bluuebunny/crossref_metadata_2025_split huggingface dataset:
Excitation Intensity and Temperature Dependent Photoluminescence Behavior of Silicon Nanoparticles
Authors: Werwa E. , Seraphin A. A. , Kolenbrander K. D. 
Published: April 2011

AbstractThe luminescence properties of silicon nanoparticles have been studied as a function of the excitation light intensity, the temporal Nature of the excitation source, and of sample temperature. The excitation intensity dependence of the luminescence was found to depend strongly on the temporal Nature of the excitation source. Under high intensity excitation from a pulsed 355 nm source, the photoluminescence (PL) intensity saturates and the peak PL wavelength shifts to the blue at room temperature. This behavior persists at reduced temperature. In contrast, under high intensity excitation using a cw 488 nm source at room temperature, the PL intensity saturates but does not shift in wavelength. At reduced temperatures, there is no saturation of luminescence intensity with high intensity cw excitation. These differences indicate that photogenerated carrier recombination occurs via different pathways depending on the temporal profile of the excitation, with cw excited samples following the expected Auger pathway while pulsed samples exhibit a state filling mechanism. Auger models for the pulsed behavior are found to be inconsistent with the experimental data. The temperature dependence of the PL from a pulsed excited sample for a constant excitation intensity was also monitored. The variation of the peak emission wavelength was found to be similar in magnitude to that observed for amorphous silicon, suggesting that structural disorder may play a role in the luminescence. The change in emission intensity was fairly weak, indicating enhanced carrier confinement, as would be expected in a quantum confined system.
Raman Scattering and Photoluminescence of Spontaneously Ordered Ga0.5In0.5P Alloy
Authors: Li G. H. , Liu Z. X. , Han H. X. , Wang Z. P. , Dong J. R. , Wang Z. G. 
Published: April 2011

AbstractSamples of the spontaneously ordered Ga0.5In0.5P alloys were grown by the MOCVD method on [001]-oriented GaAs substrates. The thickness of the epitaxal layer is about 2 μm. Raman scattering and photoluminescence spectra have been measured at room temperature. The result from photoluminescence measurement indicates that the direct-band gaps of the spontaneously ordered samples are lower than that of the disordered sample. Three scattering peaks have been observed in the Raman spectra, corresponding to the GaP-like LO, InP-like LO and InP-like TO modes in the alloys, respectively. The frequencies of the GaP- and InP-like LO modes increase with the decrease of the band-gap of the ordered alloys. It is related to the formation of the (GaP)1/(InP)1 monolayer superlattice along [111] direction in the ordered alloys. The polarization properties of the ordered alloys are similar to those of the bulk III-V semiconductors with the zinc-blende structure.
Growth and Characterization of Zn1-xMnxSe1-ySy Epilayers and Related Heterostructures
Authors: Flack F. S. , Samarth N. , Semendy F. 
Published: April 2011

AbstractWe describe the growth of (Zn,Mn)(S,Se) epitaxial layers of high structural quality on (100) GaAs substrates. Double crystal x-ray diffraction (DCXRD) measurements indicate that quaternary epilayers nearly lattice-matched with GaAs are characterized by DCXRD curves with a foil width at half maximum in the range 30 – 60 arc seconds. Photoluminescence (PL) spectroscopy is employed to map the variation of the energy gap of the quartenary alloys over a wide range of alloy compositions. Finally, temperature dependent PL is used to examine the viability of (Zn,Mn)(S,Se) alloys as confining layers for ZnSe and (Zn,Cd)Se quantum wells. While efficient exciton confinement is demonstrated through the observation of robust PL from such quantum wells up to high temperatures, the renormalization of the quartenary band gap by spin fluctuations leads to a rapid decrease in confinement with increase in temperature.
Raman Scattering and Photoluminescence Measurements on II-VI Semiconductor Nanocrystals as a Function of Pressure and Particle Size
Authors: Schroeder John , Lee Mierie , Silvestri Markus R. , Hwang Lih-Wen , Persans Peter D. 
Published: April 2011

