Crystalline Interfaces Research Articles

Percolation of Low‐Dimensional Water at Crystalline Interfaces Mediates Fluid Migration in Subducting Slabs

AbstractDuring subduction, metamorphic dehydration reactions in the downgoing slab release fluids, generating fluid overpressure. It has been suggested that fluid is driven to flow upward by buoyancy, but a sufficiently high permeability allowing formation of a fluid percolation network is required. Traditionally, fluid percolation has been identified based on the textural equilibrium assumption by measuring the dihedral angle at the triple junction of grains. According to this theory, grain boundaries generally cannot be infiltrated by fluid, and only the grain edge can form a fluid flow channel. We argue that this theory is insufficient because we have found that water from fluid can be adsorbed into the crystalline interface, that is, a layered mineral interlayer, a crack, or a grain boundary. The high pressure in a subducting slab drives water adsorption into the crystalline interface, forming a low‐dimensional fluidic phase, and thus fluid percolation is achieved. Because water adsorbed in the interface is fluidic, water diffusion drives fluid transport in the subducting slab. Due to water adsorption, fluid overpressure at the dehydration front may release, so that dehydration embrittlement may be excluded. Stable water adsorption in the subduction‐slab conditions is determined here by combining molecular dynamics simulations and thermodynamic calculations. Analysis based on simulations shows that water adsorption requires crystalline surfaces which do not form hydrogen bonds well.

Self-Assembly of Peptide-Conjugated Forklike Mesogens at Aqueous/Liquid Crystalline Interfaces: Molecular Design for Ordering Transition Induced by Specific Binding of Biomolecules.

Self-assembly of functional liquid crystals provides a powerful approach to the development of stimuli-responsive materials and interfaces. Here, we have designed and synthesized bioconjugated amphiphilic dendritic mesogens containing arginine-glycine-aspartic acid (RGD) peptide sequence to develop new biofunctional aqueous/liquid crystalline interfaces. We have found that the RGD peptide-conjugated forklike mesogens induce the homeotropic alignment of liquid crystals at the aqueous interfaces, leading to distinct optical changes caused by the specific binding of the target proteins. In contrast, no response to the target protein is observed for the interfaces prepared with the RGD peptide-conjugated single mesogen. Molecular insights into the orientation and stimuli-responsiveness of the bioconjugated mesogens at the interfaces are obtained based on measurements of the Langmuir films and self-assembled properties of these molecules. These results demonstrate that the number of rodlike cores of the bioconjugated mesogens affects the monolayer structures formed at the aqueous interface as well as the liquid crystalline properties. We propose a new molecular design of bioconjugated mesogens to couple biomolecular interactions at the aqueous interfaces with the ordering transition of the liquid crystals. These materials have the potential to tailor the responsiveness of liquid crystalline interfaces for biomolecular sensing.

Engineering the Au-Cu2 O Crystalline Interfaces for Structural and Catalytic Integration.

Precise structural control has attracted tremendous interest in pursuit of the tailoring of physical properties. Here, this work shows that through strong ligand-mediated interfacial energy control, Au-Cu2 O dumbbell structures where both the Au nanorod (AuNR)and the partially encapsulating Cu2 O domains are highly crystalline. The synthetic advance allows physical separation of the Au and Cu2 O domains, in addition to the use of long nanorods with tunable absorption wavelength, and the crystalline Cu2 O domain with well-defined facets. The interplay of plasmon and Schottky effects boosts the photocatalytic performance in the model photodegradation of methyl orange, showing superior catalytic efficiency than the AuNR@Cu2 O core-shell structures. In addition, compared to the typical core-shell structures, the AuNR-Cu2 O dumbbells can effectively electrochemically catalyze the CO2 to C2+ products (ethanol and ethylene) via a cascade reaction pathway. The excellent dual function of both photo- and electrocatalysis can be attributed to the fine physical separation of the crystalline Au and Cu2 O domains.

