Hexagonal Diffraction Pattern Research Articles

Photoaligned Liquid Crystal Devices with Switchable Hexagonal Diffraction Patterns.

Highly efficient optical diffraction can be realized with the help of micrometer-thin liquid crystal (LC) layers with a periodic modulation of the director orientation. Electrical tunability is easily accessible due to the strong stimuli-responsiveness in the LC phase. By using well-designed photoalignment patterns at the surfaces, we experimentally stabilize two dimensional periodic LC configurations with switchable hexagonal diffraction patterns. The alignment direction follows a one-dimensional periodic rotation at both substrates, but with a 60° or 120° rotation between both grating vectors. The resulting LC configuration is studied with the help of polarizing optical microscopy images and the diffraction properties are measured as a function of the voltage. The intricate bulk director configuration is revealed with the help of finite element Q-tensor simulations. Twist conflicts induced by the surface anchoring are resolved by introducing regions with an out-of-plane tilt in the bulk. This avoids the need for singular disclinations in the structures and gives rise to voltage induced tuning without hysteretic behavior.

Facile Green Synthesis of Silver Nanoparticles Using Rubus rosifolius Linn Aqueous Fruit Extracts and its Characterization

Rubus rosifolius Linn is a wild raspberry found in the Philippines. The fruit extract is found to be rich in flavonoids, terpenoids, tannins, and polyphenols. It has been shown to contain interesting profiles of antioxidants and it is not previously explored as a reducing agent, specifically in the field of bio-nano research. Some of these antioxidants have been documented to have anti-cancer potential. In this study, the fruit extracts of sampinit were used as a reducing agent for the synthesis of silver nanoparticles (AgNPs) via a ‘one-pot’ facile approach. Sampinit aqueous fruit extract-AgNPs (SAFE-AgNPs) were synthesized by reducing silver ions using sampinit extract. Optical, chemical, and morphological properties of the synthesized SAFE-AgNPs were characterized. Maximal absorption of SAFE-AgNPs was observed at 415 nm which is associated with the characteristic surface plasmon resonance profile of AgNPs. SAFE-AgNPs displayed highly stable and homogeneous nanoparticles with hydrodynamic size of 85.6 ± 0.98 nm and a zeta potential of –29.5 ± 0.96 mV. HR-TEM analysis of a single crystalline image of SAFE-AgNPs corresponds to hexagonal diffraction pattern and the AgNPs were coated with soft carbon-rich materials. EDX and XRF analysis showed that SAFE-AgNPs are chiefly composed of silver (Ag) and carbon (C), where C was largely localized on the surface of AgNPs. FTIR analysis showed that SAFE-AgNPs contains key chemical functional groups associated with sampinit-derived phytoconstituents. Overall, the biosynthesized SAFE-AgNPs produced small size, spherical shape, and monodisperse population which can be a candidate therapeutic agent for the treatment of various diseases such as cancer.

Characterization of a Disordered above Room Temperature Skyrmion Material Co8Zn8Mn4.

Topologically nontrivial spin textures host great promise for future spintronic applications. Skyrmions in particular are of burgeoning interest owing to their nanometric size, topological protection, and high mobility via ultra-low current densities. It has been previously reported through magnetic susceptibility, microscopy, and scattering techniques that CoZnMn forms an above room temperature triangular skyrmion lattice. Here, we report the synthesis procedure and characterization of a polycrystalline CoZnMn disordered bulk sample. We employ powder X-ray diffraction and backscatter Laue diffraction as characterization tools of the crystallinity of the samples, while magnetic susceptibility and Small Angle Neutron Scattering (SANS) measurements are performed to study the skyrmion phase. Magnetic susceptibility measurements show a dip anomaly in the magnetization curves, which persists over a range of approximately 305 K–315 K. SANS measurements reveal a rotationally disordered polydomain skyrmion lattice. Applying a symmetry-breaking magnetic field sequence, we were able to orient and order the previously jammed state to yield the prototypical hexagonal diffraction patterns with secondary diffraction rings. This emergence of the skyrmion order serves as a unique demonstration of the fundamental interplay of structural disorder and anisotropy in stabilizing the thermal equilibrium phase.

