Hexagonal Voids Research Articles

Lifshitz Transition in a Single-Atom-Thick GdxYb1-xPb3 Kagome Compound on Si(111).

Lanthanide (Ln) elements Gd and Yb alloyed with a Pb monolayer on the Si(111) substrate form LnPb3 compounds having the same crystal structure. They comprise a single-atom-thick Pb layer arranged in a slightly distorted kagome lattice with Ln atoms filling the hexagonal voids. They have similar electronic band structures except for the Fermi level position, which varies between the divalent Yb- and trivalent Gd-containing compounds by ∼0.47 eV. The ability to create a 2D solid solution with the unified continuous Pb layer and hexagonal voids randomly filled with either Gd or Yb atoms allows precise control of the Fermi level position. Small alteration of the Fermi level triggers drastic changes in the Fermi surface topology due to the Lifshitz transition, hence in the physical properties. In particular, the sheet resistance of the GdxYb1-xPb3/Si(111) system can be controllably varied over an order of magnitude range.

Rim nucleation and step-train orientation effects in SOI(111) dewetting

We demonstrate that the step-train orientation of a vicinal surface plays an important role in solid-state dewetting. Focusing on SOI(111), we observe the formation of hexagonal voids in the silicon film due to dewetting. These voids are surrounded by an asymmetric rim, which is wider on the higher-terrace side compared to the lower-terrace side. Our findings also reveal a layer-by-layer growth of the rim around the dewetting voids during the early stages, before the formation of branched structures. Dewetting fingers develop faster and are thinner in the step-down direction compared to the step-up direction. Kinetic Monte Carlo (KMC) simulations confirm our experimental observations and provide insights into the atomic details underlying rim and finger formation. During the initial stages of dewetting, the nucleation of a new rim layer is not required: Atoms leaving the dewetting void are captured by the adjacent upper step, leading to the growth of the upper terrace that thus surrounds the dewetting void. The nucleation of a new rim layer only becomes necessary when the distance between the edge of the void and the upper terrace increases. The energetics involved in the dewetting kinetics of SOI(111) are consistent with those of SOI(100).

Ordered CaCu5-type phases REPd2Zn3 (RE = Y, La–Nd, Sm, Gd–Dy)

Abstract The ternary intermetallic phases REPd2Zn3 (RE = Y, La–Nd, Sm, Gd–Dy) were synthesized from the elements by induction melting in sealed niobium ampoules followed by annealing. The CaCu5-type samples (space group P6/mmm) were characterized through Guinier powder patterns. The structures of YPd2Zn3 (a = 523.91(4), c = 430.88(3) pm, wR2 = 0.0570, 96 F 2 values, 9 variables) and NdPd2Zn3 (a = 535.08(7), c = 427.81(6) pm, wR2 = 0.0217, 123 F 2 values, 9 variables) were refined from single-crystal X-ray diffractometer data. The zinc atoms form Kagome layers in AA stacking which leave trigonal prismatic voids for the palladium and hexagonal prismatic voids for the rare earth atoms. The Zn–Zn distances within the Kagome network of YPd2Zn3 are 262 pm and the Pd–Zn distances in the Pd@Zn6 trigonal prisms are 263 pm.

Infrared nonlinear optical performances of a new sulfide β-PbGa2S4

Nonlinear optical (NLO) crystals can produce tunable lasers through the direct second-harmonic generation (SHG) process, which is of great significance in modern laser technology. Herein, a new chalcogenide β-PbGa2S4 (1) crystallized in space group of Pna21 was synthesized via high-temperature solid-state reactions. Compound 1 is composed of a three-dimensional framework [Ga8S16]8− with hexagonal voids filled by Pb atoms via covalent Pb–S interactions. Importantly, it exhibits promising NLO performances, such as phase-matchable SHG efficiency (0.6×AgGaS2@1200 nm and 0.1×AgGaS2@1910 nm), moderate band-gap (2.46 eV), suitable laser-induced damage threshold (3.0×AgGaS2), and a broad IR transparency window (0.5–25.0 µm). SHG response of 1 is mainly attributable to the occupied S–3p and Ga–4p states and unoccupied S–3p and Pb–6p states.

Crystal structure of a trigonal polymorph of aqua-dioxidobis(pentane-2,4-dionato-κ2 O,O')uranium(VI).

