Evolution Of Atomic Structure Research Articles

Monitoring C–C coupling in catalytic reactions via machine-learned infrared spectroscopy

Abstract Tracking atomic structural evolution along chemical transformation pathways is essential for optimizing chemical transitions and enhancing control. However, molecule-level knowledge of structural rearrangements during chemical processes remains a great challenge. Here, we couple infrared spectroscopy as a non-invasive method to probe molecular transformations, with a machine-learned protocol to immediately map the spectroscopic fingerprints to atomistic structures. From the theoretical perspective, we demonstrate it here with the example of C–C coupling in catalytic reactions, elucidating various structural conformations along dynamic trajectories. Within the transferable application to the specific CO–CO dimerization reaction, the structural and energetic variations of the critical chemical species could be identified via infrared spectroscopy. This approach extends the power of spectroscopy from fingerprinting chemical configurations to using them for assigning dynamic structural information.

Atomic Fabrication of 2D Materials Using Electron Beams Inside an Electron Microscope.

Two-dimensional (2D) materials have garnered increasing attention due to their unusual properties and significant potential applications in electronic devices. However, the performance of these devices is closely related to the atomic structure of the material, which can be influenced through manipulation and fabrication at the atomic scale. Transmission electron microscopes (TEMs) and scanning TEMs (STEMs) provide an attractive platform for investigating atomic fabrication due to their ability to trigger and monitor structural evolution at the atomic scale using electron beams. Furthermore, the accuracy and consistency of atomic fabrication can be enhanced with an automated approach. In this paper, we briefly introduce the effect of electron beam irradiation and then discuss the atomic structure evolution that it can induced. Subsequently, the use of electron beams for achieving desired structures and patterns in a controllable manner is reviewed. Finally, the challenges and opportunities of atomic fabrication on 2D materials inside an electron microscope are discussed.

In situ revealing the dehydration and atomic structure evolution of protonated titanate nanotubes via environmental transmission electron microscopy

The precise control of nanomaterial microstructure at the atomic level relies on the comprehensive understanding of atomic structural transition during the fabrication process at the atomic level. The phase transition from protonated titanate to TiO2 is a prevalent route to synthesize low dimensional TiO2-based nanomaterials, which exhibit excellent photocatalytic, lithium-ion battery performances, while the detailed phase transition mechanism remains to be clarified due to lack of atomic-level in situ information. Herein, the atomic structural transitions from one-dimensional H2Ti3O7 (HT) to TiO2(B) and anatase TiO2 (TB and TA) nanocrystals were revealed through in situ environmental transmission electron microscopy, which exhibited a two-step phase transition at 200–600 °C. (I) H2Ti3O7 to TiO2(B) transition began via an indirect pathway at ∼200 °C: The HT (200) interlayer dehydration occurred firstly with lattice shrinkage; Then TB discretely nucleated at the dehydrated H2Ti3O7 nanotube wall with a crystallographic relationship of (200)HT∥(200)TB {[001]HT∥[001]TB}; At higher temperature, the separated nuclei grew up with defects and distorted crystal lattice among them, which then connected and jointed to a single crystalline TB nanotube by atomic rearrangement. (II) The further transition of TiO2(B) to anatase TiO2 occurred via a direct pathway above 400 °C: Scarce nucleation event of TA phase was observed, which generated within TB nanocrystal with a crystallographic relationship of (200)TB∥(002)TA {[001]TB∥[010]TA}. Once a TA nucleus formed, it grew up to a large crystal by consuming the neighbor TB nanocrystals. These findings may contribute to comprehensively understanding phase transition and precisely manipulating the atomic structure of one-dimensional TiO2 nanocrystals.

