Hexagonal Layers Research Articles

Ab initio engineering of materials with stacked hexagonal tin frameworks.

The group-IV tin has been hypothesized to possess intriguing electronic properties in an atom-thick hexagonal form. An attractive pathway of producing sizable 2D crystallites of tin is based on deintercalation of bulk compounds with suitable tin frameworks. Here, we have identified a new synthesizable metal distannide, NaSn2, with a 3D stacking of flat hexagonal layers and examined a known compound, BaSn2, with buckled hexagonal layers. Our ab initio results illustrate that despite being an exception to the 8-electron rule, NaSn2 should form under pressures easily achievable in multi-anvil cells and remain (meta)stable under ambient conditions. Based on calculated Z2 invariants, the predicted NaSn2 may display topologically non-trivial behavior and the known BaSn2 could be a strong topological insulator.

First 14-Layer Twinned Hexagonal Perovskite Ba14Mn1.75Ta10.5O42: Atomic-Scale Imaging of Cation Ordering

Formation of hexagonal perovskite with mixed cubic and hexagonal stacking of AO3 layers becomes more and more difficult when the number of layers in the stacking repeating unit increases. So far, the highest number of layers reported for twinned hexagonal perovskite is 12, with alternative 5 consecutive cubic layers and one hexagonal layer in the (ccccch)2 sequence. Here, we present the unexpected formation of a 14-layer twinned hexagonal perovskite with a stacking sequence (cccccch)2 for the BaO3 layers on the Ba14Mn1.75Ta10.5O42 (Ba8MnTa6O24) composition, the first example of twinned hexagonal perovskite with a periodicity exceeding 12-layers. The B-cation and vacancy distributions are characterized by multiple efficient and complementary techniques including neutron and synchrotron powder diffraction, scanning transmission electron microscopy-high angle annular dark field (STEM-HAADF) imaging, and electron energy loss spectroscopy (EELS) and X-ray energy dispersive spectroscopy (EDS) elemental mapping....

Helical superstructure of continuum graphene cone uncovered by TEM analysis of herringbone-striped pattern in graphitic whiskers

Cone-shaped graphitic whiskers (CGWs) are a form of pyrolytic carbon, consisting of conically stacked hexagonal carbon layers with an apex angle of ~135–140°. Under transmission electron microscopy (TEM), CGWs often exhibit herringbone-striped patterns. Bright-field (BF) and dark-field (DF) TEM images indicated that the stripes are due to periodical appearance of a strong inter-planar reflection, which is consistent with helical rotation of layers with stepwise “layer overlap”. High-resolution TEM revealed that the period was ~14–15 layers. The relationship between apex angle and stripe periodicity of CGWs could be consistently explained in terms of a helical superstructure of tightly coiled continuous graphene cone.

Integrating superconducting phase and topological crystalline quantum spin Hall effect in hafnium intercalated gallium film

Motivated by the growth of superconducting atomic hexagonal Ga layers on GaN surface we have calculated the electronic properties of Hf intercalated honeycomb Ga layers using first-principles theory. In contrast to the hexagonal Ga layers where substrate is necessary for their stability, we find the above structure to be dynamically stable in its freestanding form with small formation energy. In particular, six Dirac cones composed of Hf-dxy/dx2-y2 orbitals are observed in the first Brillouin zone, slightly below the Fermi energy. Spin-orbit coupling opens a large band gap of 177 meV on these Dirac cones. By calculating its mirror Chern number, we demonstrate that this band gap is topologically nontrivial and protected by mirror symmetry. Such mirror symmetry protected band gaps are rare in hexagonal lattice. A large topological crystalline quantum spin Hall conductance σSH ∼ −4 e2/h is also revealed. Moreover, electron-phonon coupling calculations reveal that this material is superconducting with a transition temperature Tc = 2.4 K, mainly contributed by Ga out-of-plane vibrations. Our results provide a route toward manipulating quantum spin Hall and superconducting behaviors in a single material which helps to realize Majorana fermions and topological superconductors.

