Hexagonal Unit Research Articles

Effect of Fe-In co-doping on microstructural, optical, electrical and LPG response properties of spray pyrolytic ZnO thin films

Abstract This work investigates the microstructure, optical absorption, electrical resistivity and LPG (Liquefied Petroleum Gas) response of ZnO thin film with the change in relative doping concentrations of In and Fe in the host lattice. All the films retain polycrystalline nature with hexagonal wurtzite unit cell structure. Scanning electron microscopic study reveals that regular spherical shaped nanoparticles are formed in undoped ZnO, but a compact surface morphology has obtained in case of 3 at.% In doped ZnO (IZO) thin films owing to the reduction in particle size. In the case of 1 at.% Fe co-doping in IZO, nano-flakes are formed. However, at higher Fe doping concentration, this typical particle morphology gradually changes to spherical shapes again. By comparing the energy dispersive X-ray analysis data, the formation of flakes type nanostructures is attributed to the presence of excess oxygen in the sample and it dominates the thermally activated conductivity phenomenon in the low temperature range. Fe-In co-doping results low optical transmittance and reduction in the band gap of ZnO. It also lowers the conductivity in IZO thin film. IZO thin film samples exhibit better LPG response than ZnO and Fe-In co-doped ZnO samples owing to its smaller particle size.

High fill factor microlens array fabrication using direct laser writing and its application in wavefront detection

We fabricate 100% fill factor microlens arrays (MLAs) using femtosecond laser direct writing. The array consists of periodical hexagonal plano-convex microlens units with a diameter of 9 µm. The focusing efficiency of each microlens is measured to be 92%. Combined with a CCD camera, the MLA works as a Shack-Hartmann wavefront sensor. We use it to detect wavefronts of both oblique incident plane beams and vortex beams. The experimental results match well with theoretical ones.

Band gap engineering, quantum confinement, defect mediated broadband visible photoluminescence and associated quantum States of size tuned zinc oxide nanostructures

A simple wet chemical method has been used to synthesize ZnO nanostructures. The ZnO nanocrystals exhibit hexagonal unit cell structure as revealed from the XRD pattern. The intensity difference of various diffraction peaks indicates that the growth of the ZnO nanostructure is anisotropic. Various lattice parameters, dislocation density, strain, particle size, and Zn-O bond length were also calculated. The band gap of the synthesized ZnO nanostructures was determined from the UV–vis absorption spectra using Tauc equation and the band gap of the synthesized ZnO varies from 3.34 eV to 3.57 eV by varying the growth duration. Crystallite sizes also decreased with increased growth duration. This variation of size leads to the change in band gap of the nanostructures. Thus, it is possible to tune the band gap of the nanostructures by varying the growth duration. The synthesized ZnO nanostructures exhibit visible photoluminescence due to deep level transitions. A simple weak quantum confinement model has been used to designate the observed photoluminescence emission peak associated with such transitions.

Determination of poisson’s ratio using growth temperature variations of InGaN/GaN MQW

In this study, InGaN/GaN/Al2O3 multi-quantum well (MQW) structure is investigated in 300–450 °C growth temperature range with steps of 50 °C. By the help of lattice constants at four different growth temperatures strain values are gained. Plot of strain in a- versus strain in c- is used to determine Poisson’s ratio for GaN in the structure. Poisson’s ratio is determined as 0.36. Reciprocal space mapping (RSM) technique of X-ray diffraction (XRD) method is used to gain peak positions of the sample instead of w–θ scans because it gives more accurate results. Shifts in peak positions by variations of growth temperatures are shown. Lattice parameter equations are used with τ modification angles which minimise the errors caused from tilt of c- in hexagonal unit cell. This work is original because Poisson’s ratio of GaN is determined using a natural method, variations in growth temperatures. It is also emphasised that RSM is a more sensitive method of XRD for determining structural properties of nitrite-based semiconductors.

