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Dispersions of purely semiconducting single-walled carbon nanotubes (SWCNTs) have enabled solution-processed SWCNT networks as active layers in field-effect transistors (FETs) with high carrier mobilities and excellent on/off current ratios. Although reproducibility has improved in recent years, reaching the level that is required for commercial large-scale processing remains a challenge. A key issue is the tendency of SWCNTs to aggregate over time, resulting in network inhomogeneities that cause large device performance variations. Based on the tailored formulation of colloidal inks by the choice of solvent and use of additives, we demonstrate the strong stabilization effect of phenanthroline additives on polymer-sorted (6,5) SWCNT using time-dependent near-infrared absorption spectroscopy as a fast and simple assessment tool for the aggregation rate. The addition of the N-heteropolycycle 1,10-phenanthroline significantly extends the stability of dispersions of polymer-wrapped nanotubes in toluene and hence improves the morphology of spin-coated networks even after ink storage for several days. Bottom-contact, top-gate FETs based on such networks show much higher charge carrier mobilities and drastically reduced device-to-device variations compared to devices based on SWCNT dispersions without phenanthroline. Nanotube ink formulations with small-molecule additives are an important step toward reproducible device parameters and are crucial for the translation of nanotube FETs from the laboratory to commercial applications.

The high sensitivity of carbon nanotube devices as gas sensors is generally attributed to the large surface area of the material, whereas selectivity is often imparted by the “lock-and-key” mechanism. In contrast with this picture, where little function is conferred to the underlying substrate, we demonstrate that bottom gate carbon nanotube network field-effect transistors (CNN-FETs) fabricated on a variety of polymer dielectrics, but with the same pristine carbon nanotubes, show differentiated responses to analytes. Interaction with the environment thus occurs not only directly with carbon nanotubes but is often mediated by the substrate. In a broad survey of polymers, we identify several materials suitable as the gate dielectric of printed devices and that yield a vanishing gate hysteresis in ambient air. The other two classes of polymers that induce a lagging or advancing hysteresis in CNN-FETs are found desirable for gas sensing applications. Moreover, we have tested CNN-FETs with different polymer gate dielectrics against a series of analytes ranging from common alcohols to organic solvents and amines, among others. A rich set of distinct and complementary analyte signatures could be observed, identifying the polymer gate dielectric as a differentiator for chemical discrimination in a future printed cross-reactive sensor array, that is, an electronic nose.

Although carbon nanotube transistors present outstanding performances based on key metrics, large-scale uniformity and repeatability required in printable electronics depend greatly on proper control of the electrostatic environment. Through a survey of polymer dielectric encapsulants compatible with printing processes, a simple correlation is found between the measured interfacial charge density and the onset of conduction in a transistor, providing a rational route to control the electrical characteristics of carbon nanotube transistors. Smooth and continuous balancing of the properties between unipolar p-type and n-type transport is achieved using a molar fraction series of poly(styrene-co-2-vinylpyridine) statistical copolymers combined with an electron-donating molecule. We further demonstrate the easy fabrication of a p-n diode which shows a modest rectification of 8:1.

To fully actualize the potential of boron nitride nanotubes (BNNTs), it is necessary to overcome the inherent insolubility of this nanomaterial. Drawing on the successes realized in the analogous carbon nanotube field, noncovalent functionalization with conjugated polymers offers a simple, scalable route toward the production of stable dispersions of BNNTs. 2,7-carbazoles were chosen as our core monomer based on density functional theory (DFT) predictions, which suggest superior interactions with BNNTs when compared to fluorene-BNNT interactions. Homo poly(2,7-carbazole)s and copolymers with fluorenes were synthesized and used successfully to disperse BNNTs into organic solvents. Thermogravimetric analysis and atomic force microscopy results confirm the proficiency of these polymers to disperse large amounts (> 80% by weight) of individualized BNNTs. Analysis of absorbance data shows that the choice of solvent is critical, with stability enhanced in THF compared to CHCl₃ due to the more efficient planarization of polymer chains on the surface of BNNTs, particularly for the homopolymers. The utility of these highly-soluble poly(2,7-carbazole)-BNNT complexes for printed electronics and transparent composites was demonstrated by the fabrication of simple capacitors and incorporation into poly(methyl methacrylate) composites, respectively.

