Characterization of AlGaN/GaN structures on various substrates grown by radio frequency-plasma assisted molecular beam epitaxy

The structural properties and surface morphology of AlGaN/GaN structures grown on LiGaO₂ (LGO), sapphire, and hydride vapor phase epitaxy (HVPE)-grown GaN templates are compared. AlGaN grown on LGO substrates shows the narrowest x-ray full width at half maximum (FWHM) for both symmetric 〈00.4〉 and asymmetric 〈10.5〉 reflections. Atomic force microscopy (AFM) analysis on AlGaN surfaces on LGO substrates also show the smoothest morphology as determined by grain size and rms roughness. The small lattice mismatch of LGO to nitrides and easily achievable Ga-polarity of the grown films are the primary reasons for the smoother surface of AlGaN/GaN structure on this alternative substrate. Optimizations of growth conditions and substrate preparation results in step flow growth for an AlGaN/GaN structure with 300 Å thick Al_0.25Ga_0.75N on 2.4 μm thick GaN. A high III/V flux ratio during growth and recently improved polishing of LGO substrates aids in promoting two dimensional step flow growth. The GaN nucleation layer directly on the LGO substrate showed no evidence of mixed phase cubic and hexagonal structure that is typically observed in the nucleation buffer on sapphire substrates. Cross-sectional high-resolution transmission electron microscopy (HRTEM) was performed on an AlGaN/GaN heterostructure grown on LGO. The atomic arrangement at the AlGaN/GaN interface was sharp and regular, with locally observed monolayer and bilayer steps.     

Calculation of the electric field enhancement for a degenerate diffusion process

The internal electric field produced by a constant source impurity diffusion is calculated using Fermi-Dirac statistics for the majority carrier. Numerical results indicate that treating the material as if it were nondegenerate always produces an underestimate in the electric field. The field enhancement factor which gives the ratio of the effective diffusion coefficient to the impurity diffusion coefficient valid for a degenerate diffusion process is clarified.     

Thermal denaturation of soy proteins as related to their dye-binding characteristics and functionality

The binding of Cresol Red and of Acid Orange 10 (in the absence and presence of urea) to unheated and progressively heated, defatted soy meal was compared with their NSI values, urease activities,in vitro digestibilities, unreactive lysine contents, and foaming and emulsifying capacities. These results suggested that increased amounts of Cresol Red and of Acid Orange 10 (in the presence of urea) bound to the heated samples were due to the progressive exposure of hydrophobic residues caused thermal denaturation. High statistical correlations were obtained between dye-binding, the duration of heating, and functional properties. Our results indicate that dye-binding has potential for predicting certain functional properties as well as for monitoring thermal denaturation of soy proteins.     

2,2′-OXO-1, 1 ′-azobenzene A microbially transformed allelochemical from 2,3-Benzoxazolinone: I

2,2-Oxo-1,1 -azobenzene (AZOB), a compound with strong herbicidal activity, was isolated and characterized from a soil supplemented with 2,3-benzoxazolinone (BOA). A parallel experiment with 6-methoxy-2,3-benzoxazolinone (MBOA) yielded AZOB as well as its mono-(MAZOB) and dimethoxy-(DIMAZOB) derivatives. These compounds were produced only in the presence of soil microorganisms, via possible intermediates, I and II, which may dimerize or react with the parent molecule to form the final products. In the case of MBOA, it was shown that demethoxylation precedes the oxidation step. Although BOA and 2,4-dihydroxy-1,4(2H)-benzoxazin-3-one (DIBOA) were leached out of rye residues, there were no detectable amounts of the biotransformation products in the soil. When BOA was mixed with soil and rye residue, either under field conditions or in vitro, AZOB was detected. Levels of free BOA in the soil were greatly reduced by incubation with rye residue. AZOB was more toxic to curly cress (Lepidium sativum L.) and barnyardgrass (Echinochloa crusgatti L.) than either DIBOA or BOA.Journal Article No. 12943 of the Michigan Agricultural Experiment Station.     

Passivation of pyrite due to surface treatment

Two samples of naturally occurring pyrite were treated with acetyl acetone, silanes, humic acids, lignin-NaOH mixture, and oxalic acid. The corrosion rate of pyrite was observed to decrease significantly due to the surface treatment by the chemicals used in this study. Surface characterization methods indicate that phenyl triacetoxy silane forms a passive coating a few Angstroms thick on the pyrite, thus preventing its dissolution in 1M H₂SO₄.     