The title compound, [UO2(acac)2(H2O)] consists of a uran-yl(VI) unit ([O=U=O]2+) coordinated to two monoanionic acetyl-acetonate (acac, C5H7O2) ligands and one water mol-ecule. The asymmetric unit includes a one-half of a uranium atom, one oxido ion, one-half of a water mol-ecule and one acac ligand. The coordination about the uranium atom is distorted penta-gonal-bipyramidal. The acac ligands and Ow atom comprise the equatorial plane, while the uranyl O atoms occupy the axial positions. Inter-molecular hydrogen bonding between complexes results in the formation of two-dimensional hexa-gonal void channels along the c-axis direction with a diameter of 6.7 Å. The monoclinic (P21/c space group) polymorph was reported by Alcock & Flanders [(1987). Acta Cryst. C43, 1480-1483].

The ternary platinides CaGa5Pt3 and EuGa5Pt3

Abstract The ternary platinides CaGa5Pt3 (a = 2082.5(4), b = 406.05(8), c = 739.2(1) pm) and EuGa5Pt3 (a = 2085.5(5), b = 412.75(9), c = 738.7(1) pm) were synthesized from the elements in sealed high-melting metal tubes in an induction furnace. CaGa5Pt3 and EuGa5Pt3 are isotypic with CeAl5Pt3 and isopointal with the YNi5Si3 type intermetallic phases (space group Pnma, oP36 and Wyckoff sequence c 9). The structure of EuGa5Pt3 was refined from single crystal X-ray diffractometer data: wR2 = 0.0443, 1063 F 2 values and 56 variables. The gallium and platinum atoms build up a three-dimensional [Ga5Pt3]2− polyanionic network in which the europium atoms fill slightly distorted hexagonal prismatic voids. The Ga–Pt distances within the network range from 249 to 271 pm, emphasizing the covalent bonding character. Temperature dependent magnetic susceptibility measurements indicate diamagnetism for CaGa5Pt3 and isotypic BaGa5Pt3. EuGa5Pt3 behaves like a Curie–Weiss paramagnet above 50 K with an experimental magnetic moment of 8.17(1) µB/Eu atom, indicating divalent europium. Antiferromagnetic ordering sets in at T N = 8.5(1) K. The divalent ground state of europium is confirmed by 151Eu Mössbauer spectroscopy. EuGa5Pt3 shows a single signal at 78 K with an isomer shift of −9.89(4) mm s−1. Full magnetic hyperfine splitting with a hyperfine field of 25.0(2) T is observed at 6 K in the magnetically ordered regime.

Study on Evolution of Micropipes from Hexagonal Voids in 4H-SiC Crystals by Cathodoluminescence Imaging.

This paper presents an investigation on micropipe evolution from hexagonal voids in physical vapor transport-grown 4H-SiC single crystals using the cathodoluminescence (CL) imaging technique. Complementary techniques optical microscopy, scanning electron microscopy, and energy-dispersive spectroscopy (EDS) are also used to understand the formation mechanism of hexagonal voids along with the origin of pipes from these voids. The ability of CL to image variations along the depth of the sample provides new insights on how micropipes are attached to hexagonal voids that lie deep within the bulk single crystals. CL imaging confirms that multiple micropipes can originate from a single hexagonal void. EDS mapping shows that the inside of the micropipe walls exhibits higher levels of carbon. Investigation of the seed region by optical imaging shows that improper fixing of the seed to the crucible lid is the root cause for the formation of hexagonal voids that subsequently lead to micropipe formation.

Tunable Potassium Ion Conductivity and Magnetism in Substituted Layered Ferrates

AbstractFive substituted oxohydroxoferrates K2–x(Fe,M)4O7–y(OH)y (M=Si, Ge, Ti, Mn, Ir) were synthesized in a potassium hydroxide hydroflux with a molar base‐water ratio q(K) of about 0.9. While the hexagonal prisms of K2–x(Fe,Ti)4O7–y(OH)y crystallize in P63/mcm, all other compounds form hexagonal plates with the trigonal space group P 1 m. The crystal structure of the oxohydroxoferrates resembles ß‐alumina. It consists of honeycomb layers Fe2O6] of edge‐sharing [FeO6] octahedra, where the hexagonal voids are capped by vertex‐sharing [FeO4] tetrahedra pairs. The cavities between the oxoferrate layers host the potassium ions. Depending on M, the substitution affects different iron positions and varies between 5 and 20 %. The magnetic structures of the antiferromagnetic compounds were determined by neutron powder diffraction. The potassium ion conductivity was characterized by electrochemical impedance spectroscopy at room temperature. By storing the oxohydroxoferrates in air or annealing them at 700 °C the ion conductivity was significantly increased, e. g. to 5.0 ⋅ 10−3 S cm−1 for a pressed pellet of the iridium substituted compound.

Three-dimensional detection and quantification of defects in SiC by optical coherence tomography.