A generalization of the Raman scattering (RS) spectrum, the Raman excitation map (REM) is a hyperspectral two-dimensional (2D) data set encoding vibrational spectra, electronic spectra and their coupling. Despite the great potential of REM for optical sensing and characterization with remarkable sensitivity and selectivity, the difficulty of obtaining maps and the length of time required to acquire them has been practically limiting. Here we show, with a simple setup using current optical equipment, that maps can be obtained much more rapidly than before (~ms to ~100 s now vs. ~1000 s to hours before) over a broad excitation range (here ~100 nm is demonstrated, with larger ranges straightforward to obtain), thus taking better advantage of scattering resonance. We obtain maps from different forms of carbon: graphite, graphene, purified single walled carbon nanotubes (SWCNTs) and chirality enriched SWCNTs. The relative speed and simplicity of the technique make REM a practical and sensitive tool for chemical analysis and materials characterization.

We describe a technique to prepare ensembles of neat, unbundled single-walled carbon nanotubes decorated with pure size-filtered metal nanoparticles. Polymer-encased nanotubes are drop-cast on nanoporous transmission electron microscope membrane grids, mounted within a nanoparticle-deposition apparatus, baked in situ to remove the polymer coating, and exposed to a beam of pure size-selected metal nanoparticles. Subsequent electron microscopy imaging reveals the presence of nanoparticles supported by pure suspended single-walled nanotubes. This method is promising for efficient production of prototype chemical and physical devices which require the presence of clean well-defined nanoparticle-nanotube hybrids for characterization, imaging, and applications.

Conjugated polymer extraction (CPE) is a low-cost, scalable process that can enrich single-walled carbon nanotube (SWCNT) materials in organic media. For other separation methods in aqueous phases, redox chemistry and/or pH control dramatically affect the sorting process of the SWCNTs. We have previously determined that the CPE process can be fine-tuned by adjusting the pH on the tube surface. Here, we systematically studied the effect of redox chemistry on the CPE process by adding organic p-/n-dopants. At a very strong p-/n-doping level, static repulsions dominated the interactions between the tubes and the CPE lost selectivity. When the doping level changed from a medium p-doping to a neutral state, the yield of CPE increased and the selectivity was compromised. We also observed chiral selectivity when a weak p-dopant was used. A photoluminescence excitation mapping under different titration conditions provided more insight into the doping level of the tubes relative to their diameters, chiralities, and redox potentials. We proposed a mechanism for the CPE process. The semiconducting and metallic tubes are separated because of their different solubilities, which are determined by the bundling energy between the tubes and are related to their doping level in polymer solutions.

Here, we present a metrological perspective on SWCNT optics, covering only the most essential photophysics, discussing sample properties and conditions that affect optical characterization, and the current metrological application of the most important optical characterization methods. We give a perspective on the continued development of nanotube metrology. Standards meetings, technical specifications, and other standard documents and protocols, as well as SWCNT reference materials are reviewed., Series: World Scientific Series on Carbon Nanoscience, Series: Handbook of Carbon Nano Materials

Spontaneously charged aerosol droplets, each containing a few single-walled carbon nanotubes wrapped in conjugated polymers were precipitated on a target substrate using electrostatic forces. Nanotube networks were assembled on a variety of dielectric surfaces including polymers with low surface energy: Good transistor performance was achieved in all cases. Under proper regime of electrostatic and flow fields, patterns were produced in a maskless fashion and feature sizes below 150 μm were demonstrated.

SWCNT network morphology and TFT performance of polyfluorene wrapped sc-SWCNT on different substrates is reported. The polymer/tube weight ratio and concentration impacts network formation and device performance. Hydrophilic SiO2 surfaces show stronger adsorption compared to poly-l-lysine treated SiO2, which leads to more uniform and higher density networks. TFTs with mobility up to 38 cm2/Vs with

A novel purification process for the enrichment of sc-SWCNTs that combines selective conjugated polymer extraction (CPE) with selective adsorption using silica gel, termed hybrid-CPE (h-CPE), has been developed, providing a high purity sc-SWCNT material with a significant improvement in process efficiency and yield. Using the h-CPE protocol, a greater than 5 fold improvement in yield can be obtained compared to traditional CPE while obtaining sc-SWCNT with a purity99.9% as assessed by absorption spectroscopy and Raman mapping. Thin film transistor devices using the h-CPE derived sc-SWCNTs as the semiconductor possess mobility values ranging from 10-30 cm(2) V(-1) s(-1) and current ON/OFF ratio of 10(4)-10(5) for channel lengths between 2.5 and 20 μm.