Temperature-induced structure evolution in monocrystalline and polycrystalline cobalt via molecular dynamics simulations

The structure transition of metallic melt strongly depends on temperature and significantly influences the comprehensive properties. However, observing the structure change in experiments is still challenging. Here, molecular dynamics is used to study the melting process and microstructural evolution in single crystal and polycrystal cobalt (Co). The results indicate that the melting process of single crystal structure starts from 1870 K and lasts for a very short period, while the polycrystal melts from about 1760 to 1870 K. In polycrystal Co, the melting initially occurs in grain boundaries, and the melting temperature shows a positive correlation with grain size. An interesting solidification phenomenon occurs on the surface of big grains in the beginning of melting process. The coordination number increasing from 12 to about 13.4 near melting point proves the local expansion of the first coordination shell, indicating the structural evolution from long-range order to short-range order in the continuous heating process. The common neighbor sub-cluster index and Voronoi polyhedron demonstrate the short-range icosahedron structures, while these polyhedrons become polydisperse and isolated in Co liquid. The findings ignite the investigation of the liquid structure origin of crystal materials and extend the understanding of the atomic structure evolution in melting.

Atomic-scale understanding of microstructural evolution in electrochemical additive manufacturing of metallic nickel

Atomic-level manufacturing is fundamentally concerned with the precise removal, addition, and migration of material at the atomic and close-to-atomic scale (ACS). Tip-based electrochemical deposition, a quintessential ACS electrochemical additive manufacturing technique, offers promising prospects for achieving bottom-up fabrication of metallic micro/nano structures. However, the complex physicochemical reactions involved in electrodes lead to a limited understanding of the mechanisms underlying atomic electrodeposition and structural evolution. For the first time, this study proposes electric double-layer controlled electrochemical kinetics and investigates the effect of direct current (DC) and pulse current (PC) on nickel atomic electrodeposition using molecular dynamics (MD) simulations. The findings reveal that compared to DC electrodeposition, PC electrodeposition results in more orderly deposition morphology, improved surface smoothness, reduced dislocation density, and lower crystal distortion, with these effects being particularly pronounced under low pulse duty ratio conditions. In addition, the pulse frequency significantly influences the morphology and structure of the deposit. The high pulse frequency yields smoother surfaces with local protrusions, while the low frequency favors the formation of orderly and dense structures excepting slightly increased roughness. This study provides critical insights into understanding the microscopic mechanisms of atomic-scale electrodeposition processes and achieving atomically controlled tip-based electrochemical additive manufacturing of micro/nanodevices.

Atomistic simulations of the thinning process of tantalum/copper heterostructure in wafer containing through silicon via

Through silicon via (TSV) interconnect structure plays an essential role in high-density 3-D integration. The tantalum (Ta)/copper (Cu) heterointerface is one of the significant interfaces in the wafer containing TSV. Molecular dynamics simulations are utilized to study the interface evolution and thinning mechanism of Ta/Cu heterostructure at different grinding depths during the thinning process. It is found that atomic diffusion, amorphization, phase transition, and micro-defects are typical characteristics of interfacial evolution, which become more pronounced at deeper grinding depths. The extrusion of the hard Ta layer on the soft Cu layer leads to an early appearance of interface defects, including dislocations and stacking faults. It is revealed that the large lattice mismatch of the constituent layers and obstruction of the heterointerface result in obvious stress concentration. The reduced tangential and normal forces at the interface are ascribed to atomic structure evolution and intrinsic properties of the heterostructure. Additionally, the heterointerface can hinder heat conduction between the Ta and Cu layers. These findings can provide valuable guidance for the manufacture of the wafer containing TSV.

On-the-Fly Machine Learning Force Field Study of Liquid-Al/Solid-TiB2 Interfaces.

Using the on-the-fly machine learning force field, simulations were performed to study the atomic structure evolution of the liquid-Al/solid-TiB2 interface with two different terminations, aiming to deepen the understanding of the mechanism of TiB2 as nucleating particles in an aluminum alloy. We conducted simulations using MLFF for up to 100 ps, enabling us to observe the interfacial properties from a deeper and more comprehensive perspective. The nucleation potential of TiB2 particles is determined by the formation of various ordered structures at the interface, which is significantly influenced by the termination of the TiB2 (0001) surface. The evolution of the interface during heterogeneous nucleation processes with different terminations is described using structural information and dynamic characteristics. The Ti-terminated surface is more prone to forming quasi-solid regions compared to the B-termination. Analysis of mean square displacement and vibrational density of states indicates that the liquid layer at the Ti-terminated interface is closer in characteristics to a solid compared to the B-terminated interface. We also found that on the TiB2 (0001) surface different terminations give rise to distinct ordered structures at the interfaces, which is ascribed to their different diffusion abilities.

