Effect Of Atomic Structure Research Articles

Unraveling the Oxygen Vacancy-Performance Relationship in Perovskite Oxides at Atomic Precision via Precise Synthesis.

Understanding the fundamental effect of the oxygen vacancy atomic structure in perovskite oxides on catalytic properties remains challenging due to diverse facets, surface sites, defects, etc. in traditional powder catalysts and the inherent structural complexity. Through quantitative synthesis of tetrahedral (LaCoO2.5-T), pyramidal (LaCoO2.5-P), and octahedral (LaCoO3) epitaxial thin films as model catalysts, we demonstrate the reactivity orders of active-site geometrical configurations in oxygen-deficient perovskites during the CO oxidation model reaction: CoO4 tetrahedron > CoO6 octahedron > CoO5 pyramid. Ambient-pressure Co L-edge and O K-edge XAS spectra clarify the dynamic evolutions of active-site electronic structures during realistic catalytic processes and highlight the important roles of defect geometrical structures. In addition, in situ XAS and resonant inelastic X-ray scattering spectra and density functional theory calculations directly reveal the nature of high reactivity for CoO4 sites and that the derived shallow-acceptor defect levels in the band structure facilitate the adsorption and activation of reactive gases, resulting in more than 23-fold enhancement for catalytic reaction rates than CoO5 sites.

Investigating the effect of external heat flux and atomic structure on the thermal behavior and phase change process of sodium sulfate/magnesium chloride hexahydrate and paraffin with molecular dynamics simulation

Investigating the effect of external heat flux and atomic structure on the thermal behavior and phase change process of sodium sulfate/magnesium chloride hexahydrate and paraffin with molecular dynamics simulation

Effect of atomic substitution and structure on thermal conductivity in monolayers H-MN and T-MN (M = B, Al, Ga).

Finding materials with suitable thermal conductivity (κ) is crucial for improving energy efficiency, reducing carbon emissions, and achieving sustainability. Atomic substitution and structural adjustments are commonly used methods. By comparing the κ of two different structures of two-dimensional (2D) IIIA-nitrides and their corresponding carbides, we explored whether atomic substitution has the same impact on κ in different structures. All eight materials exhibit normal temperature dependence, with κ decreasing as the temperature rises. Both structures are single atomic layers of 2D materials, forming M-N bonds, with the difference being that H-MN consists of hexagonal rings, while T-MN consists of tetragonal and octagonal rings. 2D IIIA-nitrides provide a good illustration of the impact of atomic substitution and structure on κ. On a logarithmic scale of κ, it approximates two parallel lines, indicating that different structures exhibit similar trends of κ reduction under the same conditions of atomic substitution. We analyzed the mechanisms behind the decreasing trend in κ from a phonon mode perspective. The main reason for the decrease in κ is that heavier atoms lower lattice vibrations, reducing phonon frequencies. Electronegativity increases, altering bonding characteristics and increasing anharmonicity. Reduced symmetry in complex structures decreases phonon group velocities and enhances phonon anharmonicity, leading to decreased phonon lifetimes. It's noteworthy that we found that atomic substitution and structure significantly affect hydrodynamic phonon transport as well. Both complex structures and atomic substitution simultaneously reduce the effects of hydrodynamic phonon transport. By comparing the impact of κ on two different structures of 2D IIIA-nitrides and their corresponding carbides, we have deepened our understanding of phonon transport in 2D materials. Heavier atomic substitution and more complex structures result in reduced κ and decreased hydrodynamic phonon transport effects. This research is likely to have a significant impact on the study of micro- and nanoscale heat transfer, including the design of materials with specific heat transfer properties for future applications.

