Single Crystalline Metal Research Articles

Conjugated crosslinks boost the conductivity and stability of a single crystalline metal-organic framework.

A linker molecule with four pendant thiophene functions was crystallized with Zr(iv) ions to form a semiconductive porous coordination solid (1.1 × 10-5 S cm-1). Oxidative treatment with FeCl3 guests then coupled the thiophene units to form conjugated bridges as covalent crosslinks. The resulting hybrid of a metal-organic framework and conjugated polymer featured robust crystalline order that withstood long-term air exposure and broad pH (from 0 to 12) conditions. Moreover, the homocoupled thiophene units, conjugated through sulfide links (-S-) with the linker backbone, afforded higher electronic conductivity (e.g., >2.2 × 10-3 S cm-1), which is characteristic of conductive polymer prototypes of polythiophene and polyphenylene sulfide. The crosslinked solid also exhibited proton conductivity that could be increased broadly upon H2SO4 treatment (e.g., from 5.0 × 10-7 to 1.6 × 10-3 S cm-1).

Nanoarchitectonics of Nanoporous Carbon Materials in Supercapacitors Applications.

High surface area and large pore volume carbon materials having hierarchical nanoporous structure are required in high performance supercapacitors. Such nanoporous carbon materials can be fabricated from organic precursors with high carbon content, such as synthetic biomass or agricultural wastes containing cellulose, hemicellulose, and lignin. Using recently developed unique concept of materials nanoarchitectonics, high performance porous carbons with controllable surface area, pore size distribution, and hierarchy in nanoporous structure can be fabricated. In this review, we will overview the recent trends and advancements on the synthetic methods for the production of hierarchical porous carbons with one- to three-dimensional network structure with superior performance in supercapacitors applications. We highlight the promising scope of accessing nanoporous graphitic carbon materials from: (i) direct conversion of single crystalline self-assembled fullerene nanomaterials and metal organic frameworks, (ii) hard- and soft-templating routes, and (iii) the direct carbonization and/or activation of biomass or agricultural wastes as non-templating routes. We discuss the appealing points of the different synthetic carbon sources and natural precursor raw−materials derived nanoporous carbon materials in supercapacitors applications.

Efficient photocatalytic hydrogen evolution on single-crystalline metal selenide particles with suitable cocatalysts.

It is important to improve the apparent quantum yields (AQYs) of narrow bandgap photocatalysts to achieve efficient H2 production. The present work demonstrates a particulate solid solution of zinc selenide and copper gallium selenide (denoted as ZnSe:CGSe) that evolves H2 efficiently and is responsive to visible light up to 725 nm. This material was synthesized using a flux-assisted method and was found to comprise single-crystalline tetrahedral particles. The coloading of Ni and Rh, Pt, Pd or Ru as cocatalysts further improved the photocatalytic H2 evolution rate over this photocatalyst. With the optimal coloading of a Ni–Ru composite cocatalyst, an AQY of 13.7% was obtained at 420 nm during a sacrificial H2 evolution reaction, representing the highest value yet reported for a photocatalyst with an absorption edge longer than 700 nm. The present study demonstrates that the preparation of single-crystalline particles and the rational assembly of composite cocatalysts are effective strategies that allow the efficient utilization of long wavelengths by metal selenide photocatalysts for solar fuel production.

Strongly enhanced Gilbert damping anisotropy at low temperature in high quality single-crystalline Fe/MgO (0 0 1) thin film

The anisotropic behavior of Gilbert damping provides a new dimension for controlling and tuning magnetization relaxation within the same magnetic device. The single-crystalline ferromagnetic metals with a high symmetry have been predicted to possess significantly anisotropic damping for one decade. By decreasing temperature down to T = 4.5 K, we found that the anisotropic damping factor Q, defined as the fractional difference of damping between easy and hard axis, can be approached to 60%–90% in Fe (15 nm)/MgO (0 0 1) without applied fields dependent behavior. It is much larger than Q = 10%–20% obtained at room temperature. Furthermore, it is followed by explanations on the basis of the breathing Fermi surface model giving around Q = 30%–40% when electron-phonon scattering rate is small enough. This discovery will enrich the understanding of damping mechanism, and can provide clues for searching strongly anisotropic damping within 3d transition metals and alloys.