A sustainable deposition method for diamond-like nanocomposite coatings – Insights into the evolution of atomic structure and properties

Silicon and oxygen-doped hydrogenated amorphous carbon (a-C:H:Si:O), typically termed as diamond-like nanocomposite (DLN), is amongst the most widely adapted coating materials for advanced applications. Traditionally, these coatings are deposited by plasma-enhanced chemical vapor deposition (PECVD) using organosilicon precursors, which not only restrict coating stoichiometry but also have a high environmental footprint. Here, we report the deposition of highly dense and microscopically defect-free a-C:H:Si:O coating using magnetron sputtering as a facile and sustainable deposition method that allows a fine stoichiometry control. Furthermore, we investigate the evolution of the atomic structure and the local-atomic environment when Si, O, and H atoms are successively doped into the amorphous carbon (a-C) matrix. Diffraction and spectroscopic analyses of doped coatings indicate the absence of any long-range order and an atomic-scale composite structure. The results also establish that doping of a-C leads to improved sp3 bonding, formation of Si–O-based networks, and termination of carbon dangling bonds, resulting in increased structural stability. The densification of the coating, combined with improved sp3 character, results in hardness and modulus exceeding those of PECVD-deposited coatings. These findings present a viable potential of magnetron sputtering as a straightforward and greener alternative to traditional PECVD-based methods for producing a promising coating material.

Effect of Strain Rate on Mechanical Deformation Behavior in CuZr Metallic Glass.

Tensile tests were performed on Cu64Zr36 metallic glass at strain rates of 107/s, 108/s, and 109/s via classical molecular dynamics simulations to explore the underlying mechanism by which strain rate affects deformation behavior. It was found that strain rate has a great impact on the deformation behavior of metallic glass. The higher the strain rate is, the larger the yield strength. We also found that the strain rate changes the atomic structure evolution during deformation, but that the difference in the atomic structure evolution induced by different strain rates is not significant. However, the mechanical response under deformation conditions is found to be significantly different with the change in strain rate. The average von Mises strain of a system in the case of 107/s is much larger than that of 109/s. In contrast, more atoms tend to participate in deformation with increasing strain rate, indicating that the strain localization degree is more significant in cases of lower strain rates. Therefore, increasing the strain rate reduces the degree of deformation heterogeneity, leading to an increase in yield strength. Further analysis shows that the structural features of atomic clusters faded out during deformation as the strain rate increased, benefiting more homogeneous deformation behavior. Our findings provide more useful insights into the deformation mechanisms of metallic glass.

Atomic-scale investigation on the evolution of T1 precipitates in an aged Al-Cu-Li-Mg-Ag alloy

The T1 (Al2CuLi) phase is one of the most effective strengthening nanoscale-precipitate in Al-Cu alloys with Li. However, its formation and evolution still need to be further clarified during aging due to the complex precipitation sequences. Here, a detailed investigation has been carried out on the atomic structural evolution of T1 precipitate in an aged Al-Cu-Li-Mg-Ag alloy using state-of-the-art Cs-corrected high-angle annular dark field (HAADF)- coupled with integrated differential phase contrast (iDPC)-scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDXS) techniques. An intermediate T1’ phase between T1p and T1 phase, with a crystal structure and orientation relationship consistent with T1, but exhibiting different atomic occupancy and chemical composition was found. We observed the atomic structural transformation from T1p to T1’ phase (fcc→hcp), involving only 1/12<112>Al shear component. DFT calculation results validated our proposed structural models and the precipitation sequence. Besides, the distributions of minor solute elements (Ag, Mg, and Zn) in the precipitates exhibited significant differences. These findings may contribute to a further understanding of the nucleation mechanism of T1 precipitate.

