Insulator Matrix Research Articles

Unichrom, a Novel Nuclear Matrix Protein, Binds to theArsInsulator and Canonical MARs

Eukaryotic genomic DNA is organized into loop structures by attachments to the nuclear matrix. These attachments to the nuclear matrix have been supposed to form the boundaries of chromosomal DNA. Insulators or boundary elements are defined by two characteristics: they interrupt promoter-enhancer communications when inserted between them, and they suppress the silencing of transgenes stably integrated into inactive chromosomal domains. We recently identified an insulator element in the upstream region of the sea urchin arylsulfatase (HpArs) gene that shows both enhancer blocking and suppression of position effects. Here, we report that Unichrom, originally identified by its G-stretch DNA binding capability, is a nuclear matrix protein that binds to the Ars insulator and canonical nuclear matrix attachment regions (MARs). We also show that Unichrom recognizes the minor groove of the AT-rich region within the Ars insulator, which may have a base-unpairing property, as well as the G-stretch DNA. Furthermore, Unichrom selectively interacts with poly(dG).poly(dC), poly(dA).poly(dT) and poly(dAT).poly(dAT), but not with poly(dGC).poly(dGC). Unichrom also shows high affinity for single-stranded G- and C-stretches. We discuss the DNA binding motif of Unichrom and the function of Unichrom in the nuclear matrix.

Novel Approach for Fabrication of Single-Crystalline Insulator/Si/Insulator Nanostructures

ABSTRACTDouble-barrier insulator/Si/insulator nanostructures on Si(111) were prepared using molecular beam epitaxy. Ultrathin single-crystalline Si buried in a single-crystalline insulator matrix with sharp interfaces was obtained by a novel approach based on an epitaxial encapsulated solid-phase epitaxy. As an example, we demonstrate the growth of Si buried in Gd2O3 and the incorporation of epitaxial Si islands into single-crystalline Gd2O3. The I-V characteristic of the obtained nanostructures exhibited negative differential resistance at low temperatures, however, with a strong memory effect.

Electronic conductivity of PbO2–xAg2O–0.75C system

Investigation on phase transformation and electrical transport are carried out for PbO2–xAg2O–0.75C composite alloys. When the Ag2O content decreases, a percolation effect takes place near x = 0.275 for the distribution of the metallic particles embedded in the insulator matrix. With decreasing Ag2O content, the temperature dependence of resistivity of the composites changes from a typical metallic behaviour to an insulating one. A metal–insulator transition appears in the PbO2–0.275Ag2O–0.75C composite, with decreasing temperature. The electronics in the PbO2–0.23Ag2O–0.75C composite are described by hopping conductivity in terms of the variable-range-hopping model.

Spin-dependent tunneling in dielectric LaSrMnO films with mesoscopic conducting clusters

The structure and electrical properties of laser-ablated LaSrMnO thin films containing mesoscopic clusters with metallic conductivity in an insulator matrix were studied. It is shown that the film properties, including temperature dependence of magnetoresistance and its behavior in high magnetic field, display features typical for spin-dependent tunneling.

Spin-dependent tunneling in dielectric LaSrMnO films with mesoscopic conducting clusters

The structure and electrical properties of laser-ablated LaSrMnO thin films containing mesoscopic clusters with metallic conductivity in an insulator matrix were studied. It is shown that the film properties, including temperature dependence of magnetoresistance and its behavior in high magnetic field, display features typical for spin-dependent tunneling.

Deposition technology of a new nanostructured material for reversible charge storage

Reversible charge storage effect was observed in thin amorphous hydrogenated carbon-germanium films produced by plasma enhanced chemical vapor deposition. These films consist of semiconductor grains embedded in an insulator matrix. The grains could serve as charge containers separated from one another by insulator walls. Scanning probe microscope equipped with conducting tip and cantilever is utilized for both local charge imaging and its modification. Depending on the sign of voltage applied to the tip, charge can be locally injected or removed from the film. Exploring this technique in data storage application, one can achieve information density of the order 10 14 bit/m 2.

Electrical conductivity of an insulator matrix (alumina) and conductor particle (molybdenum) composites

Different concentrations of molybdenum (from 5 to 35 vol.%) were added to alumina matrices and the resulting mixture hotpressed to produce dense metal toughened-ceramic composites. Their resulting electrical properties were measured using two-probe impedance spectroscopy, in the range 25–1000 � C. Experimental and theoretical studies were done to examine how the volume fractions and the morphology of the embedded metal particles affect the electrical behaviour of these composite materials. Due to the percolation effect, a sharp increase in the electrical conductivity of these composites was observed for compositions with molybdenum contents higher than 20 vol.%. The experimental data were fitted into the GEM equation. It was shown that the conductivity of these metal-toughened-ceramic composites is strongly affected by the amount, and less by the size of the metal particles. For low concentrations of molybdenum only one semicircle was readily identified with impedance spectroscopy analysis. This indicates that the matrix is still the main conductor phase. For higher concentrations of molybdenum, two readily resolvable arcs were present. For concentrations of metal higher than 30 vol.% the second semicircle might be an overlapping of two semicircles and therefore the impedance spectra for these composites can be resolved with three semicircles. The presence of this third semicircle can be explained in terms of molybdenum clusters. # 2002 Elsevier Science Ltd. All rights reserved.

