Unveiling a Chemisorbed Crystallographically Heterogeneous Graphene/L10-FePd Interface with a Robust and Perpendicular Orbital Moment.

A crystallographically heterogeneous interface was fabricated by growing hexagonal graphene (Gr) using chemical vapor deposition (CVD) on a tetragonal FePd epitaxial film grown by magnetron sputtering. FePd was alternately arranged with Fe and Pd in the vertical direction, and the outermost surface atom was identified primarily as Fe rather than Pd. This means that FePd has a high degree of L10-ordering, and the outermost Fe bonds to the carbon of Gr at the interface. When Gr is grown by CVD, the crystal orientation of hexagonal Gr toward tetragonal L10-FePd selects an energetically stable structure based on the van der Waals (vdW) force. The atomic relationship of Gr/L10-FePd, which is an energetically stable interface, was unveiled theoretically and experimentally. The Gr armchair axis was parallel to FePd [100]L10, where Gr was under a small strain by chemical bonding. Focusing on the interatomic distance between the Gr and FePd layers, the distance was theoretically and experimentally determined to be approximately 0.2 nm. This shorter distance (≈0.2 nm) can be explained by the chemisorption-type vdW force of strong orbital hybridization, rather than the longer distance (≈0.38 nm) of the physisorption-type vdW force. Notably, depth-resolved X-ray magnetic circular dichroism analyses revealed that the orbital magnetic moment (Ml) of Fe in FePd emerged at the Gr/FePd interface (@inner FePd: Ml = 0.16 µB → @Gr/FePd interface: Ml = 0.32 µB). This interfacially enhanced Ml showed obvious anisotropy in the perpendicular direction, which contributed to interfacial perpendicular magnetic anisotropy (IPMA). Moreover, the interfacially enhanced Ml and interfacially enhanced electron density exhibited robustness. It is considered that the shortening of the interatomic distance produces a robust high electron density at the interface, resulting in a chemisorption-type vdW force and orbital hybridization. Eventually, the robust interfacial anisotropic Ml emerged at the crystallographically heterogeneous Gr/L10-FePd interface. From a practical viewpoint, IPMA is useful because it can be incorporated into the large bulk perpendicular magnetic anisotropy (PMA) of L10-FePd. A micromagnetic simulation assuming both PMA and IPMA predicted that perpendicularly magnetized magnetic tunnel junctions (p-MTJs) using Gr/L10-FePd could realize 10-year data retention in a small recording layer with a circular diameter and thickness of 10 and 2 nm, respectively. We unveiled the energetically stable atomic structure in the crystallographically heterogeneous interface, discovered the emergence of the robust IPMA, and predicted that the Gr/L10-FePd p-MTJ is significant for high-density X nm generation magnetic random-access memory (MRAM) applications.

Relationship between diffraction peak, network topology, and amorphous-forming ability in silicon and silica.

The network topology in disordered materials is an important structural descriptor for understanding the nature of disorder that is usually hidden in pairwise correlations. Here, we compare the covalent network topology of liquid and solidified silicon (Si) with that of silica (SiO2) on the basis of the analyses of the ring size and cavity distributions and tetrahedral order. We discover that the ring size distributions in amorphous (a)-Si are narrower and the cavity volume ratio is smaller than those in a-SiO2, which is a signature of poor amorphous-forming ability in a-Si. Moreover, a significant difference is found between the liquid topology of Si and that of SiO2. These topological features, which are reflected in diffraction patterns, explain why silica is an amorphous former, whereas it is impossible to prepare bulk a-Si. We conclude that the tetrahedral corner-sharing network of AX2, in which A is a fourfold cation and X is a twofold anion, as indicated by the first sharp diffraction peak, is an important motif for the amorphous-forming ability that can rule out a-Si as an amorphous former. This concept is consistent with the fact that an elemental material cannot form a bulk amorphous phase using melt quenching technique.

Graphene-coated nanoporous nickel towards a metal-catalyzed oxygen evolution reaction.

Developing highly active electrocatalysts with low costs and long durability for oxygen evolution reactions (OERs) is crucial towards the practical implementations of electrocatalytic water-splitting and rechargeable metal-air batteries. Anodized nanostructured 3d transition metals and alloys with the formation of OER-active oxides/hydroxides are known to have high catalytic activity towards OERs but suffer from poor electrical conductivity and electrochemical stability in harsh oxidation environments. Here we report that high OER activity can be achieved from the metallic state of Ni which is passivated by atomically thick graphene in a three-dimensional nanoporous architecture. As a free-standing catalytic anode, the non-oxide transition metal catalyst shows a low OER overpotential, high OER current density and long cycling lifetime in alkaline solutions, benefiting from the high electrical conductivity and low impedance resistance for charge transfer and transport. This study may pave a new way to develop high efficiency transition metal OER catalysts for a wide range of applications in renewable energy.