Here the growth kinetics of vertically aligned carbon nanotube forests depend on the size of the patterned catalyst films from which they grow. Forests are grown using chemical vapor deposition on thin film catalyst islands patterned at the 100 μm scale on silicon wafers. The smaller the pattern, the faster the forest grows and the earlier it stops growing. Furthermore, the shape and structure of the forests, in particular the concavity of their top surface, also depend on the size of the catalyst islands. This result can be understood as a consequence of the high efficiency by which the acetylene source gas is converted into carbon nanotubes (here ∼30%) and a varying local amount of acetylene source gas available for growth. A diffusion model can explain the observed shape and structure of the forests and their growth kinetics by using experimentally measured parameters. This model also gives insight into the density and growth rate of carbon nanotube forests and suggests a mechanism that coordinates growth rates across the sample and, under certain conditions, can limit the fraction of catalyst nanoparticles that produce nanotubes.

The dynamics of the chemical vapor deposition (CVD) of single-walled carbon nanotubes (SWNTs) is extracted experimentally using in situ Raman spectroscopy. Nanotubes are grown using a thin fi lm cobalt catalyst and an ethanol precursor in a miniature hot walled reactor with optical access. Raman spectra at room temperature and at the growth temperature are compared for two growth temperatures. The evolution of the G-band, D-band, and radial breathing mode (RBM) is tracked at the growth temperature with time resolution of a few seconds. There are three identifiable phases in the evolution of the Raman signal intensity: an initial exponential increasing phase, a linear growth phase, and a saturation phase. In situ optical spectroscopy thus enables the study of nucleation, steady growth, and deactivation processes to be investigated separately in real time. The evolution curves for all bands (G, D, and RBM), when scaled, collapse onto the same curve, to within experimental uncertainty.

Vertically aligned carbon nanotube forests show various morphologies on both macro- and micro-scales. These morphologies are a result of growth mechanisms and interactions between nanotubes. By investigating these morphologies, we study both the growth mechanisms and the interactions. We examine forest morphologies in situ, dynamically during chemical vapor deposition growth and ex situ, post growth. In situ observations allow the separate characterization of nucleation, growth and termination phases, and the exploration of connections between morphology and growth. Forests systematically show different morphologies, ranging from uniform to cracked, delaminated, and periodically rippled. These are discussed in terms of the balance of forces within the forests including cohesion, adhesion, and stiffness. We propose a simplified model that predicts termination as a result of an imbalance in the forces present. We show that growth rate differences drive many morphological effects, and these differences originate in the nucleation phase due to gas diffusion., 219th ECS Meeting, 1-6 May 2011, Montreal, QC, Canada

The existence of a critical temperature T c in the chemical vapor deposition growth of carbon nanotubes, including single-walled nanotubes, is shown by in situ optical imaging of growing nanotube forests. At T c , the temperature dependence of the chemical processes and characteristics of the grown nanotubes change. The lifetime and initial growth rate show this critical temperature and are linked by a characteristic energy through the initial growth rate decay. Ex situ Raman spectroscopy shows that the number of radial breathing modes, G band, and D band show the same characteristic energies and T c .

We show how photoluminescence (PL) imaging can be used to gain fundamental spectroscopic information on the optical excitations in single-walled carbon nanotubes (SWNTs). Individual SWNTs are identified, and emission and excitation (PLE) spectra are obtained. In addition to the dominant E l i and Ej 2 transitions, we observe several resonances including the transverse Ej 2 exciton, and up to three excited excitonic bands. The (n,m) family dependence is obtained for some 25 species in the 1.1 to 1.6 pm emission range. We provide simple power series expansion to facilitate (n,in) assignment of all these peaks. This paper complements recently published work.

Physically cross-linked beta-lactoglobulin (BLG) protein gels containing theophylline and sulfamethoxazole low molecular weight drugs were prepared in 50% ethanol solution at pH 8 and two protein concentrations (6 and 7% (w/v)). Swelling behavior of cylindrical gels showed that, irrespective of the hydrated or dehydrated state of the gel, the rate of swelling was the highest in water. When the gels were exposed to water, they first showed a swelling phase in which their weight increased 3 and 30 times for hydrated and dehydrated gels, respectively, due to absorption of water, followed by a dissolution phase. The absorption of solvent was however considerably reduced when the gels were exposed to aqueous buffer solutions. The release behavior of both theophylline and sulfamethoxazole drugs from BLG gels was achieved in a time window ranging from 6 to 24 h. The drug release depended mainly on the solubility of the drugs and the physical state of the gel (hydrated or dry form). Analysis of drug release profiles using the model of Peppas showed that diffusion through hydrated gels was governed by a Fickian process whereas diffusion through dehydrated gels was governed partly by the swelling capacities of the gel but also by the structural rearrangements inside the network occurring during dehydration step. By a judicious selection of protein concentration, hydrated or dehydrated gel state, drug release may be modulated to be engineered suitable for pharmaceutical as well as cosmetics and food applications.

Creep and creep recovery, stress relaxation and small- and large-amplitude oscillatory shear experiments have been used to study the steady-state flow behaviour and the transient viscoelastic response of wheat flour dough in shear over large ranges of time, stress and strain. The results are discussed with reference to the limited body of reliable literature data. Dough does display a linear viscoelastic domain. The complex character of its non-linear viscoelastic properties is essentially due to the extremely low shear rate limit of the initial Newtonian plateau and to the onset of time-dependent flow behaviour above a certain strain threshold, which explain qualitatively the discrepancies observed in certain cases on a part of the range of the rheological variables explored, despite global self-consistency of the results. Comparison of gluten and dough linear viscoelastic properties shows that dough cannot be viewed simply as a concentrated suspension of starch granules in! the hydrated viscoelastic gluten matrix.

We demonstrate confocal, direct Raman imaging of carbon nanotubes based on the detection of the $G$-band using an electron multiplying charge coupled detector. Individual carbon nanotubes and bundles of nanotubes are observed in real-time over a large area using global illumination. We use the technique to show that suspended nanotubes can be manipulated selectively with a focused laser beam and describe the relevant physical mechanisms.

We present optical spectra from numerous, single, self-assembled InAs/InP quantum dots. More than 50 individual dots are studied that emit in the 1.1-1.6 mm wavelength range. The dots are of high optical quality as judged by the clean, single exciton emission line at low power, the resolution limited linewidth, and the brightness. Each dot exhibits similar trends in the power evolution spectra, despite large variations in height and diameter. The level splittings in the p -shell increase with decreasing height, which we interpret to be from dot elongation along the [01] direction. The evolution of the spectra with increasing power agrees well with predictions from effective bond orbital calculations. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The stable anhydrous form of α-lactose has been obtained by the dehydration of α-lactose monohydrate in methanol. An X-ray powder diffraction pattern was recorded at room temperature with a laboratory diffractometer equipped with an INEL curved sensitive detector CPS120. The starting structural model of this form was found by a Monte-Carlo simulated annealing method. The structure was obtained through Rietveld refinements and the minimization of crystalline energy for the localization of the H atoms of the hydroxyl groups. Soft restraints were applied to bond lengths and angles. Networks of O—H...O hydrogen bonds account for the crystalline cohesion. A comparison is made between the hydrogen-bond networks of this form and those of the monohydrate and hygroscopic anhydrous forms of α-lactose.

The mixed form of α/β lactose was obtained by heating amorphous α-lactose at 443 K. NMR spectroscopy determined the stoichiometry of this mixed compound to be 1/1. The X-ray powder diffraction pattern was recorded at room temperature with a sensitive curved detector (CPS 120). The structure was solved by real-space methods (simulated annealing) followed by Rietveld refinements with soft constraints on bond lengths and bond angles. The H atoms of the hydroxyl groups were localized by minimization of the crystalline energy. The cell of 1/1 α/β lactose is triclinic with the space group P1 and contains two molecules (one molecule of each anomer). The crystalline cohesion is achieved by networks of O—H...O hydrogen bonds. The width of the Bragg peaks is interpreted through a microstructural approach in terms of isotropic strain effects and anisotropic size effects.

