Metal Single Crystals Research Articles

Metallic Porous Iron Nitride and Tantalum Nitride Single Crystals with Enhanced Electrocatalysis Performance.

Altering a material's catalytic properties would require identifying structural features that deliver electrochemically active surfaces. Single-crystalline porous materials, combining the advantages of long-range ordering of bulk crystals and large surface areas of porous materials, would create sufficient active surfaces by stabilizing 2D active moieties confined in lattice and may provide an alternative way to create high-energy surfaces for electrocatalysis that are kinetically trapped. Here, a radical concept of building active metal-nitrogen moieties with unsaturated nitrogen coordination on a porous surface by directly growing metallic porous metal nitride (Fe3 N and Ta5 N6 ) single crystals at unprecedented 2 cm scale is reported. These porous single crystals demonstrate exceptionally high conductivity of 0.1-1.0 × 105 S cm-1 , while the atomic surface layers of the porous crystals are confirmed to be an Fe termination layer for Fe3 N and a Ta termination layer for Ta5 N6 . The unsaturated metal-nitrogen moieties (Fe6 -N and Ta5 -N3 ) with unique electronic structures demonstrate enhanced electrocatalysis performance and durability.

The strain hardening of micro-sized face-centered-cubic single crystal metals: A crystal plasticity study

Recent experimental and theoretical studies indicate the “external geometry” and “internal structure” are the controlling factors to characterize the size-dependent plastic flow for micro-sized single crystal metals. i.e., increasing yield strength with decreasing sample size. This paper aims to investigate the strain hardening behavior for micro-sized face-centered-cubic (FCC) metals by employing crystal plasticity approach. The size-dependent dislocation density evolution law incorporates the dimensional parameters of internal structure and external geometry in order to consider the dislocation surface annihilation and dislocation source controlled plastic mechanisms. The current shear yield strength for micro-sized FCC metals is formulated by introducing the dislocation source length in classical anisotropic hardening model of Franciosi type. Combined with the above two aspects, crystal plasticity finite element method is developed to examine the strain hardening behavior of single crystal micro-sized metals during uniaxial compression. The validity of the new theoretical formulation and modeling approach is checked by comparing the simulation results with the experimental results available in literatures. The simulation results indicate that the micro-sized FCC metals have significant size effect. Further studies are performed to evaluate the influences of the initial dislocation density and the number of dislocation sources on strain hardening behavior of micro-sized FCC metals.

Turning many into one

Metallurgy Single-crystal metal foils are valuable for their surface properties that allow for synthesis of materials like graphene. Jin et al. present a strategy for creating colossal single-crystal metal foils called “contact-free annealing” (see the Perspective by Rollett). The method relies on hanging and heating commercially available, inexpensive, cold-rolled metal foils. Almost as if by magic, the polycrystalline grains rotate and anneal into a large single-crystal sheet with a specific crystal orientation. The strategy allows for the creation of much larger and much cheaper single-crystal metal foils. Science , this issue p. [1021][1]; see also p. [996][2] [1]: /lookup/doi/10.1126/science.aao3373 [2]: /lookup/doi/10.1126/science.aav6733

Nanoscale Surface Structure-Activity in Electrochemistry and Electrocatalysis.

Nanostructured electrochemical interfaces (electrodes) are found in diverse applications ranging from electrocatalysis and energy storage to biomedical and environmental sensing. These functional materials, which possess compositional and structural heterogeneity over a wide range of length scales, are usually characterized by classical macroscopic or "bulk" electrochemical techniques that are not well-suited to analyzing the nonuniform fluxes that govern the electrochemical response at complex interfaces. In this Perspective, we highlight new directions to studying fundamental electrochemical and electrocatalytic phenomena, whereby nanoscale-resolved information on activity is related to electrode structure and properties colocated and at a commensurate scale by using complementary high-resolution microscopy techniques. This correlative electrochemical multimicroscopy strategy aims to unambiguously resolve structure and activity by identifying and characterizing the structural features that constitute an active surface, ultimately facilitating the rational design of functional electromaterials. The discussion encompasses high-resolution correlative structure-activity investigations at well-defined surfaces such as metal single crystals and layered materials, extended structurally/compositionally heterogeneous surfaces such as polycrystalline metals, and ensemble-type electrodes exemplified by nanoparticles on an electrode support surface. This Perspective provides a roadmap for next-generation studies in electrochemistry and electrocatalysis, advocating that complex electrode surfaces and interfaces be broken down and studied as a set of simpler "single entities" (e.g., steps, terraces, defects, crystal facets, grain boundaries, single particles), from which the resulting nanoscale understanding of reactivity can be used to create rational models, underpinned by theory and surface physics, that are self-consistent across broader length scales and time scales.