Constructing Amorphous–Crystalline Interfaces of Nickel–Iron Phosphides/Oxides for Oxygen Evolution Reaction

Oxygen evolution reaction (OER) is the bottleneck of electrocatalytic water splitting due to its sluggish proton-coupled four-electron-transfer kinetics. Interface engineering has been regarded as an effective strategy to promote the OER performance of transition-metal oxide (TMO) electrocatalysts. In this study, we successfully construct the amorphous–crystalline interfaces of NiFePx/NiFeOx nanosheets with substantial oxygen vacancies through controllable surface phosphorization. Experimental results confirm that the unique structure possesses fast charge transfer, abundant active sites, and enhanced intrinsic activity, resulting in the outstanding OER performance. The optimized NiFePx/NiFeOx displays a low overpotential of 238 mV at 20 mA cm–2 and excellent stability in 1 M KOH electrolyte. This study provides an effective strategy to promote the OER performance of TMOs in alkaline water splitting.

Thermal Fluctuations as a Computational Microscope for Studying Crystalline Interfaces: A Mechanistic Perspective

Interfaces such as grain boundaries are ubiquitous in crystalline materials and have provided a fertile area of research over decades. Their importance stems from the numerous critical phenomena associated with them, such as grain boundary sliding, migration, and interaction with other defects, that govern the mechanical properties of materials. Although these crystalline interfaces exhibit small out-of-plane fluctuations, statistical thermodynamics of membranes has been effectively used to extract relevant physical quantities such as the interface free energy, grain boundary stiffness, and interfacial mobility. In this perspective, we advance the viewpoint that thermal fluctuations of crystalline interfaces can serve as a computational microscope for gaining insights into the thermodynamic and kinetic properties of grain boundaries and present a rich source of future study.

Crystalline CoFeB/Graphite Interfaces for Carbon Spintronics Fabricated by Solid Phase Epitaxy

AbstractStructurally ordered interfaces between ferromagnetic electrodes and graphene or graphite are of great interest for carbon spintronics, since they allow spin‐filtering due to k‐vector conservation. By solid phase epitaxy of amorphous/nanocrystalline CoFeB at elevated temperatures, the feasibility of fabricating crystalline interfaces between a 3d ferromagnetic alloy and graphite is demonstrated, without suffering from the unwetting problem that was commonly seen in many previous studies with 3d transition metals. The films fabricated on graphite in this way are found to have a strong body‐centered‐cubic (110) texture, albeit without a unique, well‐defined in‐plane epitaxial relationship with the substrate lattice. Using various X‐ray spectroscopic techniques, it is shown that boron suppresses the formation of CoFe‐O during CoFeB deposition, and then diffuses out of the CoFe lattice. Segregation of B occurred exclusively to the film surface upon in situ annealing, and not to the interface between CoFeB and graphite. This is favorable for obtaining a high spin polarization at the hybrid CoFe/graphite crystalline interface. The Co and Fe spin moments in the crystalline film, determined by X‐ray magnetic circular dichroism, are found to be bulk‐like, while their orbital moments show an unusual giant enhancement which has yet to be understood.

Effects of interface and microstructure on the mechanical behaviors of crystalline Cu-amorphous Cu/Zr nanolaminates

In this paper, the unique interface of crystalline–amorphous nanolaminates was investigated. At the outset of the plastic deformation, dislocations are nucleated from amorphous–crystalline interfaces (triggered by shear transformation zones activities in the glass layers), gliding across the nanocrystal layer, and being absorbed at the opposite amorphous–crystalline interface. To evaluate the mechanical behavior of crystalline–amorphous nanolaminates, a model comprised of grain interior regarded as an ordered crystal phase and plastically softer amorphous–crystalline interface affected zone phase was proposed. Considering their different properties and the interplay between the two parts, the corresponding dislocation evolution behavior in crystal phase and the new strain gradient based on potential energy were studied. To describe the strain strength quantitatively, a strain-hardened law from a nanostructure characteristic length parameter and strain gradient were determined. The strain gradient in crystalline–amorphous nanolaminates and effect of the amorphous layer contribute to the mechanical behaviors of this material were found.