Phase-controllable laser thinning in MoTe2

Laser thinning of two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) has been considered a promising method to tune the bandgaps of TMDs via precise control of their thickness. However, the laser irradiation generates numerous chalcogen vacancies, which are known to cause a phase transition in polymorphic TMDs such as MoTe2. Therefore, the delicate control of the thickness and the phase during laser thinning is highly demanded to study the intrinsic properties of few-layered TMDs. Here, we report power-dependent laser thinning and phase control of semiconducting hexagonal MoTe2 (2H-MoTe2). High-resolution X-ray nano diffraction with synchrotron radiation showed that laser-thinned 2H-MoTe2 with low laser power (<2 mW) retained its hexagonal diffraction patterns with a single crystal orientation. In contrast, a phase transition to monoclinic (1T′) MoTe2 occurred during laser thinning at a high laser power level. Confocal Raman spectroscopy and atomic force microscopy (AFM) revealed that the low-power laser thinning of 2H-MoTe2 retained the crystal structure whereas high-power laser thinning created considerable amount of chalcogen vacancies and a phase transition. Power-dependent laser thinning thus provides a promising way to control the thickness and the phase of polymorphic 2D TMDs for next-generation optoelectronic devices.

Hexagonal phase into Au plate-like particles: A precession electron diffraction study

Crystalline defects can play a role in explaining speculative crystal growth mechanisms. Nevertheless, these structural features are commonly out of range for conventional diffraction characterizations. Particularly, defects in Au nanoplates (NPs) have attracted attention over the last 40 years due to the so called 1/3{422} “Forbidden Reflections” (FR). This work ends with the discussion of nanoplate defects by performing large angle tilting experiments in Transmission Electron Microscope (TEM) using several imaging techniques such as: bright/dark field (BF/DF) and the advanced Precession Electron Diffraction (PED) technique. Obtained results can relate FR with a hexagonal Au phase, presenting for the first time two hexagonal diffraction patterns on 112¯3 and 51¯4¯3¯ zone axes. This hexagonal-fcc interface is located inside of plate particles and has a hexagonal phase with a = 0.2870 nm and c = 0.7351 nm lattice parameters.

Imaging the Polymorphic Transformation in a Single Cu₆Sn₅ Grain in a Solder Joint.

In-situ observations of the polymorphic transformation in a single targeted Cu6Sn5 grain constrained between Sn-0.7 wt % Cu solder and Cu-Cu3Sn phases and the associated structural evolution during a solid-state thermal cycle were achieved via a high-voltage transmission electron microscope (HV-TEM) technique. Here, we show that the monoclinic η′-Cu6Sn5 superlattice reflections appear in the hexagonal η-Cu6Sn5 diffraction pattern upon cooling to isothermal 140 °C from 210 °C. The in-situ real space imaging shows that the η′-Cu6Sn5 contrast pattern is initiated at the grain boundary. This method demonstrates a new approach for further understanding the polymorphic transformation behavior on a real solder joint.

Toward Two‐Dimensional π‐Conjugated Covalent Organic Radical Frameworks

AbstractReported is the synthesis, characterization, and material properties of the first π‐conjugated two‐dimensional covalent organic radical framework (CORF), PTM‐CORF, based on the stable polychlorotriphenylmethyl (PTM) radical. The covalent organic framework (COF) precursor (PTM‐H‐COF) was first synthesized by liquid/liquid interfacial acetylenic homocoupling of a triethynylpolychlorotriphenylmethane monomer, and showed crystalline features with a hexagonal diffraction pattern matching that of A‐B‐C stacking. Subsequent deprotonation and oxidation of the PTM units in PTM‐H‐COF gave PTM‐CORF. Magnetic measurements revealed that the neighboring PTM radicals in the PTM‐CORF are anti‐ferromagnetically coupled each other, with a moderate exchange interaction (J=−375 cm−1). The PTM‐CORF has a small energy gap (ca. 0.88 eV) and a low‐lying LUMO energy level (−4.72 eV), and exhibits high electrocatalytic activity and durability toward the oxygen reduction reaction.