Unstable plastic deformation in bimetal

The present work aims the characterization of different types of waves generated upon the plastic flow in single-crystal metals and alloys. The propagation of new types of waves is highlighted during the linear work hardening and easy glide stages of flow. As found, the motion velocity of waves is the inverse function of the work hardening coefficient.

Characterization and Modeling of Hydrogen-Terminated MOSFETs With Single-Crystal and Polycrystalline Diamond

The mechanism of hydrogen-terminated (H-terminated) single-crystal and polycrystalline diamond metal–oxide field-effect transistors (MOSFETs) is investigated in this letter. In order to characterize the complicated p-type surface layer in H-terminated diamond MOSFETs, both p-type acceptors and the C–H dipole effect are considered in our model. Simulated hole distribution and band bending are used to illustrate the operation mechanism in the surface region. Simulated results by using the proposed model match well with both measured ${I} - {V}$ and transfer characteristics of these two kinds of diamond MOSFETs. Moreover, channel lattice temperature distribution reveals that self-heating effect-induced slight current collapse occurs in the polycrystalline diamond MOSFET at ${I}_{\text {ds}} $ over 500 mA/mm due to relatively lower thermal conductivity. Finally, the lateral electric field and hole velocity distribution under the gate electrode are given to aid future research.

Deformation Model of Magnesium Single Crystals

There are many approaches to the description and prediction of mechanical properties of metal single crystals possessing various crystal lattices. For the first time, the von Mises yield criterion is generalized here for the case of uniaxial tension/compression, shear and uniaxial compression with the axis constraint of magnesium single crystals having a hexagonal close-packed lattice. The paper presents the assessment of anisotropy factors and the correlation between the yield stress and orientation of magnesium single crystals.

Colossal grain growth yields single-crystal metal foils by contact-free annealing.

Single-crystal metals have distinctive properties owing to the absence of grain boundaries and strong anisotropy. Commercial single-crystal metals are usually synthesized by bulk crystal growth or by deposition of thin films onto substrates, and they are expensive and small. We prepared extremely large single-crystal metal foils by "contact-free annealing" from commercial polycrystalline foils. The colossal grain growth (up to 32 square centimeters) is achieved by minimizing contact stresses, resulting in a preferred in-plane and out-of-plane crystal orientation, and is driven by surface energy minimization during the rotation of the crystal lattice followed by "consumption" of neighboring grains. Industrial-scale production of single-crystal metal foils is possible as a result of this discovery.

Ultrathin Metal Crystals: Growth on Supported Graphene Surfaces and Applications.

Controlled nucleation and growth of metal clusters in metal deposition processes is a long-standing issue for thin-film-based electronic devices. When metal atoms are deposited on solid surfaces, unintended defects sites always lead to a heterogeneous nucleation, resulting in a spatially nonuniform nucleation with irregular growth rates for individual nuclei, resulting in a rough film that requires a thicker film to be deposited to reach the percolation threshold. In the present study, it is shown that substrate-supported graphene promotes the lateral 2D growth of metal atoms on the graphene. Transmission electron microscopy reveals that 2D metallic single crystals are grown epitaxially on supported graphene surfaces while a pristine graphene layer hardly yields any metal nucleation. A surface energy barrier calculation based on density functional theory predicts a suppression of diffusion of metal atoms on electronically perturbed graphene (supported graphene). 2D single Au crystals grown on supported graphene surfaces exhibit unusual near-infrared plasmonic resonance, and the unique 2D growth of metal crystals and self-healing nature of graphene lead to the formation of ultrathin, semitransparent, and biodegradable metallic thin films that could be utilized in various biomedical applications.