Controlled growth of single-crystalline metal nanowires via thermomigration across a nanoscale junction

Mass transport driven by temperature gradient is commonly seen in fluids. However, here we demonstrate that when drawing a cold nano-tip off a hot solid substrate, thermomigration can be so rampant that it can be exploited for producing single-crystalline aluminum, copper, silver and tin nanowires. This demonstrates that in nanoscale objects, solids can mimic liquids in rapid morphological changes, by virtue of fast surface diffusion across short distances. During uniform growth, a thin neck-shaped ligament containing a grain boundary (GB) usually forms between the hot and the cold ends, sustaining an extremely high temperature gradient that should have driven even larger mass flux, if not counteracted by the relative sluggishness of plating into the GB and the resulting back stress. This GB-containing ligament is quite robust and can adapt to varying drawing directions and velocities, imparting good controllability to the nanowire growth in a manner akin to Czochralski crystal growth.

Single-Crystalline Metal Oxide Nanostructures Synthesized by Plasma-Enhanced Thermal Oxidation.

To unravel the influence of the temperature and plasma species on the growth of single-crystalline metal oxide nanostructures, zinc, iron, and copper foils were used as substrates for the study of nanostructure synthesis in the glow discharge of the mixture of oxygen and argon gases by a custom-made plasma-enhanced horizontal tube furnace deposition system. The morphology and microstructure of the resulting metal oxide nanomaterials were controlled by changing the reaction temperature from 300 to 600 °C. Experimentally, we confirmed that single-crystalline zinc oxide, copper oxide, and iron oxide nanostructures with tunable morphologies (including nanowires, nanobelts, etc.) can be successfully synthesized via such procedure. A plausible growth mechanism for the synthesis of metal oxide nanostructures under the plasma-based process is proposed and supported by the nanostructure growth modelling. The results of this work are generic, confirmed on three different types of materials, and can be applied for the synthesis of a broader range of metal oxide nanostructures.

Interaction of Single-Crystalline Metal Fluorides with Titanium-Containing Melts

The wetting of solid titanium substrates with liquid lithium and magnesium fluorides was studied by the sessile drop method in 1 · 10–3 Pa vacuum at 1173 and 1553 K, respectively. Titanium and fluorine were found to actively interact in the systems to change the titanium substrate surface profile, leading to a ‘crater’ in the contact area. The ‘solid titanium–liquid magnesium fluoride’ interfacial geometry was examined and confirmed the potential to form gaseous products in the titanium–fluorine interaction. Such gaseous compounds prevent the wetting of alkaline-earth metal fluorides with titanium-containing melts at temperatures above 1400 K. This allows the materials made of alkaline-earth metal fluorides to be used as refractories for high-temperature melting, longterm homogenization, and casting of alloys with a high content of Ti, Zr, Hf, Nb, and V.