Toward Two-Dimensional π-Conjugated Covalent Organic Radical Frameworks.

Reported is the synthesis, characterization, and material properties of the first π-conjugated two-dimensional covalent organic radical framework (CORF), PTM-CORF, based on the stable polychlorotriphenylmethyl (PTM) radical. The covalent organic framework (COF) precursor (PTM-H-COF) was first synthesized by liquid/liquid interfacial acetylenic homocoupling of a triethynylpolychlorotriphenylmethane monomer, and showed crystalline features with a hexagonal diffraction pattern matching that of A-B-C stacking. Subsequent deprotonation and oxidation of the PTM units in PTM-H-COF gave PTM-CORF. Magnetic measurements revealed that the neighboring PTM radicals in the PTM-CORF are anti-ferromagnetically coupled each other, with a moderate exchange interaction (J=-375 cm-1 ). The PTM-CORF has a small energy gap (ca. 0.88 eV) and a low-lying LUMO energy level (-4.72 eV), and exhibits high electrocatalytic activity and durability toward the oxygen reduction reaction.

Two-dimensional hexagonal symmetry diffraction pattern by SiO2 photonic structures fabricated by hot embossing

Using synthesized sol–gel nanoimprint resist, large-area, ordered SiO2 cylindrical and stripe photonic nanostructures with constant aspect ratio have been fabricated by a single-step soft stamp hot embossing nanoimprint. Different from the traditional hot embossing nanoimprint technique, in our imprint process, the external force applied to the soft stamp is provided just by using our hand. Since the stress can be well released, the stamp can be easily released after the hot stage cooling down. So the optical window (K9 glass) substrates with imprinted two-dimensional SiO2 cylindrical photonic nanostructures show good light diffraction property. Also, our experiment demonstrates that with the imprinted sample annealing at 200[Formula: see text]C, the density of SiO2 will be increased and the diffraction efficiency can be further enhanced. In addition, the light splitting characteristic can still keep good for a larger (6 inch) substrate, which has a certain radian. This illustrates that this nanoimprint method can be compatible with the fluctuation of the imprinted substrates. Furthermore, as the distance between two adjacent feature nanostructures is in sub-micro scale, hexagonal symmetry diffraction pattern by the cylindrical photonic structures was realized at normal incidence of monochromatic laser ([Formula: see text]532[Formula: see text]nm). The diffraction efficiency of first order is about 11.2%. Morever, the diffraction pattern and the intensity of the first diffraction order can be modulated just by changing the incident angle of the input laser.

Gas source molecular beam epitaxy of scandium nitride on silicon carbide and gallium nitride surfaces

