Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “atomic structure”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 667 records · Page 37
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Effect of Ar ion-beam-assistance and substrate temperature on physical properties of Al-doped ZnO thin films deposited by RF magnetron sputtering

Highlights: • AZO and IAZO (303–473 K) films were grown by RF magnetron sputtering technique. • Ar ion-beam converts its kinetic energy as diffusion energy to the deposited atoms. • Structural evolution relies on diffusion energy imparted to the deposited atoms. • Surface texture densify under the aid of ion-beam and substrate temperature. - Abstract: In this study, pronounced effects of ion-beam-assistance and substrate temperature were revealed by exploring the physical properties of Al-doped ZnO thin films deposited by the RF magnetron sputtering technique. Herein, two measurable parameters, ion-beam discharge current (0.1 A) and substrate temperature (303–473 K) were adopted to regulate the structural changes in Al-doped ZnO films. Specifically, progressive structural changes were achieved based on the addition of Ar ion-beam during the film growth. From XRD analysis, we observed significant structural changes that provided an improvement in the film growth towards c-axis (002) plane. In addition, FESEM and AFM analyses demonstrated excellent surface features with the assisted Ar ion-beam. As a result of this process, we effectively modulated the binding energy of Zn 2p level. Finally, the structural investigation accurately demonstrated that the dependence of assisted ion-beam and substrate temperature linearly related to the superior visible transmittance and reduced band gap.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Targeting One- and Two-Dimensional Ta–Te Structures via Nanotube Encapsulation

Fine control over material synthesis on the nanoscale can facilitate the stabilization of competing crystalline structures. In this report we demonstrate how carbon nanotube reaction vessels can be used to selectively create one-dimensional TaTe 3 chains or two-dimensional TaTe 2 nanoribbons with exquisite control of the chain number or nanoribbon thickness and width. Transmission electron microscopy and scanning transmission electron microscopy reveal the detailed atomic structure of the encapsulated materials. Complex superstructures such as multichain spiraling and apparent multilayer moirés are observed. The rare 2H phase of TaTe 2 (1H in monolayer) is found to be abundant as an encapsulated nanoribbon inside carbon nanotubes. The experimental results are complemented by density functional theory calculations for the atomic and electronic structure, which uncovers the prevalence of 2H-TaTe 2 due to nanotube-to-nanoribbon charge transfer and size confinement. Calculations also reveal new 1T' type charge density wave phases in TaTe 2 that could be observed in experimental studies.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Atomically sharp jagged edges of chemical vapor deposition-grown WS 2 for electrocatalysis

Chemical vapor deposition (CVD)-grown 2D transition metal dichalcogenides can adopt faceted edges. To investigate how sharp these sites can be, we utilize aberration-corrected annular dark-field scanning transmission electron microscopy (ADF-STEM) to resolve the atomic structure of two-dimensional (2D) WS 2 domains that show jagged edges. Nanoscale triangular edge structures with S zigzag terminations are observed. Both the peak and valley regions exhibit near-atomic sharpness. The peaks are as sharp as two atoms in width. Highly ordered valley sites display a minimum width of three atoms, and prospective single-atom valleys appear as two-atom-wide sites in the ADF-STEM contrast. Regarding the kinetics, density-functional theory (DFT) calculations indicate that the 2-W-atom peak would not evolve into a single-W-atom peak even though the latter configuration is also stable with a high enough binding energy under the growth conditions. These results help deepen our understanding of the possible structuring at the nanoscale and the atomic-scale limits of peaks and valleys formed via intersection of two zigzag edges. The enriched edge sites lead to higher catalytic activities for the hydrogen evolution reaction (HER).

36 MATERIALS SCIENCE↗
DOE OSTI2020

Validating first-principles molecular dynamics calculations of oxide/water interfaces with x-ray reflectivity data

Metal oxide/water interfaces play a crucial role in many electrochemical and photocatalytic processes, such as photoelectrochemical water splitting, the creation of fuel from sunlight, and electrochemical CO 2 reduction. First-principles electronic structure calculations can reveal unique insights into these processes, such as the role of the alignment of the oxide electronic energy levels with those of liquid water. An essential prerequisite for the success of such calculations is the ability to predict accurate structural models of these interfaces, which in turn requires careful experimental validation. Here we report a general, quantitative validation protocol for first-principles molecular dynamics simulations of oxide/aqueous interfaces. The approach makes direct comparisons of interfacial x-ray reflectivity (XR) signals from experimental measurements and those obtained from ab initio simulations with semilocal and van der Waals functionals. The protocol is demonstrated here for the case of the Al 2 O 3 (001)/water interface, one of the simplest oxide/water interfaces. We discuss the technical requirements needed for validation, including the choice of the density functional, the simulation cell size, and the optimal choice of the thermodynamic ensemble. Our results establish a general paradigm for the validation of structural models and interactions at solid/water interfaces derived from first-principles simulations. Furthermore, while there is qualitative agreement between the simulated structures and the experimental best-fit structure, direct comparisons of simulated and measured XR intensities show quantitative discrepancies that derive from both bulk regions (i.e., alumina and water) as well as the interfacial region, highlighting the need for accurate density functionals to properly describe interfacial interactions. Our results show that XR data are sensitive not only to the atomic structure (i.e., the atom locations) but also to the electron-density distributions in both the substrate and at the interface.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Atomically imaged crystal structure and normal-state properties of superconducting C a 10 P t 4 A s 8 ( ( F e 1 – x P t x ) 2 A s 2 ) 5

Superconducting Ca 10 Pt 4 As 8 (Fe 2 As 2 ) 5 is rare because optimal superconducting transition temperature is achieved without chemical doping or pressure. Furthermore, the unclear crystal structure limits our ability to understand the structure-property relationship. Using atomically resolved scanning transmission electron microscopy and electron diffraction, we directly determine the structure of this superconductor: it forms a monoclinic structure (space group P 2 1 /n) with lattice parameters a = b = 8.76Å, c = 20.18Å, and γ = 90.5°. Compared with previously reported structures derived from diffraction experiments, the c-lattice constant is doubled due to alternating stacking of Pt 4 As 8 layers, which induces a high density of stacking faults. With the establishment of the crystal structure, stacking faults, and chemical composition, the distinctive normal-state electrical and thermal transport properties of our superconducting Ca 10 Pt 4 As 8 ((Fe 1–x Pt x ) 2 As 2 ) 5 (x ~ 0.05) single crystals can be explained.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Genetic algorithm prediction of crystal structure of metastable Si-IX phase

We performed genetic algorithm search for the atomic structure of the long Lime unsolved Si-IX phase. We found two new structures with space groups of P4(2)/m and P-4, respectively, which have lattice parameters in excellent agreement with the experimental data. The phonon calculations showed that the P4(2)/m structure exhibits a soft phonon mode, while the P-4 structure is dynamically stable. Our calculation also showed that the P-4 structure is a meta-stable structure in a pressure range from 0 to 40 GPa, The Si-IX phase could be a mixed phase consisting of the P4(2)/m and the P-4 structures. Published by Elsevier Ltd.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Computational Approaches for Clean Energy Materials

Currently, 80% of the global final energy consumption occurs in form of fuels and only 20% as electricity. On the other hand, renewable energy additions come almost exclusively in the form of electricity (dominantly photovoltaics and wind). Thus, a successful energy transition will require enormous growth in renewables, sufficient to convert excess electricity into fuels, as well as the development of non-electricity based solar fuel technologies. As much as photovoltaic capacities have grown over the past 20 years, it is far from clear that current technologies and materials are up to the task to grow from here by yet another factor 100 until 2050. Therefore, sustained research efforts on emerging inorganic semiconductors for solar electricity and fuels are essential for facing the double challenge of climate change and energy security. Computational materials science can make important contributions, guiding and supporting research activities through both materials search and discovery and through detailed studies that help to develop a mechanistic understanding of materials performance and bottlenecks. This presentation will highlight three recent computational projects with relevance for photovoltaics and solar fuels (1) Defect graph neural networks (dGNN) for materials discovery in solar thermochemical hydrogen (STCH) [1]. The dGNN approach facilitates broad and fast materials screening for defect properties. (2) Modeling highly off-stoichiometric systems by evaluating the free energy of defect interaction [2]. This approach allows quantitative prediction of H2 production in complex STCH oxides. (3) First-principles atomic structure prediction for interfaces [3]. This work showed how an atomically thin CdCl2 interlayer phase enables in principle ideal electron transport across the incommensurate SnO2/CdTe interface. [1] M.D. Witman, A. Goyal, T. Ogitsu, A.H. McDaniel, S. Lany, Nat. Comput. Sci. (2023). https://doi.org/10.1038/s43588-023-00495-2. [2] A. Goyal, M.D. Sanders, R.P. O'Hayre, S. Lany, PRX Energy 3, 013008 (2024). https://doi.org/10.1103/PRXEnergy.3.013008. [3] A. Sharan, M. Nardone, D. Krasikov, N. Singh, S. Lany, Appl. Phys. Rev. 9, 041411 (2022). https://doi.org/10.1063/5.0104008.

density functional theory↗
DOE OSTI2016

Structure and phonon spectrum of a submonolayer Ni film on the surface of Cu(100)