A sensitivity analysis of twinning crystal plasticity finite element model using single crystal and poly crystal Zircaloy

The popularity of crystal plasticity finite element method (CPFEM) models is increasing due to their ability to predict the mechanical response of crystalline materials such as metals and metal alloys more accurately than traditional continuum mechanics models. This is since the crystal plasticity models consider the effect of atomic structure, microstructural morphology, and properties of individual grains. These CPFEM models use a large number of material parameters in order to capture the mesoscale physics which comes with the downside of the tedious calibration process. In this paper, a CPFEM code was developed to include the twinning induced grain reorientation and subsequent crystallographic slip for HPC material. The developed code is incorporated in a large-scale, parallelized nonlinear solver WARP3D. A sensitivity analysis with respect to 22 material parameters was then conducted using single crystal and polycrystal representative volume element (RVE) of Zircaloy material. Loading was applied along five different crystallographic orientations for single crystal RVE and along three directions namely, rolling (RD), transverse (TD), and normal (ND) direction for polycrystal RVE. Results obtained from the sensitivity analysis were used for the calibration of material parameters for Zircaloy. Finally, developed code along with calibrated material parameters was used to investigate the effect of the hydride phase formation in Zircaloy which is a typical case observed for nuclear applications. It was found that the volume fraction of the hydride phase has a significant impact on the mechanical properties of Zircaloy.

First-principles study on the half-metallic properties of the VA group atoms adsorbed on WS2 monolayer

Adsorption of atoms on the surface of two-dimensional (2D) materials is one of the most effective ways to induce magnetic properties. In this study, the atomic structure, electronic structure, magnetic properties, and strain effects of VA group atoms (N, P, As, Sb and Bi) adsorbed on a WS2 monolayer are systematically studied using a first-principles method. After calculating the adsorption energy, it was determined that all of the VA group atoms showed a preference for being directly adsorbed above the S atoms. Based on the analysis of the orbital projection density of states and charge transfer, it appears that the group VA atoms chemisorb onto the WS2 layer. The adsorption of the VA group atoms on a WS2 monolayer will introduce 1 μB magnetic moment into the system. It is exciting that WS2 monolayer adsorbed with P, As, Sb or Bi is half-metallic with 100% spin polarization at the Fermi level. Furthermore, the magnetic properties are robust in the range of 10% strain and the magnetic moment of the system can be effectively controlled by tensile strain. In addition, when two or four atoms are adsorbed on a monolayer WS2 supercell, the adatoms show a tendency towards alignment in terms of their local magnetic moments, which may indicate a potential for ferromagnetic ordering in the system. After the adsorption of VA group atoms, monolayer WS2 exhibits structural stability, tunable magnetism under strain, 100% spin polarizability, and potential for ferromagnetism, making it a promising material for spintronic device applications.

Effect of Graphite Defect Type and Heterogeneous Atomic Doping Structure on the Electromagnetic Wave Absorption Properties of Sulfur-Doped Carbon Fibers

The heterogeneous atom-doped structure breaks the atomic-level symmetry of the local microstructure, and it and the defect design are considered to be an efficient way to fabricate microwave absorbers with a wide effective bandwidth and strong absorption capability. Herein, we have successfully designed and synthesized sulfur-doped carbon fibers by combining a strategy of introducing heterogeneous atoms with defect engineering. The analysis of Raman spectroscopy and transmission electron microscopy data reveals the correspondence between the type of graphite defects in sulfur-doped carbon fibers and their microwave absorption properties. The heterostructure formed by graphite and amorphous carbon can be considered a boundary-type defect, which is beneficial for microwave absorption. The introduction of vacancy defects and sulfur atom doping in graphite crystals breaks the symmetry of the graphite structure and leads to changes in dipole polarization. The ratio of the two sulfur-doped structures, C═S and C–S–C, is controlled to achieve the microwave absorption properties of the material in the high-frequency region. The three-dimensional structure formed by the cross-linking of one-dimensional fibers is more conducive to the formation of macroscopic eddy current losses. The strong absorption of microwaves at extremely thin thicknesses has been achieved with S-doped carbon fibers due to the synergy of multiple loss mechanisms. When the absorber thickness is only 0.98 mm, the optimized S-doped carbon fiber can reach a minimum reflection loss (RL) of −69.232 dB, and when the thickness is 1.04 mm, the widest effective bandwidth reaches 3.9 GHz (14.1–18 GHz), demonstrating a strong electromagnetic wave absorption capability.

Thermal Conductance of Copper-Graphene Interface: A Molecular Simulation.

