Hexagonal Holes Research Articles

Retarded Charge–Carrier Recombination in Photoelectrochemical Cells from Plasmon‐Induced Resonance Energy Transfer

AbstractN‐type metal oxides such as hematite (α‐Fe2O3) and bismuth vanadate (BiVO4) are promising candidate materials for efficient photoelectrochemical water splitting; however, their short minority carrier diffusion length and restricted carrier lifetime result in undesired rapid charge recombination. Herein, a 2D arranged globular Au nanosphere (NS) monolayer array with a highly ordered hexagonal hole pattern (hereafter, Au array) is introduced onto the surface of photoanodes comprised of metal oxide films via a facile drying and transfer‐printing process. Through plasmon‐induced resonance energy transfer, the Au array provides a strong electromagnetic field in the near‐surface area of the metal oxide film. The near‐field coupling interaction and amplification of the electromagnetic field suppress the charge recombination with long‐lived photogenerated holes and simultaneously enhance the light harvesting and charge transfer efficiencies. Consequently, an over 3.3‐fold higher photocurrent density at 1.23 V versus reversible hydrogen electrode (RHE) is achieved for the Au array/α‐Fe2O3. Furthermore, the high versatility of this transfer printing of Au arrays is demonstrated by introducing it on the molybdenum‐doped BiVO4 film, resulting in 1.5‐fold higher photocurrent density at 1.23 V versus RHE. The tailored metal film design can provide a potential strategy for the versatile application in various light‐mediated energy conversion and optoelectronic devices.

Preparation and Structural Characterization of Platinum Nanosheets Intercalated between Graphite Powder with High Surface Area

Synthesis of platinum nanoparticles intercalated between layers of commercially available graphite powder samples possessing surface area values ranging from 20 to 750 m2 g-1 was attempted by the insertion of platinum chloride between the layers followed by hydrogen reduction. Platinum nanosheets with 1-4 nm thickness and 5-300 nm width and hexagonal holes were obtained for the graphite powder with 20 m2 g-1 independent of the platinum loadings (5, 10, and 15 wt%). Platinum nanosheets with the similar size (1-4 nm thickness and 5-300 nm width) and hexagonal holes were formed for graphite powder with 120 and 300 m2 g-1 with only 15 wt% platinum loading, but not for 5 and 10 wt% platinum loadings. Interestingly, for the graphite powder with higher surface area of 500 and 750 m2 g-1, platinum nanosheets were not formed. Platinum nanosheets intercalated between graphite samples were evaluated for cinnamaldehyde hydrogenation in supercritical carbon dioxide solvent. High cinnamyl alcohol selectivity was obtained for the platinum nanosheets intercalated between graphite layers having surface area of 20 m2 g-1; however, hydrocinnamaldehyde selectivity was high for the platinum nanosheets intercalated between graphite samples having surface area of 120 m2 g-1.

Hexagonal PCF of honeycomb lattice with high birefringence and high nonlinearity

This paper presents an optimum design for highly birefringent micro structured photonic crystal fiber (PCF). This is designed with novel hexagonal PCF of honeycomb lattice cladding with hexagonal shaped air cavities and the kernel of the hive is formed by two oblong air holes. The dispersion characteristics of this novel structure have been analyzed analytically using Finite Element Method (FEM) and reported for dispersion and nonlinear coefficient including other fiber parameter. This structure is optimized with distinctive parameters of fiber geometry like pitch of the core, diameters of elliptical shaped air cavities, cladding region having ample of space assortment of wavelength from 0.1 m to 2.0 m also made of SF57 glass helps in reaching high birefringence and low dispersion through the optimization of the arrangement and diameter of elliptical and hexagonal air holes. The proposed model has been suggested with high nonlinearity for super continuum generation.

