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All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Two-step annealing induced structural rejuvenation: A cause for memory effect in metallic glasses

The structural origin of the memory effect is still elusive in glassy materials. Here, in this letter, the memory effect in a series of metallic glasses (MGs) is observed by two step annealing using differential scanning calorimetry (DSC) and molecular dynamics simulations. It demonstrates that the Kovacs peak can be reflected from not only the system potential energy but also the atomic dynamics, showing the memory effect could originate from the formation of more loose packing configurations around mobile atoms by structural rejuvenation in the second step of high temperature annealing. In contrast, the high-low temperature annealing protocol can only induce the monotonic relaxation. Therefore, our results pave a pathway to better understand the origin of memory effect in MGs and other glassy materials from their dynamics and local atomic structure.

Memory effect↗
DOE OSTI2018

Atomic Scale Account of the Surface Effect on Ionic Transport in Silver Hollandite

Here, nanosized electrodes for Li-ion batteries typically display improved electrochemical properties, which are generally attributed to the reduced dimensionality for lithiation. However, the intriguing roles of surface defects and disorder associated with the nanosized materials are often overlooked. Here, combining atomically resolved structural analysis with density functional theory calculations, we reveal that the formation of intrinsic oxygen vacancies near surface in silver hollandite nanorods modifies the local atomic structure and valence state. These surface reconstructions resulted from oxygen vacancies can significantly affect the diffusion pathways in what are otherwise one-dimensional (1D) tunneled structures. On the basis of energy barrier calculations, we demonstrate that the oxygen vacancies boost ionic transport through the edge sharing MnO 6 polyhedra in the a–b plane. Thus, within a single rod different from the inherent 1D tunnel diffusion in the interior, the ionic transport at oxygen vacancy decorated surfaces likely adopts a three-dimensional diffusion pathway including both tunnel and planar diffusion.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Tunable Magnetic Coupling in Graphene Nanoribbon Quantum Dots

Abstract Carbon‐based quantum dots (QDs) enable flexible manipulation of electronic behavior at the nanoscale, but controlling their magnetic properties requires atomically precise structural control. While magnetism is observed in organic molecules and graphene nanoribbons (GNRs), GNR precursors enabling bottom‐up fabrication of QDs with various spin ground states have not yet been reported. Here the development of a new GNR precursor that results in magnetic QD structures embedded in semiconducting GNRs is reported. Inserting one such molecule into the GNR backbone and graphitizing it results in a QD region hosting one unpaired electron. QDs composed of two precursor molecules exhibit nonmagnetic, antiferromagnetic, or antiferromagnetic ground states, depending on the structural details that determine the coupling behavior of the spins originating from each molecule. The synthesis of these QDs and the emergence of localized states are demonstrated through high‐resolution atomic force microscopy (HR‐AFM), scanning tunneling microscopy (STM) imaging, and spectroscopy, and the relationship between QD atomic structure and magnetic properties is uncovered. GNR QDs provide a useful platform for controlling the spin‐degree of freedom in carbon‐based nanostructures.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Charging and Heating Dynamics of Nanoparticles in Nonthermal Plasmas

The focus of this award was to understand the interactions of nanometer-sized particles with ionized gases, also called plasmas. Plasmas are widely used in the fabrication of electronic circuits such as microprocessors and memory devices, in plasma display panels, as well as in medical applications. Recently, these ionized gases are finding applications in the synthesis of advanced nanomaterials with novel properties, which are based on nanometer-sized particulate (nanoparticles) building blocks. As these nanoparticles grow in the plasma environment, they interact with the plasmas species such as electrons and ions which critically determines the nanoparticle properties. The University of Minnesota researchers conducting this project performed numerical simulations and developed analytical models that described the interaction of plasma-bound nanoparticles with the plasma ions. The plasma ions bombard the nanoparticle surface with substantial energy, which can result in the rearrangement of the nanoparticles’ atoms, giving them often desirable structures at the atomic scale. Being able to tune the ion energies allows to control the properties of nanoparticles produced in order to tailor their attributes for certain applications. For instance, when used in high efficiency light emitting devices, nanoparticles produced under high fluxes of highly energetic ions may show superior light emission to particles produced under low fluxes of less energetic ions. The analytical models developed by the University of Minnesota researchers enable the research community to easily determine the energy of ions bombarding the nanoparticles. The researchers extensively tested the validity of the analytical models by comparing them to sophisticated computer simulations based on stochastic particle modeling, also called Monte Carlo modeling, which simulated the motion of hundreds of thousands of ions and their interaction with the nanoparticle surfaces. Beyond the scientific intellectual merits, this award had significant broader impacts. Two graduate students received their doctoral degrees and both have joined a U.S. manufacturer of plasma-based semiconductor processing equipment. Four undergraduate students participated in research conducted under this grant and gained valuable hands-on laboratory experience. A middle school science teacher observed research conducted under this grant and developed three new course modules that introduce middle school students to the concepts of nanometer scale, the atomic structure of matter, and the composition of matter of different chemical elements.