Discharge and ignition mechanism of high-entropy alloy induced by crack propagation under quasi-static compressive load

In order to reveal the discharge and ignition mechanism of high-entropy alloy under quasi-static compression load, the stress, potential and ignition evolution process of high-entropy alloy with prefabricated cracks were tested. The atomic structure and dislocation evolution of high-entropy alloy during crack propagation and compression were determined by molecular dynamics simulation, and the discharge mechanism of crack tip was analyzed at the micro level. Based on the calculation of Gibbs free energy, the products in the ignition reaction process were predicted. The results of mechanical/thermal/electrical characteristics induced by compressive load show that the discharge usually occurs near the ignition moment of specimen fracture, and the maximum potential signal can reach 34.2 V. The formation of crack group, crack propagation, local stress concentration, charge accumulation and release at the crack tip jointly induce the generation of discharge signals. With the propagation of cracks, a large number of stacking faults appear and a diamond-shaped failure zone is formed. The increase of disordered atoms leads to fracture. During the compression process, a large number of dislocations induce the separation of charges, which aggravates the discharge at the crack tip. In the Hf-Zr-Ti-Ta-Nb-Cu high-entropy alloy system, the reaction product Ta2O5 is preferentially generated, while in the Ti–Zr-Hf-Cu system, Ti2O3 is preferentially generated. The tip discharge and chemical bond fracture during crack propagation induce ignition, and the chemical bond recombines with energy release.

Atomistic insights into chemical vapor deposition process of preparing silicon carbide materials using ReaxFF-MD simulation

SiC is a kind of key advanced coating material used widely. The in-depth understanding of the CVD process of preparing SiC materials at the atomic scale is beneficial for studying the CVD mechanism and then optimizing the materials. Here, a CVD model of preparing SiC materials is established based on ReaxFF-MD considering the whole pyrolysis-adsorption-desorption-growth mechanism. The accuracy of the ReaxFF potential and its parameters is verified by DFT at first. Then an energy-based Monte Carlo (EMC)-MD-CVD model is proposed based on the bonding formation energy originating from DFT. It is used to simulate the CVD process of preparing SiC materials. The CVD of MTS is studied by deposition efficiency and deposited film components at different temperatures, and it can be found it in good agreement with experimental results. Finally, the crystallization of SiC film prepared by the CVD of MTS is studied by energy evolution and atomic structure evolution. The results are helpful to understand the CVD mechanism for preparing coatings and it can be used to optimize the CVD parameters of preparing SiC materials in the future.

Ultralow Strain-Induced Emergent Polarization Structures in a Flexible Freestanding BaTiO3 Membrane.

The engineering of ferroic orders, which involves the evolution of atomic structure and local ferroic configuration in the development of next-generation electronic devices. Until now, diverse polarization structures and topological domains are obtained in ferroelectric thin films or heterostructures, and the polarization switching and subsequent domain nucleation are found to be more conducive to building energy-efficient and multifunctional polarization structures. In this work, a continuous and periodic strain in a flexible freestanding BaTiO3 membrane to achieve a zigzag morphology is introduced. The polar head/tail boundaries and vortex/anti-vortex domains are constructed by a compressive strain as low as ≈0.5%, which is extremely lower than that used in epitaxial rigid ferroelectrics. Overall, this study c efficient polarization structures, which is of both theoretical value and practical significance for the development of next-generation flexible multifunctional devices.

Revealing in situ stress-induced short- and medium-range atomic structure evolution in a multicomponent metallic glassy alloy

Deformation behaviour of multicomponent metallic glasses are determined by the evolution/reconfiguration of the short- and medium-range order (SRO and MRO) atomic structures. A precise understanding of how different atom species rearrange themselves in different stress states is still a great challenge in materials science and engineering. Here, we report a systematic and synergetic research of using electron microscopy imaging, synchrotron X-ray total scattering plus empirical potential structure refinement (EPSR) modelling to study in situ the deformation of a Zr-based multicomponent metallic glassy alloy with 5 elements. Systematic and comprehensive analyses on the characteristics of the SRO and MRO structures in 3D and the decoupled 15 partial PDFs at each stress level reveal quantitatively how the SRO and MRO structures evolve or reconfigure in 3D space in the tensile and compressive stress states. The results show that the Zr-centred atom clusters have low degree of icosahedra and are the preferred atom clusters to rearrange themselves under the tensile and compressive stresses. The Zr-Zr is the dominant atom pair in controlling the shear band's initiation and propagation. The evolution and reconfiguration of the MRO clusters under different stress states are realised by changing the connection modes between the Zr-centred atom clusters. The coordinated changes of both bond angles and bond lengths of the Zr-centred clusters are the dominant factors in accommodating the tensile or compressive strains. While other solute-centred MRO clusters only play minor roles in the atomic structure reconfiguration/evolution. The research has demonstrated a synergetic and multimodal materials operando characterization methodology that has great application potential in design and development of high performance multiple-component engineering alloys.