Photoluminescence measurements in magnetic fields up to 16T are reported from single InAs∕InP quantum dots nucleated at the apex of individual InP pyramidal nanotemplates. Clear Zeeman splitting and diamagnetic shifts of the s-shell exciton peak are observed and are correlated with the quantum-dot dimension. This behavior is interpreted in terms of a template-controlled quantum-dot diameter and is corroborated by the observed diamagnetic shifts.

The spectroscopy of single InAs/InP quantum dots emitting close to 1.55 μm is described. The dots are produced using a nanotemplate deposition technique that allows precise, a priori control of quantum dot position and electronic configuration. The experimentally observed luminescence signal from the p-shell is composed of several lines. Using exact diagonalization calculations of the emission spectra we interpret the splittings between these lines in terms of Coulomb induced, many-body renormalization of the excitonic states and a template-induced shape asymmetry of the quantum dot.

The importance of the soluble fraction in flour in determining the rheological properties of dough subjected to large deformations and its possible consequence for breadmaking performance was investigated by measuring shear and extensional viscosities of native wheat flour and reconstituted doughs using creep–recovery tests and lubricated squeezing flow tests. Standard French breadmaking wheat flour was fractionated into water-soluble and insoluble fractions. Flour defatting and puroindoline additions were also tested. The large strain properties of doughs made from these fractions were studied in shear and biaxial extension by creep–recovery and lubricated squeezing flow tests, respectively. Shear viscosity η exhibited a Newtonian plateau at low shear rate γ ˙ and a shear thinning behaviour at higher γ ˙ in the range 10−6–1 s−1. Apparent viscosity versus shear rate data could be fitted by Cross model. The results elucidated the lubricating role of the soluble fraction, and were confirmed by the bi-extensional test, although obtained over a narrower strain rate range ( e ˙ b = 10 − 3 − 1 s − 1 ), for strain values in the range (0.1–1). The specific volume of bread was found to be inversely related to bi-extensional viscosity, whereas crumb fineness could be related to the strain hardening index, in agreement with gas retention and bubble growth phenomena during proofing.

Photoluminescence (PL) and photoluminescence excitation (PLE) spectra are obtained from individual single-walled carbon nanotubes (SWNTs). Individual SWNT spectra are compared with spectra from ensembles. The PL spectrum of an individual SWNT in air at room temperature has a single asymmetric peak of width typically 10 to 15 meV, with no detected background. Both absorption and emission are strongly polarized along the tube axis. Photoluminescence excitation spectroscopy on single SWNTs clearly confirms the unique, one-to-one association of optical absorption resonances with individual emission peaks. Resonances in the PLE spectra are typically $\ensuremath{\approx}30\mathrm{meV}$ wide, with the PL intensity enhanced tenfold over nonresonant excitation. Whether for emission or absorption, the peak shape and peak width are almost the same for a single nanotube as they are for the corresponding species in a large ensemble. That is, there is no significant inhomogeneous broadening. While ensemble measurements are complicated by the superposition of many PL peaks from many different species, single nanotube spectra clearly isolate a single peak and are thus simpler to interpret.

We investigate the optical emission characteristics of InAs/InP quantum dot microcavities emitting around λ=1.5 μm . Planar and etched-pillar-type microcavities employing multi-layer dielectric Bragg mirrors are demonstrated. For planar cavities we observe a large increase in the cavity mode emission as a function of increasing temperature above 4 K , which is interpreted in terms of carrier trapping within the wetting layer. Pillar-type microcavities show the discrete mode structure expected for full three-dimensional optical confinement.

