Dielectric Character Research Articles

Mechanisms of Spectral Tuning in Blue Cone Visual Pigments: VISIBLE AND RAMAN SPECTROSCOPY OF BLUE-SHIFTED RHODOPSIN MUTANTS

Spectral tuning by visual pigments involves the modulation of the physical properties of the chromophore (11-cis-retinal) by amino acid side chains that compose the chromophore-binding pocket. We identified 12 amino acid residues in the human blue cone pigment that might induce the required green-to-blue opsin shift. The simultaneous substitution of nine of these sites in rhodopsin (M86L, G90S, A117G, E122L, A124T, W265Y, A292S, A295S, and A299C) shifted the absorption maximum from 500 to 438 nm, accounting for 2,830 cm-1, or 80%, of the opsin shift between rhodopsin and the blue cone pigment. Raman spectroscopy of mutant pigments shows that the dielectric character and architecture of the chromophore-binding pocket are specifically altered. An increase in the number of dipolar side chains near the protonated Schiff base of retinal increases the ground-excited state energy gap via long range dipole-dipole Coulomb interaction. In addition, the W265Y substitution causes a decrease in solvent polarizability near the chromophore ring structure. Finally, two substitutions on transmembrane helix 3 (A117G and E122L) act in combination with the other substitutions to alter the binding-pocket structure, resulting in stronger interaction of the protonated Schiff base group with the surrounding dipolar groups and the counterion. Taken together, these results identify the amino acid side chains and the underlying physical mechanisms responsible for a majority of the opsin shift in blue visual pigments.

A differential capacitive biosensor using polyethylene glycol to overlay the biolayer

The feasibility of immunosensors based on capacitance measurements on semiconductor-immobilized antibody-electrolyte heterostructures is being investigated. Capacitance measurements on biosensors succeed only if the successive biomolecular layers grafted onto the heterostructures are sufficiently electrically insulating and retain their recognizing ability (Jaffrezic-Renault et al., Sensors and Actuators B 15–16 (1993) 458–462; Bergveld, Biosensors and Bioelectronics 6 (1991) 55–72; Schasfoort et al., Anal. Chim. Acta 238 (1990) 323–329). However, as with other groups, our system showed no reproducible capacitance decrease on addition of antigen. The biolayer was not sufficiently insulating or did not have a suitable dielectric character. Ions were moving through or around this layer causing `shorting' of the system. In an attempt to drive the system toward exclusion of ions from the biolayer, an overlay of non-conducting polymer, in the form of polyethylene glycol (PEG), was added. Our results indicate that PEG forms an insulating layer on a bare chip surface, causing the capacitance to drop. The response was self-consistent and reproducible. On surfaces where there was protein pre-immobilized or pre-adsorbed, the PEG could not form a continuous, integral layer and a smaller drop in capacitance was observed. There is a difference in the response of single layers of protein, where only antibody is immobilized, and triple protein layers, where antibody was immobilized, antigen added and a secondary antibody bound. These results show the possibility of developing a differential capacitive biosensor.

Raman and Electrical Characterizations of a-GaAs1-xNx Thin Films Grown on c-Si(p) Substrates by N2 Reactive Sputtering

In this paper, we report on electrical properties and Raman scattering of amorphous GaAs1-x Nx deposited on p-type crystalline silicon (c-Si(p)) substrates. The aim of this study is the investigation of the dielectric character of GaAs1-x Nx in view of its applications in MIS structures. The “compound” GaAs1-x Nx is obtained by reactive RF sputtering of a GaAs target. The electrical characteristics of the a-GaAs1-x Nx /c-Si(p) heterostructures present a MIS-like structure behaviour with some imperfections. Raman scattering points out that a-GaAs1-x Nx is a wide gap material which is formed by the substitution of arsenic by nitrogen in the GaAs network. For substitution ratios below 25–30%, the material obtained would be an alloy whose vibrational behaviour is a single mode. Beyond 30%, it would be in a very inhomogeneous phase, infact polyphased, and would present a phase of hexagonal origin (GaN).