Large-scale ordering of nanoparticles using viscoelastic shear processing.

Despite the availability of elaborate varieties of nanoparticles, their assembly into regular superstructures and photonic materials remains challenging. Here we show how flexible films of stacked polymer nanoparticles can be directly assembled in a roll-to-roll process using a bending-induced oscillatory shear technique. For sub-micron spherical nanoparticles, this gives elastomeric photonic crystals termed polymer opals showing extremely strong tunable structural colour. With oscillatory strain amplitudes of 300%, crystallization initiates at the wall and develops quickly across the bulk within only five oscillations. The resulting structure of random hexagonal close-packed layers is improved by shearing bidirectionally, alternating between two in-plane directions. Our theoretical framework indicates how the reduction in shear viscosity with increasing order of each layer accounts for these results, even when diffusion is totally absent. This general principle of shear ordering in viscoelastic media opens the way to manufacturable photonic materials, and forms a generic tool for ordering nanoparticles.

Two-dimensional hexagonal layers of A N B 8–N compounds on semiconductors

Using the parabolic model of the electronic spectrum of the substrate and the low-energy approximation of the dispersion law for two-dimensional hexagonal compounds ANB8‒N, the density of states of an epitaxial layer has been investigated as a function of the band gap of the substrate, the band gap of the graphene-like compound in a free-standing state, their mutual arrangement, and the dimensionless “layer–substrate” coupling constant C. It has been shown that, when the coupling constant C exceeds critical values, the density of states of the epitaxial layer undergoes qualitative changes. Both flat and buckled epitaxial layers have been considered. Estimates of the charge redistribution due to the transformation of the density of states of the graphene-like compound have been presented.

Cu2 − xS films as counter-electrodes for dye solar cells with ferrocene-based liquid electrolytes

In this work, the application of hexagonal CuS nanoparticle layers as counter electrodes for dye sensitized solar cells has been studied. A fast, cheap and reliable deposition method was proposed for the one-step preparation of Cu2−xS layers on F-doped SnO2 within 30min through an ink-based technique. The electrodes prepared with our method were tested with iodine/iodide electrolyte, Co(II)/(III) bipyridine redox shuttle and Fe(II)/(III) ferrocene-based liquid electrolyte. The Cu2−xS layers showed high efficiency and stability with the ferrocene/ferrocenium redox couple, showing a fast charge recombination kinetic, low charge transfer resistance (Rct=0.73Ωcm2), reasonably high limiting current (11.8mAcm−2) and high stability in propylene carbonate.

The electronic structure of graphene tuned by hexagonal boron nitrogen layers: Semimetal–semiconductor transition

The electronic structure of graphene and hexagonal boron nitrogen (G/h-BN) systems have been carefully investigated using the pseudo-potential plane-wave within density functional theory (DFT) framework. We find that the stacking geometries and interlayer distances significantly affect the electronic structure of G/h-BN systems. By studying four stacking geometries, we conclude that the monolayer G/h-BN systems should possess metallic electronic properties. The monolayer G/h-BN systems can be transited from metallicity to semiconductor by increasing h-BN layers. It reveals that the alteration of interlayer distances 2.50–3.50 Å can obtain the metal–semiconductor–semimetal variation and a tunable band gap for G/h-BN composite systems. The band dispersion along [Formula: see text]–[Formula: see text] direction is analogous to the band of rhombohedral graphite when the G/h-BN systems are semiconducting.

ChemInform Abstract: Direct Preparation of High Quality Graphene on Dielectric Substrates

AbstractReview: 50 refs.