42 ENGINEERING↗
DOE OSTI2016

Precipitation in Mg-Gd-Y-Zr Alloy: Atomic-scale insights into structures and transformations

This paper reports on a comprehensive investigation, using atomic-scaled C{sub s} corrected high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM), of the precipitation in Mg-9.28Gd-2.93Y-0.35Zr, including structures of the precipitates and several structural transformation processes. The precipitation sequence is identified as: Super saturated solid solution → clusters → nucleation β{sup '} (major)/ β{sub H} (minor)→ precipitate β{sup '} (major)/ β{sub M} , β{sub T}{sup '} (minor)→β{sub 1} → β(Equilibrium). Under peak-age condition, the strengthening structure is independent defect-free β{sup '} with little interaction among each other. Under over-age condition, the β{sup '} precipitates start to interact with each other, during which coarsening and orientation transformation of β{sup '} precipitates take place. The coarsening and orientation transformation, in which some RE atoms in the precipitates need to change their positions, are achieved by β{sub M} and β{sub T}{sup '} as transitional structures. - Highlights: • The precipitation in Mg-Gd-Y-Zr is comprehensively investigated by advanced atomic-scale Cs-corrected HAADF-STEM. • Various novel precipitate structures are unraveled. • Four types of interfaces between the precipitates are fully characterized. • The structural transformation processes of the precipitates are revealed.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Tailoring Local Chemical Ordering via Elemental Tuning in High-Entropy Alloys

Due to the large multi-elemental space desired for property screening and optimization, high-entropy alloys (HEAs) hold greater potential over conventional alloys for a range of applications, such as structural materials, energy conversion, and catalysis. However, the relationship between the HEA composition and its local structural/elemental configuration is not well understood, particularly in noble-metal-based HEA nanomaterials, hindering the design and development of nano-HEAs in energy conversion and catalysis applications. In this work, we determined precise atomic-level structural and elemental arrangements in model HEAs composed of RhPtPdFeCo and RuPtPdFeCo to unveil their local characteristics. Notably, by changing just one constituent element in the HEA (Rh to Ru), we found dramatic changes in the elemental arrangement from complete random mixing to a local single elemental ordering feature. Additionally, we demonstrate that the local ordering in RuPtPdFeCo can be further controlled by varying the Ru concentration, allowing us to toggle between local Ru clustering and distinct heterostructures in multicomponent systems. Overall, our study presents a practical approach for manipulating local atomic structures and elemental arrangements in noble-metal-based HEA systems, which could provide in-depth knowledge to mechanistically understand the functionality of noble-metal-based HEA nanomaterials in practical applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Revealing Structural Evolution of Single Atom Catalysts during Electrochemical CO 2 Reduction by in Situ X-ray Absorption Spectroscopy

Investigating the structural variation of single-atom catalysts (SACs) is crucial to reveal the reaction mechanism under working conditions. Different in situ techniques, especially X-ray absorption spectroscopy (XAS), have been reported to study the structural changes in active sites. However, a systematic study of the relationship between the coordination environment and the catalytic ability of different kinds of SACs is still lacking. Herein, we established isolated transition metal atoms (Fe, Co, Ni, Cu) on N-doped carbon (M-N-C) and employed them in electrocatalytic CO 2 reduction reaction (CO 2 RR). Significantly, Ni-N-C exhibits the highest selectivity (similar to 97.9%) toward CO at -0.8 V vs RHE. In situ XAS characterization discloses the coordination number dependent catalytic performance. Further, the decreased average coordination number of Ni in Ni-N-C at the voltage point with maximum Faradaic efficiency was observed. Density functional theory further provides the possible mechanism of CO 2 -to-CO over the undercoordinated Ni-N-C structure.

