Hexagonal Plane Research Articles

Structural, Optical, and Thermoelectric Properties of the ZnO:Al Films Synthesized by Atomic Layer Deposition

Aluminum-doped zinc oxide thin films have been grown by atomic layer deposition at a temperature of 200°C. Using X-ray diffraction, it has been established that the ZnO:Al thin films exhibits the reflections from the (100), (002), (110), and (201) ZnO hexagonal phase planes. The (101) and (102) planes have also been detected by electron diffraction. The ZnO:Al thin films grow smooth with a root-mean-square roughness of Rq = 0.33 nm and characteristic nanocrystallite sizes of ~70 and ~15 nm without additional aluminum or aluminum oxide phases. The transmission at a wavelength of 550 nm with regard to the substrate has been found to be 96%. The refractive indices and absorption coefficients of the ZnO:Al thin films in the wavelength range of 250–900 nm have been determined. The maximum refractive indices and absorption coefficients have been found to be 2.09 at a wavelength of 335 nm and 0.39 at a wavelength of 295 nm, respectively. The optical band gap is 3.56 eV. The resistivity, Seebeck coefficient, and power factor of the ZnO:Al thin films are ∼1.02 × 10–3 Ω cm, –60 μV/K, and 340 μW m–1 K–2 at room temperature, respectively. The maximum power factor attains 620 μW m–1 K–2 at a temperature of 200°C.

Highly Anisotropic Thermal Transport in LiCoO2.

LiCoO2 is the prototypical cathode in lithium-ion batteries. Its crystal structure consists of Li+ and CoO2- layers that alternate along the hexagonal ⟨0001⟩ axis. It is well established that the ionic and electronic conduction are anisotropic, but little is known regarding the heat transport. We analyze the phonon dispersion and lifetimes using anharmonic lattice dynamics based on quantum-chemical force constants. Around room temperature, the thermal conductivity in the hexagonal ab plane of the layered cathode is∼6 times higher than that along the c axis. An upper limit to the average thermal conductivity at T = 300 K of 38.5 W m-1 K-1 is set by short phonon lifetimes associated with anharmonic interactions within the octahedral face-sharing CoO2- network. Observations of conductivity <10 W m-1 K-1 can be understood by additional scattering channels including grain boundaries in polycrystalline samples. The impact on thermal processes in lithium-ion batteries is discussed.

A combined experimental and quantum chemical studies on the structure and binding preferences of picolinamide based ligands with uranyl nitrate

Three N,N-substituted pyridine 2-carboxamide ligands, C5H4NCONRR′ (where, R, R′ = iC3H7 (L1); R, R′ = iC4H9 (L2); and R = H, R′ = tC4H9 (L3), as well as their coordination complexes (1–3) of uranyl nitrate have been synthesized. All the ligands and their corresponding complexes were characterized by CHN elemental analysis, infrared (FTIR) and NMR spectroscopy. The crystal structure of the complex 1 shows that the central uranium atom occupies a distorted hexagonal bipyramidal geometry, where the four oxygen atoms of the bidentate nitrate ligands with the oxygen and nitrogen atoms of the ligand L1 form the hexagonal plane and two axial oxygen atoms of uranyl occupy the trans axial position. Here, the ligand acts as a bidentate chelating ligand. The preliminary structural analysis of the complex 3 also indicates similar kind of structure as that of the complex 1. Quantum mechanical calculation at DFT level is supportive with the solid state structure obtained from X-ray crystallographic analysis.

The Effects of Three Magnons Interactions in the Magnon-Density Waves of Triangular Spin Lattices

We investigate the magnon-density waves proposed as the longitudinal excitations in triangular lattice antiferromagnets by including the cubic and quartic corrections in the large-s expansion. The longitudinal excitation spectra for the two-dimensional (2D) triangular antiferromagnetic model and quasi-one dimensional (quasi-1D) antiferromagnetic materials have been obtained for a general quantum spin number $s$. For the 2D triangular lattice model, we find a significant reduction (about 40 %) in the energy spectra at the zone boundaries due to both the cubic and quartic corrections. For the quasi-1D antiferromagnets, since the cubic term comes from the very weak couplings on the hexagonal planes, they make very little correction to the energy spectra, whereas the major correction contribution comes from the quartic terms in the couplings along the chains with the numerical values for the energy gaps in good agreement with the experimental results as reported earlier (Ref.~41).

