bluuebunny/crossref_metadata_2025_split huggingface dataset:
Fabrication and characterization of GaN nanopillar arrays
Authors: Wang Y.D. , Tripathy S. , Chua S.J. , Fonstad C.G. 
Published: April 2011

ABSTRACTVarious nanofabrication technologies are currently under investigation to realize fine patterning of III-Nitrides. Dry and wet etching techniques have been explored in the past for the fabrication of GaN-based devices. However, due to etch-induced damage, it is still a major challenge to achieve high-quality GaN-based nanostructures with high aspect ratio. In this study, GaN nanopillars were fabricated by inductively coupled plasma etching (ICP) using anodic aluminium oxide (AAO) as a mask. High-spatial resolution optical techniques were employed to characterize these pillar arrays. The average diameter and length of these pillars are about 60–70 nm and 350–400 nm, respectively. Low temperature micro-photoluminescence spectra show a red shift compared with the spectrum recorded from the as-grown GaN, indicating stress relaxation in these nanopillars. The evidence of good crystalline quality is also confirmed by micro-Raman measurement where red shift of the E2(TO) mode from GaN nanopillars suggest partial relaxation of the compressive strain.
Strong room temperature 510 nm emission from cubic InGaN/GaN multiple quantum wells
Authors: Li S.F. , As D.J. , Lischka K. , Pacheco-Salazar D.G. , Scolfaro L.M.R. , Leite J.R. , Cerdeira F. , Meneses E.A. 
Published: April 2011

ABSTRACTCubic InGaN/GaN double heterostructures and multi-quantum-wells have been grown by Molecular Beam Epitaxy on cubic 3C-SiC. We find that the room temperature photoluminescence spectra of our samples has two emission peaks at 2.4 eV and 2.6 e V, respectively. The intensity of the 2.6 eV decreases and that of the 2.4 eV peak increases when the In mol ratio is varied between X = 0.04 and 0.16. However, for all samples the peak energy is far below the bandgap energy measured by photoluminescence excitation spectra, revealing a large Stokes-like shift of the InGaN emission. The temperature variation of the photoluminescence intensity yields an activation energy of 21 meV of the 2.6 eV emission and 67 meV of the 2.4 eV emission, respectively. The room temperature photoluminescence of fully strained multi quantum wells (x = 0.16) is a single line with a peak wavelength at about 510 nm.
X-ray characterization of GaN single crystal layers grown by the ammonothermal technique on HVPE GaN seeds and by the sublimation technique on sapphire seeds
Authors: Raghothamachar Balaji , Dudley Michael , Wang Buguo , Callahan Michael , Bliss David , Konkapaka Phanikumar , Wu Huaqiang , Spencer Michael 
Published: April 2011

ABSTRACTSingle crystal GaN substrates are a more reliable, efficient and low cost alternative to sapphire and SiC substrates that are currently being used for developing nitride based devices such as LEDs, high power FETs and laser diodes. GaN growth by two techniques are investigated here. In ammonothermal growth (analogous to hydrothermal growth which is commonly used for quartz), using potassium and lithium amide as mineralizers and GaN powders and vapor grown polycrystals as nutrients, thick GaN single crystal layers have been grown on both faces of (0001) HVPE GaN seeds in nickel based autoclaves. In GaN sublimation growth, Ga is transported under a temperature gradient in an ammonia atmosphere from a GaN powder source to the sapphire substrate which has a 3–6 micron GaN epilayer deposited on it. Synchrotron white beam x-ray topography (SWBXT) was initially used to image the defect structures in the layers. High resolution x-ray diffraction experiments were also carried out and reciprocal space maps were recorded from the grown GaN layers. Characterization results will be discussed with respect to the growth conditions.
The intrinsic free carrier mobility in AlGaN/GaN quantum wells
Authors: Carosella F. , Germain M. , Farvacque J.-L. 
Published: April 2011

ABSTRACTThe aim of this contribution is to determine theoretically the maximum mobility that can be expected in AlGaN/GaN quantum wells as soon as the free carriers are only submitted to intrinsic scattering mechanisms associated with phonons and the carrier-carrier interaction. In our model, we consider that the carrier-carrier two bodies collisions do not constitute by themselves a relaxation mechanism since they conserve the momentum and the energy of the electron system. Thus, we assume that the free carriers act only through their contribution to the dynamical dielectric response of the material and, at least, through their collective behavior resulting into plasmons which, when damped, constitute now a real relaxation mechanism. The full scattering strength is connected with the imaginary part of the total reversed dielectric function including the lattice and the free carrier contributions. This approach automatically includes the scattering mechanisms associated with hybrid phonon/plasmon particles.
Synthesis of Microcrystalline Silicon Films by Low Energy Electron-Beam-Induced Deposition at Cryogenic Temperature
Authors: Sato Tetsuya , Nakagawa Kiyokazu , Aoki Yutaka , Sato Shouji 
Published: April 2011