AbstractPressure tuned photoluminescence and Raman scattering were employed to study the structural phase transition from wurtzite to rocksalt structure of CdSxSei1-x nanocrystals embedded in a glass matrix as a composite material and in the colloidal form. In both the composite and the colloidal form the increased phase stability of the wurtzite to rocksalt structure is reversible and size dependent. In contrast the phase transition in bulk semiconductors of cadmium-sulfide is sharp but for the nanocrystals it is broad and occurs at higher pressures. A size dependent study of the phase stability was conducted and a correlation between size and increase in phase stability was definitely established. For the nanocrystals in the composite form the difference in surface tension in both phases allowed for a model that predicted the tendencies observed in optical spectra. For the colloidal nanocrystals a model based on surface tension and volume effects (defects) gave good agreement in explaining the observations. The above results are discussed with respect to surface tension and deep level defects and their pressure dependence.
In-Situ Photoluminescence Investigation of the Initial Porous Silicon Formation in 0.2M NH4F (pH 3.2)
Authors: Dittrich T. , Timoshenko V. Y. , Rappich J. 
Published: April 2011

AbstractThe porous silicon (por-Si) formation in 0.2M NH4F (pH 3.2) is investigated in-situ by photoluminescence (PL). The p-type Si(100) samples are treated electrochemically in the galvanostatic regime starting from Ihe hydrogenated surface. Single pulses of a N2 laser are used to probe stroboscopically the radiative band-band recombination of the bulk c-Si and the PL of por-Si. The PL intensity of c-Si is correlated with the current density during the current-voltage scan and indicates changes of surface recombination by the onset of chemical reactions. The PL intensity of c-Si increases rapidly after switching off the anodic current while the PL intensity of por-Si is not influenced by the rapid current switch. This shows that the passivation of the surfaces of the Si nanostructures is not affected by the por-Si formation at the surface of the bulk c-Si.
Synchrotron Radiation Induced Optical Luminescence from Porous Silicon: Recent Observations
Authors: Coulthard I. , Sham T. K. , Jiang D.-T. , Tan K. H. 
Published: April 2011

AbstractPhotoluminescence from porous silicon was examined using synchrotron radiation as an excitation source. The tunability of the excitation source permitted a wide range of excitation energies ranging from VUV to X-rays. This permitted site selective excitation where specific core levels (i.e. Si-K, O-K, Si-2p) were probed. In high porosity samples, luminescence bands of both surface and bulk origins were observed. All experiments exhibited a common luminescence maximum typically in the orange-red region of the visible spectrum. At certain specific excitation energies particularly in the VUV region additional peaks related to sites with oxygen character were also observed. The VUV excitation luminescence spectra of the porous silicon remarkably resembled that of oxygen deficient amorphous silicon dioxide glasses.
Electron Microscopy of Mesoscale Arrays of Quantum-Confined CdS Nanoparticles Formed on DNA Templates
Authors: Pinizzotto Russell F. , Rho Young G. , Chen Yandong , Pirtle Robert M. , Pirtle Irma L. , Coffer Jeffery L. , Li Xin 
Published: April 2011

AbstractThis paper describes the fabrication method and initial characterization of self-assembled mesoscale arrays of quantum-confined CdS nanoparticles using DNA as a template for the overall shape. Three DNAs were used: the circular and linear forms of the plasmid pUCLeu4, and circular φ×174 RF II. In all three cases, the mesoscale lengths are consistent with the A-form of DNA. The structural signatures and crystallography were confirmed using conventional and high resolution transmission electron microscopy, and electron diffraction. Optical spectroscopy demonstrated that the particles display quantum-confinement effects. This research is a fundamental demonstration of the power of combining biochemical and solid-state processing techniques.
Ab-Initio Calculation of the Optical Properties of Silicon Quantum Wires
Authors: Ossicini Stefano , Biagini M. , Bertoni C. M. , Roma G. , Bisi O. 
Published: April 2011

AbstractWe studied the effect of H, O passivation and inter-wire interaction on the optical properties of nanoscale Si wires. We find that wires with diameters as small as 10–25 Å are active in the visible range. Inter-wire interaction leads to the presence of localized states which lower the band gap energy. The presence of dangling bonds generates broad features in the infrared region. O-Si bonds reduce the absorption threshold. These results are important for the discussions concerning absorption and luminescence in porous Si.
Excimer-Laser Crystallization of Silicon Films: Numerical Simulation of Lateral Solidification
Authors: Gupta Vikas V. , Jin Song H. , Im James S. 
Published: April 2011

