Unit Cell Orientation Research Articles

Probing Structural Perturbation in a Bent Molecular Crystal with Synchrotron Infrared Microspectroscopy and Periodic Density Functional Theory Calculations.

The range of unit cell orientations generated at the kink of a bent single crystal poses unsurmountable challenges with diffraction analysis and limits the insight into the molecular-scale mechanism of bending. On a plastically bent crystal of hexachlorobenzene, it is demonstrated here that spatially resolved microfocus infrared spectroscopy using synchrotron radiation can be applied in conjunction with periodic density functional theory calculations to predict spectral changes or to extract information on structural changes that occur as a consequence of bending. The approach reproduces well the observed trends, such as the wall effects, and provides estimations of the vibrational shifts, unit cell deformations, and intramolecular parameters. Generally, expansion of the lattice induces red-shift while compression induces larger blue-shift of the characteristic ν(C-C) and ν(C-Cl) modes. Uniform or non-uniform expansion or contraction of the unit cell of 0.1 Å results in shifts of several cm-1, whereas deformation of the cell of 0.5° at the unique angle causes shifts of <0.5 cm-1. Since this approach does not include parameters related to the actual stimulus by which the deformation has been induced, it can be generalized and applied to other mechanically, photochemically, or thermally bent crystals.

Transmission function and thermal conductivity of Si phononic crystals

We investigate phonon transport properties of Si phononic crystals by using an atomistic Green function method. By varying form factors such as thickness, orientation, and pore size of unit cells, mean-free path and associated thermal conductivity of phononic crystals are obtained. We present the empirical formula for thermal conductivity as a function of cell size and, by extrapolating results from it for realistic cell sizes, the thermal conductivity is compared with experimental ones. The formula is found to predict a nearly equal amount of the suppression in thermal conductivity to the experimental values, implying a feasible calculation method for predicting thermal properties of phononic crystals.

MAGNDATA: towards a database of magnetic structures. I. The commensurate case

A free web page under the nameMAGNDATA, which provides detailed quantitative information on more than 400 published magnetic structures, has been developed and is available at the Bilbao Crystallographic Server (http://www.cryst.ehu.es). It includes both commensurate and incommensurate structures. This first article is devoted to explaining the information available on commensurate magnetic structures. Each magnetic structure is described using magnetic symmetry,i.e.a magnetic space group (or Shubnikov group). This ensures a robust and unambiguous description of both atomic positions and magnetic moments within a common unique formalism. A non-standard setting of the magnetic space group is often used in order to keep the origin and unit-cell orientation of the paramagnetic phase, but a description in any desired setting is possible. Domain-related equivalent structures can also be downloaded. For each structure its magnetic point group is given, and the resulting constraints on any macroscopic tensor property of interest can be consulted. Any entry can be retrieved as a magCIF file, a file format under development by the International Union of Crystallography. An online visualization tool usingJmolis available, and the latest versions ofVESTAandJmolsupport the magCIF format, such that these programs can be used locally for visualization and analysis of any of the entries in the collection. The fact that magnetic structures are often reported without identifying their symmetry and/or with ambiguous information has in many cases forced a reinterpretation and transformation of the published data. Most of the structures in the collection possess a maximal magnetic symmetry within the constraints imposed by the magnetic propagation vector(s). When a lower symmetry is realized, it usually corresponds to an epikernel (isotropy subgroup) of one irreducible representation of the space group of the parent phase. Various examples of the structures present in this collection are discussed.

Structural and thermo-electrical properties of Sn–Al alloys

Thermal conductivities of pure Sn, pure Al and Sn-x wt% Al [x = 0.5, 2.2, 25, 50, 75] binary alloys were measured by radial heat flow method and were found to be 60.60, 208.80, 69.70, 80.30, 112.30, 142.00, 188.50 W/Km, respectively. The values of electrical resistivity were measured by four-point probe method and were found to be 2.90 × 10−8−3.90 × 10−7 Ωcm. The crystal structures, unit cell parameters and orientations of crystallization of the same samples were determined by X-ray diffraction. Smooth surfaces with a clear grain boundary for the samples were shown on the scanning electron microscopy micrographs. The temperature coefficients of electrical and thermal conductivity were also determined.