Infrared-assisted Synthesis of Lithium Nickel Cobalt Alumina Oxide Powders as Electrode Material for Lithium-ion Batteries

This study explores an efficient infrared (IR) heating technique to synthesize highly-crystalline LiNi0.8Co0.15Al0.05O2 (NCA) cathode materials for Li-ion batteries. One home-made IR induction reactor, equipped with medium-wave IR emitter array, is adopted to prepare the NCA powders at 700°C for a calcination period of 1–5h. Two kinds of preparation routes, ball milling and rheological-phase method, are used to prepare NCA precursors. The as-prepared NCA powders display well ordering of hexagonal two-dimensional layer structure with low degree of cation mixing under appropriate conditions: IR heating time (5h) and rheological-phase method. The NCA cathode exhibits an improved discharge capacity, fast Li+ diffusion rate, high rate capability, and good cycling stability. This improved performance mainly originates from low cation mixing, low defect level, and homogeneous particle size of NCA crystals. The carbon-coated NCA cathode offers high capacities of ca. 213 and 115mAhg−1 at 0.1 and 5C, respectively. Analyzed by the Randles-Sevcik plots, the diffusion coefficients in the NCA cathodes increase up to 1.73×10−8 and 5.82×10−9cm2s−1 for Li-extraction and Li-insertion, respectively. Accordingly, the IR heating route turns on a commercial feasibility to synthesize NCA cathode materials for Li-ion battery application.

Hydrogen storage in Li-doped fullerene-intercalated hexagonal boron nitrogen layers

New materials for hydrogen storage of Li-doped fullerene (C20, C28, C36, C50, C60, C70)-intercalated hexagonal boron nitrogen (h-BN) frameworks were designed by using density functional theory (DFT) calculations. First-principles molecular dynamics (MD) simulations showed that the structures of the Cn-BN (n = 20, 28, 36, 50, 60, and 70) frameworks were stable at room temperature. The interlayer distance of the h-BN layers was expanded to 9.96–13.59 A by the intercalated fullerenes. The hydrogen storage capacities of these three-dimensional (3D) frameworks were studied using grand canonical Monte Carlo (GCMC) simulations. The GCMC results revealed that at 77 K and 100 bar (10 MPa), the C50-BN framework exhibited the highest gravimetric hydrogen uptake of 6.86 wt% and volumetric hydrogen uptake of 58.01 g/L. Thus, the hydrogen uptake of the Li-doped Cn-intercalated h-BN frameworks was nearly double that of the non-doped framework at room temperature. Furthermore, the isosteric heats of adsorption were in the range of 10–21 kJ/mol, values that are suitable for adsorbing/desorbing the hydrogen molecules at room temperature. At 193 K (–80 °C) and 100 bar for the Li-doped C50-BN framework, the gravimetric and volumetric uptakes of H2 reached 3.72 wt% and 30.08 g/L, respectively.

Hexagonal two-dimensional layers of A N B 8–N compounds on metals

Analytical expressions for densities of states of free graphene-like A N B 8–N compounds and flat and buckled epitaxial monolayers on a metallic substrate have been obtained by the tight-binding method using a low-energy approximation. Characteristic features of the densities of states as functions of the layer–substrate coupling constant and the buckling factor have been analyzed. The energy gaps and the binding energy between the epitaxial layer and the substrate have been estimated.

Ambient Temperature Growth of Mono- and Polycrystalline NbN Nanofilms and Their Surface and Composition Analysis

This paper presents the studies of high-quality 5-nm-thin NbN films deposited by means of reactive dc magnetron sputtering at room temperature. The deposition without substrate heating offers major advantages from a processing point of view and motivates the extensive composition and surface characterization and comparison of the present films with high-quality films grown at elevated temperatures. Monocrystalline NbN films have been epitaxially grown onto hexagonal GaN buffer layers (0002) and show a distinct low defect interface as confirmed by high-resolution TEM. The critical temperature $T_{c}$ of the films on the GaN buffer layer reached 10.4 K. Furthermore, a polycrystalline structure was observed on films grown onto Si (100) substrates, exhibiting a $T_{c}$ of 8.1 K, albeit a narrow transition from the normal to the superconducting state. X-ray photoelectron spectroscopy and reflected electron energy loss spectroscopy verified that the composition of NbN was identical irrespective of applied substrate heating. Moreover, the native oxide layer at the surface of NbN has been identified as NbO 2 and, thus, is in contrast to the Nb 2O 5, usually claimed to be formed at the surface of Nb when exposed to air. These findings are of significance since it was proven the possibility of growing epitaxial NbN onto GaN buffer layer in the absence of high temperatures, hence paving the way to employ NbN in more advanced fabrication processes involving a higher degree of complexity. The eased integration and employment of liftoff techniques could, in particular, lead to improved performance of cryogenic ultrasensitive terahertz electronics.

