bluuebunny/crossref_metadata_2025 huggingface dataset:
Ferroelectric domain imaging by defect-luminescence microscopy
Authors: Dierolf V. , Sandmann C. , Kim S. , Gopalan V. , Polgar K.
Published: February 2003

In order to study the role of defects within the domain inversion process in ferroelectric LiNbO3 crystals, we investigated the optical properties of intentionally introduced Er3+ defect complexes across a 180° domain wall produced at room temperature by electric-field poling. Using site-selective excitation-emission spectroscopy for well chosen excitation energies, we found drastic differences in the Er3+ emission, which are due to a rearrangement of the defect complexes. We used these changes in a confocal luminescence microscope to image ferroelectric-domain structures. This powerful imaging method with a 700 nm, 50 ms spatial and temporal resolution can be used to study real-time dynamics of domain walls.
Prediction of domain nucleation and growth during polarization switching in ferroelectric thin films
Authors: Kim Sang-Joo , Shin Joonho , Kim Yun-Jae
Published: February 2003

A three-dimensional constitutive model for polarization switching in ferroelectric ceramics is presented and implemented into a finite element code. The developed code is applied to investigate the nucleation and growth of domains in a ferroelectric thin film. Initially the thin film is completely poled upward and its bottom electrode is grounded. Then a point on the top surface of the film is subjected to a constant positive electric potential for a certain period of time, leading to a polarization switching downward. The distribution of electric field is not homogeneous within the film and so neither the distribution of polarization. It is found that initially the switched zone is of a dagger shape but after the switched zone penetrates the film thickness it changes to the shape of a reversed cup with the lower part wider that the upper part. It is also observed that the width of switched zone increases with the period and magnitude of applied potential. The predictions are qualitatively compared with experimental observations.
Master equation approach to finding the rate-limiting steps in biopolymer folding
Authors: Zhang Wenbing , Chen Shi-Jie
Published: February 2003

A master equation approach is developed to find the rate-limiting steps in biopolymer folding, where the folding kinetics is described as a linear combination of basic kinetic modes determined from the eigenvalues and eigenvectors of the rate matrix. Because the passage of a rate-limiting step is intrinsically related to the folding speed, it is possible to probe and to identify the rate-limiting steps through the folding from different unfolded initial conformations. In a master equation approach, slow and fast folding speeds are directly correlated to the large and small contributions of the (rate-limiting) slow kinetic modes. Because the contributions from the slow modes can be computed from the corresponding eigenvectors, the rate-limiting steps can be identified from the eigenvectors of the slow modes. Our rate-limiting searching method has been tested for a simplified hairpin folding kinetics model, and it may provide a general transition state searching method for biopolymer folding.
Self-consistent analysis of the contact phenomena in low-mobility semiconductors
Authors: Preezant Yevgeni , Tessler Nir
Published: February 2003

Self-consistent solution of charge injection and charge transport in low mobility light emitting diodes (LEDs) is reported. We show that an explicit description of the contact region under the same premise as the transport equations is needed to accurately evaluate the current–voltage characteristics of polymer or small-molecule based LEDs. The results are compared to widely used models, which treat the contact region in an implicit manner.
Elastic constants of borocarbides. New approach to acoustic measurement technique
Authors: Masalitin E. A. , Fil’ V. D. , Zhekov K. R. , Zholobenko A. N. , Ignatova T. V. , Lee Sung-Ik
Published: February 2003

A new version of the phase method of determining the sound velocity is proposed and implemented. It utilizes the “Nonius” measurement technique and can give acceptable accuracy (⩽1%) in samples of submillimeter size. Measurements of the sound velocity are made in single-crystal samples of the borocarbides RNi2B2C (R=Y, Lu, Ho). The elastic constants and the Debye temperature are calculated.
Photomodulated thermoreflectance detection of hydrogen at elevated temperatures: a detection limit
Authors: Othonos Andreas , Christofides Constantinos
Published: February 2003

Laser photomodulated thermoreflectance has been used as a means of detecting, at elevated temperatures, low concentration of hydrogen using an optically thin film of palladium. Data indicate that concentrations as low as a few parts per billion can easily be detected at 100 °C. A semi-quantitative interpretation of the photothermal signal has been achieved using a Langmurian isothermic model.
Chemical composition and local structure of plasma enhanced chemical vapor-deposited Si nanodots and their embedding silica matrix
Authors: Dalba G. , Daldosso N. , Fornasini P. , Grisenti R. , Pavesi L. , Rocca F. , Franzò G. , Priolo F. , Iacona F.
Published: February 2003

X-ray absorption measurements in total electron yield mode have been carried out on Si nanodots embedded in amorphous silica produced by plasma enhanced chemical vapor deposition (PECVD). The amount of Si atoms composing the Si nanodots and the chemical composition of the amorphous host matrix has been determined thanks to the comparison with Rutherford backscattering spectrometry data. The influence of nitrogen, incorporated during the PECVD procedure, on the structure of the host silica matrix has been discussed.
Pseudozeros of multivariate polynomials
Authors: Hoffman J. , Madden James , Zhang Hong
Published: February 2003