Scandium nitride (ScN) is a group IIIB transition metal nitride semiconductor with numerous potential applications in electronic and optoelectronic devices due to close lattice matching with gallium nitride (GaN). However, prior investigations of ScN have focused primarily on heteroepitaxial growth on substrates with a high lattice mismatch of 7%–20%. In this study, the authors have investigated ammonia (NH3) gas source molecular beam epitaxy (NH3-GSMBE) of ScN on more closely lattice matched silicon carbide (SiC) and GaN surfaces (&amp;lt;3% mismatch). Based on a thermodynamic analysis of the ScN phase stability window, NH3-GSMBE conditions of 10−5–10−4 Torr NH3 and 800–1050 °C where selected for initial investigation. In-situ x-ray photoelectron spectroscopy (XPS) and ex-situ Rutherford backscattering measurements showed all ScN films grown using these conditions were stoichiometric. For ScN growth on 3C-SiC (111)-(√3 × √3)R30° carbon rich surfaces, the observed attenuation of the XPS Si 2p and C 1s substrate core levels with increasing ScN thickness indicated growth initiated in a layer-by-layer fashion. This was consistent with scanning electron microscopy (SEM) images of 100–200 nm thick films that revealed featureless surfaces. In contrast, ScN films grown on 3C-SiC (111)-(3 × 3) and 3C-SiC (100)-(3 × 2) silicon rich surfaces were found to exhibit extremely rough surfaces in SEM. ScN films grown on both 3C-SiC (111)-(√3 × √3)R30° and 2H-GaN (0001)-(1 × 1) epilayer surfaces exhibited hexagonal (1 × 1) low energy electron diffraction patterns indicative of (111) oriented ScN. X-ray diffraction ω-2θ rocking curve scans for these same films showed a large full width half maximum of 0.29° (1047 arc sec) consistent with transmission electron microscopy images that revealed the films to be poly-crystalline with columnar grains oriented at ≈15° to the [0001] direction of the 6H-SiC (0001) substrate. In-situ reflection electron energy loss spectroscopy measurements determined the band-gap for the NH3-GSMBE ScN films to be 1.5 ± 0.3 eV, and thermal probe measurements indicated all ScN films to be n-type. The four point probe sheet resistance of the ScN films was observed to increase with decreasing growth temperature and decreased with unintentional oxygen incorporation. Hg probe capacitance–voltage measurements indicated ND-NA decreased with decreasing growth temperature from 1019 to 1020/cm3 for the lowest resistivity films to ≅5 × 1016/cm3 for the highest resistivity films. In-situ ultraviolet photoelectron spectroscopy measurements additionally showed the valence band maximum moving from 1.4 to 0.8 eV below the Fermi level with decreasing growth temperature consistent with the increased resistivity and reduction in carrier concentration. These results suggest that additional reductions in ScN carrier concentrations can be achieved via continued optimization of ScN growth conditions and selection of substrate orientation and surface termination.

Colloidal graphite/graphene nanostructures using collagen showing enhanced thermal conductivity

In the present study, the exfoliation of natural graphite (GR) directly to colloidal GR/graphene (G) nanostructures using collagen (CL) was studied as a safe and scalable process, akin to numerous natural processes and hence can be termed “biomimetic”. Although the exfoliation and functionalization takes place in just 1 day, it takes about 7 days for the nano GR/G flakes to stabilize. The predominantly aromatic residues of the triple helical CL forms its own special micro and nanoarchitecture in acetic acid dispersions. This, with the help of hydrophobic and electrostatic forces, interacts with GR and breaks it down to nanostructures, forming a stable colloidal dispersion. Surface enhanced Raman spectroscopy, X-ray diffraction, photoluminescence, fluorescence, and X-ray photoelectron spectroscopy of the colloid show the interaction between GR and CL on day 1 and 7. Differential interference contrast images in the liquid state clearly reveal how the GR flakes are entrapped in the CL fibrils, with a corresponding fluorescence image showing the intercalation of CL within GR. Atomic force microscopy of graphene-collagen coated on glass substrates shows an average flake size of 350 nm, and the hexagonal diffraction pattern and thickness contours of the G flakes from transmission electron microscopy confirm ≤ five layers of G. Thermal conductivity of the colloid shows an approximate 17% enhancement for a volume fraction of less than approximately 0.00005 of G. Thus, through the use of CL, this new material and process may improve the use of G in terms of biocompatibility for numerous medical applications that currently employ G, such as internally controlled drug-delivery assisted thermal ablation of carcinoma cells.