A Titanium-Organic Framework as an Exemplar of Combining the Chemistry of Metal- and Covalent-Organic Frameworks.

A crystalline material with a two-dimensional structure, termed metal-organic framework-901 (MOF-901), was prepared using a strategy that combines the chemistry of MOFs and covalent-organic frameworks (COFs). This strategy involves in situ generation of an amine-functionalized titanium oxo cluster, Ti6O6(OCH3)6(AB)6 (AB = 4-aminobenzoate), which was linked with benzene-1,4-dialdehyde using imine condensation reactions, typical of COFs. The crystal structure of MOF-901 is composed of hexagonal porous layers that are likely stacked in staggered conformation (hxl topology). This MOF represents the first example of combining metal cluster chemistry with dynamic organic covalent bond formation to give a new crystalline, extended framework of titanium metal, which is rarely used in MOFs. The incorporation of Ti(IV) units made MOF-901 useful in the photocatalyzed polymerization of methyl methacrylate (MMA). The resulting polyMMA product was obtained with a high-number-average molar mass (26 850 g mol(-1)) and low polydispersity index (1.6), which in many respects are better than those achieved by the commercially available photocatalyst (P-25 TiO2). Additionally, the catalyst can be isolated, reused, and recycled with no loss in performance.

Partially disordered antiferromagnetism and multiferroic behavior in a frustrated Ising systemCoCl2−2SC(NH2)2

We investigate partially disordered antiferromagnetism in CoCl$_2$-2SC(NH$_2$)$_2$, in which a-b plane hexagonal layers are staggered along the c-axis, rather than stacked. A robust 1/3 state forms in applied magnetic fields which the spins are locked, varying neither as a function of temperature nor field. By contrast, in zero field, partial antiferromagnetic order occurs, in which free spins are available to create a Curie-like magnetic susceptibility. We report measurements of the crystallographic structure, and the specific heat, magnetization, and electric polarization down to $T$ = 50 mK and up to $\mu_{0}H$ = 60 T. The Co$^{2+}$ $S = 3/2$ spins are Ising-like and form distorted hexagonal layers. The Ising energy scale is well separated from the magnetic exchange, and both energy scales are accessible to the measurements allowing us to cleanly parameterize them. In transverse fields, a quantum Ising phase transition can be observed at 2 T. Finally we find that magnetic exchange striction induces changes in the electric polarization up to 3 $\mu$C/m$^2$ and single-ion magnetic anisotropy effects induce a much a larger electric polarization change of 300 $\mu$C/m$^2$.

Paracrystalline Structure of Glass‐Like Carbons

This study reports on structural characterization of a series of glass‐like carbons obtained by pyrolysis of polyfurfuryl alcohol at 600, 800, 980, and 2700°C. The atomic scale structure of the prepared materials has been studied using wide‐angle X‐ray scattering technique. The acquired diffraction data were analyzed in reciprocal space as the structure factor and in real space in the form of the pair distribution function to reveal the structural attributes such as number of hexagonal network layers, size of the layers, interlayer correlations, interlayer and interatomic distances. The parameters have different effects on the diffraction intensity and the pair distribution function and are verified in reciprocal and real diffraction space simultaneously. The obtained results show that the structure of the glass‐like carbons consists of defective graphite‐like domains which size increase with the pyrolysis temperature. The heat treatment leads to a noticeable ordering of coherently scattering domains of glass‐like carbon in directions perpendicular to graphene‐like layers. However, paracrystalline type of disorder within individual layers manifesting itself in decrease in intensity and broadening of the diffraction lines is preserved in the atomic structure even at heat‐treatment temperature of 2700°C.

