Anisotropic Geometry Research Articles

Two-Dimensional Single Crystal CdS Nanosheets: Synthesis and Properties

A one-dimensional (1D) to two-dimensional (2D) topotactic transformation has been proposed for the synthesis of 2D CdS nanosheets by promoting the lateral growth of 1D CdS nanobelts. The as-prepared nanosheets are single crystal with hexagonal structure, usually 20−100 nm in thickness, 20−100 μm in width, and up to several millimeters in length. CdS nanosheets show enhanced multiphonon responses and exciton−phonon couplings, which are correlated to their extraordinary anisotropic geometry. Photoluminescence measurement reveals three emission bands at about 1.61, 2.39, and 2.45 eV, which are attributed to the radiative transitions related to surface states, bandgap, and excitonic molecules, respectively; moreover, the formation of the surface states and the excitonic molecules are significantly enhanced due to the anisotropic geometry of the 2D nanosheets. CdS nanosheets show also pronounced photoconduction under visible light irradiation, indicating potentials in assembly of optoelectronic nanodevices.

Periodical Nanostructured Multiline Copper Films Self-Organized by Electrodeposition: Structure and Properties

Nanoscale patterned electrodeposition of metals on flat substrates has become a topic of great interest both scientifically and technologically in recent years. Here, we demonstrate the self-organized growth of extended arrays of parallel nanoscale copper wires on flat glass chips from an ultrathin aqueous electrolyte layer. Regular, periodic patterns of copper nanowires with diameters in the range of 40 nm-400 nm were grown, forming parallel arrays of alternating lines of copper and cuprous oxide at periodicities down to approx. 80 nm, depending on the deposition parameters such as voltage and pH. The resulting multiline films are investigated with respect to their structure and electrical properties using Electrical Force Microscopy, Scanning Electron Microscopy and Scanning Auger Electron Spectroscopy. The results of the experiments show a strongly anisotropic behavior of the electrical properties of multiline nanostructures corresponding to their strongly anisotropic geometry.

Multinomial inference on distributed responses in SPM

In this work, we propose statistical methods to perform inference on the spatial distribution of topological features (e.g. maxima or clusters) in statistical parametric maps (SPMs). This contrasts with local inference on the features per se (e.g., height or extent), which is well-studied (e.g. Friston et al., 1991, 1994; Worsley et al., 1992, 2003, 2004). We present a Bayesian approach to detecting experimentally-induced patterns of distributed responses in SPMs with anisotropic, non-stationary noise and arbitrary geometry. We extend the framework to accommodate fixed- and random-effects analyses at the within and between-subject levels respectively. We illustrate the method by characterising the anatomy of language at different scales of functional segregation.

Carbon-13 NOESY and equivalent protons: Methyl iodide dynamics

We have shown that proton-coupled carbon-13 2D NOESY experiments, performed on degenerate spin systems, can provide unique quantitative information about anisotropic reorientational motions and molecular geometry. Relevant theory for AX 2 and AX 3 spin systems is presented, assuming the dipole–dipole and random field relaxation mechanisms of 13C nucleus, and demonstrated on methyl iodide solution in chloroform. Agreement with experimental intensities of all the six independent peaks is very good in the whole range of mixing times (up to 45 s).

Plasmonic cloaking for irregular objects with anisotropic scattering properties

Here we extend the plasmonic cloaking technique to irregularly shaped objects with anisotropic scattering response. The scattering-cancellation approach to cloaking [A. Alù and N. Engheta, Phys. Rev. E 72, 016623 (2005)] has been extensively applied in the past to symmetrical geometries and canonical shapes. However, recent papers have raised some doubts concerning the fact that its use may not be as effective when dealing with strongly anisotropic and noncanonical geometries. Our goal here is to extend the plasmonic cloaking technique to irregular obstacles and to show that proper cloak design may provide a significant and uniform scattering reduction, independent of angle of incidence, position, and polarization of the illumination. We investigate how the volumetric effect of scattering cancellation provided by plasmonic media may drastically suppress the scattering for these irregular geometries independent of the illumination angle, and we shed some light on the physical mechanisms and the design rules at the basis of this cloaking technique when applied to objects whose scattering properties are dependent upon polarization and angle of incidence.

