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Confinement Effects (confinement + effects)
Kinds of Confinement Effects Selected AbstractsA Spherical 24,Butyrate Aggregate with a Hydrophobic Cavity in a Capsule with Flexible Pores: Confinement Effects and Uptake,Release Equilibria at Elevated Temperatures,ANGEWANDTE CHEMIE, Issue 43 2009Christian Schäffer Moleküle unter Zwang: Der Einschluss einer Anordnung aus 24,organischen Butyrat-Einheiten (siehe Bild), die mit 72,H-Atomen einen bemerkenswert großen hydrophoben Hohlraum umschließen, in einer porösen Kapsel führt zu interessanten Wechselwirkungen zwischen den eingesperrten Butyraten. Bei Temperaturerhöhung wird die ,Quarantäne" aufgehoben und die Gastspezies können leichter kommen und gehen. [source] Confinement effects on the morphology of photopatterned porous polymer monoliths for capillary and microchip electrophoresis of proteinsELECTROPHORESIS, Issue 14 2008Mei He Abstract We find that the morphology of porous polymer monoliths photopatterned within capillaries and microchannels is substantially influenced by the dimensions of confinement. Porous polymer monoliths were prepared by UV-initiated free-radical polymerization using either the hydrophilic or hydrophobic monomers 2-hydroxyethyl methacrylate or butyl methacrylate, cross-linker ethylene dimethacrylate and different porogenic solvents to produce bulk pore diameters between 3.2 and 0.4,µm. The extent of deformation from the bulk porous structure under confinement strongly depends on the ratio of characteristic length of the confined space to the monolith pore size. The effects are similar in cylindrical capillaries and D-shaped microfluidic channels. Bulk-like porosity is observed for a confinement dimension to pore size ratio >10, and significant deviation is observed for a ratio <5. At the extreme limit of deformation a smooth polymer layer ,300 nm thick is formed on the surface of the capillary or microchannel. Surface tension or wetting also plays a role, with greater wetting enhancing deformation of the bulk structure. The films created by extreme deformation provide a rapid and effective strategy to create robust wall coatings, with the ability to photograft various surface chemistries onto the coating. This approach is demonstrated through cationic films used for electroosmotic flow control and neutral hydrophilic coatings for electrophoresis of proteins. [source] Confinement effects on optical phonons in spherical, rod-, and tetrapod-shaped nanocrystals detected by Raman spectroscopyPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 2 2007Concetta Nobile Abstract Spherical, rod- and tetrapod shaped CdSe nanocrystals are investigated by Raman spectroscopy and the longitudinal-optical and surface optical phonons are observed. We find that the position of the longitudinal-optical phonon slightly red-shifts with decreasing diameter, whereas the position of the surface optical phonon depends significantly on diameter and length of the rods or the tetrapod arms. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Cover Picture: Electrophoresis 14/2008ELECTROPHORESIS, Issue 14 2008Article first published online: 23 JUL 200 Issue 14 is a regular issue including an Emphasis Section offering a series of 9 papers on ,Microfluidics and Miniaturization". These 9 research papers report on various topics including studying single DNA molecules, selective release of intracellular molecules on the single cell level, isoelectric focusing of proteins in an ordered micropillar array, sample stream focusing in a microchip, integrated microfluidic system for sensing infectious viral disease, EOF in annulus and rectangular channels, confinement effects on monolith morphology, accumulation and filtering of nanoparticles in microchannels, and carbon nanotubes disposable detectors. [source] Biomineralization: Amorphous Calcium Carbonate is Stabilized in Confinement (Adv. Funct.ADVANCED FUNCTIONAL MATERIALS, Issue 13 2010Mater. F. C. Meldrum, H. K. Christenson, et al. describe on page 2108 a device based on two crossed cylinders used to study confinement effects on precipitation of calcium carbonate. Amorphous calcium carbonate (ACC) remains stable for days even when the confining surfaces are as far as a micrometer apart. Such stabilization of ACC in vitro may have important implications for the understanding of biomineralization. [source] Effects of Phonon Confinement on Anomalous Thermalization, Energy Transfer, and Upconversion in Ln3+ -Doped Gd2O3 NanotubesADVANCED