Occurrence and elimination of in-plane misoriented crystals in AlN epilayers on sapphire via pre-treatment control

AlN epilayers are grown directly on sapphire (0001) substrates each of which has a low temperature AlN nucleation layer. The effects of pretreatments of sapphire substrates, including exposures to NH3/H2 and to H2 only ambients at different temperatures, before the growth of AlN epilayers is investigated. In-plane misoriented crystals occur in N-polar AlN epilayers each with pretreatment in a H2 only ambient, and are characterized by six 60°-apart peaks with splits in each peak in (101̄2) phi scan and two sets of hexagonal diffraction patterns taken along the [0001] zone axis in electron diffraction. These misoriented crystals can be eliminated in AlN epilayers by the pretreatment of sapphire substrates in the NH3/H2 ambient. AlN epilayers by the pretreatment of sapphire substrates in the NH3/H2 ambient are Al-polar. Our results show the pretreatments and the nucleation layers are responsible for the polarities of the AlN epilayers. We ascribe these results to the different strain relaxation mechanisms induced by the lattice mismatch of AlN and sapphire.

Generation of scalable quasi-3D metallo-dielectric SERS substrates through orthogonal reactive ion etching

We combined two orthogonal reactive ion etching strategies on monolayers of 4 μm diameter polystyrene (PS) microspheres assembled on a planar glass substrate to create geometrically diverse but highly regular quasi-3D nano-structured arrays with centimeter length scales. Reactive ion etching (RIE) with O2/CF4 was used to selectively etch PS and, thus, to adjust the morphology of the PS mask, and CHF3 RIE was then used to etch the SiO2 substrate. The dynamic combination of these two etching procedures facilitated the realization of a wide variety of 3D-corrugated surface morphologies, including pedestals, bowls, honeycombs and blossom bud like arrays. The generated structures were then evaporated with Au films of defined thicknesses to generate metallo-dielectric arrays with tunable surface roughness. Hexagonal diffraction patterns from the fabricated structures confirm the successful realization of extended periodic hexagonal structures and the optical transmission spectra showed an efficient trapping of incident light in these ultra-rough metallo-dielectric arrays. The metallo-dielectric substrates were finally optimized for the detection of the pesticide methyl parathion in a concentration as low as 1 × 10−10 M through surface enhanced Raman spectroscopy (SERS).

Plastic deformation of natural diamonds by twinning: evidence from X-ray diffraction studies

AbstractA pink-purple diamond crystal from the Internatsional'naya kimberlite pipe (Siberia) was studied by single-crystal X-ray diffraction techniques using an area detector. Direct indexing of the diffraction pattern suggested a primitive hexagonal unit cell [ahex = 2.513(4), chex = 6.172(11) Å], instead of the well known face-centred cubic unit cell (acub ∼3.567 Å). Theoretical considerations and diffraction pattern simulation showed that the hexagonal diffraction pattern is the result of the superposition of two diffraction patterns with cubic symmetry due to spinel-law twinning along (111). These data are in good agreement with previous analyses of deformation microtwins in natural pink-purple diamonds using electron paramagnetic resonance spectroscopy and optical microscopy. The results suggest that natural epigenetic plastic deformation of diamonds occurs not only by dislocation slipping but also as a result of mechanical twinning.

Observation of unconventional optical diffraction in α-Fe2O3/Si two-dimensional photonic crystal slab fabricated by sol-gel method

The α-Fe2O3/Si two-dimensional (2D) photonic crystal (PC) with hexagonal honeycomb structure was fabricated by self-assembly and solution-dipping method based on colloidal monolayer template with 780 and 470 nm diameter polystyrene (PS) spheres. Optical diffraction measurement was carried out under wavelengths of 800, 532 and 400 nm. An interesting dispersion phenomenon with a hexagonal diffraction pattern composed of six elliptical spots is observed under specific wavelengths. Both theoretical and experimental calculations reveal that the diffraction can only occur when the incident wavelength is smaller than \(\sqrt{3}/2\) of lattice constant, and both the shape and distribution of these spots can be modified by the PC lattice constant and the incident wavelength. All these results reveal that optical properties of these 2D PC slabs are different from conventional diffractive devices.

Switchable photonic crystal for polymer dispersed liquid crystal

Single-prism systems are used to fabricate electrically switchable Photonic Crystal (PhC) structures in Polymer Dispersed Liquid Crystal (PDLC) films. The optical configuration is simple, stable and repeatable. A good agreement between theoretical and experimental results is obtained for the PhC structures. Hexagonal far-field diffraction patterns and electrical switching characteristics are also investigated. The smallest droplets of liquid crystals are 10 nm in diameter. The switching voltage can be decreased to 13.3 V/μm.