Mechanical failure analysis of thin film transistor devices on steel and polyimide substrates for flexible display applications

The crack onset strain (COS) of 4-level thin film transistor (TFT) devices on both steel foils and thin polyimide (PI) films was investigated using tensile experiments carried out in situ in an optical microscope. Cracks initiated first within the SiO 2 insulator layer for both types of substrates. The COS was found to be equal to 1.15% and 0.24% for steel and PI, respectively. The influence of loading direction on failure of the TFT stack with anisotropic geometry was moreover found to be considerable, leading to recommendations for backplane design. The large difference in critical strain of the SiO 2 layer on the two substrates was analyzed using an energy release rate approach, and found to result from differences in layer/substrate mechanical contrast and in internal stress state. Based on this analysis a correlation between layer/substrate elastic contrast and tensile failure behavior was devised.

Integration of Microstrip Patch Antenna With Polycrystalline Silicon Solar Cell

The implementation of a polycrystalline silicon solar cell as a microwave groundplane in a low-profile, reduced-footprint microstrip patch antenna design for autonomous communication applications is reported. The effects on the antenna/solar performances due to the integration, different electrical conductivities in the silicon layer and variation in incident light intensity are investigated. The antenna sensitivity to the orientation of the anisotropic solar cell geometry is discussed.

Fabrication of Polymer Nanocavities with Tailored Openings

A templated fabrication of open nanocavities is reported, where rational control of partial polymer attachment on sacrificial metal cores introduces openings in the polymer shells. This approach provides a facile means to modify the structural features of polymer nanocavities by manipulating the surface chemistry of colloidal nanoparticles. In particular, the anisotropic geometry of gold nanorods is exploited to promote the anisotropic polymer attachment, such that two diametric openings occurred in the polymer shell. After etching the gold nanorods, this approach yields open nanochannels that are tunable in both diameter and length. The synthetic scope of the anisotropic core/shell nanoparticles is expanded, supporting the previously proposed mechanism. We demonstrate that reducing the symmetry of nano-objects could open up new ways to create structural features using simple assembly and etching techniques. The thermostability of the open polymer nanostructures is also investigated.

A new simulation code for particle diffusion in anisotropic, large-scale and turbulent magnetic fields

A new Monte Carlo code is presented that simulates the scattering processes of energetic particles in turbulent magnetic fields. The growing number of analytical models for anisotropic turbulence geometries gives rise to the need of fast and adaptable simulation codes that, in order to be able to judge the accuracy of the results, calculate the estimated mean errors of the transport parameters. Furthermore, the need to understand the interplay of scattering in anisotropic large-scale (such as the solar magnetic field) and turbulent (such as the Maltese cross-like structured solar wind turbulence) magnetic fields is accounted for with the calculation of the off-diagonal elements of the diffusion tensor.

Complex anisotropy in D″ beneath the eastern Pacific from SKS–SKKS splitting discrepancies

Although observations of seismic anisotropy in the lowermost mantle are abundant, the mechanism which generates anisotropy in the D″ layer is not well understood. Most observational constraints on D″ anisotropy come from the splitting of shear waves that propagate (nearly) horizontally through the lowermost mantle. However, anisotropy in the D″ layer can also give rise to discrepancies in shear wave splitting for SKS and SKKS phases for the same event-station pair, as these phases sample different regions of the lowermost mantle. Here I report observations of strongly discrepant SKS–SKKS splitting observed at broadband stations in western Mexico and California. In particular, strong SKKS splitting with fast polarization directions near ~ 60° and delay times up to ~ 3 s is observed for a group of raypaths that sample a region of the D″ layer beneath the eastern Pacific Ocean. A comparison of SKKS splitting with SKS splitting observed from the same events, as well as with SKS splitting observed at a variety of backazimuths at each station, suggests that the anomalous anisotropic structure is in the lowermost mantle. My preferred model for the unusual anisotropic geometry in this region is large shear deformation in D″ at the edge of a region where slab material impinges upon the core–mantle boundary, resulting in lattice preferred orientation of lower mantle minerals.