The pseudozero set of a system

f
f

of polynomials in

n
n

complex variables is the subset of

C

n

\mathbf {C}^n

which is the union of the zero-sets of all polynomial systems

g
g

that are near to

f
f

in a suitable sense. This concept is made precise, and general properties of pseudozero sets are established. In particular it is shown that in many cases of natural interest, the pseudozero set is a semialgebraic set. Also, estimates are given for the size of the projections of pseudozero sets in coordinate directions. Several examples are presented illustrating some of the general theory developed here. Finally, algorithmic ideas are proposed for solving multivariate polynomials.
Constructing complete tables of quartic fields using Kummer theory
Authors: Cohen Henri , Diaz y Diaz Francisco , Olivier Michel
Published: February 2003

We explain how to construct tables of quartic fields of discriminant less than or equal to a given bound in an efficient manner using Kummer theory, instead of the traditional (and much less efficient) method using the geometry of numbers. As an application, we describe the computation of quartic fields of discriminant up to

10
7

10^7

, the corresponding table being available by anonymous ftp.
Loyalty and scepticism among members of a professional union: the case of the Union of Librarians in Denmark
Authors: Ole Pors Niels
Published: February 2003

Presents some of the main results from a comprehensive survey among Danish librarians and their activities within, and attitudes towards, the Danish Union of Librarians. Analysis of librarians’ responses to the survey indicates that both occupational setting and age play a significant role in forming attitudes among members. The age structure appears to have the greatest significance. The employment of different statistical tools, like factor and cluster analysis, make it possible to classify librarians as either sceptical or loyal in relation to their union. The two groups differ with regard to activities, age, employment, and general professional orientation. The groups also differ in relation to how they perceive their union ought to prioritise its services. The future for all the Danish unions probably depends on how successful they are in creating loyalty and satisfaction among members. This task seems to be difficult because the members differ much in relation to an individualistic and a communalistic view of the profession in society.
Team leadership activities in different project phases
Authors: Weinkauf Katharina , Hoegl Martin
Published: February 2003

Flexible and highly integrated team‐based organizational designs are often seen as the necessary response to increasing technological and market complexities and dynamics. Building on the extant literature on team leadership, we specify and categorize leadership activities that are seen as critical to teamwork and performance in project teams. Based on an empirical study on a large‐scale project comprising 39 teams, we show how these leadership activities are carried out in the course of a project. The results indicate that individual teams did change in how strongly they performed the various leadership activities. Implications for theory and practice are discussed.
Entrepreneurial traits of undergraduate students at selected South African tertiary institutions
Authors: Louw L. , van Eeden S.M. , Bosch J.K. , Venter D.J.L.
Published: February 2003

Developments in the global and national economies as well as the labour market, have made it necessary that more attention be paid to entrepreneurship and the updating of curricula presented by tertiary institutions. For this purpose reliable and valid information is required. The primary objectives of this article are to report on the levels of students’ entrepreneurial traits, to establish whether these traits are interrelated, and to determine the extent of the impact that demographic variables have on these entrepreneurial traits. A convenience sampling method (n = 1,215) was used. The best developed entrepreneurial traits observed included: “Competing against self‐imposed standards”, “Self‐confidence” and “Dealing with Failure”. Statistically significant relationships were also identified between the entrepreneurial traits of students and the tertiary institution attended, and students’ gender, race and age.
The role and impact of culture on South Pacific island entrepreneurs
Authors: Saffu Kojo
Published: February 2003

This comparative study explores the relevance and applicability of the characteristics of entrepreneurs espoused in the western entrepreneurship literature to indigenous entrepreneurs. Using five South Pacific island countries as a case in point, the literature reviewed shows that culture impacts on the characteristics of entrepreneurs from these countries and accounts for differences between the characteristics of the Pacific island entrepreneurs and the characteristics found in the Western entrepreneurship literature. In the light of the influence of culture, perhaps a new list of characteristics that indigenous entrepreneurs in the South Pacific island countries would require to succeed is warranted. An integrative model of cultural dimension and characteristics of Pacific island entrepreneurs is provided. Propositions are advanced for the study of culture as a moderating influence on entrepreneurial characteristics elsewhere, especially indigenous entrepreneurs from developing countries.
Synchronized Vibrations of a Circular Cylinder in Cross Flow at Supercritical Reynolds Numbers1
Authors: Kawamura Tsutomu , Nakao Toshitsugu , Takahashi Masanori , Hayashi Masaaki , Murayama Kouichi , Gotoh Nobuho
Published: February 2003

Synchronized vibrations of a circular cylinder in a water cross flow at supercritical Reynolds numbers were measured. Turbulence intensities were varied to investigate the effect of the Strouhal number on the synchronization range. Self-excited vibration in the drag direction due to symmetrical vortex shedding began only when the Strouhal number was about 0.29, at a reduced velocity of 1.1. The reduced velocities at the beginning of lock-in vibrations caused by Karman vortex shedding decreased from 1.5 to 1.1 in the drag direction and from 2.7 to 2.2 in the lift direction, as the Strouhal number increased from 0.29 to 0.48.
Yield Pressure Measurements and Analysis for Autofrettaged Cannons
Authors: Underwood John H. , Moak David B. , Audino Michael J. , Parker Anthony P.
Published: February 2003