Microstructure evolution of poly[tetrafluoroethylene-co-(perfluoropropylvinylether)] films under uniaxial deformation

Abstract This paper presents an investigation of semi-crystalline poly[tetrafluoroethylene -co-(perfluoropropylvinylether)] (PFA) films uniaxially drawn at two temperatures. The extent of crystal unit cell orientation increased significantly with drawing at both draw temperatures as expected, but crystallinity was found to increase with draw ratio only at the lower draw temperature, remaining unchanged at the higher temperature. Considering the evolution of small-angle X-ray scattering patterns as a function of deformation, a model for the changes in PFA lamellar microstructure on drawing was developed. The observed mechanical α and γ relaxations of PFA were assigned to relaxation of rigid and mobile amorphous segments, respectively. Drawing at 200 °C to a draw ratio of 7 leads to a ∼40 °C increase in the α transition temperature compared to the unoriented film, and the findings support the key role of molecular orientation in suppressing the cooperative motions of non-crystalline segments in drawn polymers.

Self-Assembly of Tetraphenyldibenzoperiflanthene (DBP) Films on Ag(111) in the Monolayer Regime.

Tetraphenyldibenzoperiflanthene (DBP) is a promising candidate as a component of highly efficient organic photovoltaic cells and organic light-emitting diodes. The structural properties of thin films of this particular lander-type molecule on Ag(111) were investigated by complementary techniques. Highly ordered structures were obtained, and their mutual alignment was characterized by means of low-energy electron diffraction (LEED). Scanning tunneling microscopy (STM) images reveal two slightly different arrangements within the first monolayer (ML), both describable as specific herringbone patterns with two molecules per unit cell whose dibenzoperiflanthene framework is parallel to the surface. In contrast, single DBP molecules in the second ML were imaged with much higher intramolecular resolution, resembling the shape of the frontier orbitals in the gas phase as calculated by means of density functional theory (DFT). Further deposition leads to the growth of highly ordered bilayer islands on top of the first ML with identical unit cell dimensions and orientation but slightly inclined molecules. This suggests that the first ML acts as a template for the epitaxial growth of further layers. Simultaneously, a significant number of second-layer molecules mainly located at step edges or scattered over narrow terraces do not form highly ordered aggregates.

Influence of the structural orientation on the mechanical properties of selective laser melted Ti6Al4V open-porous scaffolds

In order to reduce the implant stiffness, open-porous designs in two geometrical dimensions were investigated (twisted design). Scaffolds were fabricated by an SLM process using Ti6Al4V and material properties were determined by conducting uniaxial compression testing. Furthermore, the unit cell was tested in different orientations to determine the mechanical properties.The elastic modulus of the scaffolds varied between 3.4 and 26.3GPa and scaffolds porosity ranged from 54 to 60%. Linear relationship was found between compression strain, compressive strength and modulus of elasticity. Results show a clear influence of the unit cell orientation on elastic modulus, compressive strength and strain. Moreover, the specific yielding that characterizes the relationship between modulus of elasticity and porosity under consideration of the cell orientation could be described in functional terms. We successfully generated scaffolds with mechanical properties close to those of human cortical bone. The influence on the unit cell orientation demonstrated the necessity of considering the load direction in the evaluation of bone scaffolds.

Topological defect lasers

We introduce a topological defect to a regular photonic crystal defect cavity with anisotropic unit cell. Spatially localized resonances are formed and have high quality factor. Unlike the regular photonic crystal defect states, the localized resonances in the topological defect structures support powerflow vortices. Experimentally we realize lasing in the topological defect cavities with optical pumping. This work shows that the spatially inhomogeneous variation of the unit cell orientation adds another degree of freedom to the control of lasing modes, enabling the manipulation of the field pattern and energy flow landscape.

