Direct Observation of Inherent Atomic‐Scale Defect Disorders responsible for High‐Performance Ti1−xHfxNiSn1−ySby Half‐Heusler Thermoelectric Alloys

Structural defects often dominate the electronic‐ and thermal‐transport properties of thermoelectric (TE) materials and are thus a central ingredient for improving their performance. However, understanding the relationship between TE performance and the disordered atomic defects that are generally inherent in nanostructured alloys remains a challenge. Herein, the use of scanning transmission electron microscopy to visualize atomic defects directly is described and disordered atomic‐scale defects are demonstrated to be responsible for the enhancement of TE performance in nanostructured Ti_1?x Hf_x NiSn_1?y Sb_y half‐Heusler alloys. The disordered defects at all atomic sites induce a local composition fluctuation, effectively scattering phonons and improving the power factor. It is observed that the Ni interstitial and Ti,Hf/Sn antisite defects are collectively formed, leading to significant atomic disorder that causes the additional reduction of lattice thermal conductivity. The Ti_1?x Hf_x NiSn_1?y Sb_y alloys containing inherent atomic‐scale defect disorders are produced in one hour by a newly developed process of temperature‐regulated rapid solidification followed by sintering. The collective atomic‐scale defect disorder improves the zT to 1.09 ± 0.12 at 800 K for the Ti_0.5Hf_0.5NiSn_0.98Sb_0.02 alloy. These results provide a promising avenue for improving the TE performance of state‐of‐the‐art materials.     

The Relation between Chemical Bonding and Ultrafast Crystal Growth

Glasses are often described as supercooled liquids, whose structures are topologically disordered like a liquid, but nevertheless retain short‐range structural order. Structural complexity is often associated with complicated electron‐charge distributions in glassy systems, making a detailed investigation challenging even for short‐range structural order, let alone their atomic dynamics. This is particularly problematic for lone‐pair‐rich, semiconducting materials, such as phase‐change materials (PCMs). Here, this study shows that analytical methods for studying bonding, based on the electron‐charge density, rather than a conventional atomic pair‐correlation‐function approach, allows an in‐depth investigation into the chemical‐bonding network, as well as lone pairs, of the prototypical PCM, Ge₂Sb₂Te₅ (GST). It is demonstrated that the structurally flexible building units of the amorphous GST network, intimately linked to the presence of distinctly coexisting weak covalent and lone‐pair interactions, give rise to cooperative structural‐ordering processes, by which ultrafast crystal growth becomes possible. This finding may universally apply to other PCMs.     

Spectral analysis of the electronic structure of γ-B<Subscript>28</Subscript>

Elemental boron and boron-rich compounds have been the subjects of intense theoretical and practical interest and seem to be surrounded by an almost unceasing controversy with respect to their fundamental atomic and electronic structures. The sensitivity of boron to the presence of impurities leads to exceptional difficulty in preparing material samples of a specific purity or stoichiometry. Hence, the detailed effort to map out and understand the atomic and electronic structures of these materials, which has been painstakingly performed by numerous researchers over many decades, has often been fraught with missteps. However, when a theoretical or experimental method finally has proven to be successful in dealing with boron and its compounds it is often heralded as a triumph of science and is thus linked with the development of a deeper and more fundamental understanding of the physics of atomic interaction. This focused review article provides an analysis of the current state of understanding the atomic and electronic structures of elemental boron and boron-rich compounds with a strong bent towards the parts of the story related to the γ-B₂₈ modification of pure boron. Also, while many techniques have been instrument-al, the focus here will be on the results obtained from experimental and theoretical/computational methods of core level spectroscopy.     

The two-phase region at Cd3Mg

The two-phase mixture of ordered and disordered phases associated with the order-disorder transformation in alloys based on Cd₃Mg has been identified metallographically in single crystal specimens. The morphology of the two-phase structure is such that the disordered phase forms lamellae in an ordered matrix. The habit plane on which the lamellae form has been established by two-surface analysis and the orientation relationship determined using back-reflection Laue technique. The results are shown to be consistent with the notion that the habit plane of the lamellae is determined by minimum atomic mismatch in the interface between lamellae and matrix. Previously obtained hardness data are explained in terms of the conditions required to produce the two-phase structures.     