Yield pressure corresponding to a small permanent OD strain was measured in quasi-static laboratory tests of autofrettaged ASTM A723 steel cannon pressure vessels. Yield pressure was found to be a consistent ratio of the yield strength measured from specimens located in close proximity to the area of observed yielding. Yield pressure measurements for dynamic cannon firing with typically a 5-ms pressure pulse duration gave 14% higher yield pressures, attributed to strain rate effects on plastic deformation. Calculated Von Mises yield pressure for the laboratory test conditions, including the Bauschinger-modified ID residual stress and open-end vessel conditions, agreed with measured yield pressure within 3–5%. Calculated yield pressure was found to be insensitive to the value of axial residual stress, since axial stress is the intermediate value in the Von Mises yield criterion. A description of yield pressure normalized by yield strength was given for autofrettaged A723 open-end pressure vessels over a range of wall ratio and degree of autofrettage, including effects of Bauschinger-modified residual stress. This description of yield pressure is proposed as a design procedure for cannons and other pressure vessels.
Blue-emitting ZnS:Ag,Al phosphors with low defect density for high-voltage field-emission displays
Authors: Kajiwara K. , Hida T. , Tanaka K.
Published: February 2003

In recent years, deterioration of luminescence efficiency of blue-emitting ZnS:Ag,Al powder phosphors has been investigated for high-voltage (e.g., 6–10 kV) field-emission displays (FEDs). It has been found by cross-sectional transmission electron microscopy that the degree of crystallinity of ZnS host crystal within the electron penetration depth is one of the important factors in order to obtain ZnS:Ag,Al phosphors having a longer lifetime of luminescence efficiency. ZnS:Ag,Al phosphors with lower defect density N0 as compared with a P22B ZnS:Ag,Al phosphor for color-TV use have been developed by using a suitable firing method with subsequent annealing and surface treatments. As a result, ZnS:Ag,Al phosphors with lower N0 were successfully prepared, and also FED tubes having a longer lifetime in blue luminescence were made by using ZnS:Ag,Al phosphors with lower N0. In addition, it was revealed that both the initial luminescence efficiency and the thermal quenching point of ZnS:Ag,Al phosphors with lower N0 are considerably higher than those of a P22B ZnS:Ag,Al phosphor.
Field emission characteristics of GaN roughened with H2 plasma
Authors: Kimura Chiharu , Yamamoto Tomohide , Sugino Takashi
Published: February 2003

Silicon (Si) doped gallium nitride (GaN) layers are grown on sapphire substrates with aluminum nitride (AlN) buffer layers by metalorganic chemical vapor deposition. As-grown GaN surfaces are roughened with hydrogen (H2) plasma produced by supplying microwave power. The surface of GaN treated with H2 plasma is observed with atomic force microscopy, and field emission characteristics are measured. The turn-on average electric field between the GaN and anode electrode is estimated to be as low as 12.4 V/μm.
Three-dimensional simulations of field emission through an oscillating barrier from a (10,0) carbon nanotube
Authors: Mayer A. , Miskovsky N. M. , Cutler P. H.
Published: February 2003

We present three-dimensional simulations of field emission through an oscillating barrier from an ideal open (10,0) carbon nanotube without adsorption by using a transfer-matrix methodology. By introducing pseudopotentials for the representations of carbon atoms and by repeating periodically a basic unit of the nanotube, band-structure effects are manifested in the distributions of energies. The total-energy distribution of the emitted electrons present features which are related essentially to the gap of the semiconducting (10,0) nanotube and to stationary waves in the structure. The current enhancement due to photon-stimulation reaches a saturation plateau for photon energies larger than 5 eV and decreases for high energies. Maximal enhancement is achieved at a photon energy around 8 eV, one electron being then emitted for every 20 photons crossing the nanotube.
Field emitter using multiwalled carbon nanotubes grown on the silicon tip region by microwave plasma-enhanced chemical vapor deposition
Authors: Wong Y. M. , Kang W. P. , Davidson J. L. , Wisitsora-at A. , Soh K. L. , Fisher T. , Li Q. , Xu J. F.
Published: February 2003

Carbon nanotubes (CNTs) were selectively grown on the proximity region of silicon tips by microwave plasma-enhanced chemical vapor deposition. The silicon substrate with silicon conical tips was sputtered with palladium to act as a nanocluster catalytic center for CNT nucleation. Curled and randomly oriented CNTs with diameters ranging from 150 to 200 nm were observed to grow selectively on surrounding areas of the silicon tips. Electron field emission tests show a low turn-on field of 3.2 V/μm. An emission current of 8.0 μA was achieved at ∼6.3 V/μm. These results show the ability to grow CNTs selectively on the silicon tip region from a catalytic metal covered surface, which may have practical applications.
Lateral field emitter arrays with high emission currents and wide operation region by high field activation
Authors: Lee Jae-Hoon , Lee Myoung-Bok , Hahm Sung-Ho , Lee Jung-Hee , Seo Hwa-Il , Kwon Dae-Hyuk , Kim Jin-Sup , Choi Kyu-Man
Published: February 2003