Analysis of electronical properties of Bismuth and Silicene antidot in the presence of strain using the four-orbital tight-binding method

We have performed a tight-binding calculation of the band structure of Bi/Si sheets on which hexagonal holes are embedded. We named this kind of structure as Bismuth/Silicene antidot lattice (BAL/SAL). We have considered the semi-empirical sp3 tight-binding model on which the nearest and next-nearest neighbors of atoms have been taken into account. Our numerical results show that the size of holes in the two-dimensional sheets as the antidot lattice points are effective on the band structures and also transport properties of the system. Also, we have observed that BALs are almost conductors with very small band gaps for the large radius of circles, but about SALs for the large radius of holes, we obtain a considerable band gap, and conductivity is reduced. Besides, we have seen that, for the different radius of holes in the monolayer of Silicene, the presence of spin-orbit coupling in our band gaps obtained by tight-binding is not very important. On the contrary to Silicene antidot lattice, SOC interaction has a considerable effect on the band gap for Bismuth antidot lattice for the various radiuses of holes in their monolayer. Also, we have investigated the effects of the distance between holes on BAL/SAL band gaps and their band structures. Finally, we have obtained the band gap of Bismuth/Silicene antidot under strain, and we have compared results of sheet and antidot in the presence of strain.

Experimental and computational analysis of the in situ tensile deformation of 2D honeycomb lattice structures in Ni single crystals

A ~12 μm thick film of single crystal Ni was shaped with a focused ion beam (FIB) instrument to form a set of tensile samples with hexagonal holes of sides ~3 μm long and ~2.5 μm thick arranged in a honeycomb lattice structure. The plane normal of the film was <001> and the tensile axis was parallel to <100>. The samples were then tested in tension in situ inside a scanning electron microscope. The tensile properties, viz., yield strength and peak strength observed in the lattice sample, were higher than those of a standard sample by about 30%, while the strain to fracture decreased drastically. Finite element (FE) modelling, utilising size effect corrections and the modified Gurson model, was performed to elucidate the response of the microlattice, and it was found that the changes in the mechanical properties could be explained by the stress distribution around the hexagonal holes and localised sample size effects. These models were corroborated by observations of the crystal orientation using electron backscatter diffraction (EBSD) maps. The assumption of void formation as a mechanism of failure in the Gurson model was validated experimentally by transmission electron microscopy, which showed nano-voids in the thin bands formed by localised slip.

First-principles on the energy band mechanism for modifying conduction property of graphene nanomeshes

By means of first-principles electronic structure calculations, the ordered graphene nanomeshes with patterned hexagonal vacancy holes are theoretically studied to explore the modification mechanism of electrical conduction on graphene atomic monolayers. According to pseudopotential plane wave first-principles scheme based on density functional theory, the band structures of graphene nanomeshes are calculated to analyze the electrical conductance in correlation with the superlattice symmetry and vacancy hole magnetism. Based on the structural features and topological magnetism of Y-shaped nodes between the nanopores on the atomic monolayer of graphene, the graphene nanomeshes are classified into three types. The quadruplet degeneracy and splitting of electronic states at Brillouin zone center are investigated by comparing the band structures of graphene nanomeshes and analogical superlattices. The effects of inversion symmetry and supercell size on the opening band-gap at Dirac cone are elaborately analyzed with the consideration of antiferromagnetic coupling and hydrogen passivation at the magnetic edge of nanopores on graphene nanomeshes. The band-structure calculation results indicate that the (3&lt;i&gt;m&lt;/i&gt;, 3&lt;i&gt;n&lt;/i&gt;) (&lt;i&gt;m&lt;/i&gt; and &lt;i&gt;n&lt;/i&gt; are integers) superlattices have fourfold degenerate electronic states at center point of Brillouin zone, which can be effectively splitted by regularly arranging porous atomic vacancy to make the (3&lt;i&gt;m&lt;/i&gt;, 3&lt;i&gt;n&lt;/i&gt;) nanomesh, resulting in adjustable band-gap no matter whether or not the sublattices keeping in equivalence. In the nanomeshes formed by patterned holes with magnetic edge, the antiferromagnetic coupling adds a quantum parameter to the inversion symmetry so as to break the sublattice equivalence, opening band-gap at the twofold degenerate &lt;i&gt;K&lt;/i&gt; point. Nevertheless, the hydrogen passivation at the edge of magnetic nanopores will convert the magnetic graphene nanomeshes into non-magnetic and eliminate the band-gap at &lt;i&gt;K&lt;/i&gt; point. The band-gap of graphene nanomeshes could also be controlled by changing the density of nanopores, suggesting a graphene nanomaterial with adjustable band-gap that can be designed by controlling the mesh pore spacing. The graphene nanomeshes represent a new mechanism of forming band-gap and thus promise a strategy for achieving special electrical properties of graphene nanostructures. These results also theoretically demonstrate that the nano-graphene is a prospective candidate with flexibly adjustable electrical properties for realizing multivariate applications in new-generation nano-electronics.

