Atomic Mechanisms Research Articles

Deciphering the atomistic mechanism underlying highly tunable piezoelectric properties in perovskite ferroelectrics via transition metal doping

Piezoelectricity, a fundamental property of perovskite ferroelectrics, endows the materials at the heart of electromechanical systems spanning from macro to micro/nano scales. Defect engineering strategies, particularly involving heterovalent trace impurities and derived vacancies, hold great potential for adjusting piezoelectric performance. Despite the prevalent use of defect engineering for modification, a comprehensive understanding of the specific features that positively impact material properties is still lacking, this knowledge gap impedes the advancement of a universally applicable defect selection and design strategy. In this work, we select perovskite KTa1−xNbxO3 single crystals with orthorhombic phase as the matrix and introduce Fe and Mn elements, which are commonly used in “hard” ferroelectrics as dopants. We investigate how transition-metal doping modifies piezoelectric properties from the perspective of intrinsic polarization behaviors. Interestingly, despite both being doped into the B-site as an acceptor, Mn doping enhances the local structural heterogeneity, greatly bolstering the piezoelectric coefficient beyond 1000 pC/N, whereas Fe doping tends to stabilize the polarization, leading to a substantial improvement in the mechanical quality factor up to 700. This work deciphers the diverse impacts of transition metal impurities on regulating polarization structures and modifying piezoelectric properties, providing a good paradigm for strategically designing perovskite ferroelectrics.

Ultra-large springback bending strain and its atomistic mechanism in Ni nanowires

Springback is one of the most interesting mechanical phenomena for bent metals after stress/strain release. Most previous studies investigate the springback behaviors of metals only with low bending angles; rarely do studies investigate these metals under large bending angles. Here, we investigated the springback behaviors of twin-structured Ni nanowires (NWs) at various bending angles through in-situ experiments and molecular dynamics (MD) simulations. We discovered that the springback angle and deformation mode of twin-structured Ni NWs can be divided into three stages, which have rarely been reported. The results revealed that, as the maximum bending angle increases, the plasticity of the NWs transfers from partial dislocations parallel to the twin boundaries (TBs) to extended and full dislocations on multiple slip systems, and then to grain boundary (GB) generation. Unlike previous studies which suggested that deformation-induced dislocations and GBs are irreversible, our results show that these defects are reversible upon unloading. The different recoverable abilities of these deformation-induced defects lead to a bending-angle-dependent springback phenomenon.

Atomic adsorption mechanism and initial growth of 2D GaN on graphene, h-BN and MoS2: A first principles study

Van der Waals epitaxial growth strategy on two-dimensional substrates presents a promising way for synthesizing hexagonal gallium nitride (h-GaN). This study investigates the most stable adsorption configurations of gallium (Ga) and nitrogen (N) atoms on graphene, h-BN, and MoS2 surfaces and further study the initial formation of GaN islands using the first-principles method. The interaction modes between Ga and N atoms with the substrates are analyzed using the Electron Localization Function (ELF), revealing the microscopic adsorption mechanism. Furthermore, diffusion behaviors of Ga and N atoms on the three substrates are evaluated via Climbing Image Nudged Elastic Band (CI-NEB) calculations, which shows that Ga diffuses via a sliding mechanism, while N diffusion involves bond-breaking and re-bonding events. N atom diffusion in the z-direction on these substrates are identified for the first time and the corresponding energy barriers are determined. Additionally, the growth morphology of 2D GaN on MoS2 is predicted by analyzing the competition between adatom-substrate and adatom-adatom interactions, and the possible morphology of initial 2D GaN islands is investigated. Our findings reveal that MoS2 is a favorable substrate for the growth of 2D h-GaN, providing theoretical insights to guide future experimental efforts.