Copper is often used as a heat-dissipating material due to its high thermal conductivity. In order to improve its heat dissipation performance, one of the feasible methods is to compound copper with appropriate reinforcing phases. With excellent thermal properties, graphene has become an ideal reinforcing phase and displays great application prospects in metal matrix composites. However, systematic theoretical research is lacking on the thermal conductivity of the copper-graphene interface and associated affecting factors. Molecular dynamics simulation was used to simulate the interfacial thermal conductivity of copper/graphene composites, and the effects of graphene layer number, atomic structure, matrix length, and graphene vacancy rate on thermal boundary conductance (TBC) were investigated. The results show that TBC decreases with an increase in graphene layers and converges when the number of graphene layers is above five. The atomic structure of the copper matrix affects the TBC, which achieves the highest value with the (011) plane at the interface. The length of the copper matrix has little effect on the TBC. As the vacancy rate is between 0 and 4%, TBC increases with the vacancy rate. Our results present insights for future thermal management optimization based on copper matrix composites.

Unveiling the mechanism of charge compensation in Li2RuxMn1−xO3 by tracking atomic structural evolution

The relationship between the structural evolution and redox of Li-rich transition-metal layered oxides (LLOs) cathodes remains ambiguous, obstructing the development of high-performance lithium-ion (Li+) battery. Herein, the coherent effects of local atomic and electronic structure in Li2RuxMn1−xO3 (LRMO) with a wide voltage window (1.3–4.8 V) is identified by in situ X-ray absorption fine spectroscopy (XAFS) and chemometrics. We not only skillfully separated the redox active structures to track the electrochemical path, but also visualized the coupling mechanism between the evolution of Ru-Ru dimer and the (de) excitation of cations and anions. Furthermore, introducing manganese triggers the “heterogeneity” of coordination environment and electronic structure between Ru and Mn after discharge to 3 V. The change of thermodynamic and kinetic paths affects the relithiation, and further leads to the hysteresis of the anion activation structure relaxation of Li2Ru0.4Mn0.6O3 relative to Li2RuO3 (LRO). Additionally, it is demonstrated that the high charge cut-off voltage restrains the relaxation of anionic active structure in LRO from a new perspective through comparative experiments. Our work associates the evolution of atomic structure with charge compensation and negative electrochemical reactions such as voltage hysteresis (VH) and capacity attenuation, deepening the understanding electrochemical reaction mechanism of LLOs during the first cycle and providing a theoretical support for the further design and synthesis of high-efficiency cathodes.

Atomic Structure Calculations of Landé g Factors of Astrophysical Interest with Direct Applications for Solar Coronal Magnetometry

Abstract We perform a detailed theoretical study of the atomic structure of ions with ns 2 np m ground configurations and focus on departures from LS coupling, which directly affect the Landé g factors of magnetic dipole lines between levels of the ground terms. Particular emphasis is given to astrophysically abundant ions formed in the solar corona (those with n = 2,3) with M1 transitions spanning a broad range of wavelengths. Accurate Landé g factors are needed to diagnose coronal magnetic fields using measurements from new instruments operating at visible and infrared wavelengths, such as the Daniel K. Inouye Solar Telescope. We emphasize an explanation of the dynamics of atomic structure effects for nonspecialists.

Bandgap Shrinkage and Charge Transfer in 2D Layered SnS2 Doped with V for Photocatalytic Efficiency Improvement.

Effects of electronic and atomic structures of V-doped 2D layered SnS2 are studied using X-ray spectroscopy for the development of photocatalytic/photovoltaic applications. Extended X-ray absorption fine structure measurements at V K-edge reveal the presence of VO and VS bonds which form the intercalation of tetrahedral OVS sites in the van der Waals (vdW) gap of SnS2 layers. X-ray absorption near-edge structure (XANES) reveals not only valence state of V dopant in SnS2 is ≈4+ but also the charge transfer (CT) from V to ligands, supported by V Lα,β resonant inelastic X-ray scattering. These results suggest V doping produces extra interlayer covalent interactions and additional conducting channels, which increase the electronic conductivity and CT. This gives rapid transport of photo-excited electrons and effective carrier separation in layered SnS2 . Additionally, valence-band photoemission spectra and S K-edge XANES indicate that the density of states near/at valence-band maximum is shifted to lower binding energy in V-doped SnS2 compare to pristine SnS2 and exhibits band gap shrinkage. These findings support first-principles density functional theory calculations of the interstitially tetrahedral OVS site intercalated in the vdW gap, highlighting the CT from V to ligands in V-doped SnS2 .

