Showing papers on "Shell (structure) published in 2012"

Core/shell colloidal semiconductor nanoplatelets.

Abstract: We have recently synthesized atomically flat semiconductor colloidal nanoplatelets with quasi 2D geometry. Here, we show that core/shell nanoplatelets can be obtained with a 2D geometry that is conserved. The epitaxial growth of the shell semiconductor is performed at room temperature. We report the detailed synthesis of CdSe/CdS and CdSe/CdZnS structures with different shell thicknesses. The shell growth is characterized both spectroscopically and structurally. In particular, the core/shell structure appears very clearly on high-resolution, high-angle annular dark-field transmission electron microscope images, thanks to the difference of atomic density between the core and the shell. When the nanoplatelets stand on their edge, we can precisely count the number of atomic planes forming the core and the shell. This provides a direct measurement, with atomic precision, of the core nanoplatelets thickness. The constraints exerted by the shell growth on the core is analyzed using global phase analysis. The co...

Direct Imaging the Upconversion Nanocrystal Core/Shell Structure at the Subnanometer Level: Shell Thickness Dependence in Upconverting Optical Properties

Abstract: Lanthanide-doped upconversion nanoparticles have shown considerable promise in solid-state lasers, three-dimensional flat-panel displays, and solar cells and especially biological labeling and imaging. It has been demonstrated extensively that the epitaxial coating of upconversion (UC) core crystals with a lattice-matched shell can passivate the core and enhance the overall upconversion emission intensity of the materials. However, there are few papers that report a precise link between the shell thickness of core/shell nanoparticles and their optical properties. This is mainly because rare earth fluoride upconversion core/shell structures have only been inferred from indirect measurements to date. Herein, a reproducible method to grow a hexagonal NaGdF4 shell on NaYF4:Yb,Er nanocrystals with monolayer control thickness is demonstrated for the first time. On the basis of the cryo-transmission electron microscopy, rigorous electron energy loss spectroscopy, and high-angle annular dark-field investigations ...

Tuning light absorption in core/shell silicon nanowire photovoltaic devices through morphological design.

Abstract: Subwavelength diameter semiconductor nanowires can support optical resonances with anomalously large absorption cross sections, and thus tailoring these resonances to specific frequencies could enable a number of nanophotonic applications. Here, we report the design and synthesis of core/shell p-type/intrinsic/n-type (p/i/n) Si nanowires (NWs) with different sizes and cross-sectional morphologies as well as measurement and simulation of photocurrent spectra from single-NW devices fabricated from these NW building blocks. Approximately hexagonal cross-section p/i/n coaxial NWs of various diameters (170–380 nm) were controllably synthesized by changing the Au catalyst diameter, which determines core diameter, as well as shell deposition time, which determines shell thickness. Measured polarization-resolved photocurrent spectra exhibit well-defined diameter-dependent peaks. The corresponding external quantum efficiency (EQE) spectra calculated from these data show good quantitative agreement with finite-diff...

Shell Thickness Dependent Photocatalytic Properties of ZnO/CdS Core–Shell Nanorods

Abstract: Core/shell nanorod arrays of ZnO/CdS have been synthesized with varying shell thickness and their shell thickness dependent photocatalytic properties have been investigated. Core/shell nanorod arrays of core diameter of 100 nm with variable shell thickness (10–30 nm) are synthesized by varying the concentration of the citric acid. XRD analysis reveals that tensile strain is obtained for ZnO nanorods and the compressive strain is obtained for core/shell nanorods. The UV–visible absorption spectra of the core/shell nanorod arrays show a red shift of the band edge of uncoated ZnO with shell growth. Steady-state photoluminescence (PL) spectra of the core/shell nanorod arrays show red shift of emission band with the increase in shell thickness. Decay kinetics indicate that the average lifetime (⟨τ⟩) of the core/shell nanorod arrays is larger than that of the uncoated ZnO nanorods due to charge separation. I–V studies show a 16-fold enhancement in current using the ZnO/CdS core/shell nanorod arrays having CdS s...

Refined beam elements with only displacement variables and plate/shell capabilities

