bluuebunny/crossref_metadata_2025_split huggingface dataset:
Roles of Surface Termination in Photoluminescence Mechanisms of Porous Si
Authors: Suda Y. , Obata K. , Kumagai A. , Koshida N. 
Published: April 2011

AbstractThe relationship between the oxidation states and PL properties and the effects of H/O termination exchange on the PL properties of PS have been investigated using synchrotron radiation photoemission spectroscopy (SR-PES), Auger electron spectroscopy (AES), Fourier transform infrared (FTIR), and photoluminescence (PL) techniques. The energy band gap, the peak energy and FWHM of the PL spectrum are almost unchanged by the oxidation process and by the H/O termination exchange. After the oxidation, the PL peak intensity decreased, suggesting the creation of nonradiative centers. In the H/O termination exchange experiment, the PL peak intensity decreased by more than 65% upon annealing. However, it recovered the initial PL intensity by oxygen exposure. These results suggest that the surface termination itself functions to eliminate the nonradiative centers without depending on the termination species of hydrogen or oxygen, and that the skeletal structure of PS crystallites is important in the PL mechanisms.
Fabrication and Characterization of Light Emitting Porous Silicon and Polymer Nanocomposites
Authors: Duttagupta S. P. , Fauchet P. M. , Chen X. L. , Jenekhe S. A. 
Published: April 2011

AbstractWe report the fabrication of nanocomposites by the infiltration of polymers into porous silicon. Polymers such as polyamide, polystyrene, PMMA, and PVC were chosen because they are commonly available and have been extensively studied. The pore-filling was accomplished by either diffusion of the polymer molecules into porous silicon or in-situ polymerization of the monomer. The Vickers hardness arid the thermal conductivity of the samples were measured. There was a difference in the nanocomposite characteristics depending on whether the samples were as-anodized or had been annealed in oxygen. By infiltrating polyamide into an as-anodized sample, a 42% increase in hardness and a 24% increase in thermal conductivity were observed at room temperature, without any degradation of luminescence.
Localisation of Carriers in Porous Silicon
Authors: Mihalcescu I. , Vial J. C. , Romestain R. 
Published: April 2011

AbstractWe analyze the intensity and decay time evolution of the porous silicon luminescence upon anodic oxidation, aging, chemiral thinning and temperature variation. Strong analogies are pointed out for the photoluminescence intensity as well as for the photoluminescence decay shape evolution. They are interpreted by the variation of the extension of the carrier wavefunction induced by the modification of potential barrier efficiencies. No additional assumption such as hopping of carriers was necessary to explain the decay shapes well fitted by stretched exponential. On the contrary our observations and our simple model are in favor of a strong localization of carriers. Some experimental results are revisited within the frame of this model.
Room Temperature Band-Edge Luminescence from Silicon Grains Prepared by the Recrystallization of Mesoporous Silicon
Authors: Moore Karen L. , Tsybeskov Leonid , Fauchet Philippe M. , Hall Dennis G. 
Published: April 2011

AbstractRoom-temperature photoluminescence (PL) peaking at 1.1 eV has been found in electrochemically etched mesoporous silicon annealed at 950°C. Low-temperature PL spectra clearly show a fine structure related to phonon-assisted transitions in pure crystalline silicon (c-Si) and the absence of defect-related (e.g.P-line) and impurity-related (e.g.oxygen, boron) transitions. The maximum PL external quantum efficiency (EQE) is found to be better than 0.1% with a weak temperature dependence in the region from 12K to 400K. The PL intensity is a linear function of excitation intensity up to 100 W/cm2. The PL can be suppressed by an external electric field ≥ 105 V/cm. Room temperature electroluminescence (EL) related to the c-Si band-edge is also demonstrated under an applied bias ≤ 1.2 V and with a current density ≈ 20 mA/cm2. A model is proposed in which the radiative recombination originates from recrystallized Si grains within a non-stoichiometric Si-rich silicon oxide (SRSO) matrix.
Light Emission from Intrinsic and Doped Silicon-Rich Silicon Oxide: from the Visible to 1.6 ΜM
Authors: Tsybeskov L. , Moore K. L. , Fauchet P. M. , Hall D. G. 
Published: April 2011