Direct Imaging of Single Dopant Atoms in a Buried Crystalline Interface by Scanning Transmission Electron Microscopy

Aberration-corrected scanning transmission electron microscopy is becoming a very powerful tool to directly image dopant atoms within buried crystalline interfaces. Here, we demonstrate direct imaging of individual dopant atoms in an alumina interface. The focused electron beam transmitted through the off-axis crystals clearly highlights the individual yttrium atoms located on the monoatomic layer interface plane. Not only is their unique two-dimensional ordered positioning directly revealed with atomic precision, but local disordering at the single atom level, which has never been detected by the conventional approaches, is also uncovered. The ability to directly probe individual atoms within buried interface structures will be powerful for characterizing internal interfaces in many advanced materials and devices.

Atomic-scale imaging of individual dopant atoms in a buried interface

Determining the atomic structure of internal interfaces in materials and devices is critical to understanding their functional properties. Interfacial doping is one promising technique for controlling interfacial properties at the atomic scale, but it is still a major challenge to directly characterize individual dopant atoms within buried crystalline interfaces. Here, we demonstrate atomic-scale plan-view observation of a buried crystalline interface (an yttrium-doped alumina high-angle grain boundary) using aberration-corrected Z-contrast scanning transmission electron microscopy. The focused electron beam transmitted through the off-axis crystals clearly highlights the individual yttrium atoms located on the monoatomic layer interface plane. Not only is their unique two-dimensional ordered positioning directly revealed with atomic precision, but local disordering at the single-atom level, which has never been detected by the conventional approaches, is also uncovered. The ability to directly probe individual atoms within buried interface structures adds new dimensions to the atomic-scale characterization of internal interfaces and other defect structures in many advanced materials and devices.

Growth of Perovskites with Crystalline Interfaces on Si(100)

ABSTRACTThe main challenges involved in the growth of an epitaxial oxide film with a crystalline interface to silicon are reviewed: (1) structural matching of the oxide and semiconductor lattices; (2) thermodynamic energy stabilization at the semiconductor–oxide interface, and (3) kinetic control over oxygen motion throughout the deposition process. We report on how this approach can be used to grow epitaxial perovskites of high structural quality from the (Ba, Sr)(Zr,Ti)O3 family with crystalline interfaces on Si (100) by molecular-beam epitaxy.

Static Frictional Forces at Crystalline Interfaces

A statistical thermodynamic description of the atomic force microscope is developed and used to compute the force of static friction for a single-atom tip on a hexagonal close-packed substrate surface under constant load in vacuum. The substrate atoms are taken to be independent isotropic harmonic oscillators, and the tip−substrate interaction is taken to be Lennard-Jones (12,6). Movement of the tip is treated as a quasistatic (reversible) process. The force of static friction (i.e., the maximum of the component of the force parallel to the direction of movement of the tip) is computed by the Monte Carlo technique for several crystallographic directions (paths) and found to be strongly anisotropic. The frictional force is minimum along a particular path where rows of substrate atoms form a “groove”; it is up to 2 orders of magnitude greater for the path perpendicular to the groove. The dependence of the frictional force on the hardness of the substrate (as measured by the force constant of the substrate harmonic potential) and on temperature was examined for these two extreme paths. For hard substrates the frictional force is nearly linear with load. As the substrate gets softer, or as the temperature increases at fixed hardness, the frictional force declines. The results of the computations are correlated with recent experimental observations on the sliding of nanocrystals of MoO3 over an MoS2 substrate.

Near-equilibrium dynamics of crystalline interfaces with long-range interactions in (1+1)-dimensional systems.

The dynamics of a one-dimensional crystalline interface model with long-range interactions is investigated. In the absence of randomness, the linear response mobility decreases to zero when the temperature approaches the roughening transition from above, in contrast to a finite jump at the critical point in the Kosterlitz-Thouless (KT) transition. In the presence of substrate disorder, there exists a phase transition into a low-temperature pinning phase with a continuously varying dynamic exponent $z>1$. The expressions for the non-linear response mobility of a crystalline interface in both cases are also derived.