Magnetic and intergranular transport properties in manganite/alumina composites

We report the magnetic and transport properties of La 2/3Ca 1/3MnO 3/Al 2O 3 crystal composites at low insulator component ratio. Phase purity and microstructure are guaranteed by X-ray and microanalysis studies. DC electric conduction is progressive blocked by scattering in alumina regions. With an insulator percentage around 10% in volume, percolation occurs. Merging the spin polarization of the manganite and the extra disorder contribution to electric transport induced by alumina we have been able to increase the low temperature magnetoresistance (MR) nearly three times with respect to the undoped manganese perovskite. Experimental results are analyzed on the basis of an earlier theory developed for a granular ferromagnetic material in an insulator matrix.

Tunnel-type Giant Magnetoresisitance in Co–Al–Ta–O Insulated Granular System

We have investigated the tunnel-type giant magnetoresistance (TMR), the related magnetic and electric properties and the microstructure of Co–Al–Ta–O granular thin films which were prepared by the reactive sputter-deposition technique with oxygen and argon. The TMR attained a maximum value of 12.5% at room temperature under a magnetic field of 1 T for Co40Al13Ta3O44 films. The MR ratio is the largest in a granular system similar to Co–Al–O. During the sputtering, O2 gas flow was introduced at 2.2 sccm into the chamber filled with Ar at a total pressure higher than 1×10-4 Torr. The specific electric resistance of the film that showed the maximum TMR is 1.46×105 µΩ·cm. Ta forms an oxide compound similar to Ta2O5 in the insulator matrix. The effect of Ta addition on the TMR appears at around 2.8 at.%. The MR ratio increases with an increase of the fraction of fcc-Co, accompanying the decrease of the Co–O fraction and the decrease in ferromagnetic interaction between Co particles. The specimen that shows large TMR consists of nanometer-sized Co particles which are completely isolated by amorphous Al2O3 and Ta2O5 intergranular layers. The standard free energy of oxidation of Co, Al and Ta may play an important role in the formation of the microstructure of Co–Al–Ta–O granular films and in TMR improvement.

Dependence of matrix and substrate on the morphology of nanocermet thin films

The influence of matrix and substrate on the morphology of three Pt nanocermet thin films has been studied by transmission electron microscopy (TEM), X-ray specular reflectivity and secondary ion mass spectrometry (SIMS). The TEM measurements clearly evidence the presence of metallic nanoparticles inserted in an amorphous insulator matrix. The structure of the thin films in the z-direction, normal to the surface of the films, is then analysed by specular X-ray reflectivity. Using two different models to analyse the data, it is shown that although the size and the mean separation of the particles does not depend much on the nature of the insulator matrix, the substrate and the insulator matrix play a key role at the film-substrate interface. These conclusions are reinforced by the SIMS analysis which shows that the diffusion of metallic particles in the substrate is important in the presence of alumina matrix.

Onset of electrical conduction in isotropic conductive adhesives: a general theory

A new theory is introduced for the onset of electrical conduction in isotropic conductive adhesives, based on the observation that conduction is a result of the creation of conducting contacts in metal–insulator composite adhesives. The present theory resolves several prevalently contradicting issues including the onset dependency of electrical conduction on the volume fraction of filler particles, the particle size, the pressure effect, and the type of insulator matrix of an adhesive. The theory also predicts the condition for the occurrence of two percolation thresholds.

Electron spectrum of a semiconductor quantum dot influenced by an interface

The generalized boundary conditions for the envelope wave function that take into account the real structure of an interface were used to investigate the hole spectrum of the semiconductor quantum dot embedded in an insulator matrix. An essential influnece of the interface levels on the hole spectrum has been demonstrated. Such levels could exist at the top of the valuence band. It is found that boundary conditions usually applied, wherein all components of the envelope wave function vanish at the interface, can be used only in the absence of the interface levels close to the band edge. @S0163-1829~99!51644-5# To determine the electron states in semiconductor quantum dots, the envelope function approximation is applied. 1 The envelope wave functions are usually supposed to be vanished at the interface in order to describe the quantum confinement. If the intervalley or interband degeneracy of the electron spectrum occurs in the semiconductor, then each component of the envelope wave function is assumed to be vanished at the interface. 1‐3 The last statement, however, needs to be justified. Indeed, the boundary condition c50 for the proper ~not envelope! wave function arises from the solution of the

Conductive polymer blends as electrochromic materials

Conductive polymer blends were prepared mixing conductive and insulating polymers and their electrochemical and electrochromic properties were studied. Depending on the insulator matrix used, these properties are not changed. Blend containing polyacrylonitrile and poly( o-methoxyaniline) doped with p-toluene sulfonic acid presents the same electrochemical and electrochromic behavior as the pure conducting polymer. Other blends studied were those obtained by combining poly(epichlorohydrin- co-ethylene oxide) with poly( o-methoxyaniline) