Atomistic Processes Research Articles

(Invited) Insights into Atomistic Processes at Electrified Solid/Liquid Interfaces from Ab Initio Calculations

Many of the challenges we face today towards achieving a greener economy focus on processes occurring at electrified solid/liquid interfaces. Understanding how the interactions between the solid, the liquid and dissolved species are affected by the applied potential will aid our ability to achieve rational design and targeted optimization of relevant processes. Hereby, ab initio molecular dynamic simulations including explicit modelling of the aqueous environment have proven an indispensable tool to gain insights into reaction mechanisms. More approximate choices to modelling the electrolyte, such as implicit solvent models, are however also popular. The talk will discuss the suitability of approximate models for the aqueous environment to study, e.g., the stability of surface phases in the Mg/H2O system and incorporation of impurities in nano-aerogels during synthesis [1,2], by coupling DFT calculations with thermodynamic models. Furthermore, insights into reaction mechanism at electrified solid/liquid interfaces enabled by our recent developments of a thermopotentiostat [3,4] will be presented, using the example of Pt[5].[1] S.-H. Kim, S.-H. Yoo, S. Shin, A. El-Zoka, O. Kasian, J. Lim, J. Jeong, C. Scheu, J. Neugebauer, H. Lee, M. Todorova, B. Gault, Adv. Mater. 34, 2203030 (2022)[2] S.-H. Kim, S.-H. Yoo, P. Chakraborty, J. Jeong, J. Lim, A. El-Zoka, L. Stephenson, T. Hickel, J. Neugebauer, C. Scheu, M. Todorova, B. Gault, J. Am. Chem. Soc. 144, 987 (2022)[3] S. Surendralal, M. Todorova, M. Finnis, and J. Neugebauer, Phys. Rev. Lett. 120, 246801 (2018)[4] F. Deißenbeck, C. Freysoldt, M. Todorova, J. Neugebauer, and S. Wippermann, Phys. Rev. Lett. 126, 136803 (2021)[5] S. Surendralal, M. Todorova, and J. Neugebauer, Phys. Rev. Lett. 126,166802 (2021)

Process Model for SiC Oxidation for a Large Range of Conditions

A comprehensive process model for 4H‐SiC oxidation is created and calibrated against a very large collection of experimental data. The model reproduces measured oxide thickness for Si‐face, C‐face, and a‐face SiC wafers, in the temperature range 950–1500 °C, in the pressure range 0.25–4.0 atm, in the thickness range 3–1600 nm, and for SiC doping ranging between 1019 cm−3 n‐type and 1019 cm−3 p‐type. The model is based on the Massoud model: Oxidation is driven by oxidants (O2, H2O) which are present in the gas phase, diffuse through the oxide, and form SiO2 at the oxide–SiC interface. For thin oxides, the interface reaction rate includes empirical correction terms which add to the oxidation rate, and which asymptotically approach zero with increasing oxide thickness. For dry oxidation, a remarkable dependence on the O2 partial pressure is discovered: For thick oxides, the oxidation rate scales linearly with the pressure, but the correction term for thin oxides scales with the square root of the pressure. This suggests that the atomistic processes responsible for the fast initial growth of oxides involve the splitting of O2 molecules into two O atoms.

Grain boundary plasticity initiated by excess volume

Grain boundaries (GBs) serve not only as strong barriers to dislocation motion, but also as important carriers to accommodate plastic deformation in crystalline solids. During deformation, the inherent excess volume associated with loose atomic packing in GBs brings about a microscopic degree of freedom that can initiate GB plasticity, which is beyond the classic geometric description of GBs. However, identification of this atomistic process has long remained elusive due to its transient nature. Here, we use Au polycrystals to unveil a general and inherent route to initiating GB plasticity via a transient topological transition process triggered by the excess volume. This route underscores the general impact of a microscopic degree of freedom which is governed by a stress-triaxiality-based criterion. Our findings provide a missing perspective for developing a more comprehensive understanding of the role of GBs in plastic deformation.

Modeling of shock wave propagation in porous magnesium based on artificial neural network

Data transfer from the level of atomistic simulations to the level of continuum modeling is one of the urgent problems in the mechanics of materials. In this work, we perform multiple molecular dynamics (MD) simulations of the uniaxial (along different crystallographic directions) and volumetric (isotropic) compression of porous HCP magnesium single crystals in order to study the atomistic processes of the plastic deformation of anisotropic porous material and to create a reference dataset (strain dependencies of stresses, porosity and dislocation density at different temperatures and initial porosities). The MD data are used to train two artificial neural networks (ANNs): the first ANN is a surrogate of the constitutive equation of porous magnesium, while the second ANN describes the dislocation nucleation/emission as the onset of plastic flow. In contrast to the previously considered case of porous FCC aluminum, the present approach takes into account the anisotropy of the matrix material. The ANN-based constitutive equation is successfully used within the continuum mechanics model for numerical study on the shock wave structure in porous magnesium in comparison with direct MD simulation of the shock wave problem.