Quantum Mechanics/Molecular Mechanics Simulations for Chiral-Selective Aminoacylation: Unraveling the Nature of Life

Biological phenomena are chemical reactions, which are inherently non-stopping or “flowing” in nature. Molecular dynamics (MD) is used to analyze the dynamics and energetics of interacting atoms, but it cannot handle chemical reactions involving bond formation and breaking. Quantum mechanics/molecular mechanics (QM/MM) umbrella sampling MD simulations gives us a significant clue about transition states of chemical reactions and their energy levels, which are the pivotal points in understanding the nature of life. To demonstrate the importance of this method, we present here the results of our application of it to the elucidation of the mechanism of chiral-selective aminoacylation of an RNA minihelix considered to be a primitive form of tRNA. The QM/MM MD simulation, for the first time, elucidated the “flowing” atomistic mechanisms of the reaction and indicated that the L-Ala moiety stabilizes the transition state more than D-Ala, resulting in L-Ala preference in the aminoacylation reaction in the RNA. The QM/MM method not only provides important clues to the elucidation of the origin of homochirality of biological systems, but also is expected to become an important tool that will play a critical role in the analysis of biomolecular reactions, combined with the development of artificial intelligence.

Synergistic promotion mechanism and structure-function relationship of nonmetallic atoms doped carbon nanodots driving Tagetes patula L. to remediate cadmium-contaminated soils

Phytoremediation is an economical and effective strategy to remove cadmium (Cd) from polluted environments. To improve its efficiency, nanotechnology has been proposed to collaborate with hyperaccumulators in the remediation of Cd-polluted soils. However, the intricate structure-function relationship and the underlying regulatory mechanisms by which nanomaterials regulate Cd migration and conversion within the soil-plant system remained unrevealed. In this study, functional carbon nanodots (FCNs) were modified by doping with nitrogen and (or) sulfur elements. The synthesized nonmetallic atoms-doped FCNs were utilized to investigate their structure-function relationship and the regulatory mechanisms underlying their role in the phytoremediation of Cd-polluted soils by Tagetes patula L. FCNs-based nanomaterials can regulate the migration and bioaccumulation of Cd in the soil-plant system, which exhibits an obvious structural dependency. Specifically, the synergistic application of sulfur doped FCNs and Tagetes patula L. had the highest Cd removal efficiency of 53.2%, which was 20.1% higher than Tagetes patula L. alone. The uptake and migration of Cd in the soil-plant system are regulated by FCNs-based nanomaterials through both direct and indirect mechanisms, involving interfacial reactions, plant physiology regulation and environmental influence. This study not only sheds light on the fate of FCNs-based nanomaterials and Cd in the soil-plant system, but also provides innovative nanotools for reinforcing phytoremediation efficiency in contaminated soils.

Investigation of the sintering behavior of nanoparticulate SiC by molecular dynamics simulation

Sintering is a valuable processing technique utilized to create bulk materials from powder compacts. In recent years, sintering has a garnered significant attention in the research community due to its versatility and applicability in various fields, including additive manufacturing. Herein, we investigate the sintering kinetics and mechanism of SiC nanoparticulate using molecular dynamics simulations. The surface morphology, microstructure, radial distribution function, mean square displacement, atomic displacement distribution, structural transformation, and diffusion coefficient are analyzed in detail. The free surface surrounding the neck undergoes a reconstruction to eventually reach an equilibrium dihedral angle. The neck rapidly grows and aligns with the surface, causing the SiC particle to transform from an original spherical shape into a twists shape. The concentration of shear stress results in a change of the local lattice structure from the face-centered cubic phase to amorphous phase in the surface. With an increasing sintering time, the dominant diffusion mechanism gradually shifts from surface diffusion to interface diffusion. This trend leads to a rise in the atomic migration, the consumption of interface energy, and the degree of particle fusion. Ultimately, the sintered SiC particles have the spherical shape, low surface energy, and stable structure. Additionally, the high mobility of melted atom causes a gradual dissolution of SiC particle into the large particle from the surface towards the interior. This process leads to high-altitude concentration structures that facilitate atomic migration. These results shed light on an atomic sintering mechanism in SiC nanoparticulate for applications in the preparation of nanostructured materials, and additive manufacturing.