The Next Generation of Colloidal Probes: A Universal Approach for Soft and Ultra‐Small Particles

The colloidal probe technique, which is based on the atomic force microscope, revolutionizes direct force measurements in many fields, such as interface science or biomechanics. It allows for the first time to determine interaction forces on the single particle or cell level. However, for many applications, important “blind spots” remain, namely, the possibility to probe interaction potentials for nanoparticles or complex colloids with a soft outer shell. Definitely, these are colloidal systems that are currently of major industrial importance and interest from theory. The here‐presented novel approach allows for overcome the aforementioned limitations. Its applicability has been demonstrated for 300 nm sized carboxylate‐modified latex particles as well as sub‐micron core–shell particles with a soft poly‐N‐isopropylacrylamide hydrogel shell and a rigid silica core. For the latter, which until now cannot be studied by the colloidal probe technique, determined is the temperature dependency of electrosteric and adhesion forces has been determined on the single particle level.     

Strain Regulation to Optimize the Acidic Water Oxidation Performance of Atomic‐Layer IrOx

Strain regulation has become an important strategy to tune the surface chemistry and optimize the catalytic performance of nanocatalysts. Herein, the construction of atomic‐layer IrO_x on IrCo nanodendrites with tunable Ir? O bond length by compressive strain effect for oxygen evolution reaction (OER) in acidic environment is demonstrated. Evidenced from in situ extended X‐ray absorption fine structure, it is shown that the compressive strain of the IrO_x layer on the IrCo nanodendrites decreases gradually from 2.51% to the unstrained state with atomic layer growth (from ≈2 to ≈9 atomic layers of IrO_x), resulting in the variation of the Ir? O bond length from shortened 1.94 Å to normal 1.99 Å. The ≈3 atomic‐layer IrO_x on IrCo nanodendrites with an Ir? O bond length of 1.96 Å (1.51% strain) exhibits the optimal OER activity compared to the higher‐strained (2.51%, ≈2 atomic‐layer IrO_x) and unstrained (>6 atomic‐layer IrO_x) counterparts, with an overpotential of only 247 mV to achieve a current density of 10 mA cm^?2. Density functional theory calculations reveal that the precisely tuned compressive strain effect balances the adsorbate–substrate interaction and facilitates the rate‐determining step to form HOO*, thus assuring the best performance of the three atomic‐layer IrO_x for OER.     

Solid‐State Diffusional Behaviors of Functional Metal Oxides at Atomic Scale

Metal/metal oxides have attracted extensive research interest because of their combination of functional properties and compatibility with industry. Diffusion and thermal reliability have become essential issues that require detailed study to develop atomic‐scaled functional devices. In this work, the diffusional reaction behavior that transforms piezoelectric ZnO into magnetic Fe₃O₄ is investigated at the atomic scale. The growth kinetics of metal oxides are systematically studied through macro‐ and microanalyses. The growth rates are evaluated by morphology changes, which determine whether the growth behavior was a diffusion‐ or reaction‐controlled process. Furthermore, atom attachment on the kink step is observed at the atomic scale, which has important implications for the thermodynamics of functional metal oxides. Faster growth planes simultaneously decrease, which result in the predominance of low surface energy planes. These results directly reveal the atomic formation process of metal oxide via solid‐state diffusion. In addition, the nanofabricated method provides a novel approach to investigate metal oxide evolution and sheds light on diffusional reaction behavior. More importantly, the results and phenomena of this study provide considerable inspiration to enhance the material stability and reliability of metal/oxide‐based devices.     

Ultrafast Growth and Locomotion of Dandelion‐Like Microswarms with Tubular Micromotors

Dynamic assembly and cooperation represent future frontiers for next generations of advanced micro/nano robots, but the required local interaction and communication cannot be directly translated from macroscale robots through the minimization because of tremendous technological challenges. Here, an ultrafast growth and locomotion methodology is presented for dandelion‐like microswarms assembled from catalytic tubular micromotors. With ultrasound oscillation of self‐generated bubbles, such microswarms could overcome the tremendous and chaotic drag force from extensive and disordered bubble generation in single units. Tubular MnO₂ micromotor individuals headed by self‐generated oxygen bubbles are ultrasonically driven to swim rapidly in surfactant‐free H₂O₂ solutions. A large bubble core fused from multiple microbubbles is excited to oscillate and the resultant local intensified acoustic field attracts the individual micromotors to school around it, leading to a simultaneous growth of dandelion‐like microswarms. The bubble‐carried micromotor groups driven by ultrasound could swarm at a zigzag pattern with an average speed of up to 50 mm s^?1, which is validated in low H₂O₂ concentrations. Additionally, such superfast locomotion could be ultrasonically modulated on demand. The ultrafast microswarm growth and locomotion strategy offers a new paradigm for constructing distinct dynamic assemblies and rapid transmission of artificial microrobots, paving the way to a myriad of promising applications.     