Linking local microstructure to fracture location in a two-dimensional amorphous solid under isotropic strain.

Brittle fracturing of materials is common in natural and industrial processes over a variety of length scales. Knowledge of individual particle dynamics is vital to obtain deeper insight into the atomistic processes governing crack propagation in such materials, yet it is challenging to obtain these details in experiments. We propose an experimental approach where isotropic dilational strain is applied to a densely packed monolayer of attractive colloidal microspheres, resulting in fracture. Using brightfield microscopy and particle tracking, we examine the microstructural evolution of the monolayer during fracturing. Furthermore, we propose and test a parameter termed Weakness that estimates the likelihood for particles to be on a crack line, based on a quantified representation of the microstructure in combination with a machine learning algorithm. Regions that are more prone to fracture exhibit an increased Weakness value, however the exact location of a crack depends on the nucleation site, which cannot be predicted a priori. An analysis of the microstructural features that most contribute to increased Weakness values suggests that local density is more important than orientational order. Our methodology and results provide a basis for further research on microscopic processes during the fracturing process.

Polymorphic phase transition in CoCrNi medium-entropy alloy under impact loadings

Polymorphic phase transition in metallic materials under high pressure is a critical aspect of dynamic properties and has been attracting a great interest. Despite the extensive researches have been made on understanding of this phase transition in traditional single-principal element alloys, little is known about the phase transition in recently emergent multi-principal medium and high entropy alloys, especially compressed under high strain rates. In this work, based on molecular dynamic simulations, three impact loading strategies with distinct loading paths, such as single-shock, double-shock and ramp-wave loading are carried out on the single crystalline CoCrNi medium-entropy alloy (MEA) to investigate the phase transition under high strain-rate compression. Careful characterizations show that the phase transition of CoCrNi MEA is loading-path dependent, as evidenced by the significant differences in macroscopic pressure evolution and microscopic structural phase transition among the samples under various thermodynamic paths. An intriguing pressure “overshoot” is found and demonstrated as the characteristic of the critical structural phase transition from face-centered cubic (FCC) structure to hexagonal-close-packed (HCP) structure mediated by body-centered cubic (BCC) like clusters. We show that such loading-path dependence is attributed to the strain rate and temperature rise in the loading process, which control the evolution of microstructure and deformation field. The inherent correlation between the atomistic process of phase transition and loading strategies results in polymorphic phase transition under high strain rates. These findings shed new light on the nature of impact phase transition of multi-principal alloys.

Adsorption, dissociation, and diffusion of borazine on Pt(111)

Using first-principles calculations, we investigate the adsorption, dissociation, and diffusion of borazine, a common precursor for the growth of two-dimensional hexagonal boron nitride (h-BN), on a Pt(111) surface. Our calculations show that borazine adsorbs on Pt(111) both molecularly and dissociatively. The molecular borazine has a planar structure, whereas the dissociated borazine is either in a tilted or vertical configuration. The energy differences between the structures are only ∼0.1 eV although the dissociated ones are more stable, and the energy barriers for conversion between the structures are also of the order of 0.1 eV. For molecular adsorption, borazine migrates through direct (0.87 eV barrier) or rotational (0.67 eV barrier) mechanisms. In the dissociated state, with alternating parallel and perpendicular structures, the borazine radical can migrate via a “slinky” movement. This results in a much lower diffusion barrier of 0.2 eV. In the case of vertical diffusion, the radical migrates with an energy barrier of 0.29 eV. Our results are consistent with experimental observations of borazine dehydrogenation and may be helpful in understanding h-BN formation at room temperature. We also discuss the atomistic processes for h-BN formation at high growth temperatures.

On the role of the electric field in the last-stage sintering of ceramics: A phase-field modelling approach

“Field-assisted sintering techniques” (FAST) are one of the landmarks for ceramists nowadays. From the beginning of their discovery, their physical mechanisms are a pending question. In this paper, the growth of a collective of 3D ceramic grains (with or without an electric field) has been modelled through phase-field. The model allows switching off thermal effects from pure non-thermal electrodynamical ones.The results show that strong electric fields affect the sintering process, but, contrary to usual statements, they retard grain growth. This effect weakens as temperature rises. Therefore, the origin of FAST techniques must have a thermal component A hypothesis is proposed: local non-homogeneities of transport properties at the grains are demonstrated to induce intense temperature gradients even in stationary conditions, and it can act as driving force for flash. The intrinsic cause of electric field-induced inhomogeneities must be related to local atomistic processes, which are still unknown.