ABSTRACTWe have proposed an advanced method for formation of semiconductor thin films at substrates temperatures below 100K. We have synthesized amorphous silicon (a-Si:H) and microcrystalline silicon (μc-Si:H) films using low-energy electron-beam-induced-chemical vapor deposition (EBICVD) onto cooled substrates which adsorb source gases (SiH4 or Si2H6) at cryogenic temperature. The temperature dependence on growth rate of the films, hydrogen content and optical constants were investigated. The μc-Si:H could be formed at 40–45 K on SiO2 using He-discharged-EBICVD with SiH4. The crystallinity of silicon was evaluated by Raman scattering spectroscopy and X-ray diffraction.
Si1-xGex Nanocrystals Observed by EFTEM: Influence of the Dry and Wet Oxidation Process
Authors: Cuadras A. , Arbiol J. , Garrido B. , Morante J.R , Rodriguez A. , Rodríguez T. 
Published: April 2011

ABSTRACTWe report about the formation of Si1-xGex nanocrystals (NC) in an oxide matrix after oxidizing a polycrystalline Si0.7Ge0.3 layer in wet and dry conditions. We have characterized the NC by Energy Filtered Transmission Electron Microscopy (EFTEM). EFTEM images analyses show the evolution of the oxidation of polycrystalline Si0.7Ge0.3 layer. We observe that a layer with Ge-rich nanoclusters is formed. For wet oxidations we find Ge-rich Si1-xGex nanocrystals while for dry oxidations Si is oxidized faster and Ge is completely segregated. Results are contrasted with infrared and Raman spectroscopies. In conclusion, wet oxidation seems to be preferable to oxidize polycrystalline Si1-xGex to obtain Ge-rich NC embedded in an oxide matrix, as both Ge and Si are oxidized, in contrast to dry oxidation, where Ge is mainly segregated.
Quasiballistic Electron Emission from Planarized Nanocrystalline-Si Cold Cathode
Authors: Tsuchiya Yoshishige , Nakatsukasa Takuya , Mizuta Hiroshi , Oda Shunri , Kojima Akira , Koshida Nobuyoshi 
Published: April 2011

ABSTRACTMechanism of electron transport through planerized nanocrystalline-Si (nc-Si) cold cathode surface emitting devices was investigated. The energy distribution of electrons emitted from nc-Si emitter was obviously not Maxwellian, which was usually obtained at conventional cold cathode devices, but was similar to that from the nanocrystalline porous silicon diode emitter. These results strongly suggest that electrons are emitted quasiballistically from our devices and indicate that the planarized nc-Si layer play an important role in this high efficiency cold cathode emitter.
Surface Nitridation of Amorphous Carbon by Nitrogen Ion Beam Irradiation
Authors: Iwasaki Chihiro , Aono Masami , Kitazawa Nobuaki , Watanabe Yoshihisa 
Published: April 2011

ABSTRACTAmorphous carbon was irradiated with a nitrogen ion beam and changes in composition and tribological properties after surface nitridation have been studied. The nitrogen ion beam energy was varied from 0.1 to 2.0 keV under the constant ion current density. Composition and chemical bonding states near the surface were analyzed by X-ray photoelectron spectroscopy (XPS). Tribological properties were studied by a pin-on-disk type tribotester. XPS studies show that the nitrogen concentration near the surface increases after nitrogen ion irradiation. The depth profiles of nitrogen in the irradiated specimens display that the nitrogen concentration near the surface is maximized after 0.2 to 0.7 keV ion irradiation. From the pin-on-disk testing, it is found that the friction coefficient of amorphous carbon increases after irradiation by 1.5 keV nitrogen ions, while the friction coefficient does not change drastically after irradiation by 0.2 keV ions.
Simulations of Chemotaxis and Random Motility in Finite Domains
Authors: Jabbarzadeh Ehsan , Abrams Cameron F. 
Published: April 2011