The equilibrium atomic structure and the phonon spectra of a submonolayer (θ = 0.5 monolayer) Ni film deposited on the surface of Cu(100) are calculated using the potentials obtained by the embedded atom method. We consider atomic relaxation, the vibrational state density distribution on Ni and substrate atoms, and polarization of vibrational modes. Variation of the phonon spectrum upon segregation of Cu atoms on the film surface is considered. It is shown that mixing of vibrations of Ni adatoms with vibrations of substrate atoms occurs in the entire frequency range, leading to a frequency shift of the vibrational modes of the substrate and to the occurrence of new vibrational states atypical of a clean surface. The Cu(100)–c(2 × 2)–Ni structure is dynamically more stable when placed in the subsurface layer of the substrate.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Atomic and electronic structure of the SrNbO{sub 3}/SrNbO{sub 3.4} interface

We have determined the atomic-scale structure of the SrNbO{sub 3}/SrNbO{sub 3.4} interface and related it to the electronic structure. Experimentally, transmission electron microscopy observations reveal that SrNbO{sub 3} and SrNbO{sub 3.4} show orientation relationships [1{sup ¯}10]{sub SrNbO{sub 3}} // [010]{sub SrNbO{sub 3{sub .{sub 4}}}} and (110){sub SrNbO{sub 3}} // (001){sub SrNbO{sub 3{sub .{sub 4}}}}, and that their interface is coherent and atomically abrupt. Theoretically, this interface is found to be strongly anisotropic in electronic structure, and takes on quasi-one-dimensional nature. We also find that the interface impacts greatly the electron occupation of Nb d orbitals, particularly d{sub z}{sup 2} orbital. The combined study represents a relevant advance in atomically bridging structures to properties of heterointerfaces.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Combining STM, RAIRS and TPD to Decipher the Dispersion and Interactions Between Active Sites in RhCu Single-Atom Alloys

Single-atom alloys are a new class of heterogeneous catalyst in which trace amounts of metal dopants exist as individual, isolated sites in a host metal. In this study, we examine RhCu, a new single-atom alloy with a combination of scanning tunneling microscopy, reflectance absorbance infrared spectroscopy, and temperature programed desorption to understand the atomic structure of the alloy and correlate this with the behavior of CO, a common probe molecule. Here, we find that Rh alloys into Cu(111) preferentially from step edges. As such, step density plays an important role in the vibrational structure of CO on isolated Rh sites. We observe that atomically dispersed Rh sites can be close enough together to have dipole-dipole coupling interactions. Together, this combined experimental approach enables us to understand the alloying mechanism of Rh with Cu and yields important signatures of the atomic sites that are useful in benchmarking CO vibrational data.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

A Platform for Atomic Fabrication and In Situ Synthesis in a Scanning Transmission Electron Microscope

Here, the engineering of quantum materials requires the development of tools able to address various synthesis and characterization challenges. These include the establishment and refinement of growth methods, material manipulation, and defect engineering. Atomic-scale modification will be a key enabling factor for engineering quantum materials where desired phenomena are critically determined by atomic structures. Successful use of scanning transmission electron microscopes (STEMs) for atomic scale material manipulation has opened the door for a transformed view of what can be accomplished using electron-beam-based strategies. However, serious obstacles exist on the pathway from possibility to practical reality. One such obstacle is the in situ delivery of atomized material in the STEM to the region of interest for further fabrication processes. Here, progress on this front is presented with a view toward performing synthesis (deposition and growth) processes in a scanning transmission electron microscope in combination with top-down control over the reaction region. An in situ thermal deposition platform is presented, tested, and deposition and growth processes are demonstrated. In particular, it is shown that isolated Sn atoms can be evaporated from a filament and caught on the nearby sample, demonstrating atomized material delivery. This platform is envisioned to facilitate real-time atomic resolution imaging of growth processes and open new pathways toward atomic fabrication.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Breit and QED Effects in Spectra of the 4s{sup 2} {sup 1}S{sub 0}−4s4p{sup 3}P{sub 1},{sup 1}P{sub 1} Transitions in the Zn Sequence

The 4s{sup 2}–4s4p E{sub 1} transitions for Zn-like ions from Z in the range 30–92 are calculated using the multiconfi guration Dirac–Hartree–Fock (MCDHF) method. The results obtained are in good agreement with other theoretical and experimental data and demonstrate the applicability of this method to high-precision atomic structure calculations of both few-electron systems and large atomic systems, such as Zn-like ions, along the entire isoelectronic sequence. We also report herein the calculations of many-electron quantum electrodynamic (QED) and Breit effects for the {sup 3}P{sub 0}, {sup 3}P{sub 1}, {sup 3}P{sub 2}, and {sup 1}P{sub 1} states for all Zn-like ions in the range 30 ≤ Z ≤ 100.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Associating damage nucleation and distribution with grain boundary characteristics in Ta