Local structure analysis of amorphous materials by angstrom-beam electron diffraction.

The structure analysis of amorphous materials still leaves much room for improvement. Owing to the lack of translational or rotational symmetry of amorphous materials, it is important to develop a different approach from that used for crystals for the structure analysis of amorphous materials. Here, the angstrom-beam electron diffraction method was used to obtain the local structure information of amorphous materials at a sub-nanometre scale. In addition, we discussed the relationship between the global and local diffraction intensities of amorphous structures, and verified the effectiveness of the proposed method through basic diffraction simulations. Finally, some applications of the proposed method to structural and functional amorphous materials are summarized.

Dealloying Kinetics of AgAu Nanoparticles by In Situ Liquid-Cell Scanning Transmission Electron Microscopy.

Understanding the formation and evolution of bicontinuous nanoporous structure during dealloying has been one of the most challenging subjects of dealloying research. However, previous in situ investigations either suffer from insufficient spatial resolution (e.g., X-ray tomography) or lack morphology visualization and mass information (e.g., scanning tunneling microscopy). In this work, we report the kinetics of the whole course of dealloying by utilizing liquid-cell aberration-corrected scanning transmission electron microscopy. With Z-contrast imaging analysis, the in situ sub-nanoscale characterization reveals two new phenomena, an initial period of dealloying indicative of an initial length scale for bulk dealloying and a large volume shrinkage in a nanoscale alloy precursor. We explain the particle-size-dependent volume shrinkage with the formation of a dense shell and quantify the dependence with a simple geometric model. These insights into the mechanisms of dealloying will enable deliberate designs of nanoporous structures.

Fast coalescence of metallic glass nanoparticles.

The coarsening of crystalline nanoparticles, driven by reduction of surface energy, is the main factor behind the degeneration of their physical and chemical properties. The kinetic phenomenon has been well described by various models, such as Ostwald ripening and coalescence. However, the coarsening mechanisms of metallic glass nanoparticles (MGNs) remains largely unknown. Here we report atomic-scale observations on the coarsening kinetics of MGNs at high temperatures by in situ heating high-resolution transmission electron microscopy. The coarsening of the amorphous nanoparticles takes place by fast coalescence which is dominated by facet-free surface diffusion at a lower onset temperature. Atomic-scale observations and kinetic Monte Carlo simulations suggest that the high surface mobility and the structural isotropy of MGNs, originating from the disordered structure and unique supercooled liquid state, promote the fast coalescence of the amorphous nanoparticles at relatively lower temperatures.

High-temperature bulk metallic glasses developed by combinatorial methods.

Since their discovery in 19601, metallic glasses based on a wide range of elements have been developed2. However, the theoretical prediction of glass-forming compositions is challenging and the discovery of alloys with specific properties has so far largely been the result of trial and error3-8. Bulk metallic glasses can exhibit strength and elasticity surpassing those of conventional structural alloys9-11, but the mechanical properties of these glasses are critically dependent on the glass transition temperature. At temperatures approaching the glass transition, bulk metallic glasses undergo plastic flow, resulting in a substantial decrease in quasi-static strength. Bulk metallic glasses with glass transition temperatures greater than 1,000 kelvin have been developed, but the supercooled liquid region (between the glass transition and the crystallization temperature) is narrow, resulting in very little thermoplastic formability, which limits their practical applicability. Here we report the design of iridium/nickel/tantalum metallic glasses (and others also containing boron) with a glass transition temperature of up to 1,162 kelvin and a supercooled liquid region of 136 kelvin that is wider than that of most existing metallic glasses12. Our Ir-Ni-Ta-(B) glasses exhibit high strength at high temperatures compared to existing alloys: 3.7 gigapascals at 1,000 kelvin9,13. Their glass-forming ability is characterized by a critical casting thickness of three millimetres, suggesting that small-scale components for applications at high temperatures or in harsh environments can readily be obtained by thermoplastic forming14. To identify alloys of interest, we used a simplified combinatorial approach6-8 harnessing a previously reported correlation between glass-forming ability and electrical resistivity15-17. This method is non-destructive, allowing subsequent testing of a range of physical properties on the same library of samples. The practicality of our design and discovery approach, exemplified by the identification of high-strength, high-temperature bulk metallic glasses, bodes well for enabling the discovery of other glassy alloys with exciting properties.