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

Low Energy Implantation into Transition-Metal Dichalcogenide Monolayers to Form Janus Structures

Atomically-thin two-dimensional (2D) materials face significant energy barriers for synthesis and processing into functional metastable phases such as Janus structures. Here, in this study, the controllable implantation of hyperthermal species from pulsed laser deposition (PLD) plasmas is introduced as a top-down method to compositionally engineer 2D monolayers. The kinetic energies of Se clusters impinging on suspended monolayer WS 2 crystals were controlled in the <10 eV/atom range with in situ plasma diagnostics to determine the thresholds for selective top layer replacement of sulfur by selenium for the formation of high quality WSSe Janus monolayers at low (300 °C) temperatures, and bottom layer replacement for complete conversion to WSe 2 . Atomic-resolution electron microscopy and spectroscopy in tilted geometry confirm the WSSe Janus monolayer. Molecular dynamics simulations reveal that Se clusters implant to form disordered metastable alloy regions, which then recrystallize to form highly ordered structures, opening the door for low-energy implantation by PLD as a novel method to explore the synthesis of 2D Janus layers and alloys of variable composition.

2D materials↗
DOE OSTI2020

Resolving individual atoms of protein complex by cryo-electron microscopy

Breakthroughs in single-particle cryo-electron microscopy (cryo-EM) technology have made near-atomic resolution structure determination possible. Cryo-EM has resolved over four thousand structures at near-atomic resolutions (2–4 Å). It is rapidly becoming the method of choice for structure determination of membrane proteins, large assemblies, and multi-protein complexes partly because it does not require a crystal and partly because it can accommodate specimens with heterogeneous composition and/or conformation. This powerful technique is now capable of resolving protein complexes to better than 2 Å resolution3 and has been used to solve 3.7 Å resolution structure of RNA as small as ~40 kilodaltons.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2023

Ultrafast Molecular Frame Quantum Tomography

Here, we develop and experimentally demonstrate a methodology for a full molecular frame quantum tomography (MFQT) of dynamical polyatomic systems. We exemplify this approach through the complete characterization of an electronically nonadiabatic wave packet in ammonia (NH 3 ). The method exploits both energy and time-domain spectroscopic data, and yields the lab frame density matrix (LFDM) for the system, the elements of which are populations and coherences. The LFDM fully characterizes electronic and nuclear dynamics in the molecular frame, yielding the time- and orientation-angle dependent expectation values of any relevant operator. For example, the time-dependent molecular frame electronic probability density may be constructed, yielding information on electronic dynamics in the molecular frame. In NH 3 , we observe that electronic coherences are induced by nuclear dynamics which nonadiabatically drive electronic motions (charge migration) in the molecular frame. Here, the nuclear dynamics are rotational and it is nonadiabatic Coriolis coupling which drives the coherences. Interestingly, the nuclear-driven electronic coherence is preserved over longer timescales. In general, MFQT can help quantify entanglement between electronic and nuclear degrees of freedom, and provide new routes to the study of ultrafast molecular dynamics, charge migration, quantum information processing, and optimal control schemes.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2019

Monte Carlo Simulation of Borosilicate Glass Dissolution using Molecular Dynamics-Generated Glass Structures

Atomic-scale insights into the dissolution mechanisms of borosilicate glasses are critical to the development of the waste form corrosion models that inform performance assessments of nuclear waste disposal facilities. Monte Carlo (MC) simulations have been instrumental in advancing the state of knowledge of borosilicate glass dissolution; however, all such simulations to date have relied on lattices to represent the amorphous glass structure, thereby potentially introducing artifacts. Therefore, a new approach, referred to as the amorphous MC approach, has been developed whereby glass structures generated from molecular dynamics simulations are used as starting points for MC simulations of glass dissolution. MC simulations of sodium borosilicate glasses that covered a wide compositional range were performed and revealed correlations between dissolution rate and structural features of the glasses. Notably, the lattice MC approach predicted the fraction of tetrahedral boron to have a lesser influence on the dissolution rate than computed with the amorphous MC approach. The lattice MC simulations also overestimated the dissolution rate for a given glass composition. These findings highlight the importance of using truly amorphous structures in MC simulations of glass corrosion.