Construction of validated, non-redundant composite protein sequence databases

A strategy has been developed for the construction of a validated,comprehensive composite protein sequence database. Entries areamalgamated from primary source data bases by a largely automatedset of processes in which redundant and trivially differententries are eliminated. A modular approach has been adoptedto allow scientific judgement to be used at each stage of databaseprocessing and amalgamation. Source databases are assigned apriority depending on the quality of sequence validation andcommenting. Rejection of entries from the lower priority database,in each pairwise comparison of databases, is carried out accordingto optionally defined redundancy criteria based on sequencesegment mismatches. Efficient algorithms for this methodologyare embodied in the COMPO software system. COMPO has been appliedfor over 2 years in construction and regular updating of theOWL composite protein sequence database from the source databasesNBRF-PIR, SWISS-PROT, a GenBank translation retrieved from thefeature tables, NBRF-NEW, NEWAT86, PSD-KYOTO and the sequencescontained in the Brookhaven protein structure databank. OWLis part of the ISIS integrated data resource of protein sequenceand structure [Akrigg et al. (1988) Nature, 335, 745–746].The modular nature of the integration process greatly facilitatesthe frequent updating of OWL following releases of the sourcedatabases. The extent of redundancy in these sources is revealedby the comparison process. The advantages of a robust compositedatabase for sequence similarity searching and information retrievalare discussed.     

Structural Studies of Anomalous Behavior in the Silica-Alumina Gel System

This paper compares the structure ofAl₂O₃-SiO₂ xerogels prepared with Al₂O₃ contents ranging from 21 to 75 wt% (13 to 64 mol%). The 47 wt% Al₂O₃ xerogel (Al/Si ≅ 1) exhibits anomalously low surface area (≅1 m²/g) and skeletal density compared with other Al₂O₃-Si₂ compositions. Based on the results of nitrogen adsorption/condensation, helium displacement, high-resolution transmission electron microscopy (HRTEM), small-angle X-ray scattering (SAXS), and small-angle neutron scattering (SANS), we attribute the low surface area and skeletal density (density of the solid phase which is inaccessable to helium) to the formation of closed micropores, whereas the higher surface area materials exhibit a slightly coarser texture comprising open pores with radii of ≅1 nm. X-ray diffraction (XRD) and ²⁹SI and ²⁷Al magic-angle spinning nuclear magnetic resonance (MASNMR) indicate no anomalous behavior in the 47% sample on molecular length scales. HRTEM indicates the presence of a small fraction of crystallites which is supported by the SAXS results, but it is unknown if this crystallinity is related to low surface area. Low-field ¹H NMR spin-lattice relaxation measurements show that the physical structure of all of the 'wet' gels is similar implying that pore closure occurs during drying. Consistent with this idea, gel surface area and density increased significantly when the pore fluid (water) was replaced with a lower surface tension fluid (formamide, dioxane, ethanol. water/surfactant).     

Endothelial cell electrical impedance parameter artifacts produced by a gold electrode and phase sensitive detection

Frequency dependent cellular micro-impedance estimates obtained from a gold two-electrode configuration using phase sensitive detection have become increasingly used to evaluate cellular barrier model parameters. The results of this study show that cellular barrier function parameter estimates optimized using measurements obtained from this biosensor are highly susceptible to both time dependent and systematic instrumental artifacts. Based on a power spectral analysis of experimentally measured microelectrode voltages, synchronous, 60 Hz, and white Gaussian noise were identified as the most significant time dependent instrumental artifacts. The reduction of these artifacts using digital filtering produced a corresponding reduction in the optimized model parameter fluctuations. Using a series of instrumental circuit models, this study also shows that electrode impedance voltage divider effects and circuit capacitances can produce systematic deviations in cellular barrier function parameter estimates. Although the implementation of an active current source reduced the voltage divider effects, artifacts produced by coaxial cable and other circuit capacitive elements at frequencies exceeding 1 kHz still remained. Reducing time dependent instrumental fluctuations and systematic errors produced a significant reduction in cellular model barrier parameter errors and improved the model fit to experimental data.     

朗讯重新定义服务概念

朗讯科技（中国）公司借上海“2005年中国国际信息通信展览会”之机,于2005年6月21日在沪举办了“2005通信服务高层论坛-朗讯服务解决方案研讨会”,邀请国内政府机构、运营商以及企业代表与来自国际知名运营商和朗讯全球服务部（IWS）的专家们一道,共同探讨通信业务和通信服务面临的挑战、未来发展方向以及相关的服务解决方案。     

Customizing MRI-Compatible Multifunctional Neural Interfaces through Fiber Drawing

Fiber drawing enables scalable fabrication of multifunctional flexible fibers that integrate electrical, optical, and microfluidic modalities to record and modulate neural activity. Constraints on thermomechanical properties of materials, however, have prevented integrated drawing of metal electrodes with low-loss polymer waveguides for concurrent electrical recording and optical neuromodulation. Here, two fabrication approaches are introduced: 1) an iterative thermal drawing with a soft, low melting temperature (T_m) metal indium, and 2) a metal convergence drawing with traditionally non-drawable high T_m metal tungsten. Both approaches deliver multifunctional flexible neural interfaces with low-impedance metallic electrodes and low-loss waveguides, capable of recording optically-evoked and spontaneous neural activity in mice over several weeks. These fibers are coupled with a light-weight mechanical microdrive (1 g) that enables depth-specific interrogation of neural circuits in mice following chronic implantation. Finally, the compatibility of these fibers with magnetic resonance imaging is demonstrated and they are applied to visualize the delivery of chemical payloads through the integrated channels in real time. Together, these advances expand the domains of application of the fiber-based neural probes in neuroscience and neuroengineering.