FUNCTIONAL MATERIALS, Issue 4 2010Andreia G. Macedo Abstract There is a growing interest in understanding how size-dependent quantum confinement affects the photoluminescence efficiency, excited-state dynamics, energy-transfer and thermalization phenomena in nanophosphors. For lanthanide (Ln3+)-doped nanocrystals, despite the localized 4f states, confinement effects are induced mostly via electron,phonon interactions. In particular, the anomalous thermalization reported so far for a handful of Ln3+ -doped nanocrystals has been rationalized by the absence of low-frequency phonon modes. This nanoconfinement may further impact on the Ln3+ luminescence dynamics, such as phonon-assisted energy transfer or upconversion processes. Here, intriguing and unprecedented anomalous thermalization in Gd2O3:Eu3+ and Gd2O3:Yb3+,Er3+ nanotubes, exhibiting up to one order of magnitude larger than previously reported for similar materials, is reported. This anomalous thermalization induces unexpected energy transfer from Eu3+C2 to S6 crystallographic sites, at 11,K, and 2H11/2,,,4I15/2 Er3+ upconversion emission; it is interpreted on the basis of the discretization of the phonon density of states, easily tuned by varying the annealing temperature (923,1123,K) in the synthesis procedure, and/or the Ln3+ concentration (0.16,6.60%). [source] Synthesis and Lasing Properties of Highly Ordered CdS Nanowire Arrays,ADVANCED FUNCTIONAL MATERIALS, Issue 9 2007L. Cao Abstract Highly ordered large-area arrays of wurtzite CdS nanowires are synthesized on Cd-foil substrates via a simple liquid reaction route using thiosemicarbazide and Cd foil as the starting materials. The CdS nanowires are single crystals growing along the [001],direction and are perpendicular to the surface of the substrate. The characteristic Raman peaks of CdS are red-shifted and show asymmetric broadening, which is ascribed to phonon confinement effects arising from the nanoscale dimensions of the nanowires. Significantly, the uniform CdS nanowire arrays can act as laser cavities in the visible-light range, leading to bandgap lasing at ca.,515,nm with obvious modes. The high density of nuclei and the preferential growth direction induce the formation of aligned CdS nanowires on the metal substrate. [source] Thermodynamics of Nanosystems with a Special View to Charge CarriersADVANCED MATERIALS, Issue 25-26 2009Joachim Maier Abstract Basic ingredients of interfacial thermodynamics are recapitulated with a special eye on the nanometer-size regime. Questions are then briefly tackled that arise if, in heterogeneous systems, the constituent phases shrink to atomistic dimensions. Particularly helpful in this context are thermodynamic approaches, in which the introduction of interfacial tension is avoided. While the first part addresses ground structure quantities, the second part deals with questions of size and confinement effects on entropy and energy of ionic and electronic defects. These defects represent the respective excitations within this ground structure. The article emphasizes the similarities between ions and electrons manifested in the statistics rather than elaborating on the discrepancies that are primarily reflected by different densities of states and mobilities. It is, therefore, not the intention of the article to address aspects of nanoelectronics that rely on quantum transport for which many reviews are available. Nonetheless all these discussed aspects are directly relevant for both nanoionics and nanoelectronics. [source] Hydrogen surface passivation of Si and Ge nanowires: A semiempirical approachINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 13 2010A. Miranda Abstract A semiempirical nearest-neighbor tight-binding approach, that reproduces the indirect band gaps of elemental semiconductors, has been applied to study the electronic and optical properties of Si and Ge nanowires (NWs). The calculations show that Si-NWs keep the indirect bandgap whereas Ge-NWs changes into the direct bandgap when the wire cross section becomes smaller. Also, the band gap enhancement of Si-NWs showing to quantum confinement effects is generally larger than that of similar-sized Ge-NWs, confirming the larger quantum confinement effects in Si than in Ge when they are confined in two dimensions. Finally, the dependence of the imaginary part of the dielectric function on the quantum confinement within two different schemes: intra-atomic and interatomic optical matrix elements are applied. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem 110:2448,2454, 2010 [source] Raman spectroscopy of II,VI semiconductor nanostructures: CdS quantum dotsJOURNAL OF RAMAN SPECTROSCOPY, Issue 2 2003B. Schreder Abstract Information about confinement effects and dot,matrix interactions of CdS nanoparticles was obtained from resonance Raman spectroscopy. The quantum dots had diameters of 3 and 5 nm and were prepared with and without organic spacer groups. It was found that the spacer improves the quality of the nanocrystallites. No phonon confinement shift could be observed even for the small quantum dots. The linewidths of the overtone series point to a mechanism of vibrational relaxation which is dominated by the decay of the LO phonons into acoustic phonons. Copyright © 2003 John Wiley & Sons, Ltd. [source] Microstructure Development in Unsupported Thin FilmsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 4 2002Brian P. Gorman To better understand the role of the substrate in the microstructural evolution of thin films, unsupported nanocrystalline yttrium-stabilized zirconia (ZrO2:16%Y or YSZ) films were examined as a function of temperature and annealing time. Grain growth, texturing, and pinhole formation were measured using transmission electron microscopy (TEM) and electron diffraction. Films were produced and subsequently annealed on metallic grids using a previously developed technique that results in near full density films at low annealing temperatures. Microstructural evolution in these films was unique compared with constrained films. Grains were found to spheroidize much more readily, ultimately resulting in the formation of porosity and pinholes. Grain growth was found to stagnate at a size particular to each annealing temperature, presumably due to the effects of Zener pinning. It is proposed that the lack of substrate strain and confinement effects allows for the dominance of surface energetics with respect to microstructural evolution. [source] Photoluminescence properties of GaAs nanowire ensembles with zincblende and wurtzite crystal structurePHYSICA STATUS SOLIDI - RAPID RESEARCH LETTERS, Issue 7 2010B. V. Novikov Abstract Self-standing III,V nanowires (NWs) are promising building blocks for future optoelectronic devices such as LEDs, lasers, photodetectors and solar cells. In this work, we present the results of low temperature photoluminescence (PL) characterization of GaAs NWs grown by Au-assisted molecular beam epitaxy (MBE), coupled with the transmission electron microscopy (TEM) structural analysis. PL spectra contain exci- ton peaks from zincblende (ZB) and wurtzite (WZ) crystal structures of GaAs. The peaks are influenced by the quantum confinement effects. PL bands corresponding to the exciton emission from ZB and WZ crystal phases are identified, relating to the PL peaks at 1.519 eV and 1.478 eV, respectively. The obtained red shift of 41 meV for WZ GaAs should persist in thin NWs as well as in bulk materials. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Dependence of the band-gap pressure coefficients of self-assembled InAs/GaAs quantum dots on the quantum dot sizePHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 1 2007C. Kristukat Abstract We report on low-temperature photoluminescence experiments on self-assembled InAs/GaAs quantum dots under high hydrostatic pressure up to 8 GPa using a diamond anvil cell. The sample exhibits a multimodal size distribution of the quantum dots, which gives rise to a characteristic emission profile displaying up to nine clearly separable peaks attributed to the ground-state recombination from each quantum dot subensemble with different size. Structural analysis revealed that their size differs in entire monolayer steps. The measured pressure coefficients for each subensemble show a linear dependence on their zero-pressure emission energy ranging from 65 meV/GPa for the largest dots to 112 meV/GPa for the smallest ones. Pressure dependent strain simulations based on an atomistic valence-force field yield that the pressure coefficient of the InAs band-gap is strongly reduced when InAs is embedded in a GaAs matrix. Taking into account confinement effects within the envelope function approximation, the calculated pressure coefficients are in good agreement with the experimental findings. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Determination of critical points on silicon nanofilms: surface and quantum confinement effectsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 12 2008Emmanouil Lioudakis Abstract In this work, we present a comprehensive study of the optical properties of nanocrystalline silicon films with thickness varied from 5 to 30 nm. Spectroscopic ellipsometry is employed to determine the dielectric functions of these films using a structural two-layer model based on the rigorous Airy formula. Our investigation gives an important insight of the origin of critical points for direct and indirect gaps of nanocrystalline silicon films as well as the evolution of them with decreasing the film thickness. The influence of the quantum confinement effect due to the nanoscale grain size and the surface vibrations at the interface on the optical properties are examined in detail. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Structure of small II-VI semiconductor nanoparticles: A new approach based on powder diffractionPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 9 2007Franziska Niederdraenk Abstract In many cases of nanostructures a better understanding of the optical, electronic, and catalytic properties is hindered by a lack of detailed structural knowledge of sufficient quality. This is in particular true for very small nanoparticles (<5 nm) where quantum confinement effects play a dominant role. We introduce a novel method for the detailed structural analysis of such small nanoparticles, which includes not only the modeling of basic parameters like size, shape, and structure, but also allows to determine impurities and defects (like, e.g., stacking faults), stress and relaxation effects, etc. Distributions of basic parameters as the size distribution are also implemented by enabling ensemble averaging. Different examples obtained from CdS, ZnS and ZnO particles are presented and demonstrate the superiority of the new approach compared to simpler methods. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] InAsSb/InGaAs quantum nanostructures on InP (100) substrate: observation of 2.35 µm photoluminescencePHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 3 2006F. Doré Abstract A theoretical and experimental study of the electronic properties of InAsSb quantum dots (QDs) grown on InP substrate is presented. Unstrained bulk InAsSb presents a direct gap between 0.1 eV to 0.35 eV, suitable for mid infrared emitters (3,5 µm). However, strain and quantum confinement effects may limit the extension of the emission spectrum of these nanostructures towards longer wavelengths. Various combinations of barrier materials are considered in the simulations. First photoluminescence spectroscopy experiments on molecular beam epitaxy (MBE) grown samples show promising results. Triple stacks of InAs QDs embedded in a GaInAsP alloy lattice matched to InP and grown by the Stranski-Krastanov mode exhibit room temperature (RT) luminescence at about 2 µm. Emission wavelengths as long as 2.35 µm have been observed at RT in a InAsSb/InGaAs/InP structure. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] The Effects of Containment on Detonation VelocityPROPELLANTS, EXPLOSIVES, PYROTECHNICS, Issue 1 2004Clark Souers Abstract Reactive flow cylinder code runs on six explosives were made with rate constants varying from 0.03 to 70,,s,1. Six unconfined/steel sets of original ANFO and dynamite data are presented. A means of comparing confinement effects both at constant radius and at constant detonation velocity is presented. Calculations show two qualitatively different modes of behavior. For Us/Co,1.2, where Us is the detonation velocity and Co the zero-pressure sound speed in steel, we find a sharp shock wave in the metal. The shock passes through the steel and the outer wall has a velocity jump-off. For Us/Co,1.04, we find a pressure gradient that moves at the detonation velocity. A precursor pulse drives in the explosive ahead of the detonation front. The outer wall begins to move outward at the same time the shock arrives in the explosive, and the outer wall slowly and continuously increases in velocity. The Us/Co,1.2 cylinders saturate in detonation velocity for thick walls but the Us/Co<<1.04 case does not. The unconfined cylinder shows an edge lag in the front that approximately equals the reaction zone length, but the highly confined detonation front is straight and contains no reaction zone information. The wall thickness divided by the reaction zone length yields a dimensionless wall thickness, which allows comparison of explosives with different detonation rates. Even so, a rate effect is found in the detonation velocities, which amounts to the inverse 0.15,0.5 power. [source] | ||||||||||||||||