Atomic mechanisms of oxidative behavior of ferrochromium alloys by water-oxygen environment

The formation of chromium-rich oxide layers on the surface of Fe-Cr alloys significantly impacts their performance at elevated temperatures. Understanding the formation process of these oxide layers at the atomic scale is crucial for further elucidating oxidation behavior, though it presents substantial challenges. In this study, ReaxFF molecular dynamics simulations were used to investigate the oxidation behavior of Fe-Cr alloys in high temperature water vapor and oxygen environments. The results demonstrate that chromium atoms are the primary contributors to the oxidation process, with Cr atoms diffusing to the surface more readily than iron atoms, leading to uneven stress distribution and creating high-stress regions near the Cr atoms. Additionally, hydrogen atoms generated from the breakdown of water molecules infiltrate the alloy matrix, promoting the creation, movement, and clustering of cation and anion vacancies, thereby enhancing oxidation reactions in high-temperature, humid conditions. The study further analyzes electron transfer during single-atom chemical reactions and explores the linear relationship between varying Cr content (10 %–30 %) and oxidation behavior under identical environmental conditions. These findings provide an important theoretical basis for optimizing the performance of Fe-Cr alloys for applications in high temperature environments.

Size-dependent atomic strain localization mechanism in Nb/amorphous CuNb nanolayered composites

Experiments have shown that crystalline–amorphous nanolayered composites show a strong size dependence in strength and plasticity. However, the underlying mechanism remains unknown. Here, the layer thickness (h)-dependent compressive strength and plastic deformation of Nb/amorphous CuNb nanolayered composites with h ranging from 2.8 to 20 nm have been studied by molecular dynamics simulations. It is found that the strength increases monotonically with the decrease of h, which can be well captured by the refined confined layer slip model. Furthermore, the shear banding-induced strain localization was analyzed in detail based on the evolution of the von Mises strain distribution with the applied strain and a strain localization parameter that represents the deviation of the specific atomic Mises strain from the average one of all atoms. The results show that the strain localization of the composite shows a significant size dependence and its magnitude increases with the decrease in the layer thickness. The prevention of shear band propagation in thicker layer samples is attributed to that the amorphous phase is capable of forming an atomic vortex to alleviate the strain concentration caused by dislocation absorption. The thicker amorphous layers accommodate enhanced homogeneous plasticity than the thinner ones by forming a larger vortex. Note that the above size dependence of strength and strain localization in the composite agrees well with existing experimental measurements and observations in the layer thickness range considered. As a result, the present work gives a deeper insight into the understanding of the size-dependent strengthening and strain localization mechanism in the amorphous/crystalline composites.

The Ni3Al/Ni Interfacial Contribution to the Indentation Size Effect of Ni-Based Single-Crystal Superalloys

The Ni3Al/Ni Interfacial Contribution to the Indentation Size Effect of Ni-Based Single-Crystal Superalloys

The role of surface substitution in the atomic disorder-to-order phase transition in multi-component core–shell structures

Intermetallic phases with atomic ordering are highly active and stable in catalysts. However, understanding the atomistic mechanisms of disorder-to-order phase transition, particularly in multi-component systems, remains challenging. Here, we investigate the atom diffusion and phase transition within Pd@Pt-Co cubic nanoparticles during annealing, using in-situ electron microscopy and ex-situ atomic resolution element analysis. We reveal that initial outward diffusing Pd partially substitutes Pt, forming a (Pt, Pd)-Co ternary system in the surface region, enabling the phase transition at a low temperature of 400 °C, followed by shape-preserved inward propagation of the ordered phase. At higher temperatures, excessive interdiffusion across the interface changes the stoichiometric ratio, diminishing the atomic ordering, leading to obvious change in morphology. Calculations indicate that the Pd-substitute in (Pt, Pd)-Co system leads to a significantly lower phase transition temperature compared to that of Pt-Co alloy and thus a lower activation energy for atomic diffusion. These insights into atomistic behavior are crucial for future design of multi-component systems.