English

From the literature, it is found that two different criteria were followed for the prediction of transverse thermal conductivity (K2) of fiber reinforced plastic (FRP) composites. In the first criterion, the internal anisotropy of the lamina is assumed negligible and K2 is estimated using simple Fourier&rsquo;s law of 1-D heat conduction applied to representative volume element (RVE). Whereas in the second approach, an electrical analogy method is followed. To estimate the effect of internal anisotropy, through thickness thermal conductivity (K3) of an FRP lamina is determined by both the approaches through finite element method for an RVE in the auxiliary plane. The problem is modeled in ANSYS 15 software. In the present paper studies are made for various volume fractions (0.1-0.75) and for various angles (20&deg;-90&deg;) made by the section plane with the fiber axis. It is observed that the through thickness thermal conductivity is consistent in the second approach, whereas in the first approach there is considerable variation (max 8.7%) with the orientation of the unit cell.&nbsp; &nbsp; Key words: Through thickness thermal conductivity, finite element method (FEM), unit cell orientation.

Polar-axis-oriented crystal growth of tetragonal PZT films on stainless steel substrate using pseudo-perovskite nanosheet buffer layer

Lead zirconate titanate (PZT) film with polar axis orientation was grown on a SUS 316L stainless steel substrate with the help of a Ca2Nb3O10 nanosheet (ns-CN) layer that had a pseudo-perovskite-type crystal structure. The ns-CN buffer layer was supported on a platinized SUS 316L (Pt/SUS) substrate, followed by chemical solution deposition (CSD) of the PZT films with tetragonal symmetry (Zr/Ti =40/60). The PZT films consisting of c-domain, with [001]-axis orientation of the perovskite unit cell, were deposited on the ns-CN/Pt/SUS substrate owing to (i) epitaxial lattice matching between the unit cell of PZT and substrate surface and (ii) in-plane thermal stress applied to the PZT film during cooling-down step of CSD procedure. The c-domain-oriented PZT film on ns-CN/Pt/SUS substrate exhibited enhanced remanent polarization of approximately 52 μC/cm2 and lowered dielectric permittivity of approximately 230, which are superior to those of conventional PZT films with random crystal orientation and comparable to those of epitaxial PZT films grown on (100)SrRuO3//(100)SrTiO3 substrates.

Photonic crystals with topological defects

We introduce topological defect to a square lattice of elliptical cylinders. Despite the broken translational symmetry, the long-range positional order of the cylinders leads to residual photonic bandgap in the density of optical states. However, the band-edge modes are strongly modified by the spatial variation of ellipse orientation. The $\Gamma-X$ band-edge mode splits into four regions of high intensity and the output flux becomes asymmetric due to the formation of crystalline domains with different orientation. The $\Gamma-M$ band-edge mode has the energy flux circulates around the topological defect center, creating an optical vortex. By removing the elliptical cylinders at the center, we create localized defect states which are dominated by either clockwise or counter-clockwise circulating waves. The flow direction can be switched by changing the ellipse orientation. The deterministic aperiodic variation of the unit cell orientation adds another dimension to the control of light in photonic crystals, enabling creation of diversified field pattern and energy flow landscape.

Competing anisotropic microstructures of Bi2(Te0.95Se0.05)3 thermoelectric materials by Bridgman technique

In this study, Bridgman technique was used to grow Bi2(Te0.95Se0.05)3 thermoelectric materials. Pulling rates were varied at 0, 2, 4 and 8mm/h. Anisotropic thermoelectric properties were investigated by measuring Seebeck coefficients, resistivities and power factors and microstructures by Orientation Imaging Microscopy (OIM) and X-ray diffraction, in the directions perpendicular and parallel to the pulling direction. An enhanced power factor of 2.01mW/mK2 in the direction parallel to the pulling direction was obtained for the pulling rate of 2mm/h. The OIM result showed that infinitely long grains, with grain boundaries and crystal a-axis parallel to the pulling direction were responsible for the enhanced power factor. Competing effects of orientations of unit cells and grain boundaries were observed in all other samples.