Identification of second-gradient elastic materials from planar hexagonal lattices. Part II: Mechanical characteristics and model validation

Positive definiteness and symmetry of the constitutive tensors describing a second-gradient elastic (SGE) material, which is energetically equivalent to a hexagonal planar lattice made up of axially deformable bars, are analyzed by exploiting the closed form-expressions obtained in part I of the present study in the ‘condensed’ form. It is shown that, while the first-order approximation leads to an isotropic Cauchy material, a second-order identification procedure provides an equivalent model exhibiting non-locality, non-centrosymmetry, and anisotropy. The derivation of the constitutive properties for the SGE from those of the ‘condensed’ one (obtained by considering a quadratic remote displacement which generates stress states satisfying equilibrium) is presented. Comparisons between the mechanical responses of the periodic lattice and of the equivalent SGE material under simple shear and uniaxial strain show the efficacy of the proposed identification procedure and therefore validate the proposed constitutive model. This model reveals that, at higher-order, a lattice material can be made equivalent to a second-gradient elastic material exhibiting an internal length, a finding which is now open for applications in micromechanics.

ZnO nanorod array/reduced graphene oxide substrate with enhanced performance in photocatalytic degradation

A novel ZnO nanorod array/reduced graphene oxide (ZnO array/rGO) composite coating was synthesised via the combination of seed layers-growth and aqueous solution reaction for the formation of ZnO nanorod arrays, which was further combined with rGO using a simple water bath method. The results revealed that perpendicular arrays with hexagonal planes were successfully fabricated. The size and density of the arrays could be optimised by adjusting the reactant concentrations. The influence of the rGO fraction in the ZnO array/rGO composites on the photocatalytic degradation of the methylene blue under UV radiation was investigated. It was found that the photocatalytic performance could be significantly improved by adding rGO. The enhanced photocatalytic activities were attributed to the interactions between ZnO nanorod arrays and rGO nanosheets and subsequently effective separation of photo-generated electron-hole pairs via rGO as electron sinks. The ZnO array/rGO composites presented advantages including facile and green preparation, long photo-electron lifetime, easy particle agglomeration and recovery, and therefore is expected to be an ideal photocatalysts for the future utilisation.

The electronic and optical properties of monovalent atom-doped ZnO monolayers: the density functional theory

The current study deals with the structural, electronic and optical properties of monovalent element-doped ZnO monolayers using density functional theory. Specifically, we have considered structural and optical properties of monovalent ( $$\mathrm {M}=\mathrm {Li}$$ , Na and K) atom-doped ZnO monolayers. Among these monovalent elements, the substitution of Li with Zn atom maintains the hexagonal planar geometry of the ZnO monolayer, but Na and K elements protrude out from the plane of the ZnO monolayer. The Li atom binds more strongly with the ZnO sheet compared with Na and K atoms. A Li-doped ZnO monolayer shows metallic behaviour whereas Na- and K-doped ZnO monolayers show half metallic magnetic behaviour. The magnetic moment is of the order of $$1\upmu _{\mathrm{B}}$$ . The magnetic moment mainly originates from nonbonding O $$_{\mathrm{p}}$$ states. The substitution of an alkali metal element-doped ZnO monolayer leads to a red-shift in optical spectra. The dielectric constant of a monovalent element-doped ZnO sheet increases compared with that of a pristine ZnO sheet. This study provides the basis to develop opto-electronic devices using doped ZnO monolayers.