Size-dependent electronic, optical and photocatalytic properties of Ti3C2O2 quantum dots studied by first-principles calculations

Abstract Due to quantum confinement effect, quantum dots (QDs) would show totally different electronic and optical properties from two-dimensional (2D) nanosheets. For the first time, we have systemically investigated the optical, electronic and photocatalytic properties of Ti3C2O2 QDs based on the first-principles calculations. Our results show that the most stable structures of Ti3C2O2 QDs have features of hexagonal unit centered by O atom. The energy gaps of QDs with H passivation vary from 2.76 to 1.14 eV with the increase of lateral size. The lowest unoccupied molecular orbital (LUMO) is mainly contributed by Ti:3d, while the highest occupied molecular orbital (HOMO) is mainly contributed by C:2p and Ti:3d. The corresponding partial charge density at HOMO or LUMO is distributed around Ti and C atoms in the outside surface of QDs. The QDs show large absorption coefficient with the order of 105 cm−1 at visible light range, while the first absorption peaks vary from 2.8 to 1.4 eV (443–886 nm). Most importantly, the Ti3C2O2 QDs present size-dependent photocatalytic selectivity for CO2 reduction and photocatalytic activity for water splitting, caused by the shift of energy level positions of HOMO and LUMO. Our study reveal the potential of Ti3C2O2 QD in electronic and photocatalytic applications and provide new ideals into the design of photocatalysis based on QD materials.

Photoluminescence, thermal stability and structural properties of Eu3+, Dy3+ and Eu3+/Dy3+ doped apatite-type silicates

Ca2Y8(SiO4)6O2 phosphors doped with Eu3+ and Dy3+ and co-doped with Eu3+/Dy3+ were synthesized via sol-gel method. The materials showed apatite-type silicates crystallized in a hexagonal unit cell. The photoluminescence spectra of the Eu3+-doped silicates exhibited an intense red emission and the Dy3+-doped phosphors showed intense emissions in the blue (4F9/2 → 6H15/2) and yellow (4F9/2 → 6H13/2) regions. The Eu3+/Dy3+-co-doped phosphors presented improvements when compared with the spectra of the Dy3+-single doped materials by means of compensation in the red component of the emission spectrum. The chromaticity diagram presented a tunable color emission. The energy transfer from Dy3+ to Eu3+ was confirmed based on the photoluminescence intensities and decay curves. The temperature dependence of photoluminescence demonstrated that the synthesized phosphors did not reach thermal quenching, even when subjected to a temperature of 250 °C.

Self-accommodated defect structures modifying the growth of Laves phase

Laves phases, with the topologically close-packed structure and a chemical formula of AB2, constitute the largest single class of intermetallics. Planar defects in Laves phases are widely investigated, especially for stacking behavior transformations through synchroshear. Here, we report the coexistence of C14, C36 and C15 structures in MgZn2 precipitates by using atomic resolution scanning transmission electron microscopy, verifying the previously predicted Laves phase transformation sequence of C14 → C36 → C15 also applies to MgZn2. One type of stacking fault couple in precipitates has been found to alone reduce the lattice mismatch with matrix, while some other stacking fault couples need to self-accommodate with irregular planar defects (rhombic units and flattened hexagonal units), or with five-fold symmetry structures to relieve the strain concentration. Precipitates thus grow towards an equiaxed or even round morphology, rather than the plate morphology as conventionally believed. Molecular dynamics calculations are performed to support our analysis. These findings reveal the principles governing the concurrent occurrence of various defects in laves structures, acting as an update of the widely accepted perception of random occurrence of defects during crystal growth.

The Structural and Phase State of the TiAl System Alloyed with Rare-Earth Metals of the Controlled Composition Synthesized by the “Hydride Technology”

The structural state and the quantitative phase analysis of the TiAl system, alloyed with rare-earth metals synthesized using hydride technology, were studied in this work. Using the Rietveld method, the content of the major phases in the initial system Ti(50 at.%)–Al(50 at.%), as well as Ti(49 at.%)–Al(49 at.%), with alloying additions Ta, Y and Dy having a high accuracy was determined. The methods of scanning electron microscopy, transmission electron microscope and X-ray spectral microanalysis of the local areas of the structure for studying the distribution of alloying elements were used. The energies of lattices of separate phases were also determined after the full-profile specification. All the lattices of the identified structures (about 30) turned out to be stable. It was established that in the Ti(49 at.%)–Al(49 at.%) systems under study with alloying additions of metals Ta, Y and Dy, there were intermetallides composed of AlTi3, TiAl in the hexagonal, tetragonal and triclinic units. It is known that after microalloying alloys by Y and Dy metals, the mass fraction of TiAl phases increases significantly (>70%).