Selective area laser annealing of GaAs/AlxGa1−xAs quantum well infrared photodetector (QWIP) material has been investigated as a possible route towards the fabrication of two-color low-cost focal plane array devices. Tuning of the wavelength response of the material has been achieved as a consequence of the quantum well intermixing (QWI) effect. A 90 s irradiation with a continuous wave Nd:yttrium–aluminum–garnet laser, at the peak temperature of 850 °C, resulted in the 40 nm blueshift of the QW photoluminescence peak from 832 to 792 nm. This corresponded to the 0.7 μm redshift of the wavelength response of the investigated QWIP microstructure in the 8 μm optical absorption region. The amplitude of this shift is consistent with the literature data obtained for similar material processed directly by rapid thermal annealing (RTA) or by a two-step process involving particle implantation and RTA. We have examined the laser-QWI approach for direct writing of arrays of a two-band gap material. The preliminary results indicate the feasibility of this approach for fabricating linear arrays with a period of 0.8 mm., 11th Canadian Semiconductor Technology Conference, August 2003, Ottawa, Canada

We present optical spectroscopy measurements on a single InAs/InP quantum dot emitting around λ=1.55 μm. The dot is produced using a nanotemplate deposition technique that allows precise a priori control of quantum dot position and electronic configuration. Clear evidence of excitonic shell structure and many-body renormalization effects are observed.

Dynamic measurements and retardation tests (creep and creep recovery) were combined systematically to characterise the different aspects of gluten rheology. Gluten flow behaviour exhibits extreme shear thinning and time-dependent viscosity. The viscoelastic contribution to the total strain behaves linearly over large strain and stress ranges; however, the material shows physical ageing and does not exhibit time translational invariance. In spite of these difficulties, it was possible, in the case of fresh gluten samples or after the material has been submitted to mechanical loading for a period long enough, to obtain the linear viscoelastic response over seven logarithmic decades of time or frequency by combining the data from dynamic measurements and from retardation tests. The method of Kaschta was used to calculate the discrete retardation spectrum from creep recovery data; the spectrum was then used to convert recovery data into storage and loss compliance and modulus. Converted recovery data match dynamic data satisfactorily in the case of fresh gluten. Some discrepancy was observed in the case of gluten submitted to previous mechanical history; it has been attributed to the effect of physical ageing. The storage and nonviscous loss compliance functions show that the extension of the viscoelastic plateau of gluten is very large. Their shape suggests a bimodal spectrum; however, this point would need further confirmation.

Selective area chemical beam epitaxy is used to fabricate submicron [100]-oriented InP ridges with well-defined, defect-free (011) sidefacets and (001) tops. Following the deposition of two monolayers of InAs on such nanotemplates and subsequent capping with InP, photoluminescence spectra show for wider ridges strong emission from a thin InAs quantum well and, as the ridge width is reduced, a gradual appearance of a quantum dot emission at lower energy. The method allows continuous tuning on a given sample in a single growth run of both the quantum dot density and the emission wavelength. The result is a consequence of adatom diffusion from the ridge sidefacets onto the top (001) facet, which increases the amount of InAs beyond the critical thickness for three-dimensional nucleation to occur. Compared with growth on planar InP(001) substrates, InAs self-assembled quantum dots grown on these nanotemplates are more uniform as revealed by a twofold reduction in emission linewidth at 4 K.

Liquid water-gelatin-locust bean gum (LBG) systems, in the conditions of lowest compatibility (near the isoelectric pH of the protein), were explored using confocal laser scanning microscopy and viscometry. Confocal microscopy observation proved to be a more sensitive method to assess the phase state of the systems than the usual centrifugation or viscometric ones. It showed that in fact the system is already phase-separated well below the apparent binodal line determined by the other methods. When the continuous phase is enriched in LBG, the observed microstructure is of the expected type with spherical droplets of gelatin dispersed in the LBG phase. Bicontinuous systems were observed close to the inversion point, which is clearly detected by viscosity measurements; beyond this point, the gelatin enriched continuous phase contained large inclusions of LBG, apparently due to aggregation of the LBG disperse phase.For systems with a LBG enriched continuous phase, shear thinning behaviour, similar to that of a LBG solution, was observed; the low newtonian viscosity decreased as the volume fraction of the gelatin disperse phase increased, following approximately the usual logarithmic additivity law. In the reverse case, where the continuous phase is enriched in gelatin, flow curves with an apparent yield stress and a negative deviation from the logarithmic additivity law were observed. The variation of the viscosity with the volume fraction of the disperse phase obeyed approximately the equation of Palierne for systems of two viscoelastic phases when the interface is purely elastic.The microstructure of phase-separated mixtures depends on the phase viscosity ratio, which also determines the differences in flow behaviour.