AbstractSilicon-rich silicon oxide (SRSO) films were prepared by thermal oxidation (700°C-950°C) of electrochemically etched crystalline silicon (c-Si). The annealing-oxidation conditions are responsible for the chemical and structural modification of SRSO as well as for the intrinsic light-emission in the visible and near infra-red spectral regions (2.0–1.8 eV, 1.6 eV and 1.1 eV). The extrinsic photoluminescence (PL) is produced by doping (via electroplating or ion implantation) with rare-earth (R-E) ions (Nd at 1.06 μm, Er at 1.5 μm) and chalcogens (S at ∼1.6 μm). The impurities can be localized within the Si grains (S), in the SiO matrix (Nd, Er) or at the Si-SiO interface (Er). The Er-related PL in SRSO was studied in detail: the maximum PL external quantum efficiency (EQE) of 0.01–0.1% was found in samples annealed at 900°C in diluted oxygen (∼ 10% in N2). The integrated PL temperature dependence is weak from 12K to 300K. Light emitting diodes (LEDs) with an active layer made of an intrinsic and doped SRSO are manufactured and studied: room temperature electroluminescence (EL) from the visible to 1.6 μmhas been demonstrated.
Improved Interference Filter Structures Made of Porous Silicon
Authors: Thönissen M. , Berger M. G. , Krüger M. , Theiβ W. , Hilbrich S. , Arens-Fischer R. , Billar S. , Lüth H. 
Published: April 2011

AbstractRecently passive optical devices like filter structures or waveguides based on porous silicon have attracted high interest due to their easy and cheap fabrication. We have formed interference filters using porous silicon by changing the current density during formation. For the specific design of these filter structures a calibration of the etch rate and the refractive indices is required. Therefore we have determined the effective dielectric function for different substrate doping levels and anodization current densities by fitting reflectance spectra. Based on these results different kinds of reflectance filters consisting of discrete layers (Bragg reflectors, Fabry-Perot filters) as well as filters with a continuous change of the refractive indices with depth (rugate-filters) can be realised. Furthermore we present applications of these filter structures such as anti-reflectance coatings and high quality mirrors.
A Microscopic Model for the Dielectric Function of Porous Silicon
Authors: Cruz M. , Beltran M. R. , Wang C. , Tagüeña-Martinez J. 
Published: April 2011

AbstractMicro and nano-structures have opened a new area in materials research since they present interesting phenomena such as efficient luminescence and localization of carriers. An important example of these new materials is porous silicon (PS). It is considered that the quantum confinement is an essential cause of the opto-electronic properties of PS [1], thus microscopic analysis should be performed. We have developed a supercell model to study PS with a tight-binding Hamiltonian, where an sp3s* basis set is used. In an otherwise perfect silicon structure empty columns of atoms are produced and passivated with hydrogen atoms [2]. In this work we calculate the dielectric function and compare it against experimental data for bulk c-Si, ultrathin c-Si films and PS. We discuss the importance of considering the relaxation of the electron wavevector (k) conservation in order to include disorder effects in PS.
Post-Deposition Annealing and Hydrogenation of Hot-Wire Amorphous and Microcrystalline Silicon Films
Authors: Conde J. P. , Brogueira P. , Chu V. 
Published: April 2011

AbstractAmorphous and microcrystalline silicon films deposited by hot-wire chemical vapor deposition were submitted to thermal annealing and to RF and electron-cyclotron resonance (ECR) hydrogen plasmas. Although the transport properties of the films did not change after these post-deposition treatments, the power density of a Ar+ laser required to crystallize the amorphous silicon films was significantly lowered by the exposure of the films to a hydrogen plasma. This decrease was dependent on the type of hydrogen plasma used, being the strongest for an ECR plasma with the substrate held at a negative bias, followed by an ECR hydrogen plasma with the substrate electrode grounded, and finally by an RF hydrogen plasma.
Fracture of Nanophase Ceramics: A Molecular-Dynamics Study
Authors: Nakano Aiichiro , Kalia Rajiv K. , Omeltchenko Andrey , Tsuruta Kenji , Vashishta Priya 
Published: April 2011

ABSTRACTNew multiscale algorithms and a load-balancing scheme are combined for molecular-dynamics simulations of nanocluster-assembled ceramics on parallel computers. Million-atom simulations of the dynamic fracture in nanophase silicon nitride reveal anisotropie self-affine structures and crossover phenomena associated with fracture surfaces.
Structure, Mechanical Properties, and Dynamic Fracture in Nanophase Silicon Nitride via Parallel Molecular Dynamics
Authors: Tsuruta Kenji , Omeltchenko Andrey , Nakano Aiichiro , Kalia Rajiv K. , Vashishta Priya 
Published: April 2011