D.c.-conductivity of poly( N-vinylcarbazole) containing iron(III)-phthalocyanine side-units

Iron(III)-phthalocyanine has been grafted onto poly( N-vinylcarbazole) to change the dielectric character of the polymer and improve electrical conductivity. Either unfractionated or fractionated polymer samples were submitted to grafting reactions. From a chemical point of view the grafted samples differ from each other in terms of the eventual chemical group joining the phthalocyanine unit to a carbazyl group, the content of unreacted amino groups, and the inter- or intramolecular charge-transfer-complex units formed between the donor (amino-carbazyl derived) and the acceptor (nitro-carbazyl derived) moieties. Molecular weight differences of the polymer samples produce little variation in conductivity of the grafted samples, whereas the above chemical aspects, heating rate and temperature influence the conductor mechanism. The density data currently obtained fit the Richardson-Schottky equation for dielectric materials well. The trapping parameter was calculated by applying Child's law. The activation energy values obtained can be explained attending to conformational changes that, in any sample, will be able to favour conductivity at high temperatures, between 180 and 200°C, whereas conductive electrical motions are related to the formation of charge-transfer-complexes near room temperature.

Microwave-induced oscillations in the conductance of a mesoscopic ring

We calculate the conductance of a mesoscopic ring subject to a pumping microwave field. We find that the electromagnetic field has the same effect as a superlattice structure, deforming the electronic energy spectrum of the ring into minibands. The shape of the spectrum depends crucially on the intensity of the pumping. Due to the induced miniband structure the character of the quantum oscillations of the ring conductance - Aharonov-Bohm (AB) effect - varies drastically with the amplitude of the microwave field. This is because the position of the minibands can be shifted relative to the Fermi energy by varying the field amplitude. When the Fermi level moves from a position within a miniband to a forbidden gap, the AB oscillations changes from having a metallic to a dielectric character. In the vicinity of the forbidden bands, corresponding to Bragg reflection, a new resonant tunneling channel for transport through the ring appears. We calculate the position and lineshape of the resonant conductance peaks as a function of magnetic flux and intensity of the pumping field.

Synthesis of high temperature superconducting ceramics by oxalate coprecipitation from non-aqueous solvents

Abstract This paper describes a variation of the oxalate procedure that has significant advantages. The oxalate approach is often used to produce rare earth 123 superconductors and related ceramics. A moderately high concentration of acetone changes the dielectric character of the solution significantly and in the process greatly modifies the solubility of the oxalate compounds. When oxalic acid or oxalate ions are added to a yttrium-barium-copper aqueous solution, significant amounts of the metal (typically 10–40% of either Cu or Ba depending upon the pH) are left in solution. Such losses result in poor quality material. The acetone makes the precipitation more than 99 + % complete. The oxalate precursor thus produced by this route fires to high quality superconductors in amazingly short periods of time.

Immobilization of antibodies onto a capacitance silicon-based transducer

The feasibility of a capacitive immunosensor, consisting of an antibody-layer fixed onto the top oxide layer of a heterostructure-based EIS (electrolyte insulator semiconductor) silicon has been studied. For such a biosensor, the sensitive layer that contains the active species must act as an electrically-blocking capacitor. Furthermore, the layer containing the immunospecies must keep its recognizing ability and a configuration allowing also a dielectric character. An attempt to solve such a problem consists in the antibodies (anti-α-fetoprotein) grafting onto silica through three different spacing reagents: aminosilane, cyanosilane and heteropolysilsesquioxanes membranes. Promising results have been obtained with polysiloxane membranes, but these compounds do not allow the direct detection of the antibody-antigen interaction.