Manipulation of a Single Crystal Nanowire on an Atomic Level

In recent years, metal oxide nanostructures (MO) have been intensively studied due to a wide variety of applications on multiple fields, for example in optoelectronics, gas sensing, photovoltaics, field-effect transistors, UV lasers, field emission sources etc. [1]. However, for applications, other types of structures are also interesting such as sulfides and nitrides. Here the question that raises is can we manipulate nanomaterials like nanowires on an atomic level. Knowing the exact mechanism of sulfur-oxygen exchange in the metal oxide crystal lattice can be utilized for the synthesis of metal sulfides with specific morphologies. When compared to metal oxides, single-crystalline metal sulfide nanostructures of specific morphologies are usually harder to produce than MOs. Generally, metal sulfide nanostructures represent unconventional materials in areas such as batteries or photovoltaics with yet unexploited potential, where their efficiency is mainly limited by the purity of crystalline phases and structural defects [2]. A promising path for novel synthesis route is incorporation of the sulfur in the crystal structure of a metal oxide nanoparticle, while during this transition the morphology of the nanostructure does not change. In this way, it is possible to obtain single crystal metal sulfide nanostructures from metal oxide nanostructures in the single crystal to single crystal (SCSC) transformation. A promising method to achieve such transformation in a short period of time is a treatment of metal oxide nanostructures with the plasma of sulfur-containing gas. In this research, we studied the mechanism of sulfur incorporation in the metal oxide crystal structure. The procedure was carried out by synthesis of ultra-thin metal oxide nanowires, which were then treated with sulfur-containing gases under increased temperature (in thermodynamic equilibrium conditions) and in the plasma of the same gases (under thermodynamically non-equilibrium conditions). Experiments were conducted at different temperatures and pressures to evaluate the influence of plasma on transformation process. Mechanisms involved in the interaction of sulfur-containing species with metal oxides were studied on isolated single ultra-thin metal oxide nanowire. Furthermore, changes in crystal structures and their effect on the material properties before and after sulfurization were studied. [1] Umar A. & Hahn Y. B. (Eds.). (2010), Metal Oxide Nanostructures and their Applications. American Scientific Publishers, CA, USA [2] Zavašnik, J., & Rečnik, A. (2013). Electron microscopy study of CVT grown Fe-sulphides. Journal of Crystal Growth, 367, 18-23.

Quantum Engineering of Atomically Smooth Single-Crystalline Silver Films

There is a demand for ultra low-loss metal films with high-quality single crystals and perfect surface for nanophotonics and quantum information processing. Many researches are devoted to alternative materials, but silver is by far theoretically the most preferred low-loss material at optical and near-IR frequencies. Usually, epitaxial growth is used to deposit single-crystalline silver films, but they still suffer from unpredictable losses and well-known dewetting effect that strongly limits films quality. Here we report the two-step approach for e-beam evaporation of atomically smooth single-crystalline metal films. The proposed method is based on the thermodynamic control of film growth kinetics at atomic level, which allows depositing state-of-art metal films and overcoming the film-surface dewetting. Here we use it to deposit 35–100 nm thick single-crystalline silver films with the sub-100pm surface roughness and theoretically limited optical losses, considering an ideal material for ultrahigh-Q nanophotonic devices. Utilizing these films we experimentally estimate the contribution of grain boundaries, material purity, surface roughness and crystallinity to optical properties of metal films. We demonstrate our «SCULL» two-step approach for single-crystalline growth of silver, gold and aluminum films which open fundamentally new possibilities in nanophotonics, biotechnology and superconductive quantum technologies. We believe it could be readily adopted for the synthesis of other extremely low-loss single-crystalline metal films.

A model for the indentation size effect in polycrystalline alloys coupling intrinsic and extrinsic length scales

Abstract

Scalable synthesis and defect modulation of large monolayer WS2 via annealing in H2S atmosphere/thiol treatment to enhance photoluminescence

Monolayer WS2 (1L-WS2) possesses direct band gap, which is beneficial for light emitting applications. However, high quality sample obtained via mechanical exfoliation suffers from ultralow yield of large flakes while large 2D material monolayers synthesized by chemical vapour deposition (CVD) unintentionally induce many defects, resulting in low photoluminescence (PL) intensity. Herein, we firstly control the 1L-WS2 scalable growth through H2 introduction at the beginning of the CVD synthesis. Its domain size can be increased from 1 μm up to 20 μm with higher coverage, compared to that synthesized without H2 introduction. Then, we propose scalable ways to anneal the obtained large 1L-WS2 in Ar diluted hydrogen sulfide (H2S) atmosphere at 750 °C or to treat it with (3-mercaptopropyl) trimethoxysilane (MPS). Its PL intensity can be increased to 12 times by H2S or more interestingly to 30 times by MPS, respectively. Our systematic analyses show that the drastic PL improvements are mainly attributed to the reduction of structural defects by H2S and MPS, which is facilitated via the H– bond in them. The results help to explore routes to obtaining large-size single crystalline transition metal dichalcogenides monolayer (1L-TMDCs) and intense PL emission.

Unusual Oxygen Partial Pressure Dependence of Electrical Transport of Single-Crystalline Metal Oxide Nanowires Grown by the Vapor-Liquid-Solid Process.