Two-dimensional itinerant ferromagnetism in atomically thin Fe3GeTe2.

Discoveries of intrinsic two-dimensional (2D) ferromagnetism in van der Waals (vdW) crystals provide an interesting arena for studying fundamental 2D magnetism and devices that employ localized spins1-4. However, an exfoliable vdW material that exhibits intrinsic 2D itinerant magnetism remains elusive. Here we demonstrate that Fe3GeTe2 (FGT), an exfoliable vdW magnet, exhibits robust 2D ferromagnetism with strong perpendicular anisotropy when thinned down to a monolayer. Layer-number-dependent studies reveal a crossover from 3D to 2D Ising ferromagnetism for thicknesses less than 4 nm (five layers), accompanied by a fast drop of the Curie temperature (TC) from 207 K to 130 K in the monolayer. For FGT flakes thicker than ~15 nm, a distinct magnetic behaviour emerges in an intermediate temperature range, which we show is due to the formation of labyrinthine domain patterns. Our work introduces an atomically thin ferromagnetic metal that could be useful for the study of controllable 2D itinerant ferromagnetism and for engineering spintronic vdW heterostructures5.

Low‐Temperature Eutectic Synthesis of PtTe2 with Weak Antilocalization and Controlled Layer Thinning

AbstractMetallic transition metal dichalcogenides (TMDs) have exhibited various exotic physical properties and hold the promise of novel optoelectronic and topological devices applications. However, the synthesis of metallic TMDs is based on gas‐phase methods and requires high‐temperature condition. As an alternative to the gas‐phase synthetic approach, lower temperature eutectic liquid‐phase synthesis presents a very promising approach with the potential for larger‐scale and controllable growth of high‐quality thin metallic TMD single crystals. Here, the first realization of low‐temperature eutectic liquid‐phase synthesis of type‐II Dirac semimetal PtTe2 single crystals with thickness ranging from 2 to 200 nm is presented. The electrical measurement of synthesized PtTe2 reveals a record‐high conductivity of as high as 3.3 × 106 S m−1 at room temperature. Besides, the weak antilocalization behavior is identified experimentally in the type‐II Dirac semimetal PtTe2 for the first time. Furthermore, a simple and general strategy is developed to obtain atomically thin PtTe2 crystal by thinning as‐synthesized bulk samples, which can still retain highly crystalline and exhibits excellent electrical conductivity. The results of controllable and scalable low‐temperature eutectic liquid‐phase synthesis and layer‐by‐layer thinning of high‐quality thin PtTe2 single crystals offer a simple and general approach for obtaining different thickness metallic TMDs with high melting‐point transition metal.

The Orientation of Silver Surfaces Drives the Reactivity and the Selectivity in Homo-Coupling Reactions.

Original reaction pathways can be explored in the on-surface synthesis approach where small aromatic precursors are confined to the surface of single crystal metals. The bis-indanedione molecule reacted with itself on silver surfaces in different ways, through a Knoevenagel reaction or an oxidative coupling, leading to the formation of a variety of new molecular compounds and covalently-linked 1D or 2D networks. Noteworthy, original reaction products were obtained that cannot be synthesized in traditional solvent-based chemistry. The lowest activation temperature for the homo-coupling reactions was found on the Ag(111) surface. The Ag(110) was highly selective in terms of coupling reaction type, while on Ag(100) the temperature could finely control the selectivity. The on-surface synthesis approach is shown here to be particularly efficient to produce original compounds in mild conditions, using activation temperatures as low as 200 °C. The different structures were characterized by scanning tunnelling microscopy (STM) together with X-ray photoelectron emission spectroscopy (XPS) and high-resolution electron energy loss spectroscopy (HREELS).

Superplastic Creep of Metal Nanowires from Rate-Dependent Plasticity Transition.