ABSTRACTMillion-atom molecular-dynamics (MD) simulations are performed to study the structure, mechanical properties, and dynamic fracture in nanophase Si3N4. We find that intercluster regions are highly disordered: 50% of Si atoms in intercluster regions are three-fold coordinated. Elastic moduli of nanophase Si3N4 as a function of grain size and porosity are well described by a multiphase model for heterogeneous materials. The study of fracture in the nanophase Si3N4 reveals that the system can sustain an order-of-magnitude larger external load than crystalline Si3N4. This is due to branching and pinning of the crack front by nanoscale microstructures.
Fluorophlogopite and Taeniolite: Synthesis and Nanocomposite Formation
Authors: Moore Gregory J. , Zavalij Peter Y. , Whittingham M. Stanley 
Published: April 2011

ABSTRACTSodium fluorophlogopite and lithium taeniolite have been synthesized by new routes for application in lithium batteries. The fluorophlogopite synthesized by a high temperature solid state reaction, was found to be non-water-swellable and unreactive towards several mono- and divalent ions. However it was found to readily undergo ion-exchange with both copper and iron ions, with concomitant swelling to a bilayer water state. This swelled material reacted readily with long chain amines and other molecules and ions behaving like a regular swellable silicate. A taeniolite precursor was synthesized by mild hydrothermal reactions, and annealed into a well crystalline layer solid, that reacted readily with organics to form ordered composites that have potential use as battery electrolytes and cathodes.
Synthesis and Lcst Behavior of Thermally Responsive Poly(N-Isopropylacrylamide)/Layered Silicate Nanocomposites
Authors: Messersmith Phillip B. , Znidarsich F. 
Published: April 2011

ABSTRACTStimuli responsive polymeric hydrogel composites were synthesized by room temperature copolymerization of N-isopropyl acrylamide and methylene bisacrylamide (crosslinking monomer) in an aqueous suspension of Na-montmorillonite. Hydrogels containing 3.5 weight % of montmorillonite exhibited a lower critical solution temperature (LCST) similar to unmodified PNIPAM hydrogel (approximately 32°C), and underwent a reversible 60–70% volume shrinkage when heated from ambient temperature to above the LCST. However, hydrogels containing 10 weight% montmorillonite did not exhibit a measurable LCST, and underwent considerably less shrinkage when heated. A solvent exchange reaction was used to replace the water with an acrylic monomer, which was polymerized in-situ to create a delaminated montmorillonite/polymer nanocomposite.
Production of Nanostructured Iron Oxide Particles via Aerosol Decomposition
Authors: Joutsensaari J. , Kauppinen E. I. 
Published: April 2011

ABSTRACTNanostructured iron oxide particles with average size below 100 nm were produced by aerosol decomposition method starting from an aqueous iron nitrate solution. Air, nitrogen, or mixture of hydrogen (7 %) and nitrogen were used as the carrier gas. Gas-phase particle number size distributions were determined with a differential mobility analyzer. Particle morphology and crystallinity were studied with scanning (SEM) and transmission (TEM) electron microscopes. Crystalline phase composition of the particles was studied with X-ray diffraction (XRD). The average gas-phase diameter of particles produced in air or N2 reduced from 80 to 47 nm when temperature was increased from 500 to 1100 °C. In H2 rich environment, the reduction of average size was larger, from 80 nm at 500 °C to 45 nm already at 900 °C. SEM results showed that very small crystallites (5–10 nm) were formed on the surface of the particles produced in N2 at 500 °C. When the processing temperature was increased to 700, 900 and 1100 °C, the crystallites on the particle surfaces were grown to 15–30, 30–60 and 60–180 nm, respectively. TEM results show that very small particles (<50 nm) were single crystals and larger particles were polycrystalline with crystallite size of about 50 nm at 700 °C in H2/N2. Magnetite particles were produced from aqueous iron nitrate solution at 500 °C in H2/N2 and at 900 °C in N2 according to XRD results.
Microstructure Analysis of Melt-Spun Al3Ti Intermetallics by XRD and EXAFS
Authors: Chen Jinmin , Frazier W. E. , Barrera E. V. 
Published: April 2011