In general, the electrical conductivities of n-type semiconducting metal oxide nanostructures increase with the decrease in the oxygen partial pressure during crystal growth owing to the increased number of crystal imperfections including oxygen vacancies. In this paper, we report an unusual oxygen partial pressure dependence of the electrical conductivity of single-crystalline SnO2 nanowires grown by a vapor-liquid-solid (VLS) process. The electrical conductivity of a single SnO2 nanowire, measured using the four-probe method, substantially decreases by 2 orders of magnitude when the oxygen partial pressure for the crystal growth is reduced from 10-3 to 10-4 Pa. This contradicts the conventional trend of n-type SnO2 semiconductors. Spatially resolved single-nanowire electrical transport measurements, microstructure analysis, plane-view electron energy-loss spectroscopy, and molecular dynamics simulations reveal that the observed unusual oxygen partial pressure dependence of the electrical transport is attributed to the intrinsic differences between the two crystal growth interfaces (LS and VS interfaces) in the critical nucleation of the crystal growth and impurity incorporation probability as a function of the oxygen partial pressure. The impurity incorporation probability at the LS interface is always lower than that at the VS interface, even under reduced oxygen partial pressures.

Repeatable and reproducible formation/rupture of oxygen vacancy filaments in the vicinity of a polycrystalline HfO2 surface

We theoretically investigated the dynamics of oxygen vacancies (Vo’s) and the effect of changing their charge states by performing first-principles molecular dynamics simulations at a temperature of 1000 K. Calculations were performed for two structures of HfO2: a slab with (110) surfaces and a bulk single crystal. Our studies revealed that when the charge state of the Vo’s changed from neutral (VO0) to divalent (VO2+), Vo’s repelled each other and dispersed, in both the slab and bulk structures. In contrast, when the charge state of the Vo’s changed from VO2+ to VO0, the Vo’s attracted each other and resumed their original positions only in the slab structure. Therefore, the repeatable and reproducible formation/rupture of a VO filament can occur near the crystal surface, where the symmetry of the bulk crystal is broken. This result is consistent with the experimental results demonstrating that the resistive switching of the resistive random access memory develops in polycrystalline metal oxides rather than in single crystalline metal oxides.

The migration of alkali metal (Na+, Li+, and K+) ions in single crystalline vanadate nanowires: Rasch-Hinrichsen resistivity

We report the synthesis of single crystalline alkali metal vanadate nanowires, Li-vanadate (Li4V10O27), Na-vanadate (NaV6O15), and K-vanadate (KV4O10) and their electrical properties in a single nanowire configuration. Alkali metal vanadate nanowires were obtained by a simple thermal annealing process with vanadium hydroxides(V(OH)3) nanoparticles containing Li+, Na+, and K+ ions and further the analysis of the migration of charged particles (Li+, Na+, and K+) in vanadate by measuring the conductivity of them. We found that their ionic conductivities can be empirically explained by the Rasch-Hinrichsen resistivity and interpreted on the basis of transition state theory. Our results thus indicate that the Li ion shows the lowest potential barrier of ionic conduction due to its small ionic size. Additionally, Na-vanadate has the lowest ion number per unit V2O5, resulting in increased distance to move without collision, and ultimately in low resistivity at room temperature.

Quadratic to linear magnetoresistance tuning in TmB4

The change of a material's electrical resistance (R) in response to an external magnetic field (B) provides subtle information for the characterization of its electronic properties and has found applications in sensor and storage related technologies. In good metals, Boltzmann's theory predicts a quadratic growth in magnetoresistance (MR) at low B, and saturation at high fields. On the other hand, a number of nonmagnetic materials with weak electronic correlation and low carrier concentration for metallicity, such as inhomogeneous conductors, semimetals, narrow gap semiconductors and topological insulators, two-dimensional electron gas (2DEG) show positive, non-saturating linear magnetoresistance (LMR). However, observation of LMR in single crystals of a good metal is rare. Here we present low-temperature, angle dependent magnetotransport in single crystals of the antiferromagnetic metal, TmB4. We observe large, positive and anisotropic MR(B), which can be tuned from quadratic to linear by changing the direction of the applied field. In view of the fact that isotropic, single crystalline metals with large Fermi surface (FS) are not expected to exhibit LMR, we attribute our observations to the anisotropic FS topology of TmB4. Furthermore, the linear MR is found to be temperature-independent, suggestive of quantum mechanical origin.