A lateral-type polysilicon field emission triode, consisting of two symmetric gate tips aligned by a 70° angle with respect to sharp cathode tips, was fabricated by using the local oxidation of polysilicon process. The fabricated device exhibited a low turn-on voltage of 13 V and an emission anode current of 3.6 μA/tip at VAC=50 V and VGC=open. After high field activation treatment, the turn-on voltage was reduced to 2 V and the emission anode current was increased to 9 μA/tip at the same bias condition. With changing gate bias, the emission current showed three different I–V characteristics; a negative resistance region with large transconductance of 200 μS/50 triodes, a positive resistance region with relatively small transconductance, and a zero transconductance region.
Investigation of the characteristic changes on SrTiO3:Pr,Al,Ga phosphors during low voltage electron irradiation
Authors: Kim Jin Young , You Yong Chan , Jeon Duk Young , Yu Il , Yang Hong-Gun
Published: February 2003

We have investigated the mechanism of degradation of the low voltage cathodoluminescence (CL) efficiency of SrTiO3:Pr,Al,Ga phosphors. Based on Auger electron spectroscopy and x-ray photoelectron spectroscopy study, it is understood that prolonged irradiation of the phosphor with an electron beam of low voltage and high current density causes characteristic changes [(1) accumulation of overlying carbon and (2) reduction of oxygen] to occur on the phosphor surface. These changes are responsible for the rapid degradation of low voltage CL of SrTiO3:Pr,Al,Ga phosphors. The two aforementioned changes are shown to impact CL output in an important way. An accurate accounting of the total impact of the two changes warrants assessment of the importance of other degradation mechanisms. These other degradation mechanisms include both carbon- and noncarbon-related enhanced nonradiative electron-hole recombination at surfaces.
Scanning tunneling microscopy images of III–V semiconductor alloys: Strain effects
Authors: McKay H. A. , Chen Huajie , Feenstra R. M. , Poole P. J.
Published: February 2003

Scanning tunneling microscope images of lattice-matched InGaAs/InP structures were investigated using autocorrelation analysis. Correlation lengths and correlation amplitudes were calculated from constant–current empty-state images. Theoretical STM images were calculated from a model which only considered surface displacements due to strain relaxation. By comparing model and experimental correlation lengths and amplitudes it is concluded that contrast variations in constant–current images are dominated by strain relaxation effects. Changes in probe tip geometry and applications of this technique to study clustering in III–V alloys are also discussed.
Analysis of the slope of the Fowler–Nordheim plot for field emission from n-type semiconductors
Authors: Chung M. S. , Yoon B.-G.
Published: February 2003

The field emission current densities j from n-type semiconductors are obtained using a fully exact calculation scheme and are plotted in the Fowler–Nordheim (FN) coordinates. It is found that the FN plot for a semiconductor has the nonlinear slope that originates in the emission mechanism. The slope of the FN plot has three values, each value representing the slope in the three distinct regions of field F. This implies that the tunneling probability makes three distinct contributions to the electron emission according to the value of F. It is also found that the slope is modified with the carrier concentration and the band gap.
Passivation of InP surfaces of electronic devices by organothiolated self-assembled monolayers
Authors: Schvartzman M. , Sidorov V. , Ritter D. , Paz Y.
Published: February 2003

Surface passivation with organothiolated self-assembled monolayers (SAMs) that form chemical bonds with the InP surface is described. Indium phosphide surfaces coated with thiolated SAMs were characterized by Fourier-transform infrared spectroscopy, contact angle measurements, and Auger spectroscopy. The steady state photoluminescence of InP wafers increased by a factor of 14 as a result of this surface passivation method. A decrease by one to two orders of magnitude in the dark currents of interdigitated metal–semiconductor–metal diodes and p-i-n photodiodes was obtained. The option of using SAMs that are well-wetted by standard encapsulators such as polyimides and the thermal stability of the SAMs at the polymer’s curing temperatures open the way to achieve high quality passivation and encapsulation, even when the electronic devices contain negative slope sidewalls or undercut cavities.
Thermal emissivity of carbon coated p-doped silicon stencil masks for ion projection lithography
Authors: Braun D. , Gajic R. , Kuchar F. , Korntner R. , Haugeneder E. , Loeschner H. , Butschke J. , Letzkus F. , Springer R.
Published: February 2003

The determination and optimization of the thermal emissivity is an essential topic for ion projection lithography (IPL), one of the most promising candidates for the next generation lithography. In order to determine the thermal emissivity of stencil masks, infrared transmittance and reflectance spectra have been measured in the spectral region 100–6000 cm−1. The infrared absorption could be mainly described by a simple Drude model. The high quality of the silicon masks became evident in a Fano resonance of the optical phonons with the intervalence-band absorption and the free-carrier continuum. The obtained values of 0.5 to 0.6 for the emissivities have been used to model the thermomechanical response of the IPL exposure station. Radiation cooling is sufficient to cool the stencil mask to room temperature at the desired ion fluxes which cause heat generation rates of 8.5–10 mW cm−2. At high irradiation intensities, the membrane distortions have to be minimized instead of optimizing the uniformity of the temperature distribution. For 7 mW cm−2, temperature variations of 3.5 K are needed to keep mask distortions below 10 nm.
Transient measurement of resist charging during electron beam exposure
Authors: Bai Min , Meisburger W. Dan , Pease R. Fabian W.
Published: February 2003