Poroviscoelastic Two-Dimensional Anisotropic Solution with Application to Articular Cartilage Testing

The transverse anisotropic poromechanics solution for the two-dimensional Mandel-type problem geometry is extended in this paper to account for the orthotropic nature of the porous media, thus mimicking the response of articular cartilage samples when subjected to load perturbation. The anisotropic solution presented takes into account the viscoelastic and anisotropic nature of the fluid-saturated cartilage specimen sandwiched between two impermeable rigid plates and subjected to quasi-static step loading conditions; thus simulating the unconfined compressive test responses of cartilage samples in biomechanics laboratory setups. The solution addresses the stress, fluid pressure, and displacement results due to load application through exact modeling of the intrinsic nature of the orthotropic viscoelastic matrix structure as well as the compressible interstitial fluid flow responses. Poromechanical parameter characterization and modeling of biological tissues, such as cartilage, will find this analytical solution to the two-dimensional anisotropic poroviscoelastic geometry very useful. This problem will not only serve as a benchmark for validating numerical schemes and simulations but also assist in calibrating laboratory results on biological tissues, including cyclic loadings.

Solvothermal synthesis of CdS nanowires templated by polyethylene glycol

CdS nanowires were solvothermally synthesized from Cd(NO 3) 2 and S powder using ethylenediamine as a solvent and polyethylene glycol (PEG) as a template. Hexagonal CdS with P6 3mc space group was detected using XRD and SAED, results which are in good accordance with those obtained by the simulation. SEM, TEM and HRTEM revealed the gradual development of nanowires in the [0 0 1] direction with a number of atoms aligning in a crystal lattice. Raman spectra of different products showed the fundamental and overtone modes at the same wavenumbers of 300 and 601 cm −1, respectively. Their relative intensities at different molecular weight PEG were influenced by the anisotropic geometry of the products. Their photoluminescence peaks were detected at the same wavelengths of 518 nm. A formation mechanism for CdS nanowires was also proposed to relate to the experimental results.

Solvothermal synthesis of CdS nanorods using hydroxyethyl cellulose as a template

CdS nanorods were solvothermally produced from Cd(CH 3COO) 2 and S powder using ethylenediamine (en) as a solvent and hydroxyethyl cellulose (HEC) as a template. The phase with hexagonal structure was detected using XRD and SAED, which is in perfect accordance with the results obtained by simulation. SEM, TEM and HRTEM revealed the development of nanorods with a number of atoms arranged in crystal lattices. When the appropriate amount of HEC was used, the longest nanorods, with preferential growth in the [0 0 1] direction, were produced. Raman spectra showed the fundamental and overtone modes at the same wavenumbers of 301 and 599 cm −1, respectively. Their relative intensities at each temperature were strongly influenced by the anisotropic geometry of the products. Photoluminescence caused by electron-hole recombination was detected at 470 nm, and by surface trapping induced emission at 575 nm. The formation mechanism of CdS nanorods was also proposed based on the experimental results.

Photonic band structure evolution of a honeycomb lattice in the presence of an external magnetic field

A standard plane-wave expansion technique is used to investigate the evolution of the photonic band structure of a two-dimensional honeycomb lattice composed by cylindrical shell rods with dielectric permittivities ε1 and ε2, and embedded in a background with permittivity ε3. We have considered the effect of dispersive dielectric responses as well as the influence of an externally applied magnetic field aiming to obtain efficient tunable bandgaps. Present results suggest that a combination of a doped semiconductor constituent with an anisotropic geometry, which breaks symmetry and unfolds degeneracies, provides an efficient realization of photonic systems with tunable bandgaps.

Criteria for positive and negative reflection and refraction for three-dimensional anisotropic geometries

Waves propagating in anisotropic media with a 3D geometry are considered. General mathematical criteria that make it possible to find out if reflection or refraction of waves in such media is positive or negative are formulated.

Arrested Coalescence of Particle-coated Droplets into Nonspherical Supracolloidal Structures

Colloidal and supracolloidal structures with anisotropic shape and surface chemistry are potential building blocks for the fabrication of novel materials. Droplets or bubbles are often used as templates for the assembly of particles into supracolloidal structures of spherical shape. Particle-coated droplets or bubbles have recently been shown to also retain nonspherical geometries after deformation, suggesting that the templating approach can also be used to produce supracolloidal structures with anisotropic shape. We show that partially coated droplets generated in a microcapillary device can undergo spontaneous coalescence into stable nonspherical structures. By positioning the droplets into regular arrays before coalescence, we produce anisotropic geometries with well-defined bonding angles between adjacent merged droplets. This approach allows for the fabrication of novel anisotropic supracolloidal structures with deliberately designed shapes.