Soft mold of microlens arrays fabricated by surface self-assembly

As a micro-optical element, microlens arrays (MLAs) play an important role in imaging, optical communication, lithography, and sensing. This paper reports a new method of fabricating a soft mold of MLAs by surface self-assembly. First, a hexagonal hole array mask is placed on a polydimethylsiloxane (PDMS) film in a radially pre-stretched state, and the masked PDMS film is irradiated using a scanning UV lamp in air, to form a hexagonal array of a silica-like hard film. Because of the surface self-assembly effect, this array bends inward, yielding a soft mold of concave MLAs after the pre-strain is spontaneously released. The focal length of the MLAs could be controlled in the range of 140–330 μm with a 37-μm-diameter hexagonal hole array by varying the exposure time of the ultraviolet ozone. Moreover, the diameter of the hexagonal hole array and the wrinkle wavelength, which are key influencing factors, were systematically studied. The soft mold of MLAs was applied to fabricate replica positive MLAs on planar and curved surfaces, which may have potential applications in image sensing and artificial compound eyes.

Experimental characterisation of the bound acoustic surface modes supported by honeycomb and hexagonal hole arrays

The Dirac point and associated linear dispersion exhibited in the band structure of bound (non-radiative) acoustic surface modes supported on a honeycomb array of holes is explored. An aluminium plate with a honeycomb lattice of periodic sub-wavelength perforations is characterised by local pressure field measurements above the sample surface to obtain the full band-structure of bound modes. The local pressure fields of the bound modes at the K and M symmetry points are imaged, and the losses at frequencies near the Dirac frequency are shown to increase monotonically as the mode travels through the K point at the Dirac frequency on the honeycomb lattice. Results are contrasted with those from a simple hexagonal array of similar holes, and both experimentally obtained dispersion relations are shown to agree well with the predictions of a numerical model.

Frequency analysis of hexagonal microbeam with 2D nanofiber mat

Flexibility of graphene (Gr) is partially credited to its two-dimensional (2D) molecular structure which resembles like interconnected unit of hexagonal carbon atoms ring. Therefore, to increase the flexibility of micromechanical beams, we analyze the influence of hexagonal shape of microcantilever beam without and with hexagonal hole of varying side length. To carry out the analysis, we first fabricate the hexagonal structure and perform measurement using laser vibrometer. Based on the measurement, we found that the presence of hexagonal holes make the beam flexible by two order of magnitude more than that of a conventional beam without holes. However, we found that the frequency of microcantilever beam with hexagonal shape decreases with increase in hole size. To undo the effect of hexagonal holes on microbeam frequency, we cover the microbeam with an electrospun cellulose acetate nanofiber mat (CAF). Based on the measurements, we found that the frequency can be increased more than that of a solid rectangular microbeam. Finally, we propose a finite element based methodology to model microbeam and nanofiber mat to compute their frequencies. Thus, based on the understanding developed in this work, this hierarchical structure of microcantilever beam with electrosun nanofibers can be used as a best candidate for utilizing it for the development of flexible sensors based on MEMS devices.