Strengthened PAN-based carbon fibers obtained by slow heating rate carbonization.

Large efforts have been made over the last 40 years to increase the mechanical strength of polyacrylonitrile (PAN)-based carbon fibers (CFs) using a variety of chemical or physical protocols. In this paper, we report a new method to increase CFs mechanical strength using a slow heating rate during the carbonization process. This new approach increases both the carbon sp3 bonding and the number of nitrogen atoms with quaternary bonding in the hexagonal carbon network. Theoretical calculations support a crosslinking model promoted by the interstitial carbon atoms located in the graphitic interlayer spaces. The improvement in mechanical performance by a controlled crosslinking between the carbon hexagonal layers of the PAN based CFs is a new concept that can contribute further in the tailoring of CFs performance based on the understanding of their microstructure down to the atomic scale.

Charge neutrality of quasi-free-standing monolayer graphene induced by the intercalated Sn layer

It has been confirmed by angle-resolved photoemission spectroscopy that decoupled quasi-free-standing monolayer graphene (QFMLG), obtained by Sn intercalation between the buffer layer and the 6H-SiC(0 0 0 1) substrate, is charge-neutral, i.e. the Dirac point matches with the Fermi level. By combined studies of scanning tunneling microscopy/spectroscopy and core-level/valence-band photoemission spectroscopy on this system, it has been found that the intercalated Sn atoms, bonding with the Si atoms of the top Si-C bilayer on the substrate comprise a hexagonal layer, which turns out to be metallic. Such a metallic character, which has never been found in intercalation using different elements, is a major cause of charge neutrality of QFMLG, since conduction electrons of the Sn layer compensate completely spontaneous polarization charges of 6H-SiC(0 0 0 1). This charge-neutral QFMLG is stable at a high temperature of 850 °C.

Aluminium oxynitride–hexagonal boron nitride composites with anisotropic properties

Structure of hexagonal boron nitride (h-BN) consist of hexagonal layers with strong covalent bonds and weak van der Waals bonds between them. Such specific structure and usually plate-like grains cause anisotropic properties of h-BN based materials. The aim of the present work was preparation of composites in the aluminium oxynitride–hexagonal boron nitride system with anisotropic properties. SHS technique was used to obtain complex powders with both phases synthesized in situ. Mixtures of aluminium, aluminium oxide and different amount of boron were combusted in nitrogen and the powders were hot-pressed. The h-BN grains in the composites show plate-like shapes and crystallographic orientation. The specific microstructure and texture result in anisotropy of thermal properties; thermal conductivity was few times higher in the direction perpendicular to hot-pressing force and parallel to the longer diameter of h-BN grains. The texture effect is stronger than the effect of the h-BN content.

Redox Reactions between Mn(II) and Hexagonal Birnessite Change Its Layer Symmetry.

Birnessite, a phyllomanganate and the most common type of Mn oxide, affects the fate and transport of numerous contaminants and nutrients in nature. Birnessite exhibits hexagonal (HexLayBir) or orthogonal (OrthLayBir) layer symmetry. The two types of birnessite contain contrasting content of layer vacancies and Mn(III), and accordingly have different sorption and oxidation abilities. OrthLayBir can transform to HexLayBir, but it is still vaguely understood if and how the reverse transformation occurs. Here, we show that HexLayBir (e.g., δ-MnO2 and acid birnessite) transforms to OrthLayBir after reaction with aqueous Mn(II) at low Mn(II)/Mn (in HexLayBir) molar ratios (5-24%) and pH ≥ 8. The transformation is promoted by higher pH values, as well as smaller particle size, and/or greater stacking disorder of HexLayBir. The transformation is ascribed to Mn(III) formation via the comproportionation reaction between Mn(II) adsorbed on vacant sites and the surrounding layer Mn(IV), and the subsequent migration of the Mn(III) into the vacancies with an ordered distribution in the birnessite layers. This study indicates that aqueous Mn(II) and pH are critical environmental factors controlling birnessite layer structure and reactivity in the environment.