Atomic-scale Structure Research Articles

Accelerated First-Principles Exploration of Structure and Reactivity in Graphene Oxide.

Graphene oxide (GO) materials are widely studied, and yet their atomic-scale structures remain to be fully understood. Here we show that the chemical and configurational space of GO can be rapidly explored by advanced machine-learning methods, combining on-the-fly acceleration for first-principles molecular dynamics with message-passing neural-network potentials. The first step allows for the rapid sampling of chemical structures with very little prior knowledge required; the second step affords state-of-the-art accuracy and predictive power. We apply the method to the thermal reduction of GO, which we describe in a realistic (ten-nanometre scale) structural model. Our simulations are consistent with recent experimental findings, including X-ray photoelectron spectroscopy (XPS), and help to rationalise them in atomistic and mechanistic detail. More generally, our work provides a platform for routine, accurate, and predictive simulations of diverse carbonaceous materials.

Molecular dynamics study of the effect of composition on elastic properties of silicon oxynitride films

Understanding the mechanical properties of silicon oxynitride (a-SiON), a key insulating material, is vital for electronic device design and reliability. Though the effects of fabrication conditions on a-SiON have been studied, the underlying relationship between its atomic-scale structure and mechanical properties remains unclear. This study investigates the relationship between elasticity and atomic-scale structures in a-SiON by molecular dynamics simulations with a universal graph neural network interatomic potential. The bulk modulus increases from 49 to 150 GPa with higher N content. N atoms form N2 molecules under O-rich conditions, hindering bulk modulus increase, and form an Si3N4-like network under O-poor conditions, enhancing bulk modulus. Formation energy calculations indicate N2 formation is preferable under O-rich conditions. Meanwhile, under O-poor conditions, Si–N bond formation is preferable, which reinforces a-SiON by increasing bond density. The findings suggest realizing O-poor conditions is crucial for highly elastic insulating films.

Cooperative adsorbate binding catalyzes high-temperature hydrogen oxidation on palladium.

Atomic-scale structures that account for the acceleration of reactivity by heterogeneous catalysts often form only under reaction conditions of high temperatures and pressures, making them impossible to observe with low-temperature, ultra-high-vacuum methods. We present velocity-resolved kinetics measurements for catalytic hydrogen oxidation on palladium over a wide range of surface concentrations and at high temperatures. The rates exhibit a complex dependence on oxygen coverage and step density, which can be quantitatively explained by a density functional and transition-state theory-based kinetic model involving a cooperatively stabilized configuration of at least three oxygen atoms at steps. Here, two oxygen atoms recruit a third oxygen atom to a nearby binding site to produce an active configuration that is far more reactive than isolated oxygen atoms. Thus, hydrogen oxidation on palladium provides a clear example of how reactivity can be enhanced on a working catalyst.

Direct observation of space-charge-induced electric fields at oxide grain boundaries.

Space charge layers (SCLs) formed at grain boundaries (GBs) are considered to critically influence the properties of polycrystalline materials such as ion conductivities. Despite the extensive researches on this issue, the presence of GB SCLs and their relationship with GB orientations, atomic-scale structures and impurity/solute segregation behaviors remain controversial, primarily due to the difficulties in directly observing charge distribution at GBs. In this study, we directly observe electric field distribution across the well-defined yttria-stabilized zirconia (YSZ) GBs by tilt-scan averaged differential phase contrast scanning transmission electron microscopy. Our observation clearly reveals the existence of SCLs across the YSZ GBs with nanometer precision, which are significantly varied depending on the GB orientations and the resultant core atomic structures. Moreover, the magnitude of SCLs show a strong correlation with yttrium segregation amounts. This study provides critical insights into the complex interplay between SCLs, orientations, atomic structures and segregation of GBs in ionic crystals.

Evaluation of multi-phased heterostructures in Fe-Cr-based alloys through TEM observation combined with a newly developed LRLM model

The characteristics of the multi-phased oxide-TiN-TiC-(δ-Fe) heterostructures in Fe-Cr-based alloys are essential for TiN refinement and resultant grain refinement. While the refining phenomena of TiN and as-cast grains in Fe-Cr-based alloys were well reported, the detailed refining mechanisms for TiN and δ-Fe grains remain to be further elucidated. In order to evaluate the oxide-TiN-TiC-(δ-Fe) heterostructures, three foils for transmission electron microscope (TEM) examination were prepared, denoted as the I0, R1, and MR1 samples, respectively. Here, the atomic lattice matching modes and orientation relationships (ORs) across the (Mg and /or RE bearing oxide)-TiN-TiC-(δ-Fe) heterostructures were revealed. Based on the results, the simulated multilayer /3D atomic matching models for the interfaces of each heterostructure were established, revealing the actual atomic-scale structures of the heterostructures and their formation processes originating from the oxides. Furthermore, a newly developed long-range lattice matching (LRLM) model was established to provide a new method of the OR verifications. In summary, this study does provide new insights into interfacial engineering and new ideals on refining TiN and as-cast grains in industrial production of Fe-Cr-based alloys.