Hartman–Perdok theory: influence of crystal structure and crystalline interface on crystal growth

Hartman–Perdok theory enables classification of crystal faces as F, S or K faces. Only F faces should be present on growth forms as they grow slowly according to a layer mechanism. Such a classification can be quantified for ionic crystals by the calculation of attachment energies in electrostatic point-charge models. The attachment energy of a crystal face (hkl)Ehkla, is the energy released per mole, when a new elementary growth layer, called a slice, with a thickness of dhkl crystallizes on an existing crystal face. Theoretical growth forms can be constructed by making use of the supposition that Ehkla is directly proportional to the growth rate. Often these growth forms are very similar to the morphologies of crystals grown in nature or in laboratory experiments {zircon (ZrSiO4), alkali feldspars ((K,Na)AlSi3O8), natural silicate garnets [A2+3 B3+2(SiO4)3]}. The structure of the crystalline interface is very important for the crystal growth processes. The derivation of F faces provides the atomic topology of the crystalline interface as well. Sometimes there is more than one possible surface of a slice with a thickness of dhkl, e.g. zircon (011), garnets (110) and (112), which may lead to growth via slices of thickness ½dhkl, causing a significant increase of the growth rate. Ordering of the ions which are situated on the slice boundaries may reduce the growth rate (alkali feldspars and YBa2Cu3O7 –x). Experimental crystal growth of PbCl2 shows that adsorption of OH– and H3O+ ions on special sites of the crystalline interfaces may cause habit modifications. This effect can be explained for PbCl2 in terms of adsorption of Pb(OH)Cl on {211}. The influence of the solvent on the growth can also be explained in terms of impurity adsorption on the crystalline interface. The growth of flattened PbCl2 crystals from HCl due to the reduction of the growth rates of {010} and {121} is such an example.

Comment on "Theory of Incomplete Crystals and Crystalline Interfaces" by F. Garcia-Moliner and V. R. Velasco

The relation between the discrete Surface Green's Function Matching approach of Garcia-Moliner and Velasco (Physica Scripta 34, 257 (1986)) and atom-removal or ideal construction approaches is clarified. Both approaches are shown to give identical results for an "ideal" surface in the tight-binding approximation. Here "ideal" is defined by the condition that the Hamiltonian matrix elements in the crystal fragment keep their bulk values.

Mathematical Aspects of a Geometrical Theory of Crystalline Interfaces

The mathematical aspects of the 0-lattice theory of crystalline interfaces are summarized. This theory gives the geometrical basis for describing grain and phase boundaries between arbitrary crystal structures. The 0-lattice theory is a generalised dislocation theory that produces the necessary dislocation structure for many types of boundaries including subgrain and coincidence boundaries. The essential feature of the theory is the possibility of separating the effects originating from the structures and relative orientation of the two crystals from those due to the location and orientation of the boundary. This is done by constructing a three-dimensional cell structure within the crystal lattices which are considered as interpenetrating. The boundary is then understood as a cut through this cell structure, where the cell structure itself represents the sum of all the possible boundaries for given crystal structures and relative orientation. Thus, this theory provides a way to gain a broad view of boundary problems. CRYSTALLINE INTERFACES; GRAIN BOUNDARIES; COINCIDENCE BOUNDARIES; DISLOCATIONS; 0-LATrICE THEORY

“Crystal Defects and Crystalline Interfaces” with 158 figures and a set of moireé-Models, IX

“Crystal Defects and Crystalline Interfaces” with 158 figures and a set of moireé-Models, IX

Defects at Crystalline Interfaces

Crystalline interfaces are of two basic types - interfaces in single phase solids separating grains of differing orientation (grain boundaries, sub-grain boundaries, twin boundaries), and interphase interfaces separating crystals which differ in crystal structure and/or composition, as well as relative orientation. Depending upon these variables a particular boundary will have more or less interfacial structure which can be resolved by transmission electron microscopy. The dislocations (line defects) which are imaged at boundaries by electron diffraction contrast effects may be unique to the boundary, i.e., have displacement vectors which are different from those of dislocations found in single phase materials. Analysis may be further complicated by extra diffraction effects such as superimposed Moire patterns (Fig. 1a and 2).