Abstract: This paper proposes a refined beam formulation with displacement variables only. Lagrange-type polynomials, in fact, are used to interpolate the displacement field over the beam cross-section. Three- (L3), four- (L4), and nine-point (L9) polynomials are considered which lead to linear, quasi-linear (bilinear), and quadratic displacement field approximations over the beam cross-section. Finite elements are obtained by employing the principle of virtual displacements in conjunction with the Unified Formulation (UF). With UF application the finite element matrices and vectors are expressed in terms of fundamental nuclei whose forms do not depend on the assumptions made (L3, L4, or L9). Additional refined beam models are implemented by introducing further discretizations over the beam cross-section in terms of the implemented L3, L4, and L9 elements. A number of numerical problems have been solved and compared with results given by classical beam theories (Euler-Bernoulli and Timoshenko), refined beam theories based on the use of Taylor-type expansions in the neighborhood of the beam axis, and solid element models from commercial codes. Poisson locking correction is analyzed. Applications to compact, thin-walled open/closed sections are discussed. The investigation conducted shows that: (1) the proposed formulation is very suitable to increase accuracy when localized effects have to be detected; (2) it leads to shell-like results in case of thin-walled closed cross-section analysis as well as in open cross-section analysis; (3) it allows us to modify the boundary conditions over the cross-section easily by introducing localized constraints; (4) it allows us to introduce geometrical boundary conditions along the beam axis which lead to plate/shell-like cases.

Enhanced Energy Storage and Suppressed Dielectric Loss in Oxide Core–Shell–Polyolefin Nanocomposites by Moderating Internal Surface Area and Increasing Shell Thickness

Abstract: Dielectric loss in metal oxide core/Al(2)O(3) shell polypropylene nanocomposites scales with the particle surface area. By moderating the interfacial surface area between the phases and using increasing shell thicknesses, dielectric loss is significantly reduced, and thus the energy stored within, and recoverable from, capacitors fabricated from these materials is significantly increased, to as high as 2.05 J/cm(3).

Colloidal CdSe/CdS dot-in-plate nanocrystals with 2D-polarized emission.

Abstract: We report the synthesis and properties of a novel class of nanocrystals with mixed dimensionality: a dot-in-plate core/shell nanostructure. This system was synthesized by growing a flat, disk-shaped, CdS shell on spherical CdSe cores. The anisotropic pressure induced by the shell drastically splits the first exciton fine structure in two: the “heavy hole” and “light hole” states become separated by up to 65 meV. As a result, these nanocrystals exhibit an emission strongly polarized in two dimensions, in the plane perpendicular to the wurtzite crystal c axis. We use polarization measurements on single nanocrystals and ensemble anisotropy studies to confirm the nature and position of the excitonic energy levels. These nanocrystals orient spontaneously when evaporated on a substrate, enabling a precise control of the orientation of their emission dipole.

Direct Mapping of Nuclear Shell Effects in the Heaviest Elements

Abstract: Quantum-mechanical shell effects are expected to strongly enhance nuclear binding on an “island of stability” of superheavy elements The predicted center at proton number Z = 114, 120, or 126 and neutron number N = 184 has been substantiated by the recent synthesis of new elements up to Z = 118 However, the location of the center and the extension of the island of stability remain vague High-precision mass spectrometry allows the direct measurement of nuclear binding energies and thus the determination of the strength of shell effects Here, we present such measurements for nobelium and lawrencium isotopes, which also pin down the deformed shell gap at N = 152

Synthesis, Characterization, and 3D-FDTD Simulation of Ag@SiO2 Nanoparticles for Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy

Abstract: Au-seed Ag-growth nanoparticles of controllable diameter (50–100 nm), and having an ultrathin SiO2 shell of controllable thickness (2–3 nm), were prepared for shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). Their morphological, optical, and material properties were characterized; and their potential for use as a versatile Raman signal amplifier was investigated experimentally using pyridine as a probe molecule and theoretically by the three-dimensional finite-difference time-domain (3D-FDTD) method. We show that a SiO2 shell as thin as 2 nm can be synthesized pinhole-free on the Ag surface of a nanoparticle, which then becomes the core. The dielectric SiO2 shell serves to isolate the Raman-signal enhancing core and prevent it from interfering with the system under study. The SiO2 shell also hinders oxidation of the Ag surface and nanoparticle aggregation. It significantly improves the stability and reproducibility of surface-enhanced Raman scattering (SERS) signal intensity, which is es...

3D simulations of globules and pillars formation around HII regions: turbulence and shock curvature

Abstract: We investigate the interplay between the ionization radiation from massive stars and the turbulence inside the surrounding molecular gas thanks to 3D numerical simulations. We used the 3D hydrodynamical code HERACLES to model an initial turbulent medium that is ionized and heated by an ionizing source. Three different simulations are performed with different mean Mach numbers (1, 2 and 4). A non-equilibrium model for the ionization and the associated thermal processes was used. This revealed to be crucial when turbulent ram pressure is of the same order as the ionized-gas pressure. The density structures initiated by the turbulence cause local curvatures of the dense shell formed by the ionization compression. When the curvature of the shell is sufficient, the shell collapse on itself to form a pillar while a smaller curvature leads to the formation of dense clumps that are accelerated with the shell and therefore remain in the shell during the simulation. When the turbulent ram pressure of the cold gas is sufficient to balance the ionized-gas pressure, some dense-gas bubbles have enough kinetic energy to penetrate inside the ionized medium, forming cometary globules. This suggests a direct relation in the observations between the presence of globules and the relative importance of the turbulence compared to the ionized-gas pressure. The probability density functions present a double peak structure when the turbulence is low relative to the ionized-gas pressure. This could be used in observations as an indication of the turbulence inside molecular clouds.