Correlation of the electronic structure and Li‐ion mobility with modulus and hardness in LiNi0.6Co0.2Mn0.2O2 cathodes by combined near edge X‐ray absorption finestructure spectroscopy, atomic force microscopy, and nanoindentation

AbstractThe electrochemical performance of cathode materials in Li‐ion batteries is reflected in macroscopic observables such as the capacity, the voltage, and the state of charge (SOC). However, the physical origin of performance parameters are atomistic processes that scale up to a macroscopic picture. Thus, revealing the function and failure of electrochemical devices requires a multiscale (and ‐time) approach using spectroscopic and microscopic techniques. In this work, we combine near‐edge X‐ray absorption fine structure spectroscopy (NEXAFS) to determine the chemical binding state of transition metals in LiNi0.6Co0.2Mn0.2O2 (NCM622), electrochemical strain microscopy to understand the Li‐ion mobility in such materials, and nanoindentation to relate the mechanical properties exhibited by the material to the chemical state and ion mobility. Strikingly, a clear correlation between the chemical binding, the mechanical properties, and the Li‐ion mobility is found. Thereby, the significant relation of chemo‐mechanical properties of NCM622 on a local and global scale is clearly demonstrated.

Nanoscale Wear Triggered by Stress-Driven Electron Transfer.

Wear of sliding contacts causes device failure and energy costs; however, the microscopic principle in activating wear of the interfaces under stress is still open. Here, the typical nanoscale wear, in the case of silicon against silicon dioxide, is investigated by single-asperity wear experiments and density functional theory calculations. The tests demonstrate that the wear rate of silicon in ambient air increases exponentially with stress and does not obey classical Archard's law. Series calculations of atomistic wear reactions generally reveal that the mechanical stress linearly drives the electron transfer to activate the sequential formation and rupture of interfacial bonds in the atomistic wear process. The atomistic wear model is thus resolved by combining the present stress-driven electron transfer model with Maxwell-Boltzmann statistics. This work may advance electronic insights into the law of nanoscale wear for understanding and controlling wear and manufacturing of material surfaces.

Understanding the Mechanism of Li-Mediated Nitrogen Reduction Reaction

Green ammonia production is a key environmental goal. As a crucial step towards this goal, a direct electrocatalytic N2reduction (eNRR) by protons and electrons emerges as a highly desirable alternative. However, all pragmatic attempts to develop such an electrochemical route have so far been hindered by invariably low selectivities and large overpotentials. The most reliable method presently is the lithium-mediated eNRR (LiNR) in non-aqueous electrolytes.1 Despite significant progress made to achieve high FE and current density2, 3,the mechanism of LiNR is yet not fully understood due to so many intricate processes involved ranging from the atomic scale to the macroscopic scale. Moreover, most Li-NRR studies have the limitations of using a sacrificial solvent as proton donors and difficulties in scaling up production in batch reactors. The talk will, address the long-term stability and high activity of PtAu in the HOR process by suppressing the unwanted THF oxidation and improving the tolerance towards CO-poison. Moreover, we investigate the mechanism of the LiNR from the ab initio atomistic reaction process, mean-field microkinetics, to the mass-transport simulations, explicitly achieving an improved understanding the dynamic change of the solid-electrolyte interphase in different LiNR reaction conditions and the effect on the performance.Reference:1 J. Choi et al, Nat. Commun. 11, 1–10 (2020)2 K. Li, et al, Science 374, 1593-1597 (2021)3 S. Li, et al, Joule, 6, 2083-2101 (2022)

The role of atomistic processes in growth of Cu–Ni metallic/bimetallic nanoparticles

Controlling the morphology of non-noble bimetallic nanocrystals can provide an excellent opportunity to improve performance and activity in catalytic reactions. Although several studies have focused on the overall macroscopic description of the synthesis process, identifying the leading factors in a typical crystal growth process at the atomic scale is still challenging. Here we report the results of atomic scale calculations on the shape evolution of bimetallic Cu–Ni nanoparticle growth using molecular static and dynamic simulations. Our calculations show that statistical analysis of space and time characteristics of single atom diffusion mechanisms and their energy barriers provide sound guidance for fabricating end products with specific shapes and architecture in a growth process.