Long-Range Ordered Amorphous Atomic Chains as Building Blocks of a Superconducting Quasi-One-Dimensional Crystal

Crystalline and amorphous structures are two of the most common solid-state phases. Crystals having orientational and periodic translation symmetries are usually both short-range and long-range ordered, while amorphous materials have no long-range order. Short-range ordered but long-range disordered materials are generally categorized into amorphous phases. In contrast to the extensively studied crystalline and amorphous phases, the combination of short-range disordered and long-range ordered structures at the atomic level is extremely rare and so far has only been reported for solvated fullerenes under compression. Here, a report on the creation and investigation of a superconducting quasi-1D material with long-range ordered amorphous building blocks is presented. Using a diamond anvil cell, monocrystalline (TaSe₄)₂I is compressed and a system is created where the TaSe₄ atomic chains are in amorphous state without breaking the orientational and periodic translation symmetries of the chain lattice. Strikingly, along with the amorphization of the atomic chains, the insulating (TaSe₄)₂I becomes a superconductor. The data provide critical insight into a new phase of solid-state materials. The findings demonstrate a first ever case where superconductivity is hosted by a lattice with periodic but amorphous constituent atomic chains.     

A fast and robust hybrid method for block‐structured mesh deformation with emphasis on FSI‐LES applications

The present work introduces an efficient technique for the deformation of block‐structured grids occurring in simulations of fluid–structure interaction (FSI) problems relying on large‐eddy simulation (LES). The proposed hybrid approach combines the advantages of the inverse distance weighting (IDW) interpolation with the simplicity and low computational effort of transfinite interpolation (TFI), while preserving the mesh quality in boundary layers. It is an improvement over the state‐of‐the‐art currently in use. To reach this objective, in a first step, three elementary mesh deformation methods (TFI, IDW, and radial basis functions) are investigated based on several test cases of different complexities analyzing not only their capabilities but also their computational costs. That not only allows to point out the advantages of each method but also demonstrates their drawbacks. Based on these specific properties of the different methods, a hybrid methodology is suggested that splits the entire grid deformation into two steps: first, the movement of the block‐boundaries of the block‐structured grid and second, the deformation of each block of the grid. Both steps rely on different methodologies, which allows to work out the most appropriate method for each step leading to a reasonable compromise between the grid quality achieved and the computational effort required. Finally, a hybrid IDW‐TFI methodology is suggested that best fits to the specific requirements of coupled FSI‐LES applications. This hybrid procedure is then applied to a real‐life FSI‐LES case. Copyright © 2016 John Wiley & Sons, Ltd.     

Disordered Conformation with Low Pii Helix in Phosphoproteins Orchestrates Biomimetic Apatite Formation

The interplay between noncollagenous proteins and biomineralization is widely accepted, yet the contribution of their secondary structure in mineral formation remains to be clarified. This study demonstrates a role for phosvitin, an intrinsically disordered phosphoprotein, in chick embryo skeletal development, and using circular dichroism and matrix least‐squares Henderson–Hasselbalch global fitting, unravels three distinct pH‐dependent secondary structures in phosvitin. By sequestering phosvitin on a biomimetic 3D insoluble cationic framework at defined pHs, access is gained to phosvitin in various conformational states. Induction of biomimetic mineralization at near physiological conditions reveals that a disordered secondary structure with a low content of P_II helix is remarkably efficient at promoting calcium adsorption, and results in the formation of biomimetic hydroxyapatite through an amorphous calcium phosphate precursor. By extending this finding to phosphorylated full‐length human recombinant dentin matrix protein‐1 (17‐513 AA), this bioinspired approach provides compelling evidence for the role of a disordered secondary structure in phosphoproteins in bone‐like apatite formation.     