Understanding the time-dependent mechanical behavior of nanomaterials such as nanowires is essential to predict their reliability in nanomechanical devices. This understanding is typically obtained using creep tests, which are the most fundamental loading mechanism by which the time-dependent deformation of materials is characterized. However, due to existing challenges facing both experimentalists and theorists, the time-dependent mechanical response of nanowires is not well-understood. Here, we use atomistic simulations that can access experimental time scales to examine the creep of single-crystal face-centered cubic metal (Cu, Ag, Pt) nanowires. We report that both Cu and Ag nanowires show significantly increased ductility and superplasticity under low creep stresses, where the superplasticity is driven by a rate-dependent transition in defect nucleation from twinning to trailing partial dislocations at the micro- or millisecond time scale. The transition in the deformation mechanism also governs a corresponding transition in the stress-dependent creep time at the microsecond (Ag) and millisecond (Cu) time scales. Overall, this work demonstrates the necessity of accessing time scales that far exceed those seen in conventional atomistic modeling for accurate insights into the time-dependent mechanical behavior and properties of nanomaterials.

Shape Control of Electrochemically Deposited Metal Films and Nanostructures through Additive Effects

The use of electrolyte additives to affect nanocrystallite shape and film morphology in electroless and electrodeposited metal films is presented. Copper electrodeposited onto gold results in preferential square pyramidal copper nanostructure morphologies in the presence of the organic additive malachite green (MG). We have employed in-situ and ex situ methods to investigate the additive effects, including nonlinear optical spectroscopy, electrochemical scanning tunneling microscopy, linear sweep and cyclic voltammetry, atomic force microscopy (AFM), scanning electron microscopy (SEM), and X-ray diffraction (XRD). These shape-controlled additive effects are supported by additive adsorption energy calculations, which indicate preferential interactions, and differential growth kinetics on different facets of the film's growing nanostructures during electrodeposition. Facet-specific interactions have also been exploited in the epitaxial electroless deposition of noble metals onto single crystal Ag(100) substrates. Here, we examine the role of specific anions in determining the resulting film and nanostructure morphologies via SEM and high resolution transmission electron microscopy (HRTEM). These effects have been exploited to yield large area patterned, and shape-controlled nanoarrays of single crystal metal nanostructures for plasmonic and metamaterial applications. These approaches offer new and cost effective routes to achieve shape-controlled surface nanostructure.

A Facile Space-Confined Solid-Phase Sulfurization Strategy for Growth of High-Quality Ultrathin Molybdenum Disulfide Single Crystals.

Single-crystal transition metal dichalcogenides (TMDs) and TMD-based heterojunctions have recently attracted significant research and industrial interest owing to their intriguing optical and electrical properties. However, the lack of a simple, low-cost, environmentally friendly, synthetic method and a poor understanding of the growth mechanism post a huge challenge to implementing TMDs in practical applications. In this work, we developed a novel approach for direct formation of high-quality, monolayer and few-layer MoS2 single crystal domains via a single-step rapid thermal processing of a sandwiched reactor with sulfur and molybdenum (Mo) film in a confined reaction space. An all-solid-phase growth mechanism was proposed and experimentally/theoretically evidenced by analyzing the surface potential and morphology mapping. Compared with the conventional chemical vapor deposition approaches, our method involves no complicated gas-phase reactant transfer or reactions and requires very small amount of solid precursors [e.g., Mo (∼3 μg)], no carrier gas, no pretreatment of the precursor, no complex equipment design, thereby facilitating a simple, low-cost, and environmentally friendly growth. Moreover, we examined the symmetry, defects, and stacking phase in as-grown MoS2 samples using simultaneous second-harmonic-/sum-frequency-generation (SHG/SFG) imaging. For the first time, we observed that the SFG (peak intensity/position) polarization can be used as a sensitive probe to identify the orientation of TMDs' crystallographic axes. Furthermore, we fabricated ferroelectric programmable Schottky junction devices via local domain patterning using the as-grown, single-crystal monolayer MoS2, revealing their great potential in logic and optoelectronic applications. Our strategy thus provides a simple, low-cost, and scalable path toward a wide variety of TMD single crystal growth and novel functional device design.