Silicon carbide (SiC) is widely used in high power electronic devices. However, defects on the SiC significantly reduce the yield and decrease the performance of SiC. Accurate detection of the defects is essential in the process control. We demonstrated a noninvasive three-dimensional (3D) defect detection method for SiC using optical coherence tomography (OCT). Defects including the triangular defects, hexagonal voids, grain boundaries, and carrot defects were inspected and analyzed on SiC wafers. The 3D images of defects acquired with OCT provided detailed information on the 3D structures and dimensions of defects, and the locations and orientations of the defects inside the wafers. This technique was not only useful for rapid defect screening in the process control, it was also extremely helpful in understanding the formation mechanism of these defects in SiC.

NiO nanofibers interspersed sponge based low cost, multifunctional platform for broadband UV protection, ultrasensitive strain and robust finger-tip skin inspired pressure sensor

We report for the first time a low cost, tactile, high performance, multifunctional sensing platform for applications namely broadband Ultraviolet (UV) filter, strain sensor and finger-tip based pressure sensor. Eco-friendly, light weight and mechanically stable melamine sponge (MS) was chemically engineered using electrospun NiO nanofibers for facile fabrication of the tactile sensor. Detailed characterization studies revealed NiO nanofibers interspersed within the structure of hexagonal voids present in melamine sponge. Under optimal conditions, the UV filter exhibited a remarkable UPF (UV Protection Factor) of 87.7, the strain sensor exhibited a gauge factor of 34 with a maximum strain withstanding capability of 76.3% and the pressure sensor displayed a wide dynamic sensing range of 50 N–700 N with a sensitivity 3.75 kPa−1 which are significantly higher than similar sensors fabricated using advanced methodologies. Furthermore, the device was found to be highly robust and displayed excellent stability up to 500 cycles of bending. The mechanism of the multifunctional sensor can be attributed to the controllable deformation of NiO nanofibers distributed across the melamine sponge upon external strain and pressure. The sensor was successfully demonstrated as strain sensor and pressure sensor for wearable motion detection and ultra-sensitive virtual keyboard applications respectively. The strategy employed here paves way towards integrating multiple devices to develop versatile, clean room-free, low-cost, multifunctional sensors useful in consumer electronics and healthcare.

Template-assisted 2D self-assembled chiral Kagomé network for selective adsorption of coronene.

Solvent molecules (n-tridecane and n-tetradecane) act as templates by coadsorption to direct the assembly of asymmetric 2-bisthienyl-4,6-diamino-1,3,5-triazine into chiral Kagomé networks. Furthermore, guest molecules (coronene) fill in the hexagonal voids by replacing the solvent molecules due to the selective and competitive adsorption behavior.

Optimization of a novel external fixator for orthopaedic applications

The use of external fixation devices is a very common method for the treatment of bone fractures. However, these fixators present some limitations in terms of mobility, significant risk of infection, and induce pain and discomfort. Moreover, they are also not fully customized to suit individual patients. To avoid these limitations, this paper presents a novel patient-specific external fixator developed using reverse engineering, finite element analysis and additive manufacturing. The fixator was designed based on a set of computer tomography (CT) scan images of a patient and optimized considering different thickness values and materials. New lightweight designs were produced through a manual process (regular distribution of circular and hexagonal voids) and topology optimization. Different polymeric materials (Polylactic acid (PLA); Acrylonitrile butadiene styrene (ABS) and Polyamide (PA)) were also considered for the fabrication of these designs. It was found that although both PLA and ABS allow to meet the design requirements, and that the best mechanical properties were obtained with fixators made of PLA. Results also showed that the best results in terms of mechanical performance and weight reduction was obtained with topology optimization.

2D Optical Gratings Based on Hexagonal Voids on Transparent Elastomeric Substrate.

A chromatic vectorial strain sensor constituted by hexagonal voids on transparent elastomeric substrate has been successfully fabricated via soft colloidal lithography. Initially a highly ordered 1.6 microns polystyrene spheres monolayer colloidal crystal has been realized by wedge-shaped cell method and used as a suitable mold to replicate the periodic structure on a polydimethylsiloxane sheet. The replicated 2D array is characterized by high periodicity and regularity over a large area, as evidenced by morphological and optical properties obtained by means of SEM, absorption and reflectance spectroscopy. In particular, the optical features of the nanostructured elastomer have been investigated in respect to uniaxial deformation up to 10% of its initial length, demonstrating a linear, tunable and reversible response, with a sensitivity of 4.5 ± 0.1 nm/%. Finally, it has been demonstrated that the specific geometrical configuration allows determining simultaneously the vectorial strain-stress information in the x and y directions.