Methanol-assisted NO oxidation under an oxygen atmosphere for efficient denitration: Experiments and ReaxFF-MD simulations

The development of selective catalytic reduction technologies for controlling nitrogen oxides (NOx) emissions in flue gas is hampered by high equipment and catalyst costs. In this study, a catalyst-free and efficient denitration process using methanol and oxygen as reactants was developed. Various gas atmospheres with differing concentrations were investigated to remove NO without catalysts. The addition of methanol significantly enhanced NO removal efficiency, reaching 94.6 % at 550 °C, with 55.1 % of NO converted to NO2, which can be absorbed by downstream alkali solutions. Reactive force field molecular dynamics (ReaxFF-MD) simulations were employed to analyze the atomic level NO oxidation mechanism in the CH3OH/NO/O2/H2O homogeneous reaction system. The simulation revealed the generation pathway of three key oxidizing intermediates (OH, HO2, and H2O2) and their roles in NO oxidation. This study offers critical insights into developing a cost-effective denitration process, highlighting the potential for improved NOx emission control.

Energetics of Point Defects in D0a Ag3Sn: First‐Principles Calculations

The Ag3Sn intermetallic compound is commonly found and acts as a strengthening factor in the key lead‐free solders based on Sn‐Ag‐Cu. The Ag3Sn phase exhibits also an asymmetric off‐stoichiometry, which necessitates the existence of constitutional point defects. The presence of point defects can, in general, sensitively affect the plastic deformation of materials and the strengthening role of Ag3Sn in particular. To understand the nature of the point defects and the compositional stability of Ag3Sn, the relaxed structures and the energies of single point defects (vacancies and antisite defects) in all three sublattices of Ag3Sn are calculated at 0 K using ab initio density functional theory (DFT) and the supercell method. The first‐principles‐based calculations predict that the AgSn antisite defect (Ag substitution for Sn) has the smallest enthalpy of formation, thus indicating that the AgSn antisite defect is the dominant point defect in Ag3Sn. The point defect concentrations as a function of Sn content are estimated using the DFT defect energies and the Wagner–Schottky model. The compositional dependence of the point defect concentrations gives new insights on the atomistic mechanisms of formation of nonstoichiometric Ag3Sn.

Experimental and molecular simulation study of CO2 adsorption in ZIF-8: Atomic heat contributions and mechanism

We successfully synthesised ZIF-8 using the solvothermal method at room temperature to study CO2 adsorption storage at 273 and 298 K up to 35 bar. Characterisation methods such as BET, SEM-EDS, XRD, and TGA were used to measure the physical and composition properties of ZIF-8. Grand canonical Monte Carlo (GCMC) simulation was conducted to compare with experimental data and get inside of the CO2 adsorption mechanism by calculating the isosteric heat and its fluid–fluid and solid–fluid contributions. The second was also split into fluid–solid atom contributions to understand in detail the interaction between CO2 and ZIF-8. The analyses revealed that there are three main stages during the CO2 adsorption gas–solid atom contributions, developing, pore-filling and densification. During the developing and pore-filling stages the largest fluid–solid atom contribution to the isosteric heat is CO2-C2 interactions, indicating that the CO2 is adsorbed close to the hexagonal windows of the ZIF-8 structure, while during the densification stage the largest contribution is CO2-N interactions. Where C2 and N refers to C-atom and N-atom, respectively in NCH group of the solid framework. This is because CO2 changes its orientation to be able to accommodate more molecules in the pore cavity. This work provides a better understanding of the adsorption mechanism of CO2 on ZIF-8 and shows how molecular simulation can be used to improve the understanding gas adsorption storage on metal–organic frameworks.

Multi-scale structure and reinforcement mechanisms of multi-element composite ceramic coatings

Ceramic coating is of significant importance for improving metallic materials in terms of mechanical properties or durability, which is commonly encountered in the field of machinery, civil engineering, and aerospace, etc. However, ceramic materials optimization is a great challenge due to the complex multi-scale design from atomic to macro level. This paper investigates the multiscale characteristics of multi-element composite ceramic coating, including electronic properties, 3D pore structure, and engineering performances and gives their bottom-up connections, via first-principles calculations and multiscale experiments. The doping of Ti, Zr, and Ce in the alumina-based ceramic crystal improves the overlap of electronic clouds between oxygen and metal atoms, which modifies the atomic charges and enhances the ionic bonding. In terms of microstructure, it reveals the mechanisms of phase transformation toughening effect of ZrO2 and the grain refinement and grain boundary purification effects of CeO2, which facilitates the amelioration of pore structure and macro mechanical properties. It provides multiscale information on the phase stability of multi-element alumina-based ceramics, shedding light on the fundamental atomic level mechanisms that play a crucial role in customizable functional designs