ABSTRACTIn an effort to expand the composition range over which Al3Ti is stable, various amounts of niobium were substituted for titanium and processed by melt-spinning. Several samples were annealed both at 600°C and 1000°C for 24 hours. The effects of processing parameters such as wheel speed, the amount of niobium, and annealing temperatures on the structure were investigated by XRD and EXAFS. XRD showed that for all the samples the only structure present was DO22-The DO22 structure was stable even after the high temperature heat treatments. By means of EXAFS, niobium atoms were observed to occupy titanium sites in the DO22 structure. Furthermore, in the unannealed samples, increasing wheel speed of the melt spinning process or the niobium concentration tended to distort the crystal structure. It was observed that Ti EXAFS had different results from the Nb EXAFS beyond their occupying similar sites, which suggested there may exist some composition zones, i.e. rich Nb zone or rich Ti zones, although the structures present were still DO22. The samples were found to experience different distortions as a function of annealing temperatures.
Texture Studies of Gamma Titanium Aluminide Sheets Produced by Melt Overflow Rapid Solidification
Authors: Weaver M. L. , Garmestani H. , Das G. 
Published: April 2011

ABSTRACTTexture analyses have been conducted on gamma titanium aluminide (γ-TiAI) strips produced using the recently developed plasma melt overflow process. The results indicated that <101] deformation textures persisted in the γ-phase while <0002>, <1010> and <1020> texture components were all observed in the α2-phase. After annealing at 1065°C/48 hrs., the γ-phase textures did not change while the α2-phase changed from a basal to a <1010> texture. It is suggested that the texture development in direct cast γ strips produced using this technique are a direct result of lattice rotations and residual stresses caused by nonuniform cooling.
Microstructural Changes During Long-Term Tension Creep of Two-Phase γ-Titanium Aluminide Alloys
Authors: Oehring M. , Ennis P. J. , Appel F. , Wagner R. 
Published: April 2011

ABSTRACTLong-term tension creep tests were performed on a Ti-48 at.% Al-2 at.% Cr alloy in order to assess the material behaviour under the intended service conditions for structural parts in turbine engines. Deformation processes and microstructural changes were investigated by TEM on a specimen loaded to 140 MPa for 5988 h at 700 °C. At lamellar boundaries the emission of interfacial dislocations was observed and is thought to contribute significantly to the high primary creep rate of the material. Under the creep conditions gliding dislocations apparently become locked by the heterogeneous formation of precipitates along their cores. Lamellar interfaces revealed ledges which indicates that they migrate during creep.
Deformation Mechanisms Responsible for the Creep Resistance of Ti-Al Alloys
Authors: Morris M. A. , Lipe T. 
Published: April 2011

ABSTRACTTwo γ-based Ti-Al alloys with similar grain sizes and, respectively, lamellar and duplex microstructures have been creep tested at 700°C and constant stresses ranging between 280 and 430 MPa. TEM observations have confirmed that the duplex alloy deforms by extensive mechanical twinning whose density increases with applied stress and increasing strain. The new twin interfaces subdivide the γ grains throughout the primary stage of creep. At the onset of the minimum creep rate, the twin interfaces in the duplex alloy behave in the same way as the γ/γ or the α2/γ interfaces in the lamellar alloy. However, single dislocations were also present and it appears that in both alloys the deformation process is controlled by the accumulation and emission of dislocations from the different interfaces.
Creep Deformation of a Fully Lamellar Gamma Based Titanium Aluminide Alloy
Authors: Herrouin F. , Bowen P. , Jones I. P. 
Published: April 2011

ABSTRACTA complex two phase γ-TiAl alloy, Ti-47Al-lCr-1Mn-2Ta-0.2Si (at.%) in a fully lamellar condition, has been creep tested at a stress of 200MPa and a temperature of 700°C. This simulates the in-service operating conditions for several potential gas turbine aero engine applications where creep resistance is a design limiting material property. The results have indicate that reduction in lamellae thickness and avoidance of feathery type microstructures contribute to improved creep resistance.
Creep in Single Crystal Ni3AI
Authors: Zhu W. , Jones I. P. , Fort D. , Smallman R. E. 
Published: April 2011

ABSTRACTSingle crystals of Ni3AI (1 at%Ta) with a compression axis of [123] were subject to creep at a stress of 550 MPa and a temperature of 520 °C. Slip trace and TEM microstructural observations reveal that primary octahedral slip is responsible for the primary creep. In the secondary stage, cube cross slip (010) is operative. There is no obvious sign of inverse creep.
Site Occupancies in Ternary C15 Ordered Laves Phases
Authors: Kotula P. G. , Anderson I. M. , Chu F. , Thoma D. J. , Bentley J. , Mitchell T. E. 
Published: April 2011