ABSTRACTRational design and selection of candidate porous biomaterials to serve as tissue engineering constructs rests on our ability to understand the influence of the porous microarchitecture on the transport of chemical species (e.g., nutrients and signaling compounds), fluid flow, and cellular locomotion and growth. We have begun to study the behavior of chemotactically mobile cells in response to unsteady signaling molecule concentration fields using a computational simulation-based model. The model couples fully time-dependent finite-difference solution of a reaction-diffusion equation for the concentration field of a generic chemoattractant to biased random walks representing individual moving cells. This model is a first step in building a quantitative, pore-level model of mass and cellular transport in porous tissue-engineered constructs. In these proceedings, we focus on our recent findings regarding the influence of flux-reactive boundary conditions in heterogeneous 2D domains on the chemotactic response of otherwise randomly moving cells. In particular, we find that, when cells are forced to “crawl” around obstacles in order to approach a point source of chemoattractant, the reactivity of the obstacle surface with respect to the chemoattractant strongly determines the morphology of the cells' paths of locomotion. Cells crawl along non-reactive surfaces and strongly avoid reactive surfaces, due to the nature of the chemoattractant concentration gradients near the surface. We show further that tuning the reactivity of the surfaces of two obstacles defining a gap can control the passage of cells through the gap. From our work, we infer the importance of a proper treatment of boundary conditions in any future pore-level quantitatve modeling of mass transport and cellular response in porous media.
Nanoscale RGD Peptide Organization Regulates Cell Proliferation and Differentiation
Authors: Hsiong Susan , Yong Lee Kuen , Alsberg Eben , Mooney David 
Published: April 2011

ABSTRACTRGD (arginine-glycine-aspartic acid) containing peptide sequences, common cell attachment sites present in many extracellular matrix (ECM) proteins, mediate many important cellular processes. The role of nanoscale organization of RGD peptides in the regulation of the adhesion, proliferation and differentiation of both preosteoblasts (MC3T3-E1) and multipotential (D1) cell lines in vitro was investigated in this study. Alginate polymer chains with varying RGD peptide degree of substitution were mixed with unmodified polymer chains at different ratios to allow variation of RGD peptide spacing in the nanometer scale, independently of the overall bulk density of peptides presented from the material. Proliferation of both cell types was observed to be closely correlated to RGD island (defined as a cluster of RGD peptides) spacing, independently of overall bulk ligand density, following cell adhesion to alginate hydrogels. Increased RGD island spacing was observed to promote spreading of MC3T3-E1 cells while simultaneously suppressing their proliferation. However, increased RGD island spacing decreased spreading of D1 cells while also decreasing proliferation. Moreover, differentiation of preosteoblasts was significantly upregulated in response to decreased RGD island spacing, whereas differentiation of multipotential cells was modestly regulated by this variable. These results demonstrate that the nanoscale organization of adhesion ligands may be an important variable in controlling cell phenotype and function. In addition, cellular responses to nanoscale ligand organization differ depending on the cell type, and this may be related to the differentiation stage of the cells.
Real-Time Salmonella Detection Using Lead Zirconate Titanate-Titanium Microcantilevers
Authors: McGovern John-Paul , Shih Wan Y. , Shih Wei-Heng , Sergi Mauro , Chaiken Irwin 
Published: April 2011

ABSTRACTCurrent methods for analysis of unknown powders in suspicious packages involve sending samples to laboratory facilities where a variety of time-consuming tests are performed. We have developed and investigated the use of a lead zirconate titanate - titanium (PZT-Ti) microcantilever for in situ detection of the common food- and water-born pathogen, Salmonella typhimurium. Using a bifunctional linking molecule to immobilize antibody on the titanium surface of the microcantilever, we can directly detect salmonella cells in suspensions of differing concentration. This novel surface functionalization technique along with the sub-nanogram sensitivity of the cantilever has allowed for direct quantification of S. typhimurium cells in suspension.
Layer-by-layer printing of cells and its application to tissue engineering
Authors: Kesari Priya , Xu Tao , Boland Thomas 
Published: April 2011