Lithium intercalation into bilayer graphene.

The real capacity of graphene and the lithium-storage process in graphite are two currently perplexing problems in the field of lithium ion batteries. Here we demonstrate a three-dimensional bilayer graphene foam with few defects and a predominant Bernal stacking configuration, and systematically investigate its lithium-storage capacity, process, kinetics, and resistances. We clarify that lithium atoms can be stored only in the graphene interlayer and propose the first ever planar lithium-intercalation model for graphenic carbons. Corroborated by theoretical calculations, various physiochemical characterizations of the staged lithium bilayer graphene products further reveal the regular lithium-intercalation phenomena and thus fully illustrate this elementary lithium storage pattern of two-dimension. These findings not only make the commercial graphite the first electrode with clear lithium-storage process, but also guide the development of graphene materials in lithium ion batteries.

Spatial heterogeneity as the structure feature for structure-property relationship of metallic glasses.

The mechanical properties of crystalline materials can be quantitatively described by crystal defects of solute atoms, dislocations, twins, and grain boundaries with the models of solid solution strengthening, Taylor strain hardening and Hall-Petch grain boundary strengthening. However, for metallic glasses, a well-defined structure feature which dominates the mechanical properties of the disordered materials is still missing. Here, we report that nanoscale spatial heterogeneity is the inherent structural feature of metallic glasses. It has an intrinsic correlation with the strength and deformation behavior. The strength and Young's modulus of metallic glasses can be defined by the function of the square root reciprocal of the characteristic length of the spatial heterogeneity. Moreover, the stretching exponent of time-dependent strain relaxation can be quantitatively described by the characteristic length. Our study provides compelling evidence that the spatial heterogeneity is a feasible structural indicator for portraying mechanical properties of metallic glasses.

Heavily Doped and Highly Conductive Hierarchical Nanoporous Graphene for Electrochemical Hydrogen Production.

Heavy chemical doping and high electrical conductivity are two key factors for metal-free graphene electrocatalysts to realize superior catalytic performance toward hydrogen evolution. However, heavy chemical doping usually leads to the reduction of electrical conductivity because the catalytically active dopants give rise to additional electron scattering and hence increased electrical resistance. A hierarchical nanoporous graphene, which is comprised of heavily chemical doped domains and a highly conductive pure graphene substrate, is reported. The hierarchical nanoporous graphene can host a remarkably high concentration of N and S dopants up to 9.0 at % without sacrificing the excellent electrical conductivity of graphene. The combination of heavy chemical doping and high conductivity results in high catalytic activity toward electrochemical hydrogen production. This study has an important implication in developing multi-functional electrocatalysts by 3D nanoarchitecture design.

Operando characterization of cathodic reactions in a liquid-state lithium-oxygen micro-battery by scanning transmission electron microscopy.

Rechargeable non-aqueous lithium-oxygen batteries with a large theoretical capacity are emerging as a high-energy electrochemical device for sustainable energy strategy. Despite many efforts made to understand the fundamental Li-O2 electrochemistry, the kinetic process of cathodic reactions, associated with the formation and decomposition of a solid Li2O2 phase during charging and discharging, remains debate. Here we report direct visualization of the charge/discharge reactions on a gold cathode in a non-aqueous lithium-oxygen micro-battery using liquid-cell aberration-corrected scanning transmission electron microscopy (STEM) combining with synchronized electrochemical measurements. The real-time and real-space characterization by time-resolved STEM reveals the electrochemical correspondence of discharge/charge overpotentials to the nucleation, growth and decomposition of Li2O2 at a constant current density. The nano-scale operando observations would enrich our knowledge on the underlying reaction mechanisms of lithium-oxygen batteries during round-trip discharging and charging and shed lights on the strategies in improving the performances of lithium-oxygen batteries by tailoring the cathodic reactions.

Three-dimensional bicontinuous nanoporous materials by vapor phase dealloying.

Three-dimensional bicontinuous open (3DBO) nanoporosity has been recognized as an important nanoarchitecture for catalysis, sensing, and energy storage. Dealloying, i.e., selectively removing a component from an alloy, is an efficient way to fabricate nanoporous materials. However, current electrochemical and liquid-metal dealloying methods can only be applied to a limited number of alloys and usually require an etching process with chemical waste. Here, we report a green and universal approach, vapor-phase dealloying, to fabricate nanoporous materials by utilizing the vapor pressure difference between constituent elements in an alloy to selectively remove a component with a high partial vapor pressure for 3DBO nanoporosity. We demonstrate that extensive elements, regardless of chemical activity, can be fabricated as nanoporous materials with tunable pore sizes. Importantly, the evaporated components can be fully recovered. This environmentally friendly dealloying method paves a way to fabricate 3DBO nanoporous materials for a wide range of structural and functional applications.