Electronic structure and ordering of multilayers of Co and Ag on Cu(111) investigated by photoelectron spectroscopy

The growth and the electronic structure of multilayers of Co and Ag on Cu(111) at room temperature have been studied with photoelectron spectroscopy and low-energy electron diffraction (LEED). The coverage range spans from Co and Ag layers between one monolayer (ML) to stacking of several monolayers. Surface states and ordered structures have been identified at room temperature. A Ag-related surface state with a binding energy of 0.30 eV is identified in normal emission in the ultraviolet photoelectron spectra when silver constitutes the top layer. Core-level binding energy shifts of Ag 3d 5/2 reflect the changing surroundings of Ag. Hexagonal diffraction patterns are observed for sandwiches of consecutive layers of Co and Ag up to 5 layers. Since no interlayer diffusion is observed in the layer-by-layer formation of the films, multilayers of consecutive silver and cobalt on Cu(111) offer preparation of sandwiched magnetic–non-magnetic structures.

Large-scale assembly of periodic nanostructures with metastable square lattices

This article reports a simple and scalable spin-coating technique for assembling non-close-packed colloidal crystals with metastable square lattices over wafer-scale areas. The authors observe the alternate formation of hexagonal and square diffraction patterns when the thickness of the colloidal crystals is gradually reduced during spin coating. No prepatterned templates are needed to induce the formation of the resulting metastable crystals with square arrangement. This bottom-up technology also enables the large-scale production of a variety of squarely ordered nanostructures that are consistent with the industry-standard rectilinear coordinate system for simplified addressing and circuit interconnection. Broadband moth-eye antireflection gratings with square lattices have been fabricated by using the shear-aligned colloidal monolayers as structural templates. Optical reflection measurement shows the squarely ordered arrays exhibit lower reflection than the nature-inspired hexagonal arrays with the similar structural parameters.

Epitaxial growth of continuous CeO 2(111) ultra-thin films on Cu(111)

Cerium oxide was grown epitaxially on a Cu(111) substrate at 520 K in an oxygen atmosphere and resulted in a high quality CeO 2(111) overlayers. The continuous single crystalline thin films were characterized by low energy electron diffraction, X-ray photoelectron spectroscopy of the Ce 3d core levels and resonant photoelectron spectroscopy of the valence band. The Ce oxide on Cu(111) grew initially in the form of islands giving a sharp (1.5 × 1.5) hexagonal diffraction pattern of the CeO 2(111) structure. It covered the substrate surface completely after deposition of a 2.5-monolayer thick overlayer. Ce 3d spectra and resonant enhancement of the valence band emission show that Ce is present in the Ce 4+ oxidation state.

The structure of the α-quartz (0 0 0 1) surface investigated using helium atom scattering and atomic force microscopy

In this paper, we present the first structural investigation of quartz using helium atom scattering (HAS). Studies were carried out on the α-quartz (0 0 0 1) surface (Z-cut). He-diffraction experiments reveal a complex hexagonal diffraction pattern. Prominent peaks are visible at positions corresponding to the periodicity of the bulk unit cell (4.90 ± 0.02 Å). In addition, weak (2 × 2) peaks and satellite peaks are observed. Two recent theoretical papers both propose a (1 × 1) reconstruction for α-quartz (0 0 0 1) with 120° symmetry for the surface unit cell: the so called dense structure. We propose a model where terraces with step heights of a multiple 1/3 of the unit cell height lead to domains rotated by ±60° relative to each other. This is supported by AFM images. The satellite peaks indicate an incommensurate super-lattice structure with a periodicity of the order of 57 Å, presumably related to strain relief mechanisms. The (2 × 2) peaks may stem from co-existing regions of a semi-dense structure, which is predicted to have a higher energy than the dense, (1 × 1) phase.