Reconfigurable topological waveguide based on honeycomb lattice of dielectric cuboids

Abstract We show that the photonic crystal (PhC) made of dielectric cuboids with their centers forming a honeycomb lattice is characterized by a ℤ 2 ${\mathbb{ℤ}}_{2}$ topological index when the longer sides of six cuboids point towards the center of hexagonal unit cell. While the C 6v symmetry regarding the center of unit cell is preserved, the C 3 symmetry regarding honeycomb sites is broken, which opens a bandgap in the Dirac dispersion of honeycomb structure and induces a band inversion between p modes and d modes. Rotating cuboids around their individual centers closes the bandgap and reopens a trivial bandgap. We discuss that this feature can be exploited for realizing a reconfigurable topological waveguide.

Tunable elastic wave propagation in planar functionally graded metamaterials

Structures made of functionally graded materials (FGM) are successful attempts to enhance the mechanical properties of homogeneous materials. On the other hand, periodically architected structures provide phenomenal opportunities to design structures much lighter than their bulk counterparts showing exceptional mechanical characteristics. In the present study, to utilize the advantages of both technologies, wave propagation properties of functionally graded metamaterials (FGMM), i.e., periodically architected structures made of FGM, are investigated for three different planar topologies, and their vibration filtering performances are analyzed. The mathematical formulations to obtain the equation of motion for the FGMM are developed using the finite element method, and Floquet–Bloch’s theorem is employed to find their dispersion curves. Periodically architected structures with hexagonal, rectangular, and triangular unit cells are considered, and the effects of the FGM on their stop-band percentages are investigated. A comparison between band structures of pure steel (St), pure alumina ( $$\hbox {Al}_{2}\hbox {O}_{3}$$ ) and $$\hbox {St}{-}\hbox {Al}_{2}\hbox {O}_{3}$$ reveals that using FGM in the periodically architected structures can greatly enhance wave propagation properties by opening new stop-band regions leading to structures with much more versatility and tunability. The material distribution is assumed to vary according to both power-law and exponential-law rules along the beam axis and thickness, and the effects of Young’s modulus ratio, density ratio, relative density, and non-negative power-law exponent are scrutinized on the bandgap properties. The results indicate that periodically architected structures made of FGM exhibit much higher percentages of stop-bands, and playing with corresponding FGM parameters can tune this value for desired engineering needs. In addition, a mathematical approach is presented to investigate the polarization of the studied FGMM in the longitudinal, transverse, and rotational directions, and to measure the effects of material distribution on the polarization of the first three branches of the dispersion curves. It is revealed that the polarization factors of the three first dispersion branches are mainly geometry-dependent and change slightly with the material distribution.

Nd3+-doped LiBaPO4 phosphors for optical temperature sensing within the first biological window: A new strategy to increase the sensitivity

In this work, powders of LiBaPO4:Nd3+ were synthesized by the sol–gel method using PVA as a chelating agent. X-ray diffraction indicates a tridymite structure with a hexagonal unit cell and space group P63. SEM images show that the samples have particles of different sizes with the majority varying between 5 and 20 μm. Under 532 nm excitation, four infrared emission bands were observed, with maxima at 749.9, 802.4, 873.2 and 953.4 nm assigned to the transitions from 4F7/2, 4F5/2, 4F3/2 to 4I9/2 and from 4F5/2 to 4I11/2. The performance of this material aiming optical thermometry based on the fluorescence intensity ratio methodology (FIR) was investigated within the range from 294 to 373 K, suitable for applications in biological systems. Under visible excitation, sensor performance is comparable to previously investigated Nd schemes. However, the sensing performance under infrared excitation was strongly enhanced by exploiting the FIR associated to the 4F5/2 → 4I11/2 and 4F3/2 → 4I9/2 transitions. The obtained relative sensitivity with this approach is one order of magnitude higher when compared with other Nd-doped materials excited in the infrared. These results suggest that the analyzed material can be used for application in temperature sensing of biological systems exploring the first biological window.

X-ray diffraction studies of ZnO:Cu thin films prepared using sol-gel method

ZnO:Cu thin films were deposited on corning glass substrate using sol gel method with different concentration of Cu (0, 1, 3, and 5%) has been done. The effect of the different Cu concentration on the structural properties of these films was studied in detail. Based on the XRD result, the ZnO thin films undoped and doped Cu are polycrystalline with hexagonal wurtzite structure and has preferred orientation is c-axis. All peaks of ZnO thin films undoped and doped Cu shows that reflection peaks associated with (100),(002) and (110) planes. The film quality was improved with the increasing of the Cu concentration. The value of the lattice constant a and c was found to be increased with increases of the Cu concentration. Changes in the lattice constant affect the bond length of ZnO (L) and the volume of one hexagonal system unit (V). The lattice parameter also affects to the crystallite size. The crystallite size was increased with Increasing of the Cu concentration and the lattice parameter. Furthermore, the strain value of the film decreases with increases of the Cu concentration. Lattice strain that has decreased is due to the cavity around of the formed film. Cavities between atoms also affect the density of the dislocation. The larger cavity between atoms on the films that makes the dislocation density becomes smaller.