Effect of interfacial structures on phonon transport across atomically precise Si/Al heterojunctions

Phonons are important carriers of energy and information in many cryogenic devices used for quantum information science and in fundamental physics experiments such as dark matter detectors. In these systems phonon behaviors can be dominated by interfaces and their atomic structures; hence, there is increasing demand for a more detailed understanding of interfacial phonon transport in relevant material systems. Previous studies have focused on understanding thermal transport over the entire phonon spectrum at and above room temperature. At ultralow temperatures, however, knowledge is missing regarding athermal phonon behavior due to the challenge in modeling the extreme conditions in microscale, heterogeneous cryogenic systems, as well as extracting single-phonon information from a large ensemble. In this paper, we delineate the effects of interfacial atomic structures on phonon transport using a combination of classical molecular dynamics (MD) and phonon wave-packet simulations, to illustrate the consistency and differences between the ensemble- and single-phonon dynamics. We consider three single-crystal Si surface reconstructions---$(1\ifmmode\times\else\texttimes\fi{}1), (\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})$ and $(7\ifmmode\times\else\texttimes\fi{}7)$---and model both experimentally observed $\mathrm{Si}(1\ifmmode\times\else\texttimes\fi{}1)/\mathrm{Al}$ interfaces and hypothesized $\mathrm{Si}(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})$/Al and $\mathrm{Si}(7\ifmmode\times\else\texttimes\fi{}7)/\mathrm{Al}$ interfaces. The overall interfacial thermal conductance calculated from non-equilibrium MD shows that for the $\mathrm{Si}(1\ifmmode\times\else\texttimes\fi{}1)/\mathrm{Al}$ system, the presence of Al twin boundaries can hinder phonon transport and reduce thermal conductance by 2--12% relative to single-crystal Al; whereas the Si $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})$ and $(7\ifmmode\times\else\texttimes\fi{}7)$ reconstructions can enhance it by 6--19%. Normal mode decomposition reveals that both the increase and decrease in conductance are related to inelastic phonon scattering. Single-phonon wave-packet simulations predict phonon transport properties consistent with non-equilibrium MD, while further suggesting that phonon polarization conversion is significant even when elastic transmission dominates, and that the interfacial structures have anisotropic impacts on atomic vibrations along different lattice directions. Our findings suggest avenues for achieving selective phonon transport via controlling interfacial structures of materials using atomically precise fabrication techniques, and that the phonon wave-packet formalism is a potentially powerful method for developing a detailed understanding of non-equilibrium phenomena in the low-temperature limit.

Effect of Local Atomic Structure on Sodium Ion Storage in Hard Amorphous Carbon.

The fundamental understanding of sodium storage mechanisms in amorphous carbon is essential to develop high-performance anode materials for sodium-ion batteries. However, the intrinsic relation between the structure of amorphous carbon and Na+ storage remains to be debated due to the difficulty in controlling and characterizing the local atomic configurations of amorphous carbon. Here we report quantitative measurements of Na+ storage in a low-temperature dealloyed hard carbon with a tunable local structure from completely disordered micropores to gradually increased graphitic order domains. The structure-capacity-potential correlation not only verifies the disputing "adsorption-intercalation" mechanisms, i.e., Na+ intercalation into local graphitic domains for the low-voltage plateaus and adsorption in fully disordered carbon for the sloping voltage profiles, but also unveils a new mechanism of Na+ adsorption on defective sites of graphitic carbon in the medium-potential sloping region. The quantitative investigations provide essential insights into the reaction mechanisms of Na+ with amorphous carbon for designing advanced sodium-ion battery anodes.