Tool path optimization for free form surface machining

This article presents a novel approach to generate optimized tool paths for free form surfaces that are commonly used in automotive, aerospace, biomedical, home appliance manufacturing and die/mold industries. The developed tool path optimization approach can handle various objectives under multiple constraints. Due to anisotropic geometry of free form surfaces, tool paths become one of the most critical factors for determining cutting forces. Here, the concept of force-minimal tool path generation is introduced and demonstrated for free form surfaces. Nowadays, process planning engineers must select the tool paths only from a set of ordinary tool paths available in CAM systems. These standard tool paths available in CAM systems are generated based on geometric computations only, not considering mechanics of processes, and most often these paths are away being optimum for free form surfaces. Here, a new methodology is introduced the first time for generating the tool paths based on process mechanics for globally minimizing the cutting forces for any given free form surface.

Anisotropic hysteresis on ratcheted superhydrophobic surfaces

We consider the equilibrium behaviour and dynamics of liquid drops on a superhydrophobic surface patterned with sawtooth ridges or posts. Due to the anisotropic geometry of the surface patterning, the contact line can preferentially depin from one side of the ratchets, leading to a novel, partially suspended, superhydrophobic state. In both this configuration, and the collapsed state, the drops show strong directional contact angle hysteresis as they are pushed across the surface. The easy direction is, however, different for the two states. This observation allows us to interpret recent experiments describing the motion of water drops on butterfly wings.

Ellipsometry를 이용한 Low-k SiOCH 박막의 유전특성에 관한 연구

We studied the dielectric characteristics of low-k SiOCH thin films by Ellipsometry. The SiOCH thin films were prepared by deposition of BTMSM precursors on p-Si wafer by CCP-PECVD method. The nano-porous structural organic/inorganic hybrid-type of SiOCH thin films correlated directly to the formation of low dielectrics close to pore(k=1). The structural groups including highly dense pores in SiOCH thin films originated the anisotropic geometry type of network structure directing to complex refractive characteristics of SiOCH single layer on the p-Si wafer. The linearly polarized beam of Xe-ramp in the range from 190 nm to 2100 nm introduced to the surface of SiOCH thin film, and the reflected beam was Elliptically polarized by complex refractive coefficients of SiOCH dipole groups. The amplitude variation <TEX>$\Psi$</TEX> and phase variation <TEX>$\Delta$</TEX> of the relative reflective coefficients between perpendicular and parallel components to the incident plane were measured by Ellipsometry. The complex optical constants n and k as well as the dielectric constant and thickness of SiOCH thin films were driven by the measured value of <TEX>$\Psi$</TEX> and <TEX>$\Delta$</TEX>.

Gravity waves signatures from anisotropic preinflation

We show that expanding or contracting Kasner universes are unstable due to the amplification of gravitational waves (GW). As an application of this general relativity effect, we consider a preinflationary anisotropic geometry characterized by a Kasner-like expansion, which is driven dynamically towards inflation by a scalar field. We investigate the evolution of linear metric fluctuations around this background, and calculate the amplification of the long-wavelength GW of a certain polarization during the anisotropic expansion (this effect is absent for another GW polarization, and for scalar fluctuations). These GW are superimposed to the usual tensor modes of quantum origin from inflation, and are potentially observable if the total number of inflationary $e$-folds exceeds the minimum required to homogenize the observable universe only by a small margin. Their contribution to the temperature anisotropy angular power spectrum decreases with the multipole $\ensuremath{\ell}$ as ${\ensuremath{\ell}}^{\ensuremath{-}p}$, where $p$ depends on the slope of the initial GW power spectrum. Constraints on the long-wavelength GW can be translated into limits on the total duration of inflation and the initial GW amplitude. The instability of classical GW (and zero-vacuum fluctuations of gravitons) during Kasner-like expansion (or contraction) may have other interesting applications. In particular, if GW become nonlinear, they can significantly alter the geometry before the onset of inflation.