Superlubricity between MoS2 Monolayers

The ultralow friction between atomic layers of hexagonal MoS₂, an important solid lubricant and additive of lubricating oil, is thought to be responsible for its excellent lubricating performances. However, the quantitative frictional properties between MoS₂ atomic layers have not been directly tested in experiments due to the lack of conventional tools to characterize the frictional properties between 2D atomic layers. Herein, a versatile method for studying the frictional properties between atomic‐layered materials is developed by combining the in situ scanning electron microscope technique with a Si nanowire force sensor, and the friction tests on the sliding between atomic‐layered materials down to monolayers are reported. The friction tests on the sliding between incommensurate MoS₂ monolayers give a friction coefficient of ≈10^?4 in the regime of superlubricity. The results provide the first direct experimental evidence for superlubricity between MoS₂ atomic layers and open a new route to investigate frictional properties of broad 2D materials.     

Computations of high‐speed compressible flows with adaptive cell‐centered finite element method

Abstract

An upwind cell‐centered finite element formulation is combined with an adaptive meshing technique to solve Navier‐Stokes equations for high‐speed inviscid and viscous compressible flows. The finite element formulation and the computational procedure are described. An adaptive meshing technique is applied to increase the analysis solution accuracy, as well as to minimize the computational time and the computer memory requirement. The efficiency of the combined method is evaluated by the examples of Mach 2.6 inviscid flow in a channel with compression and expansion ramps, Mach 6.47 inviscid and viscous flows past a cylinder, and Mach 4 viscous flow over a flat plate.     

Anisotropic Nanoclustered Carbon Phases Prepared from Fullerite C60 Under Non‐hydrostatic High Pressure

Abstract

We show that application of the non‐hydrostatic pressure to the cluster‐based molecular material, like fullerite C₆₀, provides a new opportunity to create elastically and structurally anisotropic carbon materials, including 2D polymerized rhombohedral C₆₀ and graphite‐type (sp ²) disordered atomic‐based phases. The elastic anisotropy, detected by the difference in the ultrasound velocities propagating along and across the loading axis, is directly confirmed by the results of x‐ray diffraction.     

Exploiting the Kubas Interaction in the Design of Hydrogen Storage Materials

Hydrogen adsorption and storage using solid‐state materials is an area of much current research interest, and one of the major stumbling blocks in realizing the hydrogen economy. However, no material yet researched comes close to reaching the DOE 2015 targets of 9 wt% and 80 kg m^?3 at this time. To increase the physisorption capacities of these materials, the heats of adsorption must be increased to ～20 kJ mol^?1. This can be accomplished by optimizing the material structure, creating more active species on the surface, or improving the interaction of the surface with hydrogen. The main focus of this progress report are recent advances in physisorption materials exhibiting higher heats of adsorption and better hydrogen adsorption at room temperature based on exploiting the Kubas model for hydrogen binding: (η²‐H₂)–metal interaction. Both computational approaches and synthetic achievements will be discussed. Materials exploiting the Kubas interaction represent a median on the continuum between metal hydrides and physisorption materials, and are becoming increasingly important as researchers learn more about their applications to hydrogen storage problems.     

Computational method for atomistic homogenization of nanopatterned point defect structures

The development of an approximation method that rigorously averages small‐scale atomistic physics and embeds them in large‐scale mechanics is the principal aim of this work. This paper presents a general computational procedure based on homogenization to average frozen nanoscale atomistics and couple them to the equations of continuum hyperelasticity. The proposed application is to nanopatterned systems in which complex atomic configurations are organized in a repeating periodic array. The finite element method is used to solve the equations at the large scale, but the small‐scale equation is representative of lattice‐statics. The method is predicated on a quasistatic zero‐temperature assumption and, through homogenization, leads to a coupled set of variational equations. The numerical procedure is presented in detail, and 2‐D examples of ultra thin film layers of carbon one atom thick are shown to illustrate its applicability. Homogenization naturally gives rise to an inner displacement term with which point defects are explicitly modelled and their non‐linear interactions with global states of multiaxial strain are studied. Published in 2004 by John Wiley & Sons, Ltd.     