Accelerated mapping of electronic density of states patterns of metallic nanoparticles via machine-learning

Within first-principles density functional theory (DFT) frameworks, it is challenging to predict the electronic structures of nanoparticles (NPs) accurately but fast. Herein, a machine-learning architecture is proposed to rapidly but reasonably predict electronic density of states (DOS) patterns of metallic NPs via a combination of principal component analysis (PCA) and the crystal graph convolutional neural network (CGCNN). With the PCA, a mathematically high-dimensional DOS image can be converted to a low-dimensional vector. The CGCNN plays a key role in reflecting the effects of local atomic structures on the DOS patterns of NPs with only a few of material features that are easily extracted from a periodic table. The PCA-CGCNN model is applicable for all pure and bimetallic NPs, in which a handful DOS training sets that are easily obtained with the typical DFT method are considered. The PCA-CGCNN model predicts the R2 value to be 0.85 or higher for Au pure NPs and 0.77 or higher for Au@Pt core@shell bimetallic NPs, respectively, in which the values are for the test sets. Although the PCA-CGCNN method showed a small loss of accuracy when compared with DFT calculations, the prediction time takes just ~ 160 s irrespective of the NP size in contrast to DFT method, for example, 13,000 times faster than the DFT method for Pt147. Our approach not only can be immediately applied to predict electronic structures of actual nanometer scaled NPs to be experimentally synthesized, but also be used to explore correlations between atomic structures and other spectrum image data of the materials (e.g., X-ray diffraction, X-ray photoelectron spectroscopy, and Raman spectroscopy).

The magnetic properties of amorphous AlxFe100−x alloys investigated by the atomic structure, magnetoresistance and anomalous Hall effect

We report on experimental results of the specific resistivity ρ, Hall resistivity ρH and magnetoresistance MR of rapidly quenched AlxFe100−x binary amorphous alloys. Atomic structure investigation done by transmission electron microscopy at room temperature verifies that samples are in the pure amorphous state with a high coordination number 8<Nc<12. Systematic changes in the resistivity, magnetoresistance and Hall effect were observed with the varying Fe content. At room temperature, samples with Fe content ⩾40% show negative magnetoresistance, are p-type and show the presence of anomalous Hall effect, while samples with Fe content ⩽25% show positive magnetoresistance, are n-type and show no anomalous Hall effect contribution. Our experimental results are discussed in the concept of electronic stabilization Hume-Rothery amorphous alloys triggered by the hybridization effect of the Fe-d and the Al-s,p electrons. Previous literature results have predicted that when the Fe-d states are full, no ferromagnetic properties should be observed in the AlxFe100−x alloys, and this is for concentrations x⩾75, which is in agreement with our results, because the appearance of an anomalous contribution in the Hall effect would be a direct evidence of ferromagnetic order in the AlxFe100−x amorphous alloys.

Analytic description of the above-threshold detachment in the adiabatic limit

Strong-field above-threshold detachment (ATD) is analyzed in the low-frequency (or adiabatic) limit. The explicit form of ATD amplitude is obtained within a developed low-frequency approximation. We show that in the low-frequency limit, the atomic structure effects are introduced in the ATD amplitude through the $T$ matrix, which in the limiting case coincides with the amplitude of elastic electron scattering. The rescattering part of ATD amplitude is evaluated in terms of the closed real classical electron trajectories. We further discuss the behavior of ATD amplitude near the caustic rescattering energies (classical energies for which two trajectories merge into a single one) of photoelectrons. Comparison of developed analytic theory with the numerical solution of time-dependent Schr\"odinger equation is presented for linearly polarized and time-delayed bicircular fields. Based on the analytical description, we predict an enhancement of ATD yield for time-delayed bicircular fields, which is similar to previous findings for the high harmonic yield [M.V. Frolov et al., Phys. Rev. Lett. 120, 263203 (2018)].

Effect of Sheet Size and Atomic Structure on the Antibacterial Activity of Nb-MXene Nanosheets

Two-dimensional (2D) MXenes have demonstrated outstanding antimicrobial properties owing to their unique physiochemical properties and ultrathin lamellar structure. However, the relationship betwee...

Near-perfect healing natures of silver five-fold twinned nanowire

Healing nanogap of metal nanowire is essential for the manufacturing and maintaining of nanoelectronic devices. Recent experimental study has demonstrated the feasibility of nanohealing in silver nanowire by using plasmonic heating. Here we report for the first time the effect of atomic lattice structures on the healing behavior and mechanism of silver nanowires due to plasmon, by using molecular dynamics simulation with a self-limited strategy. Three fundamental steps of the repair process are identified based on atomic behaviors, i.e. melting, recrystallizing, and healing. The five-fold twinned nanowires (FFTNW) is easier to get perfect healing than the single crystal ones. The near-perfect healing natures of FFTNW are mainly attributed to the non-linear stress distribution of cross section and the inherent five-fold twinned boundaries, which limit dislocation nucleating and provide self-healing nature. Our results provide a new microscopic perspective and understanding for the plasmonic nanohealing technology.