ABSTRACTSite occupancies in three C15-structured AB2(X) Laves phases have been determined with Atom Location by CHanneling Enhanced MIcroanalysis (ALCHEMI). In NbCr2(V), the results are consistent with exclusive site occupancies of Nb for the A sublattice and Cr and V for the A sublattice. The B-site occupancy of V can be interpreted in terms of electronic structure. In NbCr2(Ti), the results are consistent with Ti partitioning mostly to the A sites with some anti-site defects likely. In HfV2(Nb), the results are consistent with Nb partitioning between the A and A sites. The results of the ALCHEMI analyses of these ternary C15 Laves phase materials are discussed with respect to previously determined phase diagrams and first-principles total energy and electronic structure calculations.
Creep of Anomalous Ni3Ga
Authors: Lunt M J , Sun Y Q 
Published: April 2011

ABSTRACTNi3Ga is among a number of LI2ordered intermetallic alloys whose yield stress increases with temperature. In this work we have examined the creep strength of [123] and [001] oriented Ni3Ga specimens in the temperature regime of the yield stress anomaly and confirmed that the creep strength shows the normal rapid decrease with temperature. Inverse creep occurs in the [001] specimens where slip is on the {111} planes only. [123] specimens exhibit steady-state creep and slip line and TEM observations have shown slip on the cube plane and dislocations of both <110> and <100> Burgers vectors are present. We have carried out creep tests of prestrained [123] samples and demonstrated that the storage of primary <110> {111} screw dislocations, locked in the Kear-Wilsdorf configuration, has no effect on creep.
Characterization of thin Polymeric Nanofoam films by Transmission Electron Microscopy and Small Angle Neutron Scattering
Authors: Briber R. M. , Fodor J. S. , Russell T. P. , Miller R. D. , Carter K. R. , Hedrick J. L. 
Published: April 2011

ABSTRACTThin film polymer nanofoams are produced from triblock copolymers of a fluorinated polyimide, 3F/PMDA (derived from pyromelletic dianhydride (PMDA) and 1,1-bis(4-aminophenyl)-1-phenyl-2,2,2-trifluoroethane (3F)) as the center block and polypropylene oxide (PO) as the end blocks. The nanofoam is produced using a three step process: 1.) spin casting the triblock copolymer onto a silicon substrate, 2.) thermal treatment in an Argon atmosphere to imidize the center block and 3.) thermal treatment in air to degrade the PO domains and form nanoscale voids. This process was characterized using both transmission electron microscopy (TEM) and small angle neutron scattering (SANS). For the TEM studies, Ruthenium tetroxide staining was used to enhance the contrast between the polyimide (PI) matrix and the PO microdomains or voids, which permitted a more detailed view of the microstructure of both the foamed and unfoamed materials. From the two dimensional Fourier transform of the micrographs the spatial correlation between the PO microdomains in the unfoamed material and between the voids in the foam were found. An interdomain separation distance of -37 nm was observed. SANS was performed to follow the imidization and foaming processes both in-situ and on a Si substrate. The SANS results indicated that the films are homogeneous when spun from solution and that the microphase separation of the PO domains occurs during the imidization step. The subsequent foaming step leaves the morphology generally intact with the PO domains converting to voids. A peak was found in the SANS curve with a spacing of about 26 nm, which is qualitative agreement with the TEM data.
Block Copolymer Microdomain Morphologies by Phase Detection Imaging
Authors: Leclère Ph. , Yu J. M. , Lazzaroni R. , Dubois Ph. , JéRôme R. , Brédas J. L. 
Published: April 2011

ABSTRACTAtomic Force Microscopy with Phase Detection Imaging is used to study the surface microdomain morphology of thick (i.e., ca. 2 mm) films of triblock copolymers, such as polymethylmethacrylate - block - polybutadiene - block - polymethylmethacrylate copolymers prepared by a well-taylored two-step sequential copolymerization promoted by a 1,3-diisopropenylbenzene based difunctional anionie initiator. By means of this new scanning probe microscopy technique, it is shown that the surface exhibits a segregated microphase structure, corresponding to the different types of components predicted theoretically by thermodynamic processes. We investigate the relationships between the size and characteristics of the microdomain structure as a function of the molecular parameters of the constituent polymers. Our data illustrate the interest of Phase Detection Imaging in the elucidation of surface phase separation in block copolymers.