Vertex-Fed Hexagonal Antenna with Low Cross-Polarization Levels

Probe-fed hexagonal patch antenna suffers from impedance mismatch especially when feed is located at one of the vertices of polygon. Hexagonal planar antennas also suffer from high cross polar levels. This paper proposes a method to overcome impedance mismatch and to increase co-polarization level by reducing ground plane. Vertex feeding is demonstrated in this paper to establish improvement in impedance values more clearly. The impedance at any vertex of a hexagonal patch is too high when compared to the characteristic impedance of the probe. Three vertex-fed hexagonal antennas are developed to demonstrate the effect of ground plane reduction within C-Band. The proposed technique can be optimized to match the impedance and achieve good return loss where monopole radiation characteristics are not an issue. A vertex-fed reduced ground hexagonal antenna is proposed here that operates around 5 GHz with a frequency span of 600 MHz. Thus, the proposed antenna is quite suitable for indoor wireless LAN (UNII-1) applications.

In situ [2 + 3] cycloaddition synthesis, crystal structures, strong SHG responses and fluorescence properties of three novel Zn coordination polymers

We report on the synthesis of two acentric and one centrosymmetric Zn metal–organic complexes with 3-tetrazolemethyl pyridine spacers obtained in situ by hydrothermal routes. X-ray diffraction structural analysis reveals that they have the same equivalent nodes but with dissimilar topologies. The two acentric frameworks [Zn(Tzmp)Cl]n (1) and ([Zn(Tzmp)Br]n (2), HTzmp = 3-tetrazolemethyl pyridine) are isomorphism which exhibit an acentric 3D framework with (10, 3)-b net called “ths”, while the centrosymmetric complex ([Zn(Tzmp)N3]n (3)) features a distinctive 2D sheet with Shubnikov hexagonal plane net. Photo-luminescent studies suggest the ligand-field strength of coordinated negative ions (Cl− > Br− > N3−) has ordered adjusting effects on the emission redshift. The second harmonic generation (SHG) measurement shows that compounds 1 and 2 are nonlinear optically active, with SHG responses being 2/3 and half of the standard potassium dihydrogen phosphate (KDP), respectively.

Extended flat voltage profile of hard carbon synthesized using a two-step carbonization approach as an anode in sodium ion batteries

Hard carbon is a promising anode material for sodium ion batteries (NIBs). In this study, a two-step carbonization approach is developed to enhance the electrochemical performance of lignocellulose biomass-derived hard carbon. The first step comprises slow low-temperature pyrolysis of fir wood that produces an amorphous carbon in which hexagonal planes are embedded in the amorphous carbon region to some extent. The second step comprises high-temperature carbonization at 1300 °C, which yields a hard carbon with a high degree of graphitization, an increased layer-plane length, and a low micropore volume. Two-step carbonized hard carbon delivers a large reversible capacity of 276 mAh g−1 at 50 mA g−1 after 100 cycles and high rate capacities of 108 mAh g−1 at 1.0 A g−1 and 76.3 mAh g−1 at 2.5 A g−1. The low-voltage plateau capacity below 0.1 V is 194 mAh g−1. The results of these experiments indicate that the exceptional electrochemical performance of two-step carbonized hard carbon arises from the effective suppression of micropore formation and a good balance between the degree of graphitization and number of defect sites. High-voltage adsorption of Na+ ions in micropores inhibits Na+-ion diffusion into the graphitic region of micropore-enriched hard carbon.

Complex magnetic order in the Nd(Tb)Fe3(BO3)4 multiferroic revealed by single-crystal neutron diffraction

Magnetic structure of the substituted multiferroics-ferroborates ${\mathrm{Nd}}_{0.9}{\mathrm{Tb}}_{0.1}{\mathrm{Fe}}_{3}{({\mathrm{BO}}_{3})}_{4}$ and ${\mathrm{Nd}}_{0.8}{\mathrm{Tb}}_{0.2}{\mathrm{Fe}}_{3}{({\mathrm{BO}}_{3})}_{4}$ were determined in the framework of a self-consistent refinement of the single crystal neutron diffraction data. The small substitution of Nd for Tb leads to the reorientation of the main antiferromagnetic vector L from the basal plane towards the hexagonal axis. The reorientation takes place via an angular structure for which L does not coincide with the principal crystallographic directions and evolves with temperature due to competing magnetic anisotropies of Fe, Nd, and Tb subsystems. Our refinement at 2 K reveals the existence of distortions in the collinear antiferromagnetic Fe spin arrangement suggested before in other ferroborates. Therefore, besides the main antiferromagnetic vector L, the magnetic structure involves additional fine symmetrized combinations of spin components allowed by symmetry. They coexist with certain L components and could originate from the antisymmetric Dzyaloshinsky-Moriya Fe-Fe exchange interactions. At higher temperatures, the magnetic structure is described by the simple collinear model, where the L vector is deviated from the hexagonal plane.