Modeling and Simulation of Sintering Process Across Scales

Sintering, as a thermal process at elevated temperature below the melting point, is widely used to bond contacting particles into engineering products such as ceramics, metals, polymers, and cemented carbides. Modelling and simulation as important complement to experiments are essential for understanding the sintering mechanisms and for the optimization and design of sintering process. We share in this article a state-to-the-art review on the major methods and models for the simulation of sintering process at various length scales. It starts with molecular dynamics simulations deciphering atomistic diffusion process, and then moves to microstructure-level approaches such as discrete element method, Monte–Carlo method, and phase-field models, which can reveal subtle mechanisms like grain coalescence, grain rotation, densification, grain coarsening, etc. Phenomenological/empirical models on the macroscopic scales for estimating densification, porosity and average grain size are also summarized. The features, merits, drawbacks, and applicability of these models and simulation technologies are expounded. In particular, the latest progress on the modelling and simulation of selective and direct-metal laser sintering based additive manufacturing is also reviewed. Finally, a summary and concluding remarks on the challenges and opportunities are given for the modelling and simulations of sintering process.

Atomic Insight into the Successive Antiferroelectric-Ferroelectric Phase Transition in Antiferroelectric Oxides.

Antiferroelectrics characterized by voltage-driven reversible transitions between antiparallel and parallel polarity are promising for cutting-edge electronic and electrical power applications. Wide-ranging explorations revealing the macroscopic performances and microstructural characteristics of typical antiferroelectric systems have been conducted. However, the underlying mechanism has not yet been fully unraveled, which depends largely on the atomistic processes. Herein, based on atomic-resolution transmission electron microscopy, the deterministic phase transition pathway along with the underlying lattice-by-lattice details in lead zirconate thin films was elucidated. Specifically, we identified a new type of ferrielectric-like dipole configuration with both angular and amplitude modulations, which plays the role of a precursor for a subsequent antiferroelectric to ferroelectric transformation. With the participation of the ferrielectric-like phase, the phase transition pathways driven by the phase boundary have been revealed. We provide new insights into the consecutive phase transformation in low-dimensional lead zirconate, which thus would promote potential antiferroelectric-based multifunctional devices.

Molecular dynamics simulation of oxidation growth of ZnO nanopillars

Reactive molecular dynamics simulations have widely used in the oxidation, the decomposition and the chemisorption of the various materials to reveal the atomistic processes. The formation of native defects in ZnO nanopillars have studied by the method of the reactive force field molecular dynamics during oxidation growth. Results indicate that the formed ZnO nanopillars have short-range order structure with lots of O vacancies. The deficiency of the O atoms is due to the formed ZnO on the surface suppressing the further diffusion of the O atoms. Moreover, increasing the oxidation temperature makes more O atoms react with Zn atoms and increases the diffusion distance of O atoms, which is easy to form ordering structure and epitaxial growth.

Mechanical properties of TiO2/carboxylic-acid interfaces from first-principles calculations.

Nature forms structurally complex materials with a large variation of mechanical and physical properties from only very few organic compounds and minerals. Nanocomposites made from TiO2 and carboxylic-acids, two substances that are available to nature as well as materials engineers, can be seen as representative of a huge class of natural and bio-inspired materials. The hybrid interfaces between the two components are thought to determine the overall properties of the composite. Yet, little is known about the atomistic processes at those interfaces under load and their failure mechanisms. The present work models the stress-strain curves of TiO2/carboxylic-acid interfaces in the slow deformation limit for different facets and binding modes, employing density functional theory calculations. Contrary to former hypotheses, the interface rarely fails through a de-bonding of the molecule, but rather through a surface failure mechanism. Furthermore, a stress-release mechanism is discovered for the bi-dentate binding mode on the {101} facet. Deriving mechanical properties, such as the interface strength, strain at interface failure, and the elastic modulus, allows a comparison with experimental results. The calculated strengths and elastic moduli already agree qualitatively with properties of nanocomposites, despite the simplifications in the model consisting of periodic sandwich structures. The results presented here will help to improve these materials and can be directly integrated in multi-scale simulations, in order to reach a more accurate quantitative description.

Decoupled ultrafast electronic and structural phase transitions in photoexcited monoclinic VO2.