ABSTRACTTissues and organs exhibit distinct shapes and functions nurtured by vascular connectivity. In order to mimic and examine these intricate structure-function relationships, it is necessary to develop efficient strategies for assembling tissue-like constructs. Many of the top-down fabrication techniques used to build microelectromechanical systems, including photolithography, are attractive due to the similar feature sizes, but are not suitable for delicate biological systems or aqueous environments. A layer-by layer approach has been proposed by us to pattern functional cell structures in three dimensions. Freeform cell structures are created by the inkjet method, in which cells are entrapped within hydrogels and crosslinked on demand. The cells are viable, functional and show potential for cell maturation as exemplified by the diversion of hematopoietic stem cells into multiple cell types. These results show promise for many tissue engineering applications.
Infrared Emitting Dye and/or Two Photon Excitable Fluorescent Dye Encapsulated in Biodegradable Polymer Nanoparticles for Bioimaging
Authors: Baba Koichi , Ohulchanskyy Tymish Y. , Zheng Qingdong , Lin Tzu-Chau , Bergey Earl J. , Prasad Paras N. 
Published: April 2011

ABSTRACTNear infrared fluorescent dyes have a significant potential in biological applications. However, many of these organic dyes are hydrophobic and/or their fluorescence is quenched in aqueous media. Nanoparticle technology has been utilized to overcome the hydrophobic property of these dyes, but there remain problems in their fabrication. To overcome these problems, a novel fabrication technique for incorporating hydrophobic dyes in a biodegradable polymer nanoparticle has been developed. An infrared emitting dye (D1) and a two photon excitable fluorescent dye (BT101) were selected as prototypic dyes for incorporation. The D1 nanocrystal dye shows quenched emission in aqueous media. Upon encapsulation in a polymeric nanoparticle, its emission at 1.1 to 1.35 μm was recovered. BT101 dye doped nanoparticles demonstrated significant increase in fluorescent yield in aqueous media as compared to the free dye. In addition, several different sizes of BT101 doped polymer nanoparticles were fabricated (c.a. 35 nm to 100 nm in size). The fluorescence intensity was found to increase with the particle size. The investigation of uptake of BT101 nanocrystal and BT101 doped PLGA nanoparticles by culture cells revealed the importance of polymer coating of dye for efficient cell imaging.
Open-Framework Copper Titanosilicates
Authors: Wang Xiqu , Liu Lumei , Wang Lingbao , Jacobson Allan. J. 
Published: April 2011

ABSTRACTFour open-framework copper titanosilicates have been synthesized by hydrothermal techniques. The crystal structure of Phase 1 determined from single crystal X-ray data is closely similar to that reported for the titanosilicate ETS-4 with ca. one third TiO5 pyramids randomly replaced by CuO4 squares. Phase 2 also has the ETS-4 structure but all the TiO5 pyramids are replaced by CuO4 squares which are not randomly disordered. Phase 3 with the composition Na8CuTi3Si16O43 has the same structure as the mineral narsarsukite, a titanosilicate closely related to the microporous titanosilicate ETS-10. Narsarsukite and ETS-10 have the same straight single chains of TiO6 octahedra which are partially substituted by CuO4 squares in 3. Phase 4 with the composition K4CuTiSi8O21 contains CuO4 squares and TiO5 tetragonal pyramids that crosslink a new type of silicate double layer to form an open framework.
Synthesis and Low Temperature Physical Properties of Metal Substituted CoSr2YCu2Oy.
Authors: Shiraki Masahiro , Shimoyama Jun-ichi , Horii Shigeru , Kishio Kohji 
Published: April 2011

ABSTRACTIn the present study, hole carrier doping was attempted for CoSr2YCu2O7+δ which is a relating compound to a well known superconductor YBa2Cu3O7-δ by introduction of excess oxygen into CoO1+δ layer and cation substitutions. High valence metal substitution for cobalt and barium substitution for strontium were revealed to be able to introduce excess oxygen and generate hole carriers. Additional calcium substitution for yttrium enhanced hole carrier concentration p/Cu > 0.05 which was enough for superconductivity, however superconducting transition was not observed. This is possibly due to incorporation of cobalt in the CuO2 planes.
Ferroelectricity in SrTiO3 Ceramics Induced by Pr Doping
Authors: Durán A. , Martínez E. , Mata J. , Heiras J. , Solans X. , Siqueiros J. 
Published: April 2011