Electronic properties of plasma-deposited films prepared from tetramethylsilane

Abstract Investigations were carried out on an optical absorption, reflectance, photoluminescence, photoconductivity and d.c. conductivity of hydrogenated amorphous Si-C films prepared by plasma deposition from tetramethylsilane at various substrate temperatures Ts in the range 400–670K. The results have been interpreted in terms of the electronic structure of the thin. It was found that the dielectric character of the electronic structure is generally maintained regardless of Ts although some changes in parameters of the electronic structure, such as transport gap, optical gap and activation energy of d.c. conductivity were observed with increasing Ts. These changes are attributed to shifts in the valence and conduction states into the transport gap.

Dielectric Character Due to the Percolative Growth of Clusters in a Metal-Ceramics Mixture

The dielectric property was measured to observe growth of metallic clusters in Nb-Al2O3 sintered mixtures with Nb volume fraction f ranging from 0 to 0.15. As Nb clusters grow large, the effective dielectric constant ε* (or capacitance) increases to diverge, reflecting the occurrence of the percolation transition at around f=0.15. The power law was found to hold for ε* with a critical exponent of 0.6.

A.c. electrical behaviour of cobaltcomplex-doped polyimide films

A series of solvent-compatible cobalt-complex-doped polyimide films have been evaluated for their a.c. electrical behaviour. The doped films display different dielectric character compared to their undoped counterpart, the extent being dependent on the dopant concentration. Temperature studies show a hysteresis effect, which is attributed to moisture pick-up by surface cobalt on the film.

Far-infrared dielectric response and hopping conductivity in quasi-one-dimensional (NbSe4)3I

Complex transmittance of the quasi-one-dimensional semiconductor (NbSe4)3In in the 8-30 cm-1 range and reflectivity in the 15-650 cm-1 range were measured as a function of temperature (10-300 K) around the structural phase transition at Tc=274 K. The spectra show predominantly dielectric characters with a rich phonon structure between 20 and 300 cm-1. Several new lines gradually appear below Tc in qualitative agreement with the factor-group analysis. The submillimetre electronic conductivity along the chain direction is appreciably higher than the DC conductivity in the whole temperature range which suggests a hopping mechanism of the electron transport. This is even more directly documented by the strong increase in submillimetre permittivity with temperature. The important role of permittivity for proving the hopping transport is stressed.

Peculiarities of dielectric properties of LixNa1−xTayNb1−yO3solid solutions

Abstract Ceramic specimens of Li x Na1−x Ta y Nb1−y O3 (LNTN), where 0.02 ≤ x ≤ 0.17 and 0 ≤ y ≤ 0.55, have been synthesized to a density more than 97 percent of the theoretical one. Their dielectric properties have been measured as functions of temperature, frequency and composition. Measurements have been carried out in strong and weak fields and with constant bias voltage. Hysteresis loops have been investigated. The LNTN dielectric properties character is determined by the presence of morphotropic boundary, by translation invariance disturbance and by a change in the tendency to spontaneous polarization caused by a composition change.

Characterization of Lead Scandium Niobate Ceramics Prepared by Precipitation Method

The synthesized monosized PSN complex precursor powder was produced using the methods of the controlled hydrolysis of Nb(OC{sub 2}H{sub 5}){sub 5} and the growth of scandium and lead carbonates on the as-hydrolyzed Nb{sub 2}O{sub 5} particle surface. Calcining the carbonates to oxides and sintering at different temperatures with various cooling rates produced dense ceramics. The relation between dielectric relaxation character and structural ordering of the sintered PSN ferroelectric ceramics was investigated.