Charging of resist films during electron beam exposure has been believed to cause significant (up to 500 nm) pattern displacement. However, our own modeling and direct measurements of pattern displacement have suggested that the pattern displacement is less than 20 nm under typical mask making conditions (10 keV electrons exposing 400 nm polymeric resist on chromium). Here we describe the use of a custom-designed secondary electron collector to measure surface potential during electron beam exposure. We observed essentially no surface potential change (less than 0.2 V) on poly(methylmethacrylate) (PMMA) resists up to 1 μm thick, when exposed by 10 and 25 keV electrons. However, significant charging occurs when the beam energy is reduced to 5 or 3 keV, and the thickness of the PMMA resist film is comparable with, or larger than, the maximum beam penetration depth.
Emission stability of a field emitter array observed by an emission microscope
Authors: Miyamoto Nobuo , Adachi Hiroshi , Nakane Hideaki , Yamane Khoichi
Published: February 2003

Emission stability is the most stringent requirement for a field emitter array (FEA) in practical applications. When fluctuating events occur stochastically, the fluctuation in electron emission reduces with 1/N, where N is the number of individual cathodes. So, the total current of the FEA is expected to be stable. However, the fluctuation does not necessarily occur stochastically, and the mode of fluctuation of each individual microtip is different from tip to tip. We observed the fluctuation of each microtip by use of an emission microscope. The behaviors are classified due to the emission mode. As a result of observation, the modes of fluctuation can be classified in two types. One has a dependence IP product, which is the product of the total current and pressure, and the other has not. To explain the former fluctuation, we propose a model where ion bombardment is due to desorbed gas molecules from the gate hole side of the emitting microtip. The model successfully explains the measured results. However, the latter mode of fluctuation cannot be explained by using this model. High-energy ion bombardment from a phosphor screen may be the origin of the latter mode of fluctuation.
Fabrication of mesoscopic devices using atomic force macroscopic electric field induced oxidation
Authors: Lee F. K. , Wen G. H. , Zhang X. X. , Tsui O. K. C.
Published: February 2003

We demonstrate the fabrication of mesoscopic devices on aluminum, titanium, and silicon-on-insulator thin films by using atomic force microscopic electric field induced oxidation together with selective wet etching. The fabricated device structure is a percolating network consisting of conducting dots (50–100 nm in diameter) randomly distributed within an area of 1×1 μm2. Details on how to fabricate the network structure and the making of electrical contacts to the device will be focused upon. Good agreement between results from transport measurement of an aluminum test sample we made and data from the literature warrants reliability of our sample fabrication technique.
Diffusion barrier integrity evaluation by ellipsometric porosimetry
Authors: Shamiryan D. , Baklanov M. R. , Maex K.
Published: February 2003

In order to reduce resistance and capacitance copper wiring is used along with materials possessing a low dielectric constant (low-k materials). A diffusion barrier is used in Cu/low-k integration scheme to prevent Cu diffusion into low-k materials. A film integrity evaluation method is needed for barrier characterization. The method should be able not only to test the barrier integrity but also to find low-density defects (so called “killer defects”). We propose fast, simple, and accurate evaluation of diffusion barriers by ellipsometric porosimetry (EP). This method can be used for testing barriers with respect to solvent penetration. When a barrier contains pores, an adsorptive (toluene) penetrates through these pores and condenses in the low-k dielectric film (e.g., porous glass or organic polymer). The condensation changes optical characteristics of the low-k film that is measured by EP. Low-density killer defects are visualized by toluene trapped around the defect beneath the barrier. We have evaluated a Ta(N) barrier on a porous glass and a WCN barrier on an organic polymer low-k films. Our experiments show that EP is able to test barrier integrity and find killer defects. If a barrier deposited on a porous film is noncontinuos, then EP is able to determine barrier optical constants and thickness as well as to estimate barrier porous structure.
Metal–oxide–semiconductor field-effect transistor-structured Si field emitter array with a built-in ring gate lens
Authors: Nagao M. , Nicolaescu D. , Matsukawa T. , Kanemaru S. , Itoh J. , Sato T. , Sato Y. , Wada N.
Published: February 2003

We propose a metal–oxide–semiconductor field-effect transistor (MOSFET)-structured Si field emitter array (FEA) with a ring gate FET. A ring-shaped MOSFET gate electrode surrounds the Si FEA and this electrode plays two roles. The first is to control the emission current, and the other is to focus the electron beam emitted from the FEA. The emission current can be controlled by the voltage of the ring gate. We evaluated focusing properties of the ring gate FET structured Si FEA by observing the emission patterns on an anode phosphor screen. The detailed electron trajectory simulation qualitatively agreed with the experimental results. The simulation revealed that the new device has better focusing properties than a conventional FEA with an in-plane lens electrode.
Influence of the optimal etching conditions of silicon substrates on field-electron emission from amorphous-diamond films
Authors: Chen Jian , Xu N. S. , She J. C. , Chen Jun , Deng S. Z.
Published: February 2003

The effect of chemical etching of silicon substrates on the field-electron emission (FEE) from amorphous-diamond (a-D) films deposited on silicon has been studied systematically and thoroughly. It is found that the greater the roughness of the silicon substrate, the better the FEE performance of the a-D/silicon emitter. For a sodium hydroxide concentration between 20% and 40%, an etching temperature in the range of 60–80 °C, and an etching time of 15–30 min, etching of the silicon substrate surface was found to be optimal for FEE from the a-D/silicon emitter. To explain the experimental results, qualitative analysis of the enhanced electric field caused by nanoprotrusions was done.
Monte Carlo simulation of gel formation and surface and line-edge roughness in negative tone chemically amplified resists
Authors: Patsis G. P. , Glezos N. , Gogolides E.
Published: February 2003