AbstractWe have utilized a recently developed transient two-dimensional model for simulating localized beam-induced melting and solidification of thin silicon films on SiO2. Specifically, by tailoring the lateral beam profile, we simulate those situations that are encountered in the artificially-controlled superlateral growth (ACSLG) method, in which various techniques are utilized to irradiate the sample in preselected regions of a silicon film. The spatially and temporally localized character of heating is simulated by introducing a time-dependent two-dimensional heat-source function. The evolution of melt-creation and ensuing solidification is studied as a function of incident energy density and film thickness. The results show two distinct types of behavior as a function of incident energy density: at low energy densities, partial melting and predominantly vertical solidification occur; while at high energy densities, complete melting of the irradiated portion of the film is followed by rapid lateral solidification.
Purely Intrinsic Poly-Silicon Films by Hot Wire Chemical Vapor Deposition
Authors: Rath J. K. , Feenstra K. F. , Ruff D. , Meiling H. , Schropp R. E. I. 
Published: April 2011

AbstractPoly-silicon films have been prepared by hot-wire chemical vapor deposition (HWCVD) from hydrogen diluted silane gas at a low temperature (430 °C). The optical gap of the poly-silicon films is 1.1 eV, though with a higher optical absorption than c-Si. The grains have a preferential orientation (220) perpendicular to the substrate with an average crystallite size of 70 nm. The crystalline volume fraction is 95% with complete coalescence of grains. Large structures up to 0.5 μm could be observed in the AFM micrograph. The activation energy (0.54 eV) and the low carrier concentration (1011 cm−3) indicate a fully intrinsic nature of the films. The μτ product of carriers is 7.1×10−7 cm2V−1 whereas the ambipolar diffusion length (LD) is 334 nm. The excellent photo-conductive properties are attributed to the low (∼1017 cm−3) defect density. The HWCVD poly-silicon films showed a very small temperature dependence of mobility, indicating negligible trapping of carriers at the grain boundaries. Preliminary n-i-p cells incorporating poly-silicon i-layer yielded 3.15 % efficiency.
Synthesis and Laser Spectroscopy of Monoclinic Eu3+: Y2O3 Nanocrystals
Authors: Bihari Bipin , Tissue Brian M. 
Published: April 2011

ABSTRACTGas-phase condensation of CO2 laser-heated EU3+:Y2O3 ceramics produces monoclinic-phase nanocrystalline material. Transmission electron microscopy shows particle diameters in the range 7–30 nm for particles quenched at 60 °C under 400 Torr of nitrogen atmosphere. The optical spectra of nanocrystals produced from 0.1% Eu3+:Y2O3 starting material have narrow lines, and the 5D0 lifetimes are 1.8, 1.2 and, 1.3 ms for the three Eu3+ cation sites. Nanocrystals obtained from 0.7 –5 % Eu3+:Y2O3 starting material show line broadening and the presence of Eu2O3 secondary phase.
Atomistic Study of Crack Propagation and Dislocation Emission in Cu-Ni Multilayers
Authors: Clinedinst Jeff , Farkas Diana 
Published: April 2011

ABSTRACTWe present atomistic simulations of the crack tip configuration in multilayered Cu-Ni materials. The simulations were carried out using molecular statics and EAM potentials. The atomistic structure of the interface was studied first for a totally coherent structure. Cracks were simulated near a Griffith condition in different possible configurations of the crack plane and front with respect to the axis of the layers. Results show that interface effects predominately control the mechanical behavior of the system studied.
Synthesis of Oxide-Coated Metal Clusters
Authors: Crane Robert A. , Matthews Jonathan T. , Andres Ronald P. 
Published: April 2011

ABSTRACT“Fish-eye” particles consisting of metal clusters (Ag, Cu) a few nanometers in diameter encapsulated within a thin layer (-1 nm) of silica are produced using aerosol synthesis procedures. We present a method for predicting stable “fish-eye” nanostructures and describe synthesis techniques for producing significant quantities of silica-encapsulated metal nanoparticles.For many metal/oxide pairs, gas phase formation of oxide encapsulated metal particles is thermodynamically favorable. Using known surface free energies and binary phase diagrams, it is possible to predict whether SiO2-encapsulated metal clusters will form in the gas phase. Two conditions which must be satisfied are: 1) that the surface free energy of the metal is higher than that of Si; and 2) that the metal composition in the particle is greater than the eutectic composition in the metal/Si phase diagram. Ag-SiO2 and Cu-SiO2 are two examples of systems which readily form “fish-eye” structures.Two types of gas phase cluster sources are used at Purdue for producing encapsulated metal nanoparticles. The Multiple Expansion Cluster Source (MECS) is a well established apparatus which produces small quantities (- 50 mg/hr) of very uniform materials using resistive heating for evaporation. The new Arc Cluster Evaporation Source (ACES) offers much higher production rates (>1 g/hr) using DC arc evaporation. These two cluster sources make possible the study of a unique class of materials.
Transition Dynamics in Ferroelectrics with Ordered Nanoregions
Authors: Siny LG. , Katiyar R. S. , Lushnikov S. G. 
Published: April 2011