Rational design concept of single crystalline metal oxide nanowires

Rational design concept of single crystalline metal oxide nanowires

Nontrivial scaling exponents of dislocation avalanches in microplasticity

Intermittency during plastic flow is one example of avalanches in critically evolving systems. Such fluctuations are typically assessed statistically with scale-free distributions. Theory and simulations have studied this behavior in detail, generally arguing for two prominent models (mean-field approach or a jamming-unjamming picture) that are characterized via distinctly different avalanche scaling exponents. In this paper, the authors show experimentally how scaling exponents for the same single crystalline metal can admit a variety of scaling exponents that encompass both models. Depending on both intrinsic and extrinsic factors, their experiments reveal how the scaling exponents are nontrivial and therefore not universal.

Direct growth of vertically aligned carbon nanotubes on stainless steel by plasma enhanced chemical vapor deposition

Applications such as energy storage and field emission devices demand direct growth of vertically aligned carbon nanotube (VACNT) arrays on conducting metal substrates, which has remained a challenge for a long time. Here, we report a controlled growth of VACNT arrays on a stainless steel (SS) substrate via dc plasma enhanced chemical vapor deposition without the addition of a metal catalyst layer. TEM and AFM examinations revealed the occurrence of nano-hills formed on the SS surface during heating process in an NH3 environment. This is a key step for uniform growth of VACNTs. The VACNTs have uniform length and diameter, good alignment, and uniform coverage over the substrate surface, superior to reported results. The particle on the tip of most of the VACNTs was found to be single crystalline Fe metal, although a few other VACNTs were found to have an alloy of Fe, Ni, Mn, and Cr at the tips. This highlights the dominating catalytic behavior of Fe over Ni, Cr, and Mn, which are the elemental composition of the nano-hills. The high quality VACNTs may find unique applications such as electron field emitters and dense nanoscale anode materials for lithium ion batteries.

Size effect for achieving high mechanical performance body-centered cubic metals and alloys

Submicron and nanostructured body-centered cubic (BCC) metals exhibit unusual mechanical performance compared to their bulk coarse-grained counterparts, including high yield strength and outstanding ductility. These properties are important for their applications in micro-, nano- and even atomic-scale devices as well as for their usages as components for enhancing the performances of structural materials. One aspect of the unusual mechanical properties of small-sized BCC metals is closely related to their dimensional confinement. Decreasing the dimensions of single crystalline metals or the grain sizes of polycrystalline metals contributes significantly to the strengthening of the small-sized BCC metals. In the last decade, significant progress has been achieved in understanding the plasticity and deformation behaviors of small-sized BCC metals. This paper aims to provide a comprehensive review on the current understanding of size effects on the plasticity and deformation mechanisms of small-sized BCC metals. The techniques used for in situ characterization of the deformation behavior and mechanical properties of small-sized samples are also presented.

Growth study of para-sexiphenyl on Ag(001): From single molecule to crystalline film

In this work we investigate the formation of crystalline organic thin films for p-sexiphenyl (6P) on Ag(001) with a combination of STM, LEED, TPD, SFM, UPS, XRD and DFT. For weakly interacting van-der-Waals-dominated systems of small organic molecules on single crystalline metals we demonstrate the complex epitaxial evolution from sub-monolayer to bulk structure. In the delicate interplay between molecule-substrate and intermolecular interactions we identify the oligo-phenyl's flexibility of central importance for the film growth. We observe a change from a planar to a twisted molecular conformation and, accompanying, a molecular realignment during the formation of a dense monolayer as the result of enhanced 2-dimensional intermolecular interaction. A second molecular realignment occurs collectively for whole domains with the formation of 3-dimensional bilayer structures. Finally the bilayer acts as a template for the epitaxial growth of the molecular thin film, determining the azimuthal orientation and a single contact plane for the organic crystallites.