Thickness dependent correlation between structural and optical properties of textured CdSe thin film

Nano-crystalline CdSe thin films of different thicknesses under sub-micron range were deposited on glass substrate via thermal evaporation route. A gradual deterioration in film crystallinity confirmed by XRD line profile analysis has been accompanied by a reduction in Cd to Se molar ratio as the film thickness decreases. A coordinated microstructural and crystallographic orientation distribution analysis explicitly demonstrated that CdSe tends to grow in nano-sized columns with hexagonal c-axis parallel to its growth direction on glass substrate. A thickness dependence of structural evolution was discussed in terms of aspect ratio of the columnar structure and dispersion in orientation of hexagonal (002) basal plane. The variation in the spectra of optical constants [n(λ), k(λ)] obtained from Swanepoel envelop method was interpreted in terms of crystallographic defects arising from stoichiometric disorder which was also accounted for the observed thickness dependent shifts in band gap and valence band split energy. The bathochromic shifts in dielectric and energy loss functions, optical conductivity, skin depth and cut-off energy were discussed in detail along with the variations in their spectral shapes in connection with the dispersion in the real and imaginary parts of complex refractive index, which might shed a new light upon holistic comprehension of thickness dependent optical properties of other chalcogenide semiconducting thin films.

Ice formation and solvent nanoconfinement in protein crystals.

Ice formation within protein crystals is a major obstacle to the cryocrystallographic study of protein structure, and has limited studies of how the structural ensemble of a protein evolves with temperature in the biophysically interesting range from ∼260 K to the protein-solvent glass transition near 200 K. Using protein crystals with solvent cavities as large as ∼70 Å, time-resolved X-ray diffraction was used to study the response of protein and internal solvent during rapid cooling. Solvent nanoconfinement suppresses freezing temperatures and ice-nucleation rates so that ice-free, low-mosaicity diffraction data can be reliably collected down to 200 K without the use of cryoprotectants. Hexagonal ice (Ih) forms in external solvent, but internal crystal solvent forms stacking-disordered ice (Isd) with a near-random stacking of cubic and hexagonal planes. Analysis of powder diffraction from internal ice and single-crystal diffraction from the host protein structure shows that the maximum crystallizable solvent fraction decreases with decreasing crystal solvent-cavity size, and that an ∼6 Å thick layer of solvent adjacent to the protein surface cannot crystallize. These results establish protein crystals as excellent model systems for the study of nanoconfined solvent. By combining fast cooling, intense X-ray beams and fast X-ray detectors, complete structural data sets for high-value targets, including membrane proteins and large complexes, may be collected at ∼220-240 K that have much lower mosaicities and comparable B factors, and that may allow more confident identification of ligand binding than in current cryocrystallographic practice.

Strain-driven thermodynamic stability and electronic transitions in ZnX (X = O, S, Se, and Te) monolayers

Semiconducting Zn chalcogenide monolayers are important members of the 2D family of materials due to their unique electronic properties. In this paper, we focus on strain-modulated electronic properties of monolayers of ZnX, with X being O, S, Se, and Te. ZnO and ZnS monolayers have a hexagonal graphene-like planar structure, while ZnSe and ZnTe monolayers exhibit slightly buckled silicene and germanene-like structures, respectively. Density functional theory calculations find the hexagonal ZnO monolayer to be dynamically stable. However, ZnS, ZnSe, and ZnTe monolayers are predicted to be less stable with small imaginary frequencies. The application of tensile strain to these monolayers, interestingly, yields stability of dynamically less stable structures together with the modification in the nature of the bandgap from direct to indirect. For a tensile strain of about 8%, a closure of the bandgap in ZnTe is predicted with the semiconductor-metal transition. The results, therefore, find strain-induced stability and modification in electronic properties of monolayers of Zn chalcogenides, suggesting the use of these monolayers for novel device applications.