Research on controllable processing technology of microsphere cavity inside silicon substrates utilizing thermoelectric coupling effect

Micro cavity structures are extensively utilized in semiconductor micro- and nanosensor devices, especially spherical microcavity, whose high Q value not only significantly improves the sensitivity of the sensors, but also enhances their reliability in complex environments. The integration of this structure not only optimizes the performance of the sensor, but also provides the possibility for high-precision detection. In this study, a thermoelectric coupling method for controllable migration of microsphere cavity inside silicon materials is proposed in order to achieve stable formation of the internal microsphere cavity structure. The directional migration mechanism of atoms on the surface of microsphere cavities in silicon substrates under an electric field is explored using a phase field model. The model indicates that changes in the total free energy density induce a solid-gas phase transition on the surface of the microsphere cavity. It is shown that the migration velocity of the microsphere cavity increases proportionally with the electric field strength, and the migration distance increases by approximately 9% for every 10% increase in electric field strength. The migration direction aligns with the direction of the electric field. Simulation results validate the theoretical accuracy, the feasibility of controllable migration by thermoelectric coupling effect in conductive materials through experimental studies. This study provides novel methods and insights for fabricating high-quality spherical cavity in silicon materials and preparing highly sensitive micro- and nanosensor devices.

Mechanism of metastable krypton atom preparation via laser-induced ionization

Preparation of metastable Kr atoms in the 5s[3/2]2 level via laser-induced ionization has been achieved. The temporal evolution of the intensity of Kr atomic spectral lines at 760.15 nm, 811.29 nm, and 431.96 nm was used to elucidate the production mechanisms of metastable Kr atoms. These mechanisms primarily involve two processes: the “excitation + radiation” process, dominated by multiphoton excitation and initial plasma-induced electron impact excitation, and the “ion-electron recombination” process, governed by avalanche ionization. The decay time constants of Kr atomic spectral lines, corresponding to the “excitation + radiation” and “ion-electron recombination” processes respectively, were obtained experimentally under both strong and weak ionization conditions. The experiments revealed delay in preparations of metastable Kr atoms between these two processes. To reduce the loss of metastable Kr atoms and effectively utilize their peak concentration, we drew inspiration from metastable rare gas lasers and proposed the “cycling” idea to keep metastable Kr atoms produced by these two processes as synchronized as possible. We used 811.29 nm laser to excite metastable Kr atoms generated rapidly during the “excitation + radiation” stage to the 5p[5/2]3 level. The Kr atoms returned to the 5s[3/2]2 level through spontaneous radiation, merging with metastable Kr atoms that were slowly produced during the “ion-electron recombination” stage. We hope that the “cycling” idea can shorten the delay in preparations of metastable Kr atoms from both processes and enhance the peak concentration of metastable Kr atoms. However, the experimental results didn't meet expectations, as we observed a decrease in the 811.29 nm fluorescence after laser excitation, attributed to the accumulation of 5p[5/2]3 level Kr atoms. These atoms undergo energy pooling to populate the 4d’[3/2]1 and 5d[7/2]3 levels, followed by absorption of 811.29 nm laser energy leading to photoionization. Reducing the concentration of 5p[5/2]3 level Kr atoms helps mitigate the reionization issue.

In Situ Atomic‐Scale Experiments Reveal the Atomistic Mechanisms of Grain Boundary Plasticity

AbstractGrain boundary (GB) plastic deformation is a critical deformation mode that can significantly affect the microstructure and mechanical properties of metals. Given its importance, numerous studies have been conducted over the past few decades to investigate the atomistic mechanism of GB plasticity, which includes GB migration, GB sliding, and grain rotation. These three deformation modes typically occur simultaneously and interact cooperatively. Despite substantial advancements in understanding the atomic‐scale mechanisms of GB plasticity, there is a scarcity of review papers addressing the in situ atomic‐scale mechanisms of GB plasticity. This work aims to provide a comprehensive overview of the atomistic mechanisms of GB plasticity in metals, detailing modes predicted by theoretical models and molecular dynamics (MD) simulations, as well as discussing mechanisms verified by in situ atomic‐scale experiments. Additionally, it examines the factors influencing GB deformation. The authors intend for this review to serve not only as a valuable resource for researchers in the field of nanocrystalline (NC) metals but also as a textbook for educating graduate students.