Honeycomb‐Inspired Surface‐Patterned Cu@CuO Composite Current Collector for Lithium‐Ion Batteries

The interface design between the current collector and active materials has a non‐negligible effect on the performance of lithium‐ion batteries (LIBs). Inspired by the honeycomb with a large specific surface area, this work validates the use of a surface‐patterned cellular Cu@CuO composite current collector with a hexagonal blind hole array and a nanostructured film of CuO nanoflowers for LIBs. This unique structure is synthesized by lithography and solution immersion. Mesocarbon microbead graphite powders are used as the active materials. Results show that the battery with the cellular Cu@CuO current collector maintains a reversible charge capacity of 354.1 mAh g−1 at 100 mA g−1 after 200 cycles. Compared with the Cu‐based current collector with a hexagonal blind hole array and a bare Cu current collector, the new pattern gains a performance improvement in the reversible capacity by 27.9% and 153.2%, respectively. The excellent electrochemical property of the cellular Cu@CuO current collector can be ascribed to strengthened adhesion with the active materials, reduced contact resistance, and improved Li‐ion diffusion capability. Due to the enhanced electrochemical performance, facile preparation processes, and cost‐effective raw materials, the new current collector shows great potential to replace the conventional current collector of energy storage systems.

Extremely high birefringent and low loss microstructure optical waveguide: Design and analysis

In this paper a new kind of porous core microstructure fiber is proposed and analyzed for the efficient transmission of terahertz signal. We use hexagonal type air hole instead of conventional circular air hole to produce hexagonal structure in cladding. Rectangular shaped air holes are introduced in the core to provide higher birefringence than other structures. Full vector finite element method is used to investigate the guiding characteristics of that hybrid waveguide. The numerical analysis ensures that, the proposed fiber offers extremely high birefringence of 0.075 and ultralow effective material loss of 0.045 cm−1 simultaneously at optimum designing condition. Along with the excellent guiding characteristics this fiber exhibits very high power fraction of 47% and very low dispersion variation of 0.17 ps/THz/cm over 600 GHz frequency range. The proposed fiber can be easily fabricated by using the ongoing technology. We hope this microstructure fiber will be a good candidate in different polarization maintaining applications.

Fabrication of grating coupled GaAs/AlGaAs quantum well infrared photodetector on an Si substrate

The grating coupled GaAs/AlGaAs quantum well infrared photodetectors (QWIPs) are integrated onto Si substrates using metal wafer bonding and epitaxial lift-off process. The 1 μm depth of hexagonal hole structure of grating was formed. The energy-dispersive x-ray spectroscopy results confirmed that the grating coupled QWIP is successfully mounted on an Si substrate. By evaluating the Raman spectra, PL, and surface roughness of bonded QWIP samples, the authors found that the grating does not induce any change in the optical or structural characteristics of actual QWIP layers. The dark current–voltage characteristics show a nearly identical dark current level between grating coupled QWIP and nongrating QWIP. The photocurrent spectrum shows that the peak photocurrent intensity of grating coupled QWIP is about 16 times higher than that of nongrating QWIP. This indicates that the grating effectively contributes to an increase in the light absorption of QWIP, showing large room for improvement of QWIP performance by further optimization of a grating structure.

Ultrathin and easy-processing photonic crystal absorbing layers to enhance light absorption efficiency of solar cells

We report the design and optimization of photonic crystal (PhC) structures within a GaAs or InAs absorption layer in thin film solar cells. In the PhC structure, hexagonal cylinder hole scatterers with the same upper and lower surfaces are used, and the cases of air and silica filled within these hexagonal hole scatterers are discussed, respectively. We designed and optimized the PhC absorption layer structures with four different conditions: the bulk materials comprising the absorption layer are either GaAs or InAs, and the hexagonal hole PhC scatterers are filled with either air or silica. The simulation results indicate that the absorptivity of the absorption layer can be greatly improved by using a PhC structure within the absorption layer. For height H = 0.20 µm, the maximum absorptivity of the GaAs absorption layer is 79.51%, while the maximum absorptivity of the InAs absorption layer is 96.57%. In addition, the absorptivity of the PhC structured absorption layer is less affected by the light incident angle, as the absorptivity of the structure is above 65% even when the light incident angle is 70°. Meanwhile, the porous PhC structure within the absorption layer is ideal for the filling of quantum dots, which has little effect on the absorptivity of light but can greatly improve the photoelectric conversion efficiency.