Crystallization process and growth mechanism for hexagonal prism of strontium hydroxyapatite by urea hydrolysis

Strontium hydroxyapatite (SrHAp) is not only used as drug carrier material for bone disease treatment, but also has application prospect in bone tissue engineering. But SrHAp is prepared with by-products strontium hydrogen phosphate (SrHP) and strontium phosphate (TSrP), which requires accurate process optimizations and adjustments to circumvent the undesirable morphology products. The integral hexagonal prism crystals of SrHAp were successfully synthesized by urea hydrolysis at ambient pressure and without any strong acids and bases. Similarly, variable by-products are classified and investigated for their effects on crystallization process of strontium hydroxyapatite via monitoring the urea hydrolysis time-dependent products. The crystallization process of SrHAp can be divided into four stages based on the crystallographic results for the samples acquired by XRD and SEM studies during the entire synthesis processes. In the first stage, SrHP is formed (0–0.17 h); secondly, SrHP is converted into SrHAp (0.17–4 h); TSrP is obtained (4–8 h) at the third stage; and at last, TSrP is transformed into SrHAp (8–24 h), especially the amount of SrHAp obtained at 24 h was the sum of the second and fourth stages. Simultaneously, the growth mechanism of SrHAp crystals was proposed, considered of the formation energy of Gibbs-Thomson crystal plane and Born-Lande lattice energy. By calculating the interfacial energy γ(h k l) of different crystal planes, the γ(1 0 0) of hexagonal prism plane family m{1 0 0} is 1.3 × 10−5U0, the γ(0 0 1) of parallel double-plane family c{0 0 1} is 7.5 × 10−6U0, the normal growth rate of Ic{0 0 1} ≫ Im{1 0 0}. Accordingly, the growth mechanism for the preferential c axis growth maintaining the hexagonal prism shape is derived.

Phase-dependent shear-induced order of nanorods in isotropic and nematic wormlike micelle solutions.

Small angle X-ray scattering with in situ shear was employed to study the assembly and ordering of dispersions of gold nanorods within wormlike micelle solutions formed by the surfactant cetylpyridinium chloride (CPyCl) and counter-ion sodium salicylate (NaSal). Above a threshold CPyCl concentration but below the isotropic-to-nematic transition of the micelles, the nanorods self-assembled under quiescent conditions into isotropically oriented domains with hexagonal order. Under steady shear at rates between 0.5 and 7.5 s-1, the nanorod assemblies acquired macroscopic orientational order in which the hexagonal planes were coincident with the flow-vorticity plane. The nanorods could be re-dispersed by strong shear but re-assembled following cessation of the shear. In the nematic phase of the micelles at higher surfactant concentration, the nanorods did not acquire hexagonal order but instead formed smectic-like layers in the gradient-vorticity plane under shear. Finally, at still higher surfactant concentration, where the micelles form a hexagonal phase, the nanorods showed no translational ordering but did acquire nematic-like order under shear due to alignment in the flow. Depletion forces mediated by the wormlike micelles are identified as the driving mechanism for this sequence of nanorod ordering behaviors, suggesting a novel mechanism for controlled, reconfigurable assembly of nanoparticles in solution.

Controlled Symmetry Breaking in Colloidal Crystal Engineering with DNA

The programmed crystallization of particles into low-symmetry lattices represents a major synthetic challenge in the field of colloidal crystal engineering. Herein, we report an approach to realizing such structures that relies on a library of low-symmetry Au nanoparticles, with synthetically adjustable dimensions and tunable aspect ratios. When modified with DNA ligands and used as building blocks for colloidal crystal engineering, these structures enable one to expand the types of accessible lattices and to answer mechanistic questions about phase transitions that break crystal symmetry. Indeed, crystals formed from a library of elongated rhombic dodecahedra yield a rich phase space, including low-symmetry lattices (body-centered tetragonal and hexagonal planar). Molecular dynamics simulations corroborate and provide insight into the origin of these phase transitions. In particular, we identify an unexpected asymmetry in the DNA shell, distinct from both the particle and lattice symmetries, which enables directional, nonclose-packed interactions.