Grains in Selectively Grown MoS2 Thin Films

Transition metal dichalcogenides (TMDCs) have recently been studied using various synthesis methods, such as chemical vapor deposition for large‐scale production. Despite the realization of large‐scale production with high material quality, a range of approaches have been made to solve the patterning issue of TMDCs focusing on the application of integrated devices; however, patterning is still under study to accurately represent nanoscale‐sized patterns, as well as the desired positions and shapes. Here, an insulating substrate is treated selectively with O₂ plasma, and MoS₂ growth is induced in the superhydrophilic area. Selectively well‐grown MoS₂ patterns are confirmed by atomic force microscopy and Raman and photoluminescence spectroscopy. In addition, the grain size, according to the growth size, and grain boundary are analyzed by annual dark field transmission electron microscopy (TEM) and spherical aberration‐corrected scanning TEM to confirm the selective growth. An analysis of the device performance and the optical properties reveals an enhancement with increasing grain size. This method presents the path of the growth technique for patterning, as well as the direction that can be applied to devices and integrated circuits.     

TiO2@Perylene Diimide Full‐Spectrum Photocatalysts via Semi‐Core–Shell Structure

A semi‐core–shell structure of perylene diimide (PDI) self‐assembly coated with TiO₂ nanoparticles is constructed, in which nanoscale porous TiO₂ shell is formed and PDI self‐assembly presented 1D structure. A full‐spectrum photocatalyst is obtained using this structure to resolve a conundrum—TiO₂ does not exhibit visible‐light photocatalytic activity while PDI does not exhibit ultraviolet photocatalytic activity. Furthermore, the synergistic interaction between TiO₂ and PDI enables the catalyst to improve its ultraviolet, visible‐light, and full‐spectrum performance. The interaction between TiO₂ and PDI leads to formation of some new stacking states along the Π–Π stacking direction and, as a consequence, electron transfer from PDI to TiO₂ suppresses the recombination of e^?/h⁺ and thus improves photocatalytic performance. But the stronger interaction in the interface between TiO₂ and PDI is not in favor of photocatalytic performance, which leads to rapid charge recombination due to more disordered stacking states. The study provides a theoretical direction for the study of core–shell structures with soft materials as a core, and an idea for efficient utilization of solar energy.     

An Atomistic Tomographic Study of Oxygen and Hydrogen Atoms and their Molecules in CVD Grown Graphene

The properties and growth processes of graphene are greatly influenced by the elemental distributions of impurity atoms and their functional groups within or on the hexagonal carbon lattice. Oxygen and hydrogen atoms and their functional molecules (OH, CO, and CO₂) positions' and chemical identities are tomographically mapped in three dimensions in a graphene monolayer film grown on a copper substrate, at the atomic part‐per‐million (atomic ppm) detection level, employing laser assisted atom‐probe tomography. The atomistic plan and cross‐sectional views of graphene indicate that oxygen, hydrogen, and their co‐functionalities, OH, CO, and CO₂, which are locally clustered under or within the graphene lattice. The experimental 3D atomistic portrait of the chemistry is combined with computational density‐functional theory (DFT) calculations to enhance the understanding of the surface state of graphene, the positions of the chemical functional groups, their interactions with the underlying Cu substrate, and their influences on the growth of graphene.     

Graphoepitaxy of High‐Quality GaN Layers on Graphene/6H–SiC

The implementation of graphene layers in gallium nitride (GaN) heterostructure growth can solve self‐heating problems in nitride‐based high‐power electronic and light‐emitting optoelectronic devices. In the present study, high‐quality GaN layers are grown on patterned graphene layers and 6H–SiC by metalorganic chemical vapor deposition. A periodic pattern of graphene layers is fabricated on 6H–SiC by using polymethyl methacrylate deposition and electron beam lithography, followed by etching using an Ar/O₂ gas atmosphere. Prior to GaN growth, an AlN buffer layer and an Al_0.2Ga_0.8N transition layer are deposited. The atomic structures of the interfaces between the 6H–SiC and graphene, as well as between the graphene and AlN, are studied using scanning transmission electron microscopy. Phase separation of the Al_0.2Ga_0.8N transition layer into an AlN and GaN superlattice is observed. Above the continuous graphene layers, polycrystalline defective GaN is rapidly overgrown by better quality single‐crystalline GaN from the etched regions. The lateral overgrowth of GaN results in the presence of a low density of dislocations (≈10⁹ cm⁻²) and inversion domains and the formation of a smooth GaN surface.