ABSTRACTRaman scattering was used to study two model relaxor ferroelectrics, PbMg1/3Nb2/3O3 (PMN) with the 1:2 stoichiometric composition of Mg2+ and Nb5+ ions in the oxygen octahedrons and PbSc1/2Ta1/2O3 (PST) with the 1:1 stoichiometric composition of Sc3+ and Ta5+ ions. In spite of a different stoichiometric ratio the Raman spectra of both materials are consistent with the Fm3m space symmetry which implies the existence of similar 1:1 ordered clusters at least in nanoscale regions. The spectra show some anomalous features in the temperature range preceding a ferroelectric state, namely a broad central peak appears in PMN and a complex structure develops from the initially singlet line in PST. Those phenomena are considered as the dynamic features in course of evolution of the relaxors to a ferroelectric state. The preceding phase is characterized by a breakdown in the selection rules for Raman scattering, so some points in the Brillouin zone can contribute to the light scattering spectra. Comparing all available data, one can assume the determinant role of heterophase fluctuations in that process. The fluctuations in a special preceding phase are caused by a competition between two phases, namely between the ferroelectric phase and an additional nonpolar phase.
Consolidation and Evolution of Physical-Mechanical Properties of Nanocomposite Materials Based on High-Melting Compounds
Authors: Andrievski R. A. , Kalinnikov G. V. , Urbanovich V. S. 
Published: April 2011

ABSTRACTNanocomposite materials of TiN/TiB2 system are prepared both by high pressure sintering of ultrafine powders and the films deposition using magnetron sputtering. Properties and structure of obtained paniculate and film materials derived from hardness measurements as well as TEM and SEM analyses are discussed and compared in detail.
Heterogeneous Nanocomposite Materials Based on Liquid Crystals and Porous Media
Authors: Sinha G. P. , Aliev F. M. 
Published: April 2011

ABSTRACTAn effective way of preparing a variety of liquid crystal based nanocomposite materials is to disperse LC in porous media with different porous matrix structure, pore size and shape. We present the results of investigations of quasiequilibrium and dynamical properties of nematic and smectic liquid crystals (LC) dispersed in porous matrices with randomly oriented, interconnected pores (porous glasses) and parallel cylindrical pores (Anopore membranes) by light scattering, photon correlation and dielectric spectroscopies. Confining LC to nanoscale level leads to quantitative changes in physical properties and appearance of new behavior which does not exist in either of the components. Relaxation of director fluctuations which is characterized by single relaxation time in the bulk LC are transformed to a process with a spectrum of relaxation times in pores, which includes extremely slow dynamics typical for glass formers. Existence of developed interface in these materials leads to new dielectric properties such as an appearance of a low frequency relaxation of the polarization and modification of dipole rotation.
A New Route to the Preparation of Nanophase Composites via Layered Double Hydroxides
Authors: Isupov V. P. , Tarasov K. A. , Mitrofanova R. P. , Chupakhina L. E. 
Published: April 2011

ABSTRACTA promising route to the preparation of nanophase composites with fine particles of transition metals via layered double hydroxides has been shown on the derivatives of Li-Al double hydroxide, LADH-X, where interlayer X anions are complexes of transition metals. Thermal decomposition of such materials in vacuum or an inert gas leads to dehydration and dehydroxylation of the hydroxide matrix and to the collapse red/ox process of the complex anion. The latter results in the carbonisation of the samples and in the appearance of nanoscale (20–500Å) metal particles.
The Yield Stress of the Fully-Lamellar Microstructure
Authors: Sun Y. Q. 
Published: April 2011

ABSTRACTThis paper is an inquiry into the relationship between the yield stress and the two length parameters in the fully-lamellar polycrystalline microstructure, the grain-size dCB and the lamellar thickness dLM. Deformation in the multilayer structure is assumed to proceed by dislocations propagating in the formation of a succession of mutually interacting pileups, blocked at the lamellar interfaces and piled-up ultimately against the grain boundary. An important case suggested is a yield stress independent of the grain size, sensitive only to the lamellar spacing.
Fracture Toughness of Stoichiometric, Non-Stoichiometric and Ternary-Alloyed Al2Ti
Authors: Ma J. C. , Lukitsch M. J. , Ambrow C. E. , Benci J. E. 
Published: April 2011