Piezoelectric shell structures as wearable energy harvesters for effective power generation at low-frequency movement

Abstract: This paper discusses a flexible energy harvester that consists of a polyvinylidene fluoride film attached to a curved substrate in a shell shape for harvesting energy from human motion. The proposed harvester effectively converts mechanical energy into electrical energy during the fast state transition of the shell structure. The results of an experiment demonstrated that shell structures with various curvatures produce high output potential and consequently offer high output power in comparison to a simple flat structure. The single shell structure generates an output power of 0.87 mW at a folding angle of 80° and a folding and unfolding frequency of 3.3 Hz. In addition, fabric with embedded piezoelectric shell structures was designed as an energy harvester in a wearable platform. The fabric, worn on the elbow joints and fingers, generates a high output power of 0.21 mW in spite of slow and irregular motion.

Three-dimensional simulations of globule and pillar formation around HII regions: turbulence and shock curvature

Abstract: Aims. We investigate the interplay between the ionization radiation from massive stars and the turbulence inside the surrounding molecular gas using three-dimensional (3D) numerical simulations.Methods. We used the 3D hydrodynamical code HERACLES to model an initial turbulent medium that is ionized and heated by an ionizing source. Three different simulations were performed with different mean Mach numbers (1, 2, and 4). A non-equilibrium model for the ionization and the associated thermal processes was chosen. This turned out to be crucial when turbulent ram pressure is on the same order as the ionized-gas pressure.Results. The density structures initiated by the turbulence cause local curvatures of the dense shell formed by the ionization compression. When the curvature of the shell is sufficient, the shell collapses in on itself to form a pillar, while a smaller curvature leads to the formation of dense clumps that are accelerated with the shell and therefore remain in the shell during the simulation. When the turbulent ram pressure of the cold gas is sufficient to balance the ionized-gas pressure, some dense-gas bubbles have enough kinetic energy to penetrate the ionized medium, forming cometary globules. This suggests that there is a direct relation in the observations between the presence of globules and the relative significance of the turbulence compared to the ionized-gas pressure. The probability density functions present a double peak structure when the turbulence is low relative to the ionized-gas pressure. This could be interpreted in observations as an indication of the turbulence inside molecular clouds.

Surface free energies for nematic shells.

Abstract: We propose a continuum model to describe the molecular alignment in thin nematic shells. By contrast with previous accounts, the two-dimensional free energy, aimed at describing the physics of thin films of nematics deposited on curved substrates, is not postulated, but it is deduced from the conventional three-dimensional theories of nematic liquid crystals. Both the director and the order-tensor theories are taken into account. The so-obtained surface energies exhibit extra terms compared to earlier models. These terms reflect the coupling of the shell extrinsic curvature with the nematic order parameters. As expected, the shape of the shell plays a key role in the equilibrium configurations of nematics coating it.

The Study of Surface Plasmon in Au/Ag Core/Shell Compound Nanoparticles

Abstract: Au/Ag core/shell nanoparticles are fabricated by laser-ablating Ag plates in Au colloid solution. The absorption band is found to blue shift with increasing ablation time. Mie theory calculations show that the shift is caused by the increase of the Ag shell thickness. The average Ag shell thickness can be determined from the measured absorption peak. Using the plasmon hybridization approach, we show that the absorption band around 510 nm originates from an anti-bonding mode ω −+ caused by the interaction between a bonding Ag shell mode ω −− and Au sphere mode ω S-Au. The blue shift of the ω −+ mode with the increase of Ag shell thickness is also well predicted by the hybridization theory.

Synthetic Architecture of Multiple Core–Shell and Yolk–Shell Structures of (Cu2O@)nCu2O (n = 1–4) with Centricity and Eccentricity

Abstract: In this work, we describe a seed-mediated approach with successive Ostwald ripening to synthesize various multiple-shell core–shell and yolk–shell structures of (Cu2O@)nCu2O (n = 1–4). In particular, the structure formed in a previous step can serve as a newer-generation seed for a subsequent shell growth; a total of 20 representative structures have been thus synthesized in a step-by-step manner. With an increase in shell number n, in principle, 2n shelled products can be fabricated, taking into account the centricity and eccentricity in their geometric symmetry. Synthetic chemistry and functions of chemical additives have also been investigated to explain the formation mechanism and to ensure the uniformity of the product. It has been found that symmetric or asymmetric Ostwald ripening during the hollowing process can be manipulated by controlling stirring conditions. Furthermore, optical properties of the resultant samples can be closely related to structural aspects of the products, such as the overal...