Kerisit, Sebastien N.↗
DOE OSTI2013

Surface Structure of Lithium Doped Potassium Tantalate (KLT) using Helium Atom Scattering.

The structures of the (001) surface of potassium tantalate doped with nominally 2, 4 and 7% lithium have been investigated using high resolution helium atom scattering. The surfaces were prepared by cleaving single crystal samples in situ under UHV conditions. Diraction measurements in the region around the He specular re ection angle soon after cleaving yielded specular peaks initially with broad shoulders. However, over a period of about an hour, the widths of these specular peaks decreased markedly as the shoulders diminished into the background. Drift spectra measurements of the stabilized surfaces revealed that the step heights separating surface terraces were predominantly multiples of the cubic unit cell dimension, about 4A, rather than multiples of a half unit cell, 2A, expected from the cleaving of these crystals. Together these results suggest that after cleaving these surfaces are rapidly modied by migration of ions to the surface from the near-surface. Further, half-order diraction peaks were observed in the <100> azimuth a short time after cleaving samples at room temperature, indicating that domains with (21) structure had formed.

Fatema, Rifat↗
DOE OSTI2022

Solving the structure of “single-atom” catalysts using machine learning – assisted XANES analysis

We show that "single-atom” catalysts (SACs) have demonstrated excellent activity and selectivity in challenging chemical transformations such as photocatalytic CO 2 reduction. For heterogeneous photocatalytic SAC systems, it is essential to obtain sufficient information of their structure at the atomic level in order to understand reaction mechanisms. In this work, a SAC was prepared by grafting a molecular cobalt catalyst on a light-absorbing carbon nitride surface. Due to the sensitivity of the X-ray absorption near edge structure (XANES) spectra to subtle variances in the Co SAC structure in reaction conditions, different machine learning (ML) methods, including principal component analysis, K-means clustering, and neural network (NN), were utilized for in situ Co XANES data analysis. As a result, we obtained quantitative structural information of the SAC nearest atomic environment thereby extending the NN-XANES approach previously demonstrated for nanoparticles and size-selective clusters.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Evolution of Structure-Reactivity Correlations for the Hydrogen Abstraction Reaction by Chlorine Atom

Empirical structure-reactivity correlations are developed for log k298, the gas-phase rate constants for the reaction (Cl + HCR3 ClH + CR3 ). It has long been recognized that correlation with rH is weak. The poor performance of the linear Evans-Polanyi formulation s illustrated and was little improved by adding a quadratic term, e.g., by making its slope smoothly dependent on rH [ ( rH rHmin) / ( rHmax rHmin)]. The polar effect ( -Cl---H---CR3 +) has also been long discussed but there is no formalization of this dependence based on widely available independent variable(s). Using the sum of Hammett constants for the R substituents also gave at best modest correlations, either for para or for its dissection into F (field/inductive) and R (resonance) effects. Much greater success was achieved by combining these approaches with the preferred independent variable set being either [( rH)2, rH, F, and R] or [ , rH, F, and R]. For 64 rate constants which span 7 orders of magnitude, these correlation formulations give r2 > 0.87 and a mean unsigned deviation of <0.5 log k units, with even better performance if primary secondary, and tertiary reaction centers are treated separately.