Self-assembled patches in PtSi/n-Si (111) diodes

Using the effect of the temperature on the capacitance–voltage (C–V) and conductance–voltage (G/ω–V) characteristics of PtSi/n-Si (111) Schottky diodes the profile of apparent doping concentration (NDapp), the potential difference between the Fermi energy level and the bottom of the conduction band (Vn), apparent barrier height (ΦBapp), series resistance (Rs) and the interface state density Nss have been investigated. From the temperature dependence of (C–V) it was found that these parameters are non-uniformly changed with increasing temperature in a wide temperature range of 79–360 K. The voltage and temperature dependences of apparent carrier distribution we attributed to the existence of self-assembled patches similar the quantum wells, which formed due to the process of PtSi formation on semiconductor and the presence of hexagonal voids of Si (111).

Controlled formation of nanostructures on MoS2 layers by focused laser irradiation

MoS2 nanostructures, i.e., nanoribbons, nano-mesh, etc., may open different prospect of applications in nano-electronic and opto-electronic devices and sensors. However, the fabrication of these complicated nanostructures can be executed by using standard nano-patterning techniques such as lithography, printing, etc. Nevertheless, these standard techniques involve affluent multistep processes to optimize scalability, form factors and accuracy in the feature size. Herein, we demonstrate the fabrication of unique nano-structures on MoS2, such as nano-ribbons and nano-mesh, by a simple one-step process of direct laser writing using 532 nm low power focused laser. The minimum power required to etch a MoS2 layer for a 532 nm laser is found to be ∼6.95 mW and the minimum void size observed is ∼300 nm, which is very close to the diffraction limit of the laser used. Both the experimental and computational results have shown that the voids induced by laser etching always take a hexagonal or triangular shape, which can be used to define crystal orientation of the MoS2 flake. Investigation shows that the periphery of hexagonal voids lies on S atoms, whereas for triangular voids, it lies on Mo atoms of the MoS2 crystal. In-depth AFM and Raman analysis show that the etching rate is tunable by controlling the laser power and the exposure time.

Is borophene a suitable anode material for sodium ion battery?

Abstract 2D Boron is an atomically thin layer of boron with both light weight and metallicity, two kinds of potential structures are proposed, namely, a close-packed triangular B layer(denoted B△ hereafter) and a hexagonal voids or vacancies ( ) in B layer(denoted hereafter) (Evgeni S. et al. Nano Lett. 2012, 12, 2441–2445). Here, we use first-principles calculations to investigate the potential of borophene as an anode material for sodium-ion batteries. It is found that borophene has an adsorption energy to sodium atoms of −1.448 eV for all boron atoms in the plane ( ) and −1.731 eV for BΔ. The fully sodiated phase of borophene is Na0.083B and Na0.1B for BΔ and , respectively, which corresponds to a theoretical specific capacity of 248 mAh g−1 (496 mAh g−1 for two sides) for BΔ and 298 mAh g−1 (596 mAh g−1 for two sides) for . The energy barrier along the furrow of corrugated borophene is only 12 meV for BΔ, which suggests that sodium diffusion on borophene can be extremely fast for BΔ. In addition, a strong directional anisotropy is observed for sodium diffusion with a 352.5 meV barrier perpendicular to the furrow of BΔ. Finally, both exhibit metallic characteristics during the entire sodiation process, which indicates that the material has excellent electronic conductivity. The findings in this work suggest that as an anode material for sodium-ion batteries, borophene can drastically boost the energy and power density of batteries.

Macroscopic shock plasticity of brittle material through designed void patterns

The rapid propagation and coalescence of cracks and catastrophic fractures, which occur often under shock compression, compromise a brittle material's design function and restrict its scope of practical application. The shock plasticity of brittle materials can be improved significantly by introducing and designing its microstructure, which can help reduce or delay failure. We used a lattice-spring model, which can describe elastic deformation and brittle fracture of modeled material accurately, to study the influence of void distributions (random, square, hexagonal, and triangular void patterns) on the macroscopic shock response and the mesoscopic deformation feature of brittle materials. Calculated results indicate that the void patterns dominate two inelastic deformation stages on the Hugoniot stress-strain curves (the collapse deformation stage and the slippage deformation stage). It shows that the strain localization is not strong and that the broken media are closer to a round bulk when the samples exist in random and triangular void patterns. This favors an increase in deformation during the slippage deformation stage. For the samples with square and hexagonal void patterns, the strain localization is strong and the broken media are closer to columnar bulks, which favors an increase in deformation during the collapse deformation stage.