Ab Initio Calculations of Carbon Bilayers with Diamond-Like Structures

Ab initio calculations of monoatomic and diatomic carbon layers composed of four-coordinated (sp3 hybridized) atoms are reported. According to the calculations, monoatomic diamond-like layers L4, L3–6, and L3–4–6 are unstable even at a temperature of 0 K. The structure of diatomic layers can be formed by the polymerization of hexagonal graphene layers. The analysis of all possible ways of cross-linking graphene layer pairs shows that only one diatomic layer can be stable. The crystal lattice of this layer belongs to the p6/mmm symmetry group (four atoms in a hexagonal unit cell, a = 2.737 A). The surface density, sublimation energy, and direct band gap values of the layer are 0.123 µg/cm2, 6.64 eV/atom, and 3.37 eV, respectively. The hexagonal structure of the diamond-like layer is predicted to be stable up to 250 K at normal pressure. The powder XRD pattern of the diamond-like layers is calculated to be used for their experimental identification in carbon materials.

Synthesis, Photophysical and Supramolecular Properties of a π-Conjugated Molecular Crown Containing a Pentagonal Unit: A Model Compound for Fullerene C240

The total synthesis of carbon onions is a significant challenge in the fields of materials science and organic chemistry. To date, the synthesis of even a fragment of fullerene C240 and its smallest carbon onion C60@C240 remains poorly explored. Herein, we demonstrate a bottom-up strategy to produce a novel π-extended molecular crown-shaped molecule (MC3) containing curved pentagonal and hexagonal units. This molecular crown represents a curved model compound for fullerene C240 and is fully characterized by NMR, mass spectrometry, UV-vis absorption, and from its emission spectra. Its supramolecular host–guest interaction with fullerene C60 is also investigated. MC3 and C60@MC3 can potentially be employed as seeds or templates for the bottom-up synthesis of fullerene C240 and the carbon onion C60@C240, respectively.

Experimental and numerical research on foam filled re-entrant cellular structure with negative Poisson's ratio

Abstract Mechanical behaviors of a new foam filled 2D re-entrant hexagonal honeycomb with negative Poisson's ratio under planar compression have been investigated in this paper. Re-entrant hexagonal unit cell specimens were fabricated by aluminum alloy and polyurethane foam. The deformation modes and force-displacement curves of the unit cell specimens under compression were carried out experimentally and numerically, and a good agreement was observed between experimental results and numerical simulation results. Subsequently, mechanical behaviors of foam filled and void re-entrant hexagonal honeycomb under planar compression were observed by numerical simulation. Compared to the void honeycomb, the foam filled honeycomb has a higher specific energy absorption capability due to a higher plateau stress. According to the mechanism of auxetic effect, the re-entrant honeycomb would move inward when subjected to compression along the orthogonal direction, which cause a biaxial compression of the foam and an increasing of stiffness/strength/energy absorption capability. The present investigations provide thorough insight into the strengthened re-entrant hexagonal cellular honeycomb, and could be used in the development of novel light weight smart functional structures.

Design and analysis of a shape-transformable polyhedral mechanism with transformation between a truncated tetrahedron and a tetrahedron

The aim of this study is to present a novel shape-transformable polyhedral mechanism namely, the TrT–T mechanism, which can transform between a truncated tetrahedron and a common tetrahedron. The conceptual model of the TrT–T mechanism is proposed and described. For one-degree-of-freedom objective, a hexagonal planar linkage is chosen as a hexagonal interlinked unit to develop a solid model and fabricate a prototype. Mobility analysis is carried out based on screw theory. The inverse and forward kinematics are investigated, and the singularity is analyzed. The trajectories and velocities of the hinge points are described and explored. The prototype and simulation results demonstrate the feasibility of the TrT–T mechanism for shape transformation and validate the correctness of the mobility analysis. As a novel polyhedral linkage, the TrT–T mechanism can be potentially applied to modules with shape-transformation ability for constructing reconfigurable and modular robots.