A molecular-based representation of a negative tone chemically amplified resist was developed and studied using stochastic simulations. The gel formation mechanism resulting from the reaction–diffusion phenomena in the polymer matrix during the postexposure bake step, as well as the surface and line-edge roughness formation were investigated in two dimensions. Graph theory computational techniques were employed to determine the gel cluster size and its relation to the fraction of the total number of chemically and physically cross-linked chains. Results of the simulation include the maximum acid diffusion range, the cross-link density, the surface and line edge roughness, versus exposure dose and photoacid generator concentration.
Comparative Genetic Diversity of the
narG
,
nosZ
, and 16S rRNA Genes in Fluorescent Pseudomonads
Authors: Delorme Sandrine , Philippot Laurent , Edel-Hermann Veronique , Deulvot Chrystel , Mougel Christophe , Lemanceau Philippe
Published: February 2003

The diversity of the membrane-bound nitrate reductase (
narG
) and nitrous oxide reductase (
nosZ
) genes in fluorescent pseudomonads isolated from soil and rhizosphere environments was characterized together with that of the 16S rRNA gene by a PCR-restriction fragment length polymorphism assay. Fragments of 1,008 bp and 1,433 bp were amplified via PCR with primers specific for the
narG
and
nosZ
genes, respectively. The presence of the
narG
and
nosZ
genes in the bacterial strains was confirmed by hybridization of the genomic DNA and the PCR products with the corresponding probes. The ability of the strains to either reduce nitrate or totally dissimilate nitrogen was assessed. Overall, there was a good correspondence between the reductase activities and the presence of the corresponding genes. Distribution in the different ribotypes of strains harboring both the
narG
and
nosZ
genes and of strains missing both genes suggests that these two groups of strains had different evolutionary histories. Both dissimilatory genes showed high polymorphism, with similarity indexes (Jaccard) of between 0.04 and 0.8, whereas those of the 16S rRNA gene only varied from 0.77 to 0.99. No correlation between the similarity indexes of 16S rRNA and dissimilatory genes was seen, suggesting that the evolution rates of ribosomal and functional genes differ. Pairwise comparison of similarity indexes of the
narG
and
nosZ
genes led to the delineation of two types of strains. Within the first type, the similarity indexes of both genes varied in the same range, suggesting that these two genes have followed a similar evolution. Within the second type of strain, the range of variations was higher for the
nosZ
than for the
narG
gene, suggesting that these genes have had a different evolutionary rate.
Cloning, Sequencing, and Expression of the Chitinase Gene
chiA74
from
Bacillus thuringiensis
Authors: Barboza-Corona J. Eleazar , Nieto-Mazzocco Elizabeth , Velázquez-Robledo Rocio , Salcedo-Hernandez Rubén , Bautista Mayela , Jiménez Beatriz , Ibarra Jorge E.
Published: February 2003

The endochitinase gene
chiA74
from
Bacillus thuringiensis
serovar kenyae strain LBIT-82 was cloned in
Escherichia coli
DH5αF′. A sequence of 676 amino acids was deduced when the gene was completely sequenced. A molecular mass of 74 kDa was estimated for the preprotein, which includes a putative 4-kDa signal sequence located at the N terminus. The deduced amino acid sequence showed high degree of identity with other chitinases such as ChiB from
Bacillus cereus
(98%) and ChiA71 from
Bacillus thuringiensis
serovar pakistani (70%). Additionally, ChiA74 showed a modular structure comprised of three domains: a catalytic domain, a fibronectin-like domain, and a chitin-binding domain. All three domains showed conserved sequences when compared to other bacterial chitinase sequences. A ca. 70-kDa mature protein expressed by the cloned gene was detected in zymograms, comigrating with a chitinase produced by the LBIT-82 wild-type strain. ChiA74 is active within a wide pH range (4 to 9), although a bimodal activity was shown at pH 4.79 and 6.34. The optimal temperature was estimated at 57.2°C when tested at pH 6. The potential use of ChiA74 as a synergistic agent, along with the
B. thuringiensis
insecticidal Cry proteins, is discussed.
Symbiotic and Genetic Diversity of
Rhizobium galegae
Isolates Collected from the
Galega orientalis
Gene Center in the Caucasus
Authors: Andronov E. E. , Terefework Z. , Roumiantseva M. L. , Dzyubenko N. I. , Onichtchouk O. P. , Kurchak O. N. , Dresler-Nurmi A. , Young J. P. W. , Simarov B. V. , Lindström K.
Published: February 2003