Multi-objective crashworthiness optimization of square and octagonal bitubal structures including different hole shapes

This paper investigates the effects of hole shapes and dimensions on crashworthiness performance of the square and octagonal bitubal structures under impact loading. Several holes with three different shapes including rectangle, hexagon and ellipse are considered on the walls of the square and octagonal structures. For these structures, crashworthiness indicators SEA and PCF are numerically obtained using finite element code LS-DYNA. Based on the results, a significant improvement on the crashworthiness capacity is achieved by employing the technique of creating holes on the walls of structures; such that the hexagonal holes created on the square and octagonal bitubal structures improve the crashworthiness capacity by 60% and 42%, respectively in comparison to the corresponding conventional structures. Interestingly, all the holed structures show less PCF compared to those without hole. In the next phase, the hole dimensions are optimized using NSGA-II (non-dominated sorting genetic algorithm) and ANNs (artificial neural networks) methods to find the optimal structures. The SAW (simple additive weight) method is then implemented on the optimal structures, and the square and octagonal structures having the hexagonal holes are finally found as the best and superior energy absorbing devices. The effect of oblique loading on crushing behavior of the holed structures is also considered. According to the results, the holes created on the structure walls improve the crashworthiness capacity of structures under oblique loading, as well.

A kinetic Monte Carlo model for the growth and etching of graphene during chemical vapor deposition

Controlled growth and etching have been employed experimentally to achieve diverse graphene patterns via chemical vapor deposition (CVD). However, a theoretical framework that is able to accurately predict the controlled growth and etching kinetics based on their underlying mechanisms is not yet available. In this work, we formulate a kinetic Monte Carlo model to predict the pattern evolution, edge structures and growth rate of graphene during CVD. In the model, the energetic parameters are obtained from previous first-principles calculations and experimental results. We show that diverse patterns fabricated in experiments, such as dodecagon with a hexagonal hole, double hexagonal rings, honeycomb-like networks and nanoribbons, can all be reproduced by the model. Beyond this, we also demonstrate that the model is able to predict the attainment of any artificially designed patterns. Therefore, the proposed model is not only able to capture the competitive growth and etching kinetics, but also provide a useful tool for the design and synthesis of diverse graphene patterns.

An optimized resistive CNT-based gas sensor with a novel configuration by top electrical contact

The paper proposes a new fabrication method of a resistive gas sensor made of vertically aligned carbon nanotubes (CNT) by the perforated electrode. This new configuration can be also utilized with vertically aligned nanomaterial such as ZnO wires. This method contains two aspects, which are getting top electrical contact from MWCNTs and devising hexagonal pattern holes on the sensor surface to ease the gas flow through the sensor. Reaching ambient molecules to these dense CNT forests is always of the essence; also, the top electrical contact of the CNTs is a fabrication challenge. The high sensitivity of proposed CNT-based sensor is attributed to their collective large porous surface along with etched circular holes located at the apexes and center of a hexagonal pattern on sensor top contact. For optimal sensor performance, the simulation approach by ANSYS-FLUENT software has optimized circular holes diameters and distances. The fabrication process proposes an oxygenated plasma treated CNTs with top electrical contact devised by a certain simple method for the very first time. Top electrical contact has been fabricated possible by the procedure of immersing the CNTs in viscosity liquid and providing electrical contact using conductive paste. Well-aligned multi-walled carbon nanotubes (MWCNT) have been grown on glass substrate placed into DC-PECVD unit bearing approximately temperature of 350 °C. In this paper, the resistivity of carbon nanotubes at different temperatures for the ambient gas and linear sensitivity to oxygen gas has been investigated. The sensors with larger diameter nanotubes are more sensitive. Etching patterned holes with 200 μm diameter enhances the sensitivity up to 50%.