ABSTRACTPolycrystalline stoichiometric Al2Ti was produced via casting or powder metallurgy and further processed yielding material in six conditions. The fracture toughness of the six material conditions was determined from the critical load to initiate cracks with a Vickers indenter. The results show a strong dependence on material condition. Powder processed Al2Ti exhibits the highest fracture toughness value among the material conditions studied.Polycrystalline non-stoichiometric Al2±yTi1±y and ternary-alloyed Al2Ti + X were prepared in as-cast and cast & annealed conditions. Each material condition exhibited a multiphase microstructure. The composition of the phases present in each alloy was analyzed with SEM/EDS. Fracture toughness values of cast & annealed stoichiometric and non-stoichiometric binary alloys are 20–30% greater than as-cast stoichiometric Al2Ti. For the ternary alloys, the fracture toughness values show a strong dependence on the ternary element used and heat treatment condition. The fracture toughness values of three hot forged ternary alloys were also determined.
Nanocrystalline TiAl Compacts Prepared by Hddr and Hot Pressing
Authors: Aoki K. , Itoh Y. , Kawamura Y. , Inoue A. , Masumoto T. 
Published: April 2011

ABSTRACTNanocrystalline TiAl powder was formed by the modified HDDR (hydrogenation-disproportion-dehydrogenaton-recombination) method utilizing ball milling in a hydrogen atmosphere. That is, TiAl compound powder decomposed into TiH2 and Al-rich TiAl powders by mechanical grinding in a hydrogen atmosphere (HD process). Dehydrogenaton-recombination (realloying) resulted in the formation of nanocrystalline TiAl powder when heated to about 700 K in an argon atmosphere (DR process). Almost fully dense nanocrystalline TiAl compacts were prepared by a combination of HDDR and hot-pressing at 873 K which is lower than the usual consolidation temperature by about 300 K. The TiAl compact thus made was brittle in the as-pressed state but showed compressive ductility after annealing.
Al-21Ti-23Cr High-Temperature Protective Coating on TiAl Intermetallic Compounds by RF Magnetron Sputtering
Authors: Park J. Y. , Lee H. N. , Park S. W. , Oh M. H. , Wee D. M. 
Published: April 2011

ABSTRACTTi-48A1 specimens were coated with Al-21Ti-23Cr film at 200 W, 0.8 Pa and 573 K by RF magnetron sputtering. The oxidation behavior of the coated specimens was investigated through isothermal and cyclic oxidation tests, and the tensile deformation properties of the coated specimens were also investigated before and after oxidation. The isothermal and cyclic oxidation curves showed that the Al-21Ti-23Cr film was very effective in decreasing the oxidation rate of Ti-48A1. This excellent oxidation resistance is attributable to the formation of a protective Al2O3 layer on the surface of the Al-21Ti-23Cr film. It was found from the results of the tensile test that the protective Al2O3 layer on the surface of the Al-21Ti-23Cr film enabled the Ti-48A1 to maintain its tensile properties in an oxidizing environment.
Transient Creep Behavior of γ-TiAl Polycrystals
Authors: Viguier B. , Bonneville J. , Spätig P. , Martin J. L. 
Published: April 2011

ABSTRACTTwo types of transient creep experiments performed along stress-strain curves are described and successfully applied to γ TiAl polycrystals at room temperature. They allow to determine activation volumes in good agreement with those measured through successive load relaxation tests. In addition, the combination of the latter method and the present ones provides relevant values of the plastic strain hardening coefficient. This latter parameter is found to exhibit similar values in transient as well as during constant strain rate tests.
Annealing of Cold-Rolled Fe-40A1 Single Crystals
Authors: Yang Y. , Baker I. 
Published: April 2011

ABSTRACTSingle crystals of Fe-40A1 were cold-rolled to plastic strains in the range 5% to 48%. Discs cut from the rolled crystals at different rolling strains were heated at 10 K/min in a differential scanning calorimeter from room temperature to 973 K. Three exothermic peaks were observed in the temperature ranges of 440–550 K, 610–650 K, and 860–930 K, all the peaks shifting to lower temperatures with increasing strain. The origins of these peaks are discussed in terms of the disorder and vacancies introduced during rolling.