Asymptotic and numerical homogenization methods applied to fibrous viscoelastic composites using Prony’s series

The paper focuses on the evaluation of the effective properties of linear viscoelastic composites with a periodic structure, containing long cylindrical fibers of circular cross-section and for two different cell arrangements: square and hexagonal unit cells. For this purpose, we use the two-scale asymptotic homogenization method (AHM) and a numerical homogenization method (NHM). Based on the correspondence principle, the local functions and the relaxation overall properties are obtained in explicit form by the AHM using the Prony series. Additionally, the NHM is established for a three-dimensional representative cell, and the problem is solved under appropriate boundary conditions, by using the Finite Element Method. The numerical results obtained by the AHM and NHM are compared and verified with other theoretical approaches. The comparisons show a good agreement and a benchmark for further experimental and theoretical investigations.

Effect of Ga content on magnetic properties of BaFe12−xGaxO19/epoxy composites

The article reports about the effect of Ga content on magnetic properties of 30 wt% BaFe12−xGaxO19/epoxy(x = 0.1, 0.3, 0.6, 0.9 and 1.2) composites with uniform and aligned (due to the action of external magnetic field under preparation) distribution of hexaferrite filler in epoxy matrix. The XRD investigations of BaFe12−xGaxO19/epoxy (x = 0.1, 0.3, 0.6, 0.9 and 1.2) composites have revealed that the alignment degree of a crystalline hexaferrite phase with a hexagonal unit cell of crystal structure and P63/mmc space group in composites with uniform filler distribution is near 0.5 and for the composites with aligned filler distribution it is 1 for the direction along the applied magnetic field and it is 0 for the direction across the applied magnetic field, respectively. The investigation of the hysteresis loops of BaFe12−xGaxO19/epoxy (x = 0.1–1.2) composites with the aligned distribution of filler for both directions to the applied magnetic field has shown that magnetic properties across the alignment direction are superior in comparison with similar measurements taken on composites with uniform filler distribution. It was shown that anisotropy of magnetic properties of BaFe12−xGaxO19/epoxy (x = 0.1–1.2) composites is observed for all the investigated composites with the aligned distribution of the filler. Squareness ratio Ms along/Ms across decreases with Ga content increase, while the value Hc along/Hc across approaches to 1. Such behavior of magnetic characteristics of the investigated composites indicates clearly the decrease in magnetocrystalline anisotropy of Ga-substituted hexaferrite under Ga content increase.

Rietveld analysis and negative dielectric behavior of perovskite-like La1–xEuxMnO3 system

In this article, we report the results of the Rietveld and impedance analysis of Eu doped La1–xEuxMnO3 (LEMO; x = 0.10, 0.30, and 0.50) perovskite manganites synthesized by solid-state reaction method. Structural studies through Rietveld analysis confirm that a mixed-phase nature of all LEM samples having a hexagonal unit cell structure with a less detectable impurity (Eu2O3) ceramics. The patterns are zanalyzed in detail with structural distortion and strain in LEM manganites ceramics. Dielectric constant and modulus are found to increase with increasing Eu concentration. The samples LEM-0.10 shows the negative capacitance at low frequency and temperature, respectively.

Crystal Chemistry of the Microporous Zirconosilicate Na6Zr3[Si9O27], a Product of High-Temperature Transformation of Catapleiite, and Its Ag-Exchanged Form

The crystal structure of the Ag-exchanged form of the zirconosilicate with the simplified formula (Na6-2xCax▯x)Zr3[Si9O27] with x < 1 (the idealized formula Na6Zr3[Si9O27]), a product of thermal transformation of catapleiite, ideally Na2Zr[Si3O9]·2H2O, was studied using single crystal X-ray diffraction data. The crystal structure of Na6Zr3[Si9O27] is based on a heteropolyhedral framework built by nine-membered tetrahedral rings [Si9O27] and isolated [ZrO6] octahedra. This zirconosilicate demonstrates high exchange capacity to Ag (experiment with 1 M AgNO3 aqueous solution, 250 °C, 30 days). Its Ag-exchanged form with the simplified formula (Ag5Ca0.5)Zr3[Si9O27] is characterized by a significant distortion of the heteropolyhedral framework and strongly disordered arrangement of extra-framework cations (Ag) which results in the doubling of a parameter of the hexagonal unit cell [a = 23.3462(3), c = 10.10640(10) Å, V = 4770.45(13) Å3] and space group P63cm. Ag+ cations preferably occupy the sites that are close to the Na sites in Na6Zr3[Si9O27].