Non-Schmid effects and finite wavelength instabilities in single crystal metals

A long standing postulate in crystal plasticity of metals, known as Schmid law, states that yielding commences once the resolved shear stress on a slip plane reaches a critical value. While non-Schmid effects have previously been reported experimentally (mostly in alloys) and in molecular-dynamics simulations, we examine the validity of this assumption through phonon stability analysis. We subject four distinct single crystal metals to a combined shear-hydrostatic deformation and identify the onset of plasticity with the onset of an instability. We find significant shear-normal coupling in single crystal metals, reflecting non-Schmid effects in defect nucleation. Also, it is a widespread assumption in the literature (Liu et al., 2010; Van Vliet et al., 2003) that the instabilities in single crystal metals are of long wavelength. In contrast, we show that short wavelength instabilities are abundant. Our results illustrate the potential pitfalls of relying on the widely used elastic stability analysis for investigating defect nucleation.

Single-crystal metal growth on amorphous insulating substrates

Metal structures on insulators are essential components in advanced electronic and nanooptical systems. Their electronic and optical properties are closely tied to their crystal quality, due to the strong dependence of carrier transport and band structure on defects and grain boundaries. Here we report a method for creating patterned single-crystal metal microstructures on amorphous insulating substrates, using liquid phase epitaxy. In this process, the patterned metal microstructures are encapsulated in an insulating crucible, together with a small seed of a differing material. The system is heated to temperatures above the metal melting point, followed by cooling and metal crystallization. During the heating process, the metal and seed form a high-melting-point solid solution, which directs liquid epitaxial metal growth. High yield of single-crystal metal with different sizes is confirmed with electron backscatter diffraction images, after removing the insulating crucible. Unexpectedly, the metal microstructures crystallize with the [Formula: see text] direction normal to the plane of the film. This platform technology will enable the large-scale integration of high-performance plasmonic and electronic nanosystems.

Graphene on nickel (100) micrograins: Modulating the interface interaction by extended moiré superstructures

Interaction with the substrate strongly affects the electronic/chemical properties of supported graphene. So far, graphene grown by chemical vapor deposition (CVD) on catalytic single crystal transition metal surfaces - mostly 3-fold close-packed - has mainly been studied. Herein, we investigated CVD graphene on a polycrystalline nickel (Ni) substrate, focusing in particular on (100) micrograins and comparing the observed behavior with that on single crystal Ni(100) substrate. The symmetry-mismatch leads to moiré superstructures with stripe-like or rhombic-network morphology, which were characterized by atomically-resolved scanning tunneling microscopy (STM). Density functional theory (DFT) simulations shed light on spatial corrugation and interfacial interactions: depending on the misorientation angle, graphene is either alternately physi- and chemisorbed or uniformly chemisorbed, the interaction being modulated by the (sub)nanometer-sized moiré superstructures. Ni(100) micrograins appear to be a promising substrate to finely tailor the electronic properties of graphene at the nanoscale, with relevant perspective applications in electronics and catalysis.

Growth of 2 cm metallic porous TiN single crystals

Metallic porous single crystals would significantly provide enhanced functionalities owing to their structural coherence that reduces the electronic and photonic scattering effects.

The metal-ionic liquid interface as characterized by impedance spectroscopy and in situ scanning tunneling microscopy.

We summarize our results of electrochemical measurements carried out on inert or close-to-inert metals in ionic liquids, with the aim to explore the metal|ionic liquid interface structure. To this we used electrochemical methods: cyclic voltammetry, impedance spectroscopy, potential of zero total charge measurements and structure-sensitive techniques, such as in situ scanning tunneling spectroscopy. The studied systems were mostly single crystals of noble metals in imidazolium-based ionic liquids. The two main findings are: (i) in the potential window where no Faradaic reactions occur, the interfacial capacitance exhibits a frequency dependence due to double-layer rearrangement processes and (ii) in certain cases ordered anion and cation structures exist at the interface.