This paper explores the relationship between the genetic diversity of rhizobia and the morphological diversity of their plant hosts.
Rhizobium galegae
strains were isolated from nodules of wild
Galega orientalis
and
Galega officinalis
in the Caucasus, the center of origin for
G. orientalis
. All 101 isolates were characterized by genomic amplified fragment length polymorphism fingerprinting and by PCR-restriction fragment length polymorphism (RFLP) of the rRNA intergenic spacer and of five parts of the symbiotic region adjacent to
nod
box sequences. By all criteria, the
R. galegae
bv. officinalis and
R. galegae
bv. orientalis strains form distinct clusters. The
nod
box regions are highly conserved among strains belonging to each of the two biovars but differ structurally to various degrees between the biovars. The findings suggest varying evolutionary pressures in different parts of the symbiotic genome of closely related
R. galegae
biovars. Sixteen
R. galegae
bv. orientalis strains harbored copies of the same insertion sequence element; all were isolated from a particular site and belonged to a limited range of chromosomal genotypes. In all analyses, the Caucasian
R. galegae
bv. orientalis strains were more diverse than
R. galegae
bv. officinalis strains, in accordance with the gene center theory.
Geographical Variation in Ribotype Profiles of
Escherichia coli
Isolates from Humans, Swine, Poultry, Beef, and Dairy Cattle in Florida
Authors: Scott Troy M. , Parveen Salina , Portier Kenneth M. , Rose Joan B. , Tamplin Mark L. , Farrah Samuel R. , Koo Andrew , Lukasik Jerzy
Published: February 2003

Waters impacted by fecal pollution can exact high risks to human health and can result in financial losses due to closures of water systems used for recreation and for harvesting seafood. Identifying the sources of fecal pollution in water is paramount in assessing the potential human health risks involved as well as in assessing necessary remedial action. Recently, various researchers have used the ribotyping method to identify sources of bacterial indicators (
Escherichia coli
and enterococci) in environmental waters. While these studies have identified genotypic differences between human- and animal-derived indicators that are capable of differentiating organisms isolated from humans and various animal hosts, most have focused on organisms collected from a confined geographic area and have not addressed the question of whether these ribotype profiles are watershed specific or if they can be applied universally to organisms from other geographic locations. In this study,
E. coli
isolates were obtained from humans, beef cattle, dairy cattle, swine, and poultry from locations in northern, central, and southern Florida and were subjected to ribotyping analysis. The intent was to determine (i) if ribotype profiles are capable of discriminating the source of
E. coli
at the host species level and (ii) if the resulting fingerprints are uniform over an extended geographic area or if they can be applied only to a specific watershed. Our research indicated that, using a single restriction enzyme (
Hin
dIII), the ribotyping procedure is not capable of differentiating
E. coli
isolates from the different animal species sampled in this study. Results indicate, however, that this procedure can still be used effectively to differentiate
E. coli
as being either human or animal derived when applied to organisms isolated from a large geographic region.
A Combined Model To Predict the Functionality of the Bacteriocin-Producing
Lactobacillus sakei
Strain CTC 494
Authors: Leroy Frédéric , De Vuyst Luc
Published: February 2003

The use of bacteriocin-producing lactic acid bacteria for improved food fermentation processes seems promising. However, lack of fundamental knowledge about the functionality of bacteriocin-producing strains under food fermentation conditions hampers their industrial use. Predictive microbiology or a mathematical estimation of microbial behavior in food ecosystems may help to overcome this problem. In this study, a combined model was developed that was able to estimate, from a given initial situation of temperature, pH, and nutrient availability, the growth and self-inhibition dynamics of a bacteriocin-producing
Lactobacillus sakei
CTC 494 culture in (modified) MRS broth. Moreover, the drop in pH induced by lactic acid production and the bacteriocin activity toward
Listeria
as an indicator organism were modeled. Self-inhibition was due to the depletion of nutrients as well as to the production of lactic acid. Lactic acid production resulted in a pH drop, an accumulation of toxic undissociated lactic acid molecules, and a shift in the dissociation degree of the growth-inhibiting buffer components. The model was validated experimentally.
Genetic Manipulation ofLactococcus lactisby Using Targeted Group II Introns: Generation of Stable Insertions without Selection
Authors: Frazier Courtney L. , San Filippo Joseph , Lambowitz Alan M. , Mills David A.
Published: February 2003

Despite their commercial importance, there are relatively few facile methods for genomic manipulation of the lactic acid bacteria. Here, the lactococcal group II intron, Ll.ltrB, was targeted to insert efficiently into genes encoding malate decarboxylase (mleS) and tetracycline resistance (tetM) within theLactococcus lactisgenome. Integrants were readily identified and maintained in the absence of a selectable marker. Since splicing of the Ll.ltrB intron depends on the intron-encoded protein, targeted invasion with an intron lacking the intron open reading frame disrupted TetM and MleS function, and MleS activity could be partially restored by expressing the intron-encoded protein intrans. Restoration of splicing from intron variants lacking the intron-encoded protein illustrates how targeted group II introns could be used for conditional expression of any gene. Furthermore, the modified Ll.ltrB intron was used to separately deliver a phage resistance gene (abiD) and a tetracycline resistance marker (tetM) intomleS, without the need for selection to drive the integration or to maintain the integrant. Our findings demonstrate the utility of targeted group II introns as a potential food-grade mechanism for delivery of industrially important traits into the genomes of lactococci.
Engineering of Carbon Distribution between Glycolysis and Sugar Nucleotide Biosynthesis in
Lactococcus lactis
Authors: Boels Ingeborg C. , Kleerebezem Michiel , de Vos Willem M.
Published: February 2003