ABSTRACTFerroelectricity was induced in SrTiO3 by the Sr exchange for Pr ion in the A site of the perovskite ABO3-type structure. X-ray diffraction patterns show single phase crystalline structure in the SrxPr1-xTiO3 compound for x=0, 0.025, 0.050, 0.075 and 0.1 compositions. Rietveld refinement shows that the unit cell volume decreases with the increasing of Pr content as a consequence of the difference between Sr/Pr ionic radii. Furthermore, high-temperature differential thermoanalysis (DTA) displays a small anomaly at about 118 °C which is probably due to the Pr ion producing a distortion of the perovskite structure via an off-center site. This deformation in the lattice induces a measurable polar behavior of the solid solution. Dielectric permittivity (ε vs T) measurements display a well defined peak at about 238 °C. Furthermore, a well defined hysteresis loop at 30 °C with a remnant polarization that tends to decrease with increasing Pr concentration is observed. Both experimental results confirm the ferroelectric state induced by the Pr ion.
Reductive Intercalation of Vanadyl Layered Perovskites
Authors: Neiner Doinita , Sweany Ray L. , Golub Vladimir , Wiley John B. 
Published: April 2011

ABSTRACTA new series of triple layered Ruddlesden-Popper compounds of the general formula (LixVO)La2Ti3O10, where X = 0.83, 0.93, 1.4 and 1.8, have been prepared. Lithium is intercalated into the layered perovskite in a reductive manner to produce new mixed valence compounds. The structure of the parent, (VO)La2Ti3O10, is maintained upon intercalation. Above a certain lithium content, the magnetic behavior of (LixVO)La2Ti3O10 changes from paramagnetic, for X = 0.83, 0.93, 1.4, to a magnetically ordered material for (Li1.8VO)La2Ti3O10.
Femtosecond Laser Micromachining of Periodical Structures in Si <100>
Authors: El-Bandrawy Mohamed , Gupta Mool C. 
Published: April 2011

ABSTRACTA frequency doubled femtosecond Ti: sapphire laser at a wavelength of 400 nm, a pulse width of 160 fs, and a repetition rate of 1 kHz was used with a computer controlled galvo head to write periodical structures in Si <100>. Laser pulses of ∼130 nJ were focused using an objective lens of 0.65 NA. Laser parameters were optimized for efficient submicron ablation, yielding 700 nm wide by 600 nm deep lines. 1-D and 2-D periodical structures of 5 and 5x5 micron periods, respectively, were fabricated and examined using optical and atomic force microscopy. The quality of the 1-D and 2-D structures was highly depended on the light polarization orientation with respect to micromachining direction. With optimized fs laser parameters, high quality 1-D and 2-D periodical structures were obtained, which would have applications in optical devices.
Quantification of Environmental Stress Cracking in Polymer Blends through Eyring Modeling
Authors: Hopson Peyton L. , Moore Robert B. 
Published: April 2011

ABSTRACTThe effect of blending polycarbonate (PC) with an amorphous copolyester (PCTG) and a crystallizable polyester (PBT) on the environmental stress cracking (ESC) resistance was studied. The determination of the ESC resistance for the blend was accomplished through tensile testing in a fluid environment utilizing an Eyring-type activated process to describe ESC. It was found that the miscible blend, PC/PCTG, displayed a rule of mixtures for ESC resistance to all fluids tested except ether resistance. The immiscible blend, PC/PBT, displayed a significant negative deviation from the rule of mixtures for ESC resistance, except for ether resistance, which has been attributed to the development of stress sites for craze initiation at the interface between the blend components on the surface of the test sample. The differences in ether resistance compared to the trends found for the fluid ESC resistance in this study were attributed to possible changes in crystallization for the samples tested in ether. The data suggests that strongly swelling fluids, e.g. diethyl ether in the presence of PC, may cause densification from polymer crystallization resulting in voids that facilitate in the initiation and growth of crazes.
Morphological effects on Glass Transitions in Immiscible Polymer Blends
Authors: Thirtha Vivek M. , Lehman Richard L. , Nosker Thomas J. 
Published: April 2011

ABSTRACTThis paper describes the effects of structures on the glass transition of glassy polymers blended with a semi-crystalline polymer. Immiscible blends of PS/PP and PS/HDPE were prepared from commercially available polymers using melt processing and extrusion without additives. The weight fractions of the components were varied from 0 to 1. SEM analysis of the blends showed a range of morphologies over the composition range from small inclusions at low volume concentrations through intertwined co-continuous structures at specific intermediate compositions, and a reversal of this configuration at high volume fractions. The glass transition of the glassy polymer was measured with differential scanning calorimetry using the sensitive and high resolution modulated DSC method. A systematic change in glass transition of glassy polymers is observed as a function of composition in various immiscible polymer blends. Results show that the glass transition of polystyrene increases with a reduction in volume fraction, by approximately 5.4°C in polypropylene and 6.5°C in polyethylene. Probable models which might explain this effect are mentioned.