3D-printed highly porous and reusable chitosan monoliths for Cu(II) removal

Removal of heavy metal pollution has received considerable research attention due to its severe effects on human health. In the present study, for the first time, a reusable monolithic 3D porous adsorbing filter was fabricated by integrating 3D printing technique with a natural adsorbent (chitosan), which is a low-cost and highly effective solution for heavy metal removal. The –NH2 and –OH functional groups found on the highly porous monoliths were capable of adsorbing metal ions. Further analyses revealed that the adsorption capacity increases with the filter’s surface area, with T = 25 °C and pH 5.5 representing the optimal working conditions. The maximum adsorption capacity of 13.7 mg/g (calculated by using the total mass) was measured for the filter with skewed hexagonal holes. The pseudo-first-order equation and Langmuir isotherm model presented a good fit to the experimental data, thus accurately describing the adsorption kinetics and the adsorption equilibrium. The monolithic chitosan-loaded filter exhibited a superior adsorption kinetics of 2.2 mg/(g min) for Cu(II) removal. Findings yielded by the desorption experiment also demonstrated that EDTA can desorb the metal ions from the monoliths, and its ~ 92% desorption capacity indicates high reusability. All results reported in this work indicate that 3D printing offers an alternative, cost-effective and facile approach for fabricating structured adsorbents with tunable structural and chemical properties for use in metal ion removal.

Oxidation of Suspended Graphene: Etch Dynamics and Stability Beyond 1000 °C.

We study the oxidation of clean suspended mono- and few-layer graphene in real time by in situ environmental transmission electron microscopy. At an oxygen pressure below 0.1 mbar, we observe anisotropic oxidation in which armchair-oriented hexagonal holes are formed with a sharp edge roughness below 1 nm. At a higher pressure, we observe an increasingly isotropic oxidation, eventually leading to irregular holes at a pressure of 6 mbar. In addition, we find that few-layer flakes are stable against oxidation at temperatures up to at least 1000 °C in the absence of impurities and electron-beam-induced defects. These findings show, first, that the oxidation behavior of mono- and few-layer graphene depends critically on the intrinsic roughness, cleanliness and any imposed roughness or additional reactivity from a supporting substrate and, second, that the activation energy for oxidation of pristine suspended few-layer graphene is up to 43% higher than previously reported for graphite. In addition, we have developed a cleaning scheme that results in the near-complete removal of hydrocarbon residues over the entire visible sample area. These results have implications for applications of graphene where edge roughness can critically affect the performance of devices and more generally highlight the surprising (meta)stability of the basal plane of suspended bilayer and thicker graphene toward oxidative environments at high temperature.

Influence thin-walled factor on the deflected mode of profile joints with interference

In machines and technological equipment for transmitting significant axial forces and torques, the connection of parts by interference fit is widely used. Such joints allow the transmission of significant axial forces and torques. An interference fit is used to connect various structural elements to the shaft instead of key, bolt and welded joints. They are more economical and rational in comparison with, for example, keyed joints, and the risk of a significant stress concentration in the places of keyed grooves also disappears. The strength of the connection in stationary fit is achieved due to the elastic, and sometimes also plastic deformation of the parts to be joined that occurs during the assembly process. The strength and relative immobility of such joints is ensured by the forces of friction between the mating parts, due to the magnitude of the interference fit. The aim of this work is to assess the dependence of the deflected mode of different types of profile joints with interference. The connections of round bushings with round, elliptical and hexagonal holes with shafts of the corresponding shape were investigated. Based on the ANSYS PC, the results were obtained, presented in the form of tables and graphs for the total displacements, contact pressures and equivalent stresses. The dependence of the deflected mode of the profile compounds of the indicated types on the thin-walled factor is shown. The practical significance of the obtained results consists in assessing the strength of the interference fit having a different shaft shape and their suitability for use.

Formation of the quasi-planar B50 boron cluster: topological path from B10 and disk aromaticity.

The lowest-lying isomer of the B50 boron cluster is confirmed to have a quasi-planar shape with two hexagonal holes. By applying a topological (leap-frog) dual operation followed by boron capping, we demonstrated that such a quasi-planar structure actually comes from the smallest elongated B102-, and its high thermodynamic stability is due to its inherent disk aromaticity arising from its 32 valent π electrons that fully occupy a disk configuration of [(1σ)2(1π)4(1δ)4(2σ)2(1φ)4(2π)4(1γ)4(2δ)4(1η)4]. The aromatic character of the quasi-planar B50 is further supported by a strong diatropic magnetic current flow. The sudden appearance of a quasi-planar B50 again points out that the growth pattern of pure boron clusters is still far from being completely understood.