A flexible procedure for analyzing impedance spectroscopy results: Description and illustrations

General considerations in nonlinear least squares fitting of small-signal ac frequency response data (conductive or dielectric systems) are first discussed. A very general and flexible complex nonlinear least squares fitting (CNLS) program (LOMFP) is described which runs on microcomputers of the PC and AT type. Advantages and disadvantages of LOMFP are discussed and compared to those of a recent CNLS program developed by Boukamp. LOMFP incorporates many thousands of equivalent circuits fitting possibilities and ten different distributed circuit elements which can be used in these circuits. Its fitting is both speedy and accurate, and it provides considerable insurance against confusing local minima fits with true least squares fits. It is optimized for fitting real, imaginary, or complex data of either partly conducting or purely dielectric character, and it incorporates great flexibility in handling and fitting data in different forms (different immittance levels and rectangular or polar representation). Seven different data weighting choices are provided. Exact synthetic data were generated from a typical Impedance Spectroscopy equivalent circuit, and the effects on CNLS fitting were investigated of rounding the exact data to 4, 3, and 2 decimal places. We present the results of an extensive study of the effects of transforming exact data to different immittance levels and representations and then rounding to two places or of transforming rounded 2-place data directly. We conclude that real data should not be transformed from its original measured form before carrying out CNLS fitting. Finally, the effects were investigated of various kinds of weightings on CNLS fitting of 2-place data, leading to the conclusion that in spite of its advocacy and use by Zoltowski and Boukamp, modulus weighting generally yields misleading and appreciably worse CNLS fitting results than does proportional weighting. The LOMFP program (source and executable files) is available from the first author.

Plasma polymerization of phenyl isothiocyanate

AbstractThin polymer films were obtained by plasma polymerization of phenyl isothiocyanate. Polymerizations were carried out in rf (13.56 MHz) glow discharge generated in an electrodeless flow system. It was found that this monomer produces uniform films with a wide range of thicknesses, from hundreds of nanometers to tens of micrometers. The deposition rate appeared to be dependent on the substrate distance from the monomer inlet. The chemical structure of plasma polymer was characterized by using elemental analysis, IR spectroscopy, gas chromatography, and mass spectrometry. Elemental analysis showed that the composition of polymer depends on the substrate position in the reactor. It was observed that sulphur content decreased with increasing the substrate distance from the monomer inlet, whereas nitrogen content appeared to increase. The IR data revealed significant decrease in —NCS groups content in the polymer as compared with the monomer spectrum and indicated for the appearance of new absorption bands corresponding to the CN and CH aliphatic, groups. The results account for a strong fragmentation of monomer in plasma involved in decomposition of isothiocyanate group and phenyl ring. The soluble fraction of polymeric material was examined by gas chromatography and then the separated products were analyzed by mass spectrometry. The soluble fraction was found to be composed of numerous low molecular‐weight compounds. Identification of their structure revealed the presence of residual monomer, thiophenol, cyanobenzene, diphenyl, diphenyl sulphide, diphenyl disulphide, phenyl thiocyanate, dicyanobenzene, phenatroline, and some other oligomeric products. Formation of these compounds proves high susceptibility of NCS group in the monomer towards different fragmentation reactions. The surface free energy and electrical conductivity of polymer films were evaluated. The surface free energy value was very close to those estimated for plasma polymers deposited from other benzene derivatives. The low electrical conductivity manifested by the investigated polymeric material indicated for its dielectric character. The photoelectrical measurements revealed some photoconductivity effect in this material.

MM'PO4, a new family of ferroic compounds

The two phosphates CsCoPO4 and CsZnPO4 are each shown to undergo two different phase transitions when heated above room temperature. The first one—form class 2 to presumably class 2 mm—is a ferroelastic to paraelastic transition, whereas the second one, occurring at a higher temperature—presumably from class 2 mm to class mmm—exhibit a weak dielectric character. This paper is a first account on the ferroelastic and dielectric properties of these compounds which are otherwise shown to be two particular members of a wide family of ferroic compounds of the general formula M+ M'2+ PO4 or M+ M'2+ AsO4 where M is a monovalent metal and M' a divalent metal with a tetrahedral coordination.

Spectator-ion effect on the passage of ions through membranes.