Assessment of crystal quality and unit cell orientation in epitaxial Cu₂ZnSnSe₄ layers using polarized Raman scattering.

We use polarization-resolved Raman spectroscopy to assess the crystal quality of epitaxial kesterite layers. It is demonstrated for the example of epitaxial Cu₂ZnSnSe₄ layers on GaAs(001) that "standing" and "lying" kesterite unit cell orientations (c'-axis parallel / perpendicular to the growth direction) can be distinguished by the application of Raman tensor analysis. From the appearance of characteristic intensity oscillations when the sample is rotated one can distinguish polycrystalline and epitaxial layers. The method can be transferred to kesterite layers oriented in any crystal direction and can shed light on the growth of such layers in general.

Microstructural change of degummed Bombyx mori silk: An in situ stretching wide-angle X-ray-scattering study

The microstructural change of degummed Bombyx mori silk was examined by in situ wide-angle X-ray-scattering (WAXS) with applied stretching force. WAXS patterns confirmed that the crystalline and amorphous regions coexist in the silk fibers. The crystallites with β-sheet structure have an orthorhombic unit cell with lattice parameters: a=9.10Å, b=9.71Å and c=6.80Å. The crystallite size, crystallite orientation and crystallinity were also estimated based on the WAXS patterns. The results demonstrate that the crystallite size is almost unchanged with the stretching strain. The crystallinity is approximately linearly increasing with the applied stretching force. However, the change of the unit-cell orientation degree with c-axis along the fiber axis behaves as a fast stage and an approximately unchanged stage during the in situ stretching process. All these experimental phenomena confirm that the microstructure of the degummed silk fibers can be well explained by the model of oriented β-sheet structure with a banded feature.

Crystal orientation switching in spherulites grown from miscible blends of poly(3-hydroxybutyrate) with cellulose tributyrate

Fully miscible blends over the whole composition range are obtained by melt mixing bacterial poly(3-hydroxybutyate) (PHB) and tri-substituted cellulose butyrate [cellulose tributyrate (CTB)]. Blends containing up to 50 wt % CTB are partially crystalline. Isothermal crystallization experiments show formation of PHB spherulites that grow until impingement. Depending on composition, radial growth rate is either constant or it suddenly increases in a very unusual manner leading to peculiar morphologies. In the latter case, in concomitance to the crystal growth acceleration, the sign of birefringence changes and rotation of the PHB unit cell orientation is observed. These results are discussed in terms of the influence of both composition and Tc on the relative crystallization kinetics of the two blend components. A strong effect played by the not yet crystallized CTB component that in the presence of the highly mobile PHB component forms a liquid crystal-like phase is proposed. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012

Synthesis of spatially variant lattices

It is often desired to functionally grade and/or spatially vary a periodic structure like a photonic crystal or metamaterial, yet no general method for doing this has been offered in the literature. A straightforward procedure is described here that allows many properties of the lattice to be spatially varied at the same time while producing a final lattice that is still smooth and continuous. Properties include unit cell orientation, lattice spacing, fill fraction, and more. This adds many degrees of freedom to a design such as spatially varying the orientation to exploit directional phenomena. The method is not a coordinate transformation technique so it can more easily produce complicated and arbitrary spatial variance. To demonstrate, the algorithm is used to synthesize a spatially variant self-collimating photonic crystal to flow a Gaussian beam around a 90° bend. The performance of the structure was confirmed through simulation and it showed virtually no scattering around the bend that would have arisen if the lattice had defects or discontinuities.