Facile one-step synthesis of plasmonic/magnetic core/shell nanostructures and their multifunctionality

Abstract: A very simple protocol, which involves the chemical reduction of AgNO3 and Fe(NO3)3 with ethylene glycol as reducing agent, has been developed for synthesizing Ag@Fe3O4 core/shell nanostructures in which the silver nanoparticle core was covered by a thicker layer of the Fe3O4 nanoparticle shell. The obtained Ag@Fe3O4 core/shell nanostructures simultaneously possess both strong magnetic responsiveness and tunable plasmonic properties. The plasmonic properties of the composite nanospheres are profoundly influenced by the high dielectric constant of the outer Fe3O4 shell layer and could be conveniently modulated over a broad spectral range spanning from the ultraviolet to near-infrared (NIR) regions (789 nm) by simply altering the thickness of the Fe3O4 shell. The localized surface plasmon resonances of the core/shell nanocomposites red-shifted with increasing thickness of the Fe3O4 shell. The morphology transformation of the Ag/Fe3O4 nanocomposites from core/shell structures with a continuous dense coating to flower-like nanostructures also allows the tuning of their plasmonic properties to be blue-shifted (to 510 nm). Catalytic degradation of rhodamine 6G (R6G) experiments show that the Ag/Fe3O4 composite nanostructures exhibit high catalytic activity by sodium borohydride. Due to the efficient optical response through localized surface plasmon resonances, the catalytic performance from the silver core and external magnetic manipulation from the Fe3O4 shell, such multifunctional nanoparticles will provide an opportunity for simultaneous optical detection and catalytic reduction with the additional benefit of relatively facile recovery and regeneration.

Strongly exchange coupled inverse ferrimagnetic soft/hard, mnxfe3 xo4/fexmn3 xo4, core/shell heterostructured nanoparticles

Abstract: Inverted soft/hard, in contrast to conventional hard/soft, bi-magnetic core/shell nanoparticles of MnxFe3� xO4/FexMn3� xO4 with two different core sizes (7.5 and 11.5 nm) and fixed shell thickness (� 0.6 nm) have been synthesized. The structural characterization suggests that the particles have an interface with a graded composition. The magnetic characterization confirms the inverted soft/hard structure and evidences a strong exchange coupling between the core and the shell. Moreover, larger soft core sizes exhibit smaller coercivities and loop shifts, but larger blocking temperatures, as expected from spring-magnet or graded anisotropy structures. The results indicate that, similar to thin film systems, the magnetic properties of soft/hard core/shell nanoparticles can be fine tuned to match specific applications.

3-D micromachined hemispherical shell resonators with integrated capacitive transducers

Abstract: We present a self-aligned fabrication method developed for three-dimensional (3-D) microscale hemispherical shell resonators with integrated capacitive transducers and a center post for electrical access to the shell. The self-aligned process preserves the axisymmetry for robust, balanced resonators that can potentially reach very high-Q due to suppressed anchor loss. High-Q operation of a thin polycrystalline silicon shell resonator is verified by exciting devices capacitively into a breathing resonance mode, with measured Q of 8,000 at 412 kHz in vacuum. This process can be further optimized to batch-fabricate micro-hemispherical resonator gyroscopes for portable inertial navigation.

Cell phone protector case having the combination of an interior soft silicone shell and a hard exterior shell with aligned retaining members

Abstract: A cell phone protector case comprising a combination of an interior soft silicone shell and a first and second hard shell press fit retained on opposite surfaces and sides of the interior soft silicone shell with the hard shell having matching retaining closing member mating flanges.

Thermoelastic free vibration and dynamic behaviour of an FGM doubly curved panel via the analytical hybrid Laplace–Fourier transformation

Abstract: The paper presents an analysis of functionally graded material doubly curved panels with rectangular planform under the action of thermal and mechanical loads. Based on the first-order shear deformation theory of modified Sanders assumptions, five coupled partially differential equations (PDEs) are established as equations of motion. Each thermo-mechanical property of the shell follows the power law distribution across the thickness, except Poisson’s ratio, which is kept constant through the panel. Assuming that four edges of the shell-panel are simply supported, a Navier-based solution is adopted to reduce the PDEs into time-dependent ODEs. Applying the Laplace transformation, the equations of motion are transformed into the Laplace domain. With the aid of analytical Laplace inverse method, solutions of stresses, strains, and displacements are obtained in time domain and expressed in explicit phrases. Dynamic, free vibration, and thermo-mechanical bending analysis of the panel is carried out for various geometries. Obtained results are validated with the well-known available data reported in the literature.