Enhancing piezoelectric coefficient and thermal stability in lead-free piezoceramics: insights at the atomic-scale

Given the highly temperature-sensitive nature of the polymorphic phase boundaries, attaining excellent piezoelectric coefficient with superior temperature stability in lead-free piezoceramics via direct compositional design remains a formidable challenge. We demonstrate the synergistic improvement of piezoelectric coefficient and thermal stability in lead-free piezoceramics via atomic-scale local ferroelectric structure design. Via modulation of the local Landau energy barrier at doping sites, we effectively mitigate fluctuations in piezoelectric d33. Our approach achieves an impressive d33 of ~430 pC/N with a minimal temperature fluctuation range (△d33 ~ 7%) across the room temperature to 100 °C in potassium sodium niobate ceramics. Further optimization through annealing extends this temperature up to 150 °C (△d33 ~ 8%) while maintaining a high d33 of ~380 pC/N, rivaling the performance of classic temperature stable lead zirconate titanate. This work establishes a framework for addressing the dilemma between high piezoelectric coefficient and inadequate temperature stability in lead-free piezoceramics.

Resolving the Structure of a Guanine Quadruplex in TMPRSS2 Messenger RNA by Circular Dichroism and Molecular Modeling.

The presence of a guanine quadruplex in the opening reading frame of the messenger RNA coding for the transmembrane serine protease 2 (TMPRSS2) may pave the way to original anticancer and host-oriented antiviral strategy. Indeed, TMPRSS2 in addition to being overexpressed in different cancer types, is also related to the infection of respiratory viruses, including SARS-CoV-2, by promoting the cellular and viral membrane fusion through its proteolytic activity. The design of selective ligands targeting TMPRSS2 messenger RNA requires a detailed knowledge, at atomic level, of its structure. Therefore, we have used an original experimental-computational protocol to predict the first resolved structure of the parallel guanine quadruplex secondary structure in the RNA of TMPRSS2, which shows a rigid core flanked by a flexible loop. This represents the first atomic scale structure of the guanine quadruplex structure present in TMPRSS2 messenger RNA.

Designing Electronic Structures of Multiscale Helical Converters for Tailored Ultrabroad Electromagnetic Absorption

HighlightsThe energy conversion mechanism is thoroughly analyzed, with a detailed quantitative characterization of the dissipation capacities of polarization, conduction, and magnetic loss.Inspired by DNA transcription, atom and geometry configurations co-modulating multi-scale helical converters achieve the RLmin of −63.13 dB at 1.29 mm, and the maximum RCS reduction value reach 36.4 dB m2.Orbital coupling, spin and cross polarization synergize to realize a 6.08 GHz EAB, further expanding to ultrabroad electromagnetic wave absorption of 12.16 GHz through metamaterial design.

GraphBNC: Machine Learning-Aided Prediction of Interactions Between Metal Nanoclusters and Blood Proteins.

Hybrid nanostructures between biomolecules and inorganic nanomaterials constitute a largely unexplored field of research, with the potential for novel applications in bioimaging, biosensing, and nanomedicine. Developing such applications relies critically on understanding the dynamical properties of the nano-bio interface. This work introduces and validates a strategy to predict atom-scale interactions between water-soluble gold nanoclusters (AuNCs) and a set of blood proteins (albumin, apolipoprotein, immunoglobulin, and fibrinogen). Graph theory and neural networks are utilized to predict the strengths of interactions in AuNC-protein complexes on a coarse-grained level, which are then optimized in Monte Carlo-based structure search and refined to atomic-scale structures. The training data is based on extensive molecular dynamics (MD) simulations of AuNC-protein complexes, and the validating MD simulations show the robustness of the predictions. This strategy can be generalized to any complexes of inorganic nanostructures and biomolecules provided that one generates enough data about the interactions, and the bioactive parts of the nanostructure can be coarse-grainedrationally.