Structure of crystalline CaAl2Si2O8 precipitated in a CaO–Al2O3–SiO2 glass-ceramic

Metastable CaAl2Si2O8 crystalline phase was obtained via heat treatment of CaO–Al2O3–SiO2 glass with metallic molybdenum particles as nucleating agents. Its crystal structure was determined by aberration-corrected scanning transmission electron microscopy (Cs-STEM), Raman spectroscopy, and X-ray diffraction (XRD). Although the hexagonal structure was observed by Cs-STEM and Raman spectroscopy, Rietveld analysis revealed that a triclinic structure with space group P1 provided the best description of the crystal structure. The most appropriate lattice parameters were a = 5.1163(5) Å, b = 5.1155(5) Å, c = 15.0031(15) Å, α = 91.319(3)°, β = 88.172(3)°, and γ = 120.279(3)°. The c-axis layers composing the hexagonal plane were slightly distorted a few degrees from their arrangement in the hexagonal CaAl2Si2O8 crystal.

Wide Angle Dynamically Tunable Enhanced Infrared Absorption on Large-Area Nanopatterned Graphene.

Enhancing light-matter interaction by exciting Dirac plasmons on nanopatterned monolayer graphene is an efficient route to achieve high infrared absorption. Here, we designed and fabricated hexagonal planar arrays of nanoholes and nanodisks with and without optical cavity to excite Dirac plasmons on patterned graphene and investigate the role of plasmon lifetime, extinction cross-section, incident light polarization, angle of incident light, and pattern dimensions on the light-absorption spectra. By incorporating a high-k Al2O3 layer as the gate dielectric for dynamic electrostatic tuning of the Fermi level, we demonstrate peak absorptions of 60% and 90% for the nanohole and nanodisk patterns, respectively, in the atmospheric transparent 8-12 μm infrared imaging band with high spectral tunability. Finally, we theoretically and experimentally demonstrate angular dependence of both s- and p-polarized light absorption in monolayer graphene. Our results showcase the practical usability of low carrier mobility CVD-grown graphene for wide angle infrared absorption, which is suitable for next-generation optoelectronic devices such as photodetectors, optical switches, modulators, etc.

Fabrication of Al2O3@BaFe12O19 core-shell powder by a modified heterogeneous precipitation method

Core-shell structural composites are promising to quench the desire for low-cost, lightweight and multifunctional magnetic materials, but the fabrication of highly pure shell nanoparticles with a desired morphology on a heterogeneous nuclear is still a big challenge. This study gives a successful illustration by in-situ synthesizing BaFe12O19 nanoparticles as a shell on Al2O3 core powders using BaCl2 and FeCl3 as precursors, instead of commonly used Ba(NO3)2 and Fe(NO3)3, by a modified heterogeneous precipitation process followed by calcination for crystallization. After calcination of the precipitants from the raw precursor mixture of BaCl2:FeCl3 for 1:10 at 800 °C, and for 1:8.5 at 900 °C, respectively, pure BaFe12O19 formed on pristine Al2O3, mostly with a hexagonal plane, about 100–200 nm in basal diameter and 20–100 nm in thickness. The highest saturation magnetization and coercivity of the coated Al2O3 @ BaFe12O19 powders was 23.3 emu/g and 5149 Oe, quite higher than that of the direct-mixed Al2O3-BaFe12O19 composite powder, and BaFe12O19-containing composites reported in literature, which could be attributed to the hexagonal shape of BaFe12O19 and uniform shell dispersion on Al2O3 core. The formation mechanism for the Al2O3@BaFe12O19 powders was discussed in detail.