In-Situ Atomic-Scale Observation of Brownmillerite to Ruddlesden–Popper Phase Transition Tuned by Epitaxial Strain in Cobaltites

Phase transitions involving oxygen ion extraction within the framework of the crystallographic relevance have been widely exploited for sake of superconductivity, ferromagnetism, and ion conductivity in perovskite-related oxides. However, atomic-scale pathways of phase transitions and ion extraction threshold are inadequately understood. Here we investigate the atomic structure evolution of LaCoO3 films upon oxygen extraction and subsequent Co migration, focusing on the key role of epitaxial strain. The brownmillerite to Ruddlesden–Popper phase transitions are discovered to stabilize at distinct crystal orientations in compressive- and tensile-strained cobaltites, which could be attributed to in-plane and out-of-plane Ruddlesden–Popper stacking faults, respectively. A two-stage process from exterior to interior phase transition is evidenced in compressive-strained LaCoO2.5, while a single-step nucleation process leaving bottom layer unchanged in tensile-strained situation. Strain analyses reveal that the former process is initiated by an expansion in Co layer at boundary, whereas the latter one is associated with an edge dislocation combined with antiphase boundary. These findings provide a chemo-mechanical perspective on the structure regulation of perovskite oxides and enrich insights into strain-dependent phase diagram in epitaxial oxides films.

A Coherent Pd–Pd16B3 Core–Shell Electrocatalyst for Controlled Hydrogenation in Nitrogen Reduction Reaction

AbstractThe manipulation of surface catalytic sites has rarely been explored for metal borides, and the subsurface effects on the electrocatalytic activity of the nitrogen reduction reaction (NRR) remain unknown. Herein, this work develops a core–shell nanoparticle catalyst with a Pd core that ensures high electron transfer rates and an Pd16B3 atomical shell that possess tunable active sites for regulating the NRR. The atomic structural evolution from Pd to Pd16B3 is investigated by precisely controlling the B atom diffusion, molecular rearrangement, and d–sp orbital hybridization. Pd/Pd16B3 core–shell nanocrystals exhibit an exceptional NRR performance with a high NH3 Faradaic efficiency of 30.8%, which is superior to those of pristine Pd (1.2%) and B‐doped Pd (4.8%) under identical conditions, and a yield rate of 0.81 µmol h−1 cm−2. This work discovers that the Pd16B3 shell could promote the NRR selectivity by separating the separating the hydrogen evolution reaction proceeded on hole sites and NRR proceeded on bridge sites, and the Pd core could provide the excellent conductivity to Pd16B3 shell through regulated electron interactions. Consequently, the controlled chemical ordering of palladium boride on palladium surfaces provides insight into the synthesis of advanced NRR electrocatalysts.

Influence of variation in grain boundary parameters on the evolution of atomic structure and properties of [111] tilt boundaries in aluminum

Grain boundaries (GBs) are fundamental planar defects in metallic materials, exerting a profound influence on material behaviour and properties. Understanding the influence of GB structure on material performance presents challenges due to the intricate atomic arrangements involved. To overcome inconsistencies observed in previous simulations, experimental validation is necessary. However, research on [111] tilt GBs in metals other than copper, such as aluminium, remains limited. This study aims to address these gaps by investigating the characterization of [111] tilt GBs in Al, employing advanced experimental techniques and validating the findings through simulations. By exploring a range of misorientation angles and plane inclinations, two distinct misorientation groups are identified, each exhibiting unique structural configurations. However, the structural differences resulting from plane inclination are more pronounced than those stemming from misorientation angle. Additionally, it is found that deviations from precise misorientation angles and different plane inclinations significantly influence the excess properties of GBs, such as GB energy and excess volume, etc. Furthermore, comparisons between Al and Cu structures enhance our understanding of GB behaviour in different metallic materials. This study lays the foundation for robust structure-property correlations in GBs and emphasizes the importance of considering intricate GB structural details to better understand and predict their properties accurately.