Synthesizing 1T-1H Two-Phase Mo1-xWxS2 Monolayers by Chemical Vapor Deposition.

1T-1H metal-semiconductor interfaces in two-dimensional (2D) transition-metal dichalcogenides (TMDs) play a crucial role in utilizing the band gaps of TMDs for applications in electronic devices. Although the 1T-1H two-phase structure has been observed in exfoliated 2D nanosheets and chemically or physically treated TMDs, it cannot in principle be achieved in large-scale TMD monolayers grown by chemical vapor deposition (CVD), which is a fabrication method for electronic device applications, because of the metastable nature of the 1T phase. In this study we report CVD growth of 1T-1H two phase TMD monolayers by controlling thermal strains and alloy compositions. It was found that in-plane thermal strains arising from the difference in thermal expansion coefficients between TMD monolayers and substrates can drive the 1H to 1T transition during cooling after CVD growth. Moreover, grain boundaries in the 2D crystals act as the nucleation sites of the 1T phase and the lattice strain perturbations from alloying noticeably promote the formation of the metastable 1T phase. This work has an important implication in tailoring structure and properties of CVD grown 2D TMDs by phase engineering.

Correlation between Local Structure Order and Spatial Heterogeneity in a Metallic Glass.

Although nanoscale spatial heterogeneity of metallic glasses has been demonstrated by extensive experimental and theoretical investigations, the nature of spatial heterogeneity remains poorly known owing to the absence of a structural depiction of the inhomogeneity from experimental insight. Here we report the experimental characterization of the spatial heterogeneity of a metallic glass by utilizing state-of-the-art angstrom-beam electron diffraction and scanning transmission electron microscopy. The subnanoscale electron diffraction reveals that the nanoscale spatial heterogeneity and corresponding density fluctuation have a close correlation with the local structure variation from icosahedronlike to tetragonal crystal-like order. The structural insights of spatial heterogeneity have important implications in understanding the properties and dynamics of metallic glasses.

Engineering the internal surfaces of three-dimensional nanoporous catalysts by surfactant-modified dealloying.

Tuning surface structures by bottom-up synthesis has been demonstrated as an effective strategy to improve the catalytic performances of nanoparticle catalysts. Nevertheless, the surface modification of three-dimensional nanoporous metals, fabricated by a top-down dealloying approach, has not been achieved despite great efforts devoted to improving the catalytic performance of three-dimensional nanoporous catalysts. Here we report a surfactant-modified dealloying method to tailor the surface structure of nanoporous gold for amplified electrocatalysis toward methanol oxidation and oxygen reduction reactions. With the assistance of surfactants, {111} or {100} faceted internal surfaces of nanoporous gold can be realized in a controllable manner by optimizing dealloying conditions. The surface modified nanoporous gold exhibits significantly enhanced electrocatalytic activities in comparison with conventional nanoporous gold. This study paves the way to develop high-performance three-dimensional nanoporous catalysts with a tunable surface structure by top-down dealloying for efficient chemical and electrochemical reactions.

Direct Observations of the Formation and Redox-Mediator-Assisted Decomposition of Li2 O2 in a Liquid-Cell Li-O2 Microbattery by Scanning Transmission Electron Microscopy.

Operando scanning transmission electron microscopy observations of cathodic reactions in a liquid-cell Li-O2 microbattery in the presence of the redox mediator tetrathiafulvalene (TTF) in 1.0 m LiClO4 dissolved dimethyl sulfoxide electrolyte are reported. It is found that the TTF addition does not obviously affect the discharge reaction for the formation of a solid Li2 O2 phase. The coarsening of Li2 O2 nanoparticles occurs via both conventional Ostwald ripening and nonclassical crystallization by particle attachment. During charging, the oxidation reaction at significantly reduced charge potentials mainly takes place at Li2 O2 /electrolyte interfaces and has obvious correspondence with the oxidized TTF+ distributions in the electric fields of the charged electrode. This study provides direct evidence that TTF truly plays a role in promoting the decomposition of Li2 O2 as a soluble charge-transfer agent between the electrode and the Li2 O2 .

Formation and Characterization of Hydrogen Boride Sheets Derived from MgB2 by Cation Exchange.