Photoexcitation has emerged as an efficient way to trigger phase transitions in strongly correlated materials. There are great controversies about the atomistic mechanisms of structural phase transitions (SPTs) from monoclinic (M1-) to rutile (R-) VO2 and its association with electronic insulator-metal transitions (IMTs). Here, we illustrate the underlying atomistic processes and decoupling nature of photoinduced SPT and IMT in nonequilibrium states. The photoinduced SPT proceeds in the order of dilation of V-V pairs and increase of twisting angles after a small delay of ~40 fs. Dynamic simulations with hybrid functionals confirm the existence of isostructural IMT. The photoinduced SPT and IMT exhibit the same thresholds of electronic excitations, indicating similar fluence thresholds in experiments. The IMT is quasi-instantaneously (<10 fs) generated, while the SPT takes place with time a constant of 100 to 300 fs. These findings clarify some key controversies in the literature and provide insights into nonequilibrium phase transitions in correlated materials.

Functionalized Clays for Radionuclide Sequestration: A Review

Performance assessment of nuclear waste disposal options requires implementation of effective buffer materials. Buffer materials must meet longevity requirements and scavenge challenging radioisotopes, e.g., iodine-129 (I-129) and technetium-99 (Tc-99), both long-lived, highly mobile, anionic species in oxic environments. In many proposed nuclear waste repositories, heat generating radioactive waste will be surrounded by clay buffer material, which swells to fill the gap between the waste package and the host geology, restricting radionuclide transport to diffusive processes for several hundred years. The clay can be functionalized to further the limit release of Tc-99, as pertechnetate (TcO4–), and I-129, as iodate (IO3–) and iodide (I–). Here, we review clay buffer functionalization and the potential to enhance sequestration of anionic radionuclides that have an increased risk of leaching out of a disposal facility. Functionalization with quaternary amines, nanosilver, bismuth-based materials, silane, and aluminum pillars have been demonstrated to improve the removal of targeted contaminants from solution. This review defines key research questions that remain: (i) can functionalized clay immobilize TcO4–, IO3–, and I– to the same extent as the reactive components themselves? (ii) Does functionalization affect the ability of the clay barrier to perform its safety functions of protecting the waste canister and limiting radionuclide transport? In addition to radionuclide retention, a multimodal analytical approach is required to assess functionalized clay performance, and recommended techniques are reviewed here. The key parameters that can influence radionuclide sequestration by functionalized clay are also evaluated. Theoretical approaches, including both density functional theory calculations and molecular dynamics simulations, can inform the energetics and kinetics of radionuclide interactions with functionalized clays. This combined experimental and theoretical approach can link atomistic and microscopic processes to predict the macroscopic physical properties of the functionalized clays.

New insights into the interfacial interactions of O2 and H2O molecules with PuH2 (110) and (111) surfaces from first-principles calculations

The interfacial reaction of highly active plutonium hydride in humid circumstance is of great interest in nuclear safe handling and storage, but it is poorly understood so far. In this paper, we first studied the O2 adsorption on (110) and (111) surfaces of PuH2 by first-principles DFT + U method. The results show that there are dissociative and non-dissociative adsorption of oxygen on the surfaces. We analyze the vibrational frequencies of non-dissociative oxygen adsorbed on the surfaces. It is found that the corresponding frequency of oxygen with bond length of 1.330–1.340 Å is 1094.8–1098.2 cm−1. The corresponding frequency of oxygen with bond length of 1.448–1.500 Å is 726.3–905.2 cm−1. It shows that non-dissociative oxygen could be considered as superoxide (O2−) or peroxide (O22−) species. In order to expound the atomistic evolution process of oxidized surface exposed to moist air or corrosive solution, the interactions between H2O molecules and the strongest oxygen adsorption structures were further explored. The results indicate that H2O molecules could dissociate into OH groups and H atoms, then they were captured to create Pu–O and H–O bonds. This work could provide new insights into the adsorption morphology of oxygen on hydride surface and the interaction between oxide/hydride interface and water.

Growth-rate model of epitaxial layer-by-layer growth by pulsed-laser deposition.

We present a numerical model of epitaxial thin-film growth applicable for pulsed-laser deposition on a single crystalline substrate. The model is based on rate equationsdescribing the time development of monolayer coverages and of densities of movable particles on atomically flat terraces. Numerical solution of the equationsshowed that the time dependence of surface roughness obeys a scaling law, the exponent of which depends on probabilities of various atomistic processes included in the simulation model. From the time dependence of monolayer coverages we calculated x-ray diffracted intensity in a quasiforbidden anti-Bragg reflection and showed that its oscillatory behavior is affected by these probabilities as well. The results show the possibility to study atomistic processes during the deposition from the time dependence of the anti-Bragg intensity measured during deposition.