New insights into physical origins of dynamic strain aging in Ti-2Al-2.5Zr alloy and influence on LCF and HCF behaviors

The multi-step strain aging tests were meticulously designed in this work to reveal the physical mechanisms of static strain aging (SSA) and dynamic strain aging (DSA) behaviors in Ti-2Al-2.5Zr alloy for the first time. It was revealed that the shuffling mechanism of interstitial oxygen atoms combined with the pinning effect of locally-generated cross-slip on the movement of screw dislocations were responsible for the occurrence of strain aging. Furthermore, the effects of DSA on the low-cycle fatigue (LCF) and high-cycle fatigue (HCF) properties were elucidated in Ti-2Al-2.5Zr alloy. Results showed that the sensitivity of DSA to cyclic loading was attributed to the generation of numerous residual edge dislocation segments through local cross-slip, facilitating the formation of dislocation veins which inhibited the formation of persistent slip bands (PSBs) and led to the significantly cyclic hardening. Finally, it was emphasized that the phenomenon of DSA should be carefully considered for the structural integrity assessment of Ti-2Al-2.5Zr alloy and several suggestions were provided.

Uncovering the electrochemical stability and corrosion reaction pathway of Mg (0001) surface: Insight from first-principles calculation

An understanding of the anomalously enhanced hydrogen evolution reaction (HER) of magnesium (Mg) under anodic polarisation in aqueous corrosion is paramount for a predictive theory of its corrosion and metal electrocatalysis. Previous theoretical and experimental studies have proposed that sub-surface hydride phases play a role in this behaviour but the underlying atomic mechanisms remain unclear. By constructing theoretical surface Pourbaix diagrams, based on density functional theory (DFT) calculations, we have identified the atomic structure of a sub-surface hydride phase on the Mg (0001) surface that remains electrochemically stable under significant anodic overpotentials across a wide pH range. Specifically, this stability persists up to 0.38 VSHE under mildly alkaline conditions (e.g., pH = 8), thus providing thermodynamic support for the proposed hydride-enhanced HER under anodic conditions. Reaction barrier analysis establishes that the proposed sub-surface hydride phase could promote anodic HER via a Heyrovsky pathway, based on hydrogen outward diffusion, with an energy barrier of 1.54 eV as the rate-limiting step, showing an anodic characteristic and significantly favouring external anodic polarisation. Furthermore, we have established that the surface adsorption condition, contingent on both the pH and potential, significantly influences the mechanism and kinetics of the initial corrosion of Mg.

Engineering of Single-Atomic Sites for Electro- and Photo-Catalytic H2O2 Production.

Direct electro- and photo-synthesis of H2O2 through the 2e- O2 reduction reaction (ORR) and H2O oxidation reaction (WOR) offer promising alternatives for on-demand and on-site production of this chemical. Exploring robust and selective active sites is crucial for enhancing H2O2 production through these pathways. Single-atom catalysts (SACs), featuring isolated active sites on supports, possess attractive properties for promoting catalysis and unraveling catalytic mechanisms. This review first systematically summarizes significant advancements in atomic engineering of both metal and nonmetal single-atom sites for electro- and photo-catalytic 2e- ORR to H2O2, as well as the dynamic behaviors of active sites during catalytic processes. Next, the progress of single-atom sites in H2O2 production through 2e- WOR is overviewed. The effects of the local physicochemical environments on the electronic structures and catalytic behaviors of isolated sites, along with the atomic catalytic mechanism involved in these H2O2 production pathways, are discussed in detail. This work also discusses the recent applications of H2O2 in advanced chemical transformations. Finally, a perspective on the development of single-atom catalysis is highlighted, aiming to provide insights into future research on SACs for electro- and photo-synthesis of H2O2 and other advanced catalytic applications.