate-limiting process, with activation energy of 0.44 eV. The rocking motion of the GaO4 tetrahedron aids the inter-tetrahedral proton transport, which has the activation energy of 0.068 eV.

Jalarvo, Niina H↗
DOE OSTI2015

Patterned arrays of lateral heterojunctions within monolayer two-dimensional semiconductors

The formation of semiconductor heterojunctions and their high density integration are foundations of modern electronics and optoelectronics. To enable two-dimensional (2D) crystalline semiconductors as building blocks in next generation electronics, developing methods to deterministically form lateral heterojunctions is crucial. Here we demonstrate a process strategy for the formation of lithographically-patterned lateral semiconducting heterojunctions within a single 2D crystal. E-beam lithography is used to pattern MoSe 2 monolayer crystals with SiO 2 , and the exposed locations are selectively and totally converted to MoS 2 using pulsed laser deposition (PLD) of sulfur in order to form MoSe 2 /MoS 2 heterojunctions in predefined patterns. The junctions and conversion process are characterized by atomically resolved scanning transmission electron microscopy, photoluminescence, and Raman spectroscopy. This demonstration of lateral semiconductor heterojunction arrays within a single 2D crystal is an essential step for the lateral integration of 2D semiconductor building blocks with different electronic and optoelectronic properties for high-density, ultrathin circuitry.

36 MATERIALS SCIENCE↗
DOE OSTI2024

In-situ and wavelength-dependent photocatalytic strain evolution of a single Au nanoparticle on a TiO 2 film

Photocatalysis is a promising technique due to its capacity to efficiently harvest solar energy and its potential to address the global energy crisis. However, the structure–activity relationships of photocatalyst during wavelength-dependent photocatalytic reactions remains largely unexplored because it is difficult to measure under operating conditions. Here we show the photocatalytic strain evolution of a single Au nanoparticle (AuNP) supported on a TiO 2 film by combining three-dimensional (3D) Bragg coherent X-ray diffraction imaging with an external light source. The wavelength-dependent generation of reactive oxygen species (ROS) has significant effects on the structural deformation of the AuNP, leading to its strain evolution. Density functional theory (DFT) calculations are employed to rationalize the induced strain caused by the adsorption of ROS on the AuNP surface. These observations provide insights of how the photocatalytic activity impacts on the structural deformation of AuNP, contributing to the general understanding of the atomic-level catalytic adsorption process.

14 SOLAR ENERGY↗
DOE OSTI2019

Time-resolved serial femtosecond crystallography at the European XFEL

The European XFEL (EuXFEL) is a 3.4-km long X-ray source, which produces femtosecond, ultrabrilliant and spatially coherent X-ray pulses at megahertz (MHz) repetition rates. This X-ray source has been designed to enable the observation of ultrafast processes with near-atomic spatial resolution. Time-resolved crystallographic investigations on biological macromolecules belong to an important class of experiments that explore fundamental and functional structural displacements in these molecules. Due to the unusual MHz X-ray pulse structure at the EuXFEL, these experiments are challenging. In this paper, we demonstrate how a biological reaction can be followed on ultrafast timescales at the EuXFEL. We investigate the picosecond time range in the photocycle of photoactive yellow protein (PYP) with MHz X-ray pulse rates. We show that difference electron density maps of excellent quality can be obtained. The results connect the previously explored femtosecond PYP dynamics to timescales accessible at synchrotrons. This opens the door to a wide range of time-resolved studies at the EuXFEL.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2021

OH radical reactions with the hydrophilic component of sphingolipids

In this article, using the example of model compounds, we studied the reactions resulting from the interaction of OH radicals with the hydrophilic part of sphingolipids. We compared the stopped-flow EPR spectroscopy and pulse radiolysis with optical detection methods to characterize radical intermediates formed in the reaction of OH radicals with glycerol, serinol and N-boc-serinol. Quantum chemical calculations were also performed to help interpret the observed experimental data. It was shown that H-abstraction from the terminal carbon atom is the main process that is realized for all the studied compounds. The presence of the unsubstituted amino group (–NH 2 ) is seen to completely change the reaction properties of serinol in comparison with those observed in glycerol and N-boc serinol.

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

Where does the electron go? The nature of ortho/para and meta group directing in electrophilic aromatic substitution

Electrophilic aromatic substitution as one of the most fundamental chemical processes is affected by atoms or groups already attached to the aromatic ring. The groups that promote substitution at the ortho/para or meta positions are, respectively, called ortho/para and meta directing groups, which are often characterized by their capability to donate electrons to or withdraw electrons from the ring. Though resonance and inductive effects have been employed in textbooks to explain this phenomenon, no satisfactory quantitative interpretation is available in the literature. Here, based on the theoretical framework we recently established in density functional reactivity theory (DFRT), where electrophilicity and nucleophilicity are simultaneously quantified by the Hirshfeld charge, the nature of ortho/para and meta group directing is systematically investigated for a total of 85 systems. We find that regioselectivity of electrophilic attacks is determined by the Hirshfeld charge distribution on the aromatic ring. Ortho/para directing groups have most negative charges on the ortho/para positions, while meta directing groups often possess the largest negative charge on the meta position. Our results do not support that ortho/para directing groups are electron donors and meta directing groups are electron acceptors. Most neutral species we studied here are electron withdrawal in nature. Anionic systems are always electron donors. There are also electron donors serving as meta directing groups. We predicted ortho/para and meta group directing behaviors for a list of groups whose regioselectivity is previously unknown. In addition, strong linear correlations between the Hirshfeld charge and the highest occupied molecular orbital have been observed, providing the first link between the frontier molecular orbital theory and DFRT.

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

Geometry-dependent lifetime of Interatomic coulombic decay using equation-of-motion coupled cluster method

Electronically excited atom or molecule in an environment can relax via transferring its excess energy to the neighboring atoms or molecules. The process is called Interatomic or Intermolecular coulombic decay (ICD). The ICD is a fast decay process in environment. Generally, the ICD mechanism predominates in weakly bound clusters. In this paper, we have applied the complex absorbing potential approach/equation-of-motion coupled cluster (CAP/EOMCCSD) method which is a combination of CAP and EOMCC approach to study the lifetime of ICD at various geometries of the molecules. We have applied this method to calculate the lifetime of ICD in Ne-X; X = Ne, Mg, Ar, systems. We compare our results with other theoretical and experimental results available in literature.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