In this paper, we investigate the interplay between geometric and dielectric factors in influencing the image force acting on an ion passing through a membrane, for a system having the approximate dimensions of Escherichia coli. We also study the effect of one ion in a membrane on the passage of a second ion through the membrane, by calculating the radial and angular forces experienced by the second ion due to the presence of the "spectator ion." Our conclusions follow from numerical studies on expressions obtained by solving (exactly) Laplace's equation for the model assumed in this paper. The conclusions are: (i) small changes in the dielectric constant of the membrane are far more significant in determining the image force acting on an ion in a membrane than dramatic changes in the dielectric character of the regions interior and/or exterior to the cell; (ii) a spectator ion in a membrane situated near a boundary may influence in a significant way the passage of a second ion through the middle third of the membrane. We suggest that this latter result should be taken into account in discussing the mechanism of ion migration across membranes.

The Chemical Composition and Distribution of Interstellar Grains

The chemical composition of interstellar grains is derived here on the basis of (1) the cosmic abundance of the elements; (2) the wavelength dependence of extinction and polarization; (3) the average total extinction; (4) the ratio of polarization to extinction; (5) the predominantly dielectric character of grains in the visible spectral region; and (6) infrared spectral characteristics of grains. It is indicated that the major portion of the grains, by mass, consist of core-mantle particles in the 0.1-µm size range, whose cores consist largely of silicates and whose mantles are a solid mixture of O, C, and N with H in a heterogeneous combination of simple and complex molecules with frozen free radicals. A minor constituent of the solid particles exist in the form of very small uncoated particles generally less than 10-6 cm in size whose precise composition is not certain. Inferences of the core-mantle model with respect to spatial distribution are consistent with the proposition that growth of the mantles occurs in the galactic shock region predicted by the density-wave theory. Estimates of the total visual extinction toward the galactic center and the consequent estimates of the total amount of far infrared radiation are shown to depend critically on the grain model. Variations of the ratio of far ultraviolet to visual extinction are correlated with the conditions for growth of mantles on the bare small particles which are generally prevented from accreting mantles primarily because of their extreme temperature fluctuations produced by the ultraviolet photons in the radiation field.

Spontaneous Emission of Plasma Waves in the Presence of a Finite Amplitude Wave

The spontaneous Cerenkov emission of longitudinal waves in a plasma with small but finite amplitude wave (hereafter referred as the carrier wave) is investigated. Two effects of the carrier wave are taken into account; one is the modification of the test-particle orbit and the other that of the dielectric character of the plasma. It is shown that particles trapped in the potential-energy troughs of the carrier wave emit a discrete spectrum of sideband waves (in addition to the carrier wave itself) whose frequencies are different from the carrier-wave frequency by approximately integral multiples of the bounce frequency. An untrapped particle also emits waves at several different frequencies, including the one which satisfies the ordinary Cerenkov condition, but these frequencies depend on the particle velocity. The results are compared with those reported in our previous paper where only the effect of the orbit modification is taken into consideration.

Dielectric and hydrogen-bonding effects of solvents on the base-catalysed reaction of phenyl glycidyl ether with benzoic acid

The effect of the dielectric character of a solvent on the pyridine-catalyzed reaction between phenyl glycidyl ether and benzoic acid was studied in various pure and mixed aromatic hydrocarbons such as toluene, monochlorobenzene, o-dichlorobenzene, nitrotoluene, nitrobenzene, and toluene-nitrobenzene mixtures at temperatures ranging from 80 to 180°, and is discussed in terms of the modified Kirkwood's expression. The hydrogen-bonding effect of the solvent was also investigated for this reaction in toluene-dioxan and nitrobenzene-dioxan mixtures at temperatures ranging from 80 to 100° and in various ethers such as tetrahydrofuran, anisole and di-n-butyl ether at 65°0°. A linear relationship was found between the logarithm of the third order rate constants and p K a values of solvents, and can be described by the proposed rate equation. These show that in aromatic hydrocarbons the nonspecific electrical interactions are predominant, while the hydrogen-bonding character of the solvent can explain reasonably the rate data in solvents containing ethers, and should also suggest that the activated complex or the transition state species proposed plays an important role for this reaction in these solvents.