Here, the effect of grain boundary (GB) characteristics (as represented by misorientation angle) on its dynamic response is investigated by sampling a large set of 185 GBs in Ta bi-crystals. These GBs encompass both ordered and disordered structures – typically referred to as metastable structures, in order to reflect the diversity in the local atomic structure observed in realistic GBs. The results reveal four distinct regimes in damage distribution and failure modes as a function of misorientation angle. The above trends also correlate with the variation in the corresponding void nucleation stress.

36 MATERIALS SCIENCE↗
DOE OSTI2017

A Theoretical Investigation into the Role of Surface Defects for Oxygen Evolution on RuO 2

Here, the inability of conventional theoretical models to corroborate the well-known experimental activity of RuO 2 for the oxygen evolution reaction (OER) has recently been the subject of numerous research efforts. In this study, we use density functional theory calculations to investigate the possibility that surface defects formed during dissolution are responsible for the OER activity of RuO 2 . It has been well-established experimentally that RuO 2 undergoes dissolution during OER, yet little is known about the atomic structure or catalytic reactivity of the resulting defect sites. Through simulation of point defects, steps, and kinks derived from the RuO 2 (110) surface, we discover a 0.7 eV range in the primary descriptor for OER activity, with the most active sites outperforming those at the ideal (110) surface by nearly 0.5 eV. We postulate that these variations in reactivity are due to differences in the local electronic structure, and we investigate in more detail the electronic structure of two singly coordinated sites at the same Ru atom located at a kink. Finally, we investigate possible dissolution pathways that may proceed at the RuO 2 surface under OER conditions.

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

Bulk Metallic Glass-like Scattering Signal in Small Metallic Nanoparticles

The atomic structure of Ni-Pd nanoparticles has been studied using atomic pair distribution function (PDF) analysis of X-ray total scattering data and with transmission electron microscopy (TEM). Larger nanoparticles have PDFs corresponding to the bulk face-centered cubic packing. However, the smallest nanoparticles have PDFs that strongly resemble those obtained from bulk metallic glasses (BMGs). In fact, by simply scaling the distance axis by the mean metallic radius, the curves may be collapsed onto each other and onto the PDF from a metallic glass sample. In common with a wide range of BMG materials, the intermediate range order may be fit with a damped single-frequency sine wave. When viewed in high-resolution TEM, these nanoparticles exhibit atomic fringes typical of those seen in small metallic clusters with icosahedral or decahedral order. These two seemingly contradictory results are reconciled by calculating the PDFs of models of icosahedra that would be consistent with the fringes seen in TEM. These model PDFs resemble the measured ones when significant atom-position disorder is introduced, drawing together the two diverse fields of metallic nanoparticles and BMGs and supporting the view that BMGs may contain significant icosahedral or decahedral order.

Doan-Nguyen, VVT↗
DOE OSTI2016

Sequential slip transfer of mixed-character dislocations across Σ3 coherent twin boundary in FCC metals: a concurrent atomistic-continuum study

Sequential slip transfer across grain boundaries (GB) has an important role in size-dependent propagation of plastic deformation in polycrystalline metals. For example, the Hall–Petch effect, which states that a smaller average grain size results in a higher yield stress, can be rationalised in terms of dislocation pile-ups against GBs. In spite of extensive studies in modelling individual phases and grains using atomistic simulations, well-accepted criteria of slip transfer across GBs are still lacking, as well as models of predicting irreversible GB structure evolution. Slip transfer is inherently multiscale since both the atomic structure of the boundary and the long-range fields of the dislocation pile-up come into play. In this work, concurrent atomistic-continuum simulations are performed to study sequential slip transfer of a series of curved dislocations from a given pile-up on Σ3 coherent twin boundary (CTB) in Cu and Al, with dominant leading screw character at the site of interaction. A Frank-Read source is employed to nucleate dislocations continuously. It is found that subject to a shear stress of 1.2 GPa, screw dislocations transfer into the twinned grain in Cu, but glide on the twin boundary plane in Al. Moreover, four dislocation/CTB interaction modes are identified in Al, which are affected by (1) applied shear stress, (2) dislocation line length, and (3) dislocation line curvature. Our results elucidate the discrepancies between atomistic simulations and experimental observations of dislocation-GB reactions and highlight the importance of directly modeling sequential dislocation slip transfer reactions using fully 3D models.

36 MATERIALS SCIENCE↗