Atomic observation and structural evolution of covalent organic framework rotamers

Dynamic 3D covalent organic frameworks (COFs) have shown concerted structural transformation and adaptive gas adsorption due to the conformational diversity of organic linkers. However, the isolation and observation of COF rotamers constitute undergoing challenges due to their comparable free energy and subtle rotational energy barrier. Here, we report the atomic-level observation and structural evolution of COF rotamers by cryo-3D electron diffraction and synchrotron powder X-ray diffraction. Specifically, we optimize the crystallinity and morphology of COF-320 to manifest its coherent dynamic responses upon adaptive inclusion of guest molecules. We observe a significant crystal expansion of 29 vol% upon hydration and a giant swelling with volume change up to 78 vol% upon solvation. We record the structural evolution from a non-porous contracted phase to two narrow-pore intermediate phases and the fully opened expanded phase using n-butane as a stabilizing probe at ambient conditions. We uncover the rotational freedom of biphenylene giving rise to significant conformational changes on the diimine motifs from synclinal to syn-periplanar and anticlinal rotamers. We illustrate the 10-fold increment of pore volumes and 100% enhancement of methane uptake capacity of COF-320 at 100 bar and 298 K. The present findings shed light on the design of smarter organic porous materials to maximize host-guest interaction and boost gas uptake capacity through progressive structural transformation.

Molecular dynamics simulation of deformation behavior of nanocrystal CuNiAlCo medium-entropy alloys

Applying the molecular simulation method, the melting, elastic, and plastic properties of nanocrystal CuNiAlCo medium entropy alloy are investigated in this work. The results show that the melting point in nanocrystal CuNiAlCo is decreased with decreasing crystallite size from 5.0 to 3.1 nm. Compared to the bulk modulus, the crystallite size has a discernible influence on both the shear and Young's modulus. The uniaxial tension calculations suggest that the tensile strength decreases with decreasing crystallite size, and the inverse Hall–Petch relationship is found in nanocrystal CuNiAlCo. The polyhedral template matching analysis illustrates that the FCC constitution of CuNiAlCo gradually decreases with the decrease in crystallite size. When the strain is 30%, the amorphization would occur in uniaxial tension. The atomic structural evolution of CuNiAlCo with the crystallite size of 5.0 nm under the uniaxial strain verifies that the stacking fault reconstruction and amorphization are the dominant plastic deformation mechanism for nanocrystal CuNiAlCo medium entropy alloy. The shockley dislocation plays an essential role in uniaxial tension.

Facile and Green Synthesis of Well-Defined Nanocrystal Oxygen Evolution Catalysts by Rational Crystallization Regulation.

The development of catalysts for an economical and efficient oxygen evolution reaction (OER) is critical for clean and sustainable energy storage and conversion. Nickel-iron-based (NiFe) nanostructures are widely investigated as active OER catalysts and especially shape-controlled nanocrystals exhibit optimized surface structure and electronic properties. However, the structural control from amorphous to well-defined crystals is usually time-consuming and requires multiple stages. Here, a universal two-step precipitation-hydrothermal approach is reported to prepare a series of NiFe-based nanocrystals (e.g., hydroxides, sulfides, and molybdates) from amorphous precipitates. Their morphology and evolution of atomic and electronic structure during this process are studied using conclusive microscopy and spectroscopy techniques. The short-term, additive-free, and low-cost method allows for the control of the crystallinity of the materials and facilitates the generation of nanosheets, nanorods, or nano-octahedra with excellent water oxidation activity. The NiFe-based crystalline catalysts exhibit slightly compromised initial activity but more robust long-term stability than their amorphous counterparts during electrochemical operation. This facile, reliable, and universal synthesis method is promising in strategies for fabricating NiFe-based nanostructures as efficient and economically valuable OER electrocatalysts.