chlorine atom↗
DOE OSTI2020

Investigation of the fine structure of antihydrogen

At the historic Shelter Island Conference on the Foundations of Quantum Mechanics in 1947, Willis Lamb reported an unexpected feature in the fine structure of atomic hydrogen: a separation of the 2S 1/2 and 2P 1/2 states. The observation of this separation, now known as the Lamb shift, marked an important event in the evolution of modern physics, inspiring others to develop the theory of quantum electrodynamics. Quantum electrodynamics also describes antimatter, but it has only recently become possible to synthesize and trap atomic antimatter to probe its structure. Mirroring the historical development of quantum atomic physics in the twentieth century, modern measurements on anti-atoms represent a unique approach for testing quantum electrodynamics and the foundational symmetries of the standard model. Here we report measurements of the fine structure in the n = 2 states of antihydrogen, the antimatter counterpart of the hydrogen atom. Using optical excitation of the 1S–2P Lyman-α transitions in antihydrogen, we determine their frequencies in a magnetic field of 1 tesla to a precision of 16 parts per billion. Assuming the standard Zeeman and hyperfine interactions, we infer the zero-field fine-structure splitting (2P 1/2 –2P 3/2 ) in antihydrogen. The resulting value is consistent with the predictions of quantum electrodynamics to a precision of 2 per cent. Using our previously measured value of the 1S–2S transition frequency, we find that the classic Lamb shift in antihydrogen (2S 1/2 –2P 1/2 splitting at zero field) is consistent with theory at a level of 11 per cent. Our observations represent an important step towards precision measurements of the fine structure and the Lamb shift in the antihydrogen spectrum as tests of the charge–parity–time symmetry8 and towards the determination of other fundamental quantities, such as the antiproton charge radius, in this antimatter system.

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

Probing potential energy landscapes via electron-beam-induced single atom dynamics

Structure, bonding, and chemical dynamics of reactions at surfaces and interfaces, and therefore most material properties are intrinsically tied to the energetic landscape in which each atom resides. Here, we demonstrate that a moving atom under electron beam excitation can be used to probe the energy landscape along (confined) step edges, providing information about atomic-scale potentials. In conclusion, the techniques for experimentally exploring atomic potentials holds promise for predictive atom-by-atom fabrication using electron beams.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Local Electronic And Dielectric Properties at Nanosized Interfaces (Final Report)

The behavior of grain boundaries and interfaces has been a focus of fundamental research for decades because variations of structure and composition at interfaces dictate mechanical, electrical, optical and dielectric properties in solids. Similarly, the consequence of atomic and electronic structures of surfaces to chemical and physical interactions are critical due to their implications to catalysis and device fabrication. Increasing fundamental understanding of surfaces and interfaces has materially advanced technologies that directly bear on energy considerations. Currently, exciting developments in materials processing are enabling creative new electrical, optical and chemical device configurations. Controlled synthesis of nanoparticles, semiconducting nanowires and nanorods, optical quantum dots, etc. along with a range of strategies for assembling and patterning nanostructures portend the viability of new devices that have the potential to significantly impact the energy landscape. As devices become smaller the impact of interfaces and surfaces grows geometrically. As with other nanoscale phenomena, small interfaces do not exhibit the same properties as do large interfaces. The size dependence of interface properties had not been explored and understanding at the most fundamental level is necessary to the advancement of nanostructured devices. An equally important factor in the behavior of interfaces in devices is the ability to examine the interfaces under realistic conditions. For example, interfaces and boundaries dictate the behavior of oxide fuel cells which operate at extremely high temperatures in dynamic high pressure chemical environments. These conditions preclude the characterization of local properties during fuel cell operation. The objective of this research was to determine the size and interface atomic structure dependence of the electronic properties of metal/oxide interfaces using model materials systems of noble metals on SrTiO 3 surfaces; and to develop experimental techniques to probe spatially localized properties under extreme conditions. The outcomes of this research summarized in more detail below include; Discovery of the presence of multiple size dependent transport mechanisms at nanoscale interfaces and determination of the critical size parameter associated with a transition from one to another; Determination of the effect of interface atomic structure and electronic structure at nanoscale interfaces on electronic transport across the interfaces, in particular the role of states associated with under coordinated cations enabling resonant tunneling and/or band bending; Discovery and characterization of size dependent resistive switching at nanoscale interfaces; These advances required the development of a process to produce nanosized contacts with controlled interface orientation over sizes (diameters) ranging from 20-500nm and the determination of the mechanical and electrical parameters for robust and accurate measurement of frequency dependent properties of nanoscale interfaces; Invention of a chamber that enables in situ scanning probe microscopy and spectroscopy at high temperature and reactive gas environments; and First measurement of interface properties in an operating solid oxide fuel cell, quantifying the local electrical potentials and energies associated with two reaction mechanisms.

36 MATERIALS SCIENCE↗