Anisotropic ferroelectric-shaped hysteresis loop and colossal permittivity in thermally treated rutile TiO2 single crystals

Space charge polarization is an unavoidable phenomenon in electronic components that leads to deterioration of electrical response. Hence manipulation and utilization of space charge polarization to improve electrical properties is a phenomenal task. In the present study, heating treatment produces oxygen vacancies, resulting in the crystal structures (i.e., modulated structure and intergrowth structure) and band gap are anisotropic, which in turn leads to anisotropic nonlinear polarization behavior and colossal permittivity. Quantitative analysis of the atomic-scale structure confirms the modulated structure and intergrowth structure are anisotropic. A modulated structure along a axis of rutile TiO2 with a width of six times that of a rutile crystal cell a is constructed through an intergrowth structure between rutile and srilankite TiO2 phases in rutile TiO2 single crystals, wherein space charges at the intergrowth structure give rise to ferroelectric-shaped hysteresis in heated TiO2 single crystals along [001] direction. Impedance spectra display three kinds of space charge polarizations (modulated structure, intergrowth structure, and electrode non-ohmic contact) contribute to the colossal permittivity along [001] direction. The space charge polarizations contribute to a colossal permittivity (∼45,000), high resistivity (∼1010 Ω cm), and ferroelectric-shaped P-E loop (remanent polarization: ∼17.1 μC/cm2, coercive field: ∼4.3 kV/cm) in the heated TiO2 single crystal along [001] direction, while a linear polarization behavior and permittivity value of 120 are obtained along [100] direction. The findings provide guidance for the utilization of space charge polarization in dielectric materials.

Alloy scattering to optimize carrier and phonon transport properties in PbBi2S4 thermoelectric

Ternary PbBi2S4 compound with crustal S-rich element and low lattice thermal conductivity is considered as a potential thermoelectric candidate. However, its inferior thermoelectric properties are rooted in the low electrical transport performance. Generally, enhancing electrical transport performance (power factor, PF) primarily entails optimizing the interdependent relationship between carrier mobility μ (linked to electrical conductivity σ) and effective mass m* (related to Seebeck coefficient S). In this work, we introduce the strategy of alloy scattering to independently enhance S without weakening μ and simultaneously reduce thermal conductivity, leading to a synergetic optimization of electron and phonon in PbBi2S4 thermoelectric. Heavy Sn alloying in PbBi2S4 presents uniform and orderly distribution on Pb sites as unclosed by the atomic-scale crystal structure observation. These massive Sn atom serves as scattering centers and turns the electron scattering mechanism to be dominated by alloy scattering, thus resulting in a ∼ 33% increment of S in Pb0.6Sn0.4Bi2S4. Meanwhile, Sn alloying aggravates phonon scattering further lowering lattice thermal conductivity and reaching an extremely low value of 0.34 W·m–1·K–1 at 773 K. Finally, a maximum zT of 0.68 at 773 K is obtained in Pb0.6Sn0.4Bi2S4, which is ∼ 45% higher than the pristine matrix. This study proves that the strategy of alloy scattering is effective in improving overall electrical transport properties as well as reducing lattice thermal conductivity, which paves a new way to develop high-performance thermoelectric materials.

Low-voltage single-atom electron microscopy with carbon-based nanomaterials

The properties of materials are strongly correlated with their atomic scale structures. Achieving a comprehensive understanding of the atomic-scale structure-property relationship requires advancements of imaging and spectroscopy techniques. Aberration-corrected scanning transmission electron microscopy (STEM) has seen rapid development over the past decades and is now routinely employed for atomic-scale characterization. However, quantitative STEM imaging and spectroscopy analysis at the single-atom level is challenging due to the extremely weak signals generated from individual atom, thus imposing stringent requirements for analysis sensitivity. This review discusses the development and application of low-voltage STEM techniques with single-atom sensitivity, primarily based on recent research presented on an invited talk at the 5th 2D23 SALVE Symposium, including annular dark-field (ADF) imaging, functional imaging and electron energy-loss spectroscopy (EELS) analysis. Carbon-based nanomaterials were chosen as model systems for demonstrating the capabilities of single-atom STEM imaging and EELS analysis, due to their structural stability under low accelerating voltages and their rich physical and chemical properties. Moreover, this review summarizes recent advancements and applications of low-voltage single-atom STEM imaging and spectroscopy in the study of functional materials and discusses prospects for future developments.