Polarized Raman Spectroscopy of Oligothiophene Crystals To Determine Unit Cell Orientation

Raman spectra were recorded experimentally and calculated theoretically for bithiophene, terthiophene, and quaterthiophene samples as a function of excitation polarization. Distinct spectral signatures were assigned and correlated to the molecular/unit cell orientation as determined by X-ray diffraction. The ability to predict molecular/unit cell orientation within organic crystals using polarized Raman spectroscopy was evaluated by predicting the unit cell orientation in a simulated terthiophene crystal given a random set of simulated polarized Raman spectra. Polarized Raman spectroscopy offers a promising tool to quickly and economically determine the unit cell orientation in known organic crystals and crystalline thin films. Implications of our methodologies for studying individual molecule conformations are discussed.

Crystal structure and orientation of uniaxially and biaxially oriented PLA and PP nanoclay composite films

AbstractCast films of poly(lactic acid) (PLA) and polypropylene (PP) with 2.5 and 5 wt % organo modified nanoclay were prepared and then uniaxially and biaxially hot drawn at T = 90 and 155°C, respectively, using a biaxial stretcher. The orientation of PLA and PP crystal unit cells, alignment of clay platelets, as well as the extent of intercalation and exfoliation were studied using wide angle X‐ray diffraction (WAXD). The measurement of d‐spacing of the 001 plane (normal to platelets plane) of the clay tactoids indicated the intercalation of the silicate layers for the PLA nanocomposite films, whereas the PP nanofilled films showed only dispersion of the nanoparticles (i.e., neither intercalation nor exfoliation were observed). The intercalation level of the clay platelets in PLA was almost identical for the uniaxially and biaxially drawn films. Our finding showed that the crystallite unit cell alignments are appreciably dependent on uniaxial and biaxial stretching. Moreover, the incorporation of clay to some extent influenced the orientation of the crystal unit cell axes (a, b, and c) of the oriented films. The silicate layers revealed a much higher orientation into the flow direction in the uniaxially stretched films compared to the biaxially drawn samples. In addition, the orientation of the 001 plane of nanoclays was significantly greater in the PLA compared to the PP nanoclay composite films probably due to a better intercalation and stress transfer in the former. Morphological pictograms illustrating the effects of uniaxial and biaxial stretching on the clay orientation are proposed. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Orientation, mechanical, and optical properties of poly (lactic acid) nanoclay composite films

AbstractIn this study, the orientation, mechanical as well as the optical properties of multilayer films of nanoclay filled poly (lactic acid) (PLA) as the core layer and polyethylene (PE) as the skin layer obtained from the film blowing process were investigated. The effect of nanoclay content in the range of 1–7.5 wt% was considered. The orientation of the polymers crystal unit cells, alignment of clay platelets as well as the extent of intercalation and exfoliation were studied using wide angle X‐ray diffraction (WAXD). The measurement of d‐spacing of the 001 plane (normal to platelets plane) of the clay tactoids indicated intercalation of the silicate layers in the PLA nanocomposite multilayer films. The Young modulus and tear resistance along the machine and transverse directions (MD and TD, respectively) improved significantly with the presence of the nanoparticles and percentage of nanofiller studied. The haze of the nanocomposite films decreased with clay concentration up to 5 wt% and increased by further addition of the nanoclay. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers

Wafer-Scale Growth of ITO Nanorods by Radio Frequency Magnetron Sputtering Deposition

We demonstrate synthesis of tin-doped indium oxide (ITO) nanorods on 2-in. glass wafers via radio frequency (rf)-magnetron sputtering deposition. The nanorods possessed a single-crystal structure of bixbyite, grew along the <100> orientation of the cubic unit cell, and were vertically aligned to the substrates. Height and diameter of the nanorods were as large as ∼810 nm and 40–100 nm, respectively. The morphological, structural, compositional, optical, and electrical properties of the ITO nanorods were examined with respect to growth temperature (25–500°C) and growth time (10–60 min). ITO nanorod films synthesized at 500°C exhibited excellent electrical and optical property such as a low sheet resistance (∼41 Ω/□) and high transparency in the wavelength range of visible light (i.e., ∼87% transmission at 550 nm). The facile approach to synthesize ITO nanorods at a large scale demonstrated in this work may find various applications including the fabrication of high performance optoelectronic devices.