We investigate high-field transport in bundles of single-wall carbon nanotubes (SWNTs) with multiple contacts. The bundles carry currents in excess of $250\ensuremath{\mu}\mathrm{A}$ (a current density of ${10}^{9}\mathrm{A}/{\mathrm{cm}}^{2})$ before saturation and electrical breakdown, indicating that conduction is predominantly by nanotubes on the surface of the bundle which are directly contacted by electrodes. Using a four-probe configuration, we measure the contact resistance and show that it is nearly constant as the bias voltage varies. This strongly supports the notion that electron-phonon scattering, and not a contact effect, causes current saturation at high-field. Electrical breakdown proceeds by sequential destruction of individual metallic nanotubes on the bundle surface, with steplike current drops of about $12\ensuremath{\mu}\mathrm{A}.$ At very high bias, the current-carrying capacity of the bundle increases due to field-enhanced coupling between nanotubes in the bundle.

The composition of high molecular weight (HMW) subunits of glutenin determines the gluten strength and influences the baking quality of bread wheat. Here, the effect of transgenes coding for subunits 1Ax1 and 1Dx5 was studied in two near-isogenic wheat lines differing in their HMW subunit compositions and mixing properties. The subunits encoded by the transgenes were overexpressed in the transformed lines and accounted for 50-70% of HMW subunits. Overexpression of 1Ax1 and 1Dx5 subunits modified glutenin aggregation, but glutenin properties were much more affected by expression of the 1Dx5 transgene. This resulted in increased cross-linking of glutenin polymers. In dynamic assay, the storage and loss moduli of hydrated glutens containing 1Dx5 transgene subunits were considerably enhanced, whereas expression of the 1Ax1 transgene had a limited effect. The very high strength of 1Dx5 transformed glutens resulted in abnormal mixing properties of dough. These results are discussed with regard to glutenin subunit and glutenin polymer structures.

We have used transmission electron microscopy and low temperature photoluminescence to study the growth by chemical beam epitaxy of self-assembled InAs/InP quantum dots. Square dots 30-40 nm in width were grown with a density of 1010 cm-2. By partially covering the dots with InP and exposing them to a phosphorus overpressure the mean height of the dots, and hence their emission energies, could be controlled. The emission energy was found to be strongly dependent on the amount of InP used to partially cover the InAs dots, but only weakly dependent on the phosphorus exposure time for times greater than 15 sec, 2000 International Conference on Indium Phosphide and Related Materials, 14-18 May 2000, Williamsburg, Virginia, USA

Viscoelasticity of hydrated gluten depends on composition of HMW gluten subunits (GS), size distribution of glutenin polymers, and proteinprotein interactions. Glutens extracted from four near-isogenic lines with differing HMW-GS were analyzed. Rheological properties were studied by dynamic assay in shear. Size distribution of prolamins was determined by sequential extraction and size-exclusion HPLC. Assays performed at 20°C confirmed that viscoelasticity was determined by large glutenin polymers. The abundance of large glutenin polymers depended on the HMW-GS composition of the lines. Difference of functionality linked to subunit structure was highlighted by comparing the behaviors of the 1A/1B null and 1A/1D null lines. Glutens were submitted to heating and cooling cycles, with or without an SH-blocking agent (N-ethylmaleimide [NEMI]). At 20–40°C, no irreversible changes of the mechanical properties occurred. Thermal treatment affected chain mobility, and possibly H bonds, but not the chemical ...

We present a simple shadow mask method to fabricate electrodes with nanometer scale separation. Metal wires with gaps are made by incorporating multiwall carbon nanotubes or single-wall carbon nanotube (SWNT) bundles into a trilayer electron beam lithography process. The simple, highly controllable, and scaleable method has been used to make gaps with widths between 20 and 100 nm and may be extended to gap sizes of 1 nm. We report electron transport measurements of individual SWNTs bridging nanogaps with electrode spacings of approximately 20 nm. Metallic SWNTs exhibit quantum dot behavior with an 80 meV charging energy and a 20 meV energy level splitting. We observe a strong field effect behavior in short semiconducting SWNT segments, evidence for diffusive electron transport in these samples.