Probing ferroelectric domain structures and their switching dynamics in SrBi2Ta2O9 by in-situ electric biasing in transmission electron microscopy

Lead-free SrBi2Ta2O9 (SBT) has been a promising ferroelectric material for various applications such as electronics and data storage due to its outstanding ferroelectric properties including high fatigue endurance and low leakage current. However, the atomic-scale domain structure and switching dynamics of ferroelectric SBT remain elusive. This study reveals that spontaneous polarization arises from canted bismuth-cation displacements, forming 90° and Ising-type 180° domain walls. Interestingly, topological pairs of ferroelectric vortex and antivortex connect ferroelectric boundaries where three domain walls converge. In situ electrical biasing transmission electron microscopy (TEM) reveals the dominance of 180° switching over 90°, where oxygen octahedral connectivity is protected by ferroelastic energy in the 90° domain wall. Consequently, all 180° domain walls and (anti)vortices are removed, leaving only 90° domain walls in the electrically poled states. Chemical deterioration along domain walls highlights vulnerability of SBT to ferroelectric fatigue. This study provides insight into crucial aspects for practical applications of SBT.

(Invited) OER Active Phases, Catalytic Mechanism and Reaction Centers of Ni (oxy)Hydroxide with and without Fe Impurities

Identifying atomic-scale structures of catalytically active phases, catalytic mechanism and reaction centers are crucial for establishing rigorous structure-activity relationship of highly active catalysts and for designing new catalysts with further improved performance. Due to the lack of theoretical guidance and challenges in experimental structural characterization during OER conditions, in-situ structures and stabilities of two (a-Ni(OH)2 and g-NiOOH with water and ion intercalation) out of four (oxy)hydroxide phases (another two are b-Ni(OH)2 and b-NiOOH without water and ion intercalations) remain elusive after at least half a century study, which has hindered the establishment of rigorous structure-activity relationship of Ni (oxy)hydroxides and their derivatives for the OER. As a consequence, it is still unclear regarding which phases should be the hosts and the precursors of choice for highly active catalysts. Here, by combining ab initio molecular dynamics simulation, in-situ wide-angle X-ray scattering, and electrochemical measurements, we provide direct atomic-scale and self-consistent insights into the crystal structure, water and ion intercalation, and stable potential windows of complex a-Ni(OH)2 and g-NiOOH with respect to b-phases. DFT calculations indicate that b-NiOOH and g-NiOOH with Fe impurities have similar OER activity to or even higher OER activity than that of g-NiFe LDH, with a dependence on the Fe coverage on the edges. Current work suggests that both b-NiOOH and g-NiOOH are potential hosts of highly active OER catalysts, which enlarges the phase space for the design of OER catalysts with further improved performance.We will discuss the implications of the above insights toward understanding of the active phases, reaction centers and catalytic mechanism of NiM and CoM layered double hydroxides. If time permits, we will further review our work our work regarding the promotion effects of non-covalent ligand-oxide interaction in OER and mitigation of RuO6 octahedron distortion by enhanced A-site electronegativity in pyrochlore for acidic water oxidation.

A novel slurry for atomic-scale polishing of KDP crystals

Abstract Atomic-scale surfaces and structures have been playing a significant role in the next generation of devices and products. Potassium dihydrogen phosphate (KDP) crystals are crucial in energy sectors but challenging for ultra-precision processing due to deliquescence, brittleness, and low hardness. This paper introduces a novel chemo-mechanical slurry designed for achieving atomic-scale polishing of KDP crystals. The slurry employs a combination of polyethylene glycol (PEG) and anhydrous ethanol (AE) to counter deliquescence. In addition, graphite oxide (GO) with KOH is incorporated to prevent the embedding of SiO2 abrasives and the dissolution of KDP in deionized water (DW). The mechanism underlying the formation of an ultra-smooth surface is elucidated based on the analysis of the X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) test. Response surface method (RSM) is used to optimize the slurry parameters and finally obtaining an atomic-scale surface with Sa 0.3 nm.

Thoughts on the past, present and future of UHV surface chemistry and the birth of Single-Atom Alloys

Throughout its relatively short lifetime, ultra-high vacuum (UHV) surface chemistry has progressed quickly. In the 1960′s, pioneers like Ertl and Somorjai started the field using single crystals and gained significant insight into catalytic processes by relating surface structure to reactivity. The more recent proliferation of scanning probes has significantly increased the power of the single crystal approach by enabling the atomic-scale structure of active sites to be correlated with their reactivity. In this perspective we briefly discuss how the field developed, identify some challenges, and highlight Single-Atom Alloys (SAAs), a new class of heterogeneous catalyst that was developed from a fundamental surface science approach. However, despite recent successes, funding for fundamental surface science has declined. Academic hires in the discipline are also declining in part due to the start-up costs. We make the case that fundamental UHV surface chemistry is still too young a field to be in recession.

Colossal Zero-Field-Cooled Exchange Bias via Tuning Compensated Ferrimagnetic in Kagome Metals.