We describe the effects of modulating the activities of glucokinase, phosphofructokinase, and phosphoglucomutase on the branching point between sugar degradation and the biosynthesis of sugar nucleotides involved in the production of exopolysaccharide biosynthesis by
Lactococcus lactis
. This was realized by using a described isogenic
L. lactis
mutant with reduced enzyme activities or by controlled expression of the well-characterized genes for phosphoglucomutase or glucokinase from
Escherichia coli
or
Bacillus subtilis
, respectively. The role of decreased metabolic flux was studied in
L. lactis
strains with decreased phosphofructokinase activities. The concomitant reduction of the activities of phosphofructokinase and other enzymes encoded by the
las
operon (lactate dehydrogenase and pyruvate kinase) resulted in significant changes in the concentrations of sugar-phosphates. In contrast, a >25-fold overproduction of glucokinase resulted in 7-fold-increased fructose-6-phosphate levels and 2-fold-reduced glucose-1-phosphate and glucose-6-phosphate levels. However, these increased sugar-phosphate concentrations did not affect the levels of sugar nucleotides. Finally, an ∼100-fold overproduction of phosphoglucomutase resulted in 5-fold-increased levels of both UDP-glucose and UDP-galactose. While the increased concentrations of sugar-phosphates or sugar nucleotides did not significantly affect the production of exopolysaccharides, they demonstrate the metabolic flexibility of
L. lactis.
High-Performance Liquid Chromatography Analyses of Pyoverdin Siderophores Differentiate among Phytopathogenic Fluorescent
Pseudomonas
Species
Authors: Bultreys Alain , Gheysen Isabelle , Wathelet Bernard , Maraite Henri , de Hoffmann Edmond
Published: February 2003

The relationship of pyoverdins produced by 41 pathovars of
Pseudomonas syringae
and by phytopathogenic
Pseudomonas
species was investigated. A high-performance liquid chromatography method for analyzing the culture medium proved to be superior to isoelectric focusing for detecting pyoverdin production, for differentiating slightly different pyoverdins, and for differentiating atypical from typical Fe(III)-chelated pyoverdins. Nonfluorescent strains were found in
Pseudomonas amygdali
,
Pseudomonas meliae
,
Pseudomonas fuscovaginae
, and
P. syringae. Pseudomonas agarici
and
Pseudomonas marginalis
produced typical pyoverdins. Among the arginine dihydrolase-negative fluorescent
Pseudomonas
species, spectral, amino acid, and mass spectrometry analyses underscored for the first time the clear similarities among the pyoverdins produced by related species. Within this group, the oxidase-negative species
Pseudomonas viridiflava
and
Pseudomonas ficuserectae
and the pathovars of
P. syringae
produced the same atypical pyoverdin, whereas the oxidase-positive species
Pseudomonas cichorii
produced a similar atypical pyoverdin that contained a glycine instead of a serine. The more distantly related species
Pseudomonas asplenii
and
Pseudomonas fuscovaginae
both produced a less similar atypical pyoverdin. The spectral characteristics of Fe(III)-chelated atypical pyoverdins at pH 7.0 were related to the presence of two β-hydroxyaspartic acids as iron ligands, whereas in typical pyoverdins one of the ligands is always ornithine based. The peptide chain influenced the chelation of iron more in atypical pyoverdins. Our results demonstrated that there is relative pyoverdin conservation in the amino acids involved in iron chelation and that there is faster evolution of the other amino acids, highlighting the usefulness of pyoverdins in systematics and in identification.
Oligonucleotide Microarray for the Study of Functional Gene Diversity in the Nitrogen Cycle in the Environment
Authors: Taroncher-Oldenburg Gaspar , Griner Erin M. , Francis Chris A. , Ward Bess B.
Published: February 2003

The analysis of functional diversity and its dynamics in the environment is essential for understanding the microbial ecology and biogeochemistry of aquatic systems. Here we describe the development and optimization of a DNA microarray method for the detection and quantification of functional genes in the environment and report on their preliminary application to the study of the denitrification gene
nirS
in the Choptank River-Chesapeake Bay system. Intergenic and intragenic resolution constraints were determined by an oligonucleotide (70-mer) microarray approach. Complete signal separation was achieved when comparing unrelated genes within the nitrogen cycle (
amoA
,
nifH
,
nirK
, and
nirS
) and detecting different variants of the same gene,
nirK
, corresponding to organisms with two different physiological modes, ammonia oxidizers and denitrifying halobenzoate degraders. The limits of intragenic resolution were investigated with a microarray containing 64
nirS
sequences comprising 14 cultured organisms and 50 clones obtained from the Choptank River in Maryland. The
nirS
oligonucleotides covered a range of sequence identities from approximately 40 to 100%. The threshold values for specificity were determined to be 87% sequence identity and a target-to-probe perfect match-to-mismatch binding free-energy ratio of 0.56. The lower detection limit was 10 pg of DNA (equivalent to approximately 10
7
copies) per target per microarray. Hybridization patterns on the microarray differed between sediment samples from two stations in the Choptank River, implying important differences in the composition of the denitirifer community along an environmental gradient of salinity, inorganic nitrogen, and dissolved organic carbon. This work establishes a useful set of design constraints (independent of the target gene) for the implementation of functional gene microarrays for environmental applications.