A transglutaminase from Streptoverticillium sp. was used to create new covalent intermolecular cross-links between proteins in gluten. This modification induced drastic changes in its physicochemical properties as well as in its rheological behavior. To understand these changes, we characterized the gluten extractability in acetic acid and identified the proteins of supernatant and pellet by immunoblotting using antibodies specific for each prolamin class. The proportion of soluble proteins decreased drastically after transglutaminase treatment due to the formation of large insoluble polymers as shown by SDS-PAGE. Among the constitutive proteins of gluten, the high molecular weight glutenin subunits were the most affected in the transglutaminase reaction. The rheological behavior of gluten after 18 hr of incubation with transglutaminase was studied in shear by dynamic measurements over 10-3 – 101 Hz frequency range and by creep and recovery tests. The behavior of treated glutens remained that of ...

The effects of pH, ionic strength and heat on the structure of beta-lactoglobulin (beta-lg) have been investigated by studying the intrinsic tryptophan fluorescence of the protein. Between pH 2 and 9, for sodium chloride concentrations varying from 0.0 to 0.2 M, the position of the fluorescence emission maximum at 20 degrees C remained constant at 328 nm, suggesting that the hydrophobic environment of the fluorophores remained unchanged. The fluorescence intensity increased significantly at pH 2, 7 and 9 on reducing the NaCl concentration of the solutions. The most likely explanation for this, supported by recent light scattering data, is that the presence of NaCl is necessary for beta-lg to dimerize. At the higher NaCl concentrations it was found that dimerization accompanied a reduction in fluorescence intensity. Thus, dissociation appears to reduce the self-quenching of tryptophan residues that occurs within the dimer. The fluorescence of solutions heated below the denaturation temperature reflected the state of association of the protein molecules. Above the denaturation temperature and associated with protein aggregation, an irreversible increase in intrinsic tryptophan fluorescence was observed. We also report what we believe to be the first front-face fluorescence measurements on globular protein gels, showing effects of pH and NaCl concentration.

The aim of this study was to understand the competition between phase separation and gelation during thermal treatment for BSA - HEC or CMC mixtures located in the one-phase region of the state diagram at 20 °C. We used dynamical rheological and small-angle neutron scattering (SANS) experiments to characterize the structure of the biopolymer mixtures in both sol and gel states. In the sol state, according to SANS and rheological measurements, the structure of the mixtures located in the one-phase region was heterogeneous and could be compared to colloidal suspensions with incipient flocculation. In the gel state, drastic changes occurred in the structure and the rheological behaviour of the gels obtained from the mixtures as compared to pure BSA, depending on the pH and temperature conditions but also the polysaccharide used. In particular, the addition of CMC to BSA solution led to the formation of gels with different macroscopic aspects ranging from a milky to a transparent character depending on the polymer concentration; moreover, competition between gelation and phase separation occurred depending on the pH and the temperature applied to the system.

The gluten proteins of three near-isogenic lines of the wheat Sicco, which differed only in high Mr glutenin subunit composition, were studied. Gluten was extracted and fractionated by a sequential extraction procedure with dilute HC1. The deletion of Glu-A1 and -D1 encoded subunits decreased the total content of high Mr glutenin subunits by about one-half as determined by RP-HPLC and SDS-PAGE, and considerably increased glutenin extractability in sequential extraction procedures. At the same time, dynamic measurements in shear showed that viscoelasticity was diminished markedly, which SE-HPLC analysis indicated was due to a lower content of large glutenin polymers and a higher content of gliadin. The substitution of subunits 5 + 10 normally present in Sicco by subunits 2 + 12 changed neither the total amount of high Mr glutenin subunits nor the relative amounts of individual components. Glutens differed, however, in that their viscoelasticities were lower when subunits 2 and 12 were present, lower levels of unextractable large glutenin polymers were also present. On the other hand, gluten subfractions with the same distribution of polymer size showed the same viscoelastic behaviour independent of high Mr subunit composition. A very strong correlation was found between the proportion of large glutenin polymers and the viscoelasticity of gluten subfractions. It was concluded that both the quantity of high Mr glutenin subunits and subunit composition influence gluten viscoelasticity by modifying the polymer size distribution and aggregative properties of gluten proteins.