Epitaxial growth of antimony nanofilms on HOPG and thermal desorption to control the film thickness**Project supported by the National Natural Science Foundation of China (Grant Nos. 21622304, 61674045, 11604063, and 61911540074), the National Key Research and Development Program of China (Grant No. 2016YFA0200700), the Strategic Priority Research Program and Key Research Program of Frontier Sciences and Instrument Developing Project (Chinese

Group-V elemental nanofilms were predicted to exhibit interesting physical properties such as nontrivial topological properties due to their strong spin–orbit coupling, the quantum confinement, and surface effect. It was reported that the ultrathin Sb nanofilms can undergo a series of topological transitions as a function of the film thickness h: from a topological semimetal (h > 7.8 nm) to a topological insulator (7.8 nm > h > 2.7 nm), then a quantum spin Hall (QSH) phase (2.7 nm > h > 1.0 nm) and a topological trivial semiconductor (h > 1.0 nm). Here, we report a comprehensive investigation on the epitaxial growth of Sb nanofilms on highly oriented pyrolytic graphite (HOPG) substrate and the controllable thermal desorption to achieve their specific thickness. The morphology, thickness, atomic structure, and thermal-strain effect of the Sb nanofilms were characterized by a combination study of scanning electron microscopy (SEM), atomic force microscopy (AFM), and scanning tunneling microscopy (STM). The realization of Sb nanofilms with specific thickness paves the way for the further exploring their thickness-dependent topological phase transitions and exotic physical properties.

Effect of atomic structure on the electrical response of aluminum oxide tunnel junctions

Many nanoelectronic devices rely on thin dielectric barriers through which electrons tunnel. For instance, aluminium oxide barriers are used as Josephson junctions in superconducting electronics. The reproducibility and drift of circuit parameters in these junctions are affected by the uniformity, morphology, and composition of the oxide barriers. To improve these circuits the effect of the atomic structure on the electrical response of aluminium oxide barriers must be understood. We create three-dimensional atomistic models of aluminium oxide tunnel junctions and simulate their electronic transport properties with the non-equilibrium Green's function formalism. Increasing the oxide density is found to produce an exponential increase in the junction resistance. In highly oxygen-deficient junctions we observe metallic channels which decrease the resistance significantly. Computing the charge and current density within the junction shows how variation in the local potential landscape can create channels which dominate conduction. An atomistic approach provides a better understanding of these transport processes and guides the design of junctions for nanoelectronics applications.

Design of High-Performance Co-Based Alloy Nanocatalysts for the Oxygen Reduction Reaction.

Co-based nanoalloys show potential applications as nanocatalysts for the oxygen reduction reaction (ORR), but improving their activity is still a great challenge. In this paper, a strategy is proposed to design efficient Co-M (M=Au, Ag, Pd, Pt, Ir, and Rh) nanoalloys as ORR catalysts by using density functional theory (DFT) calculations. Through the Sabatier analysis, the overpotential as a function of ΔGOH * is identified as a quantitative descriptor for analyzing the effect of dopants and atomic structures on the activity of the Co-based nanoalloys. By adopting the suitable dopants and atomic structures, ΔGOH * accompanied by overpotential could be adjusted to the optimal range to enhance the activity of the Co-based nanoalloys. With this strategy, the core-shell structured Ag42 Co13 nanoalloy is predicted to have the highest catalytic activity for ORR among these Co-based nanoalloys. To give a deeper insight into the properties of Ag-Co nanoalloys, the structure, thermal stability, and reaction mechanism of Ag-Co nanoalloys with different compositions are also studied by using molecular simulations and DFT calculations. It is found that core-shell Ag42 Co13 exhibits the highest structural and thermal stability among these Ag-Co nanoalloys. In addition, the core-shell Ag42 Co13 shows the lowest ORR reaction energy barriers among these Ag-Co nanoalloys. It is expected that this kind of strategy could provide a viable way to design highly efficient heterogeneous catalysts in extensive applications.