Exchange bias (EB) is a crucial property with widespread applications but particularly occurs by complex interfacial magnetic interactions after field cooling. To date, intrinsic zero-field-cooled EB (ZEB) has only emerged in a few bulk frustrated systems and their magnitudes remain small yet. Here, enabled by high temperature synthesis, we uncover a colossal ZEB field of 4.95 kOe via tuning compensated ferrimagnetism in a family of kagome metals, which is almost twice the magnitude of known materials. Atomic-scale structure, spin dynamics, and magnetic theory revealed that these compensated ferrimagnets originate from significant antiferromagnetic exchange interactions embedded in the holmium-iron ferrimagnetic matrix due to supersaturated preferential manganese doping. A random antiferromagnetic order of manganese sublattice sandwiched between ferromagnetic iron kagome bilayers accounts for such unconventional pinning. The outcome of the present study outlines disorder-induced giant bulk ZEB and coercivity in layered frustrated systems.

Ultrahigh energy storage performance in BNT-based binary ceramic via relaxor design and grain engineering

Dielectric capacitors attract much attention for advanced electronic systems owing to their ultra-fast discharge rate and high power density. However, the low energy storage density (Wrec) and efficiency (η) severely limit their applications. Herein, Bi0.5Na0.5TiO3-K0.5Na0.5NbO3 binary ceramic is developed to obtain excellent energy storage performance with strong relaxor behavior, together with large polarization and distorted lattice calculated by first-principles calculations. The atomic-scale local structure analysis of 0.75Bi0.5Na0.5TiO3–0.25K0.5Na0.5NbO3 ceramic reveals distorted polarization distribution and small polar nanoregion related with the enhanced relaxation and low hysteresis. Finite element analysis demonstrates ultrafine grain and increased grain boundary density following hot-pressing sintering are crucial factors in significantly enhancing the breakdown strength (Eb). As a result, ultrahigh Wrec of 12.39 J/cm3 and η of 87.1 % are achieved at a superhigh Eb of 713 kV/cm. Encouragingly, excellent performance of temperature, frequency and fatigue stability at 400 kV/cm are obtained, and high power density of ∼ 306 MW/cm3 as well as fast discharge speed of ∼ 23.3 ns are also realized. This work proposes a simple binary design with strong relaxor behavior, and highlights the potential of grain engineering to achieve outstanding energy storage performance with great stability and excellent charge-discharge property for pulse power devices.

Structural Complexities in Sodium Ion Conductive Antiperovskite Revealed by Cryogenic Transmission Electron Microscopy.

We use low-dose cryogenic transmission electron microscopy (cryo-TEM) to investigate the atomic-scale structure of antiperovskite Na2NH2BH4 crystals by preserving the room-temperature cubic phase and carefully monitoring the electron dose. Via quantitative analysis of electron beam damage using selected area electron diffraction, we find cryogenic imaging provides 6-fold improvement in beam stability for this solid electrolyte. Cryo-TEM images obtained from flat crystals revealed the presence of a new, long-range-ordered supercell with a cubic phase. The supercell exhibits doubled unit cell dimensions of 9.4 Å × 9.4 Å as compared to the cubic lattice structure revealed by X-ray crystallography of 4.7 Å × 4.7 Å. The comparison between the experimental image and simulated potential map indicates the origin of the supercell is a vacancy ordering of sodium atoms. This work demonstrates the potential of using cryo-TEM imaging to study the atomic-scale structure of air- and electron-beam-sensitive antiperovskite-type solid electrolytes.

Theoretical model for elastic modulus prediction on basis of atomic structure information: Beginning from amorphous carbon to covalent bonding materials

The mechanical property is one of the most important properties of a material and is determined by the atomic-scale structure. It is thus crucial to establish the theoretical model for the prediction of materials' mechanical properties, benefiting the material design. In our previous work, we proposed an atomic-scale structure-based model for predicting the Young's modulus of hydrogenated amorphous carbon; however, the application range of this model is too narrow to be used practically for the materials development. Therefore, in this work, an extended prediction model of Young's modulus of materials with wide applicability is successfully constructed, based on the two important inherent properties of materials: one is effective coordination number (CNeff) evaluating how densely atoms are structured and the another one is effective bond stiffness (Keff) that is first proposed here and indicates how bonding types contribute the elastic properties. Through the high-throughput molecular dynamics simulations, the predictive model of Young's modulus (E) is determined as E = 3.37Keff (CNeff - 2.0)1.5. Then we demonstrate that this model is valid for a large variety of materials including both amorphous and crystalline structures, and the accuracy is proven by comparing with other work. Overall, this fundamental work may benefit the preparation, development, and utilization of new materials.