Two-dimensional (2D) materials are promising for applications in a wide range of fields because of their unique properties. Hydrogen boride sheets, a new 2D material recently predicted from theory, exhibit intriguing electronic and mechanical properties as well as hydrogen storage capacity. Here, we report the experimental realization of 2D hydrogen boride sheets with an empirical formula of H1B1, produced by exfoliation and complete ion-exchange between protons and magnesium cations in magnesium diboride (MgB2) with an average yield of 42.3% at room temperature. The sheets feature an sp2-bonded boron planar structure without any long-range order. A hexagonal boron network with bridge hydrogens is suggested as the possible local structure, where the absence of long-range order was ascribed to the presence of three different anisotropic domains originating from the 2-fold symmetry of the hydrogen positions against the 6-fold symmetry of the boron networks, based on X-ray diffraction, X-ray atomic pair distribution functions, electron diffraction, transmission electron microscopy, photo absorption, core-level binding energy data, infrared absorption, electron energy loss spectroscopy, and density functional theory calculations. The established cation-exchange method for metal diboride opens new avenues for the mass production of several types of boron-based 2D materials by countercation selection and functionalization.

Tuning Surface Structure of 3D Nanoporous Gold by Surfactant-Free Electrochemical Potential Cycling.

3D dealloyed nanoporous metals have emerged as a new class of catalysts for various chemical and electrochemical reactions. Similar to other heterogeneous catalysts, the surface atomic structure of the nanoporous metal catalysts plays a crucial role in catalytic activity and selectivity. Through surfactant-assisted bottom-up synthesis, the surface-structure modification has been successfully realized in low-dimensional particulate catalysts. However, the surface modification by top-down dealloying has not been well explored for nanoporous metal catalysts. Here, a surfactant-free approach to tailor the surface structure of nanoporous gold by surface relaxation via electrochemical redox cycling is reported. By controlling the scan rates, nanoporous gold with abundant {111} facets or {100} facets can be designed and fabricated with dramatically improved electrocatalysis toward the ethanol oxidation reaction.

Chemical Selectivity at Grain Boundary Dislocations in Monolayer Mo1-xWxS2 Transition Metal Dichalcogenides.

Grain boundaries (GBs) are unavoidable crystal defects in polycrystalline materials and significantly influence their properties. However, the structure and chemistry of GBs in 2D transition metal dichalcogenide alloys have not been well established. Here we report significant chemical selectivity of transition metal atoms at GB dislocation cores in Mo1-xWxS2 monolayers. Different from classical elastic field-driven dislocation segregation in bulk crystals, the chemical selectivity in the 2D crystals originates prominently from variation of atomic coordination numbers at dislocation cores. This observation provides atomic insights into the topological effect on the chemistry of crystal defects in 2D materials.

Ultrastrong steel via minimal lattice misfit and high-density nanoprecipitation.

Next-generation high-performance structural materials are required for lightweight design strategies and advanced energy applications. Maraging steels, combining a martensite matrix with nanoprecipitates, are a class of high-strength materials with the potential for matching these demands. Their outstanding strength originates from semi-coherent precipitates, which unavoidably exhibit a heterogeneous distribution that creates large coherency strains, which in turn may promote crack initiation under load. Here we report a counterintuitive strategy for the design of ultrastrong steel alloys by high-density nanoprecipitation with minimal lattice misfit. We found that these highly dispersed, fully coherent precipitates (that is, the crystal lattice of the precipitates is almost the same as that of the surrounding matrix), showing very low lattice misfit with the matrix and high anti-phase boundary energy, strengthen alloys without sacrificing ductility. Such low lattice misfit (0.03 ± 0.04 per cent) decreases the nucleation barrier for precipitation, thus enabling and stabilizing nanoprecipitates with an extremely high number density (more than 1024 per cubic metre) and small size (about 2.7 ± 0.2 nanometres). The minimized elastic misfit strain around the particles does not contribute much to the dislocation interaction, which is typically needed for strength increase. Instead, our strengthening mechanism exploits the chemical ordering effect that creates backstresses (the forces opposing deformation) when precipitates are cut by dislocations. We create a class of steels, strengthened by Ni(Al,Fe) precipitates, with a strength of up to 2.2 gigapascals and good ductility (about 8.2 per cent). The chemical composition of the precipitates enables a substantial reduction in cost compared to conventional maraging steels owing to the replacement of the essential but high-cost alloying elements cobalt and titanium with inexpensive and lightweight aluminium. Strengthening of this class of steel alloy is based on minimal lattice misfit to achieve maximal precipitate dispersion and high cutting stress (the stress required for dislocations to cut through coherent precipitates and thus produce plastic deformation), and we envisage that this lattice misfit design concept may be applied to many other metallic alloys.