Assembling Coordination Frameworks of Tetrakis[meso-(3,5-biscarboxyphenyl)]-Metalloporphyrins with Polynuclear Metallic Nodes: Mechanistic Insights into the Synthesis and Crystallization Process

The article provides a deep insight into an improved synthetic methodology of crystalline framework solids with octa-topic tetrakis(3,5-dicarboxyphenyl)-zinc/cobalt-porphyrin linkers and a variety of metallic nodes, using NaOH as a modulator to enhance crystal growth. In the given conditions of the supramolecular reaction polynuclear metallic clusters were formed in situ, leading to the construction of metalloporphyrin frameworks (MPFs) of diverse architectures perforated by wide intralattice voids. Synthetic procedures with Pr3+ ions yielded two different frameworks, labeled as PrMPF-1 (1) and PrMPF-2 (2). The former is sustained by heterometallic {NaPr(COO)4}n pillared synthons, revealing tetragonal interporphyrin channel voids. In the latter the porphyrin units are intercoordinated by heterometallic tetranuclear {PrNa3(H2O)6(COO)9}3– clusters and the resulting structure exhibits hexagonal voids. Additional crystalline materials could be obtained with In3+ and Ga3+ cations as the inorganic component, yielding open 3D framework-structures InMPF (3) and GaMPF (4). These were found to be stabilized by the heterometallic In2Na(μ3-H2O)(COO)7 cluster-type and {NaGa(COO)4}n chain-type interaction synthons, respectively. When the transition metal ions Fe3+ or Mn3+ were used as the connecting reagents, homometallic trinuclear oxo-centered clusters (Fe/Mn)3(μ3-O) (COO)7 were formed in the corresponding polymeric structures FeMPF (5) and MnMPF (6). In 6, adjacent trinuclear clusters are further interconnected by a formate bridge to form covalently linked [Mn3(μ3-O)]2 dimers. Similar reactions in the presence of the NaOH additive, but with Co2+ or Zr4+, yielded two isomorphous 3D supramolecular architectures (7; NaMPF-1 and 8; NaMPF-2) with tetragonally shaped interporphyrin channel voids. Yet, in these structures the porphyrin-bridging {Na(COO)2}−n-chains were found to contain Na+ ions only. The solvent (DMF, water, and possibly dissociation products of hydrolyzed DMF) accessible voids in the metal–organic frameworks 1–8 account for >50% of the crystal volume. Mechanistic aspects of the NaOH-modulated synthetic approach are discussed, characterizing the products at each step in the synthetic course and proposing a plausible reaction mechanism.

Beam model for the elastic properties of material with spherical voids

The elastic properties of a material with spherical voids of equal volume are analysed using a new model, with particular attention paid to the hexagonal close-packed and the face-centred cubic arrangement of voids. Void fractions well above 74 \% are considered, yielding overlapping voids as in an open-cell foam and hence a connected pore structure. The material is represented by a network of beams. The elastic behaviour of each beam is derived analytically from the material structure. By computing the linear elastic properties of the beam network, the Young's moduli and Poisson ratios for different directions are evaluated. In the limit of rigidity loss a power law is obtained, describing the relation between Young's modulus and void fraction with an exponent of 5/2 for bending-dominated and 3/2 for stretching-dominated directions. The corresponding Poisson ratios vary between 0 and 1. With decreasing void fraction, these exponents become 2.3 and 1.3, respectively. The data obtained analytically and from the new beam model are compared to Finite Element simulations carried out in a companion study, and good agreement is found. The hexagonal close-packed void arrangement features very anisotropic behaviour, comparable to that of fibre-reinforced materials, This might allow for new applications of open-cell materials.

Systematic Study on the Self-Assembled Hexagonal Au Voids, Nano-Clusters and Nanoparticles on GaN (0001).

Au nano-clusters and nanoparticles (NPs) have been widely utilized in various electronic, optoelectronic, and bio-medical applications due to their great potentials. The size, density and configuration of Au NPs play a vital role in the performance of these devices. In this paper, we present a systematic study on the self-assembled hexagonal Au voids, nano-clusters and NPs fabricated on GaN (0001) by the variation of annealing temperature and deposition amount. At relatively low annealing temperatures between 400 and 600°C, the fabrication of hexagonal shaped Au voids and Au nano-clusters are observed and discussed based on the diffusion limited aggregation model. The size and density of voids and nano-clusters can systematically be controlled. The self-assembled Au NPs are fabricated at comparatively high temperatures from 650 to 800°C based on the Volmer-Weber growth model and also the size and density can be tuned accordingly. The results are symmetrically analyzed and discussed in conjunction with the diffusion theory and thermodynamics by utilizing AFM and SEM images, EDS maps and spectra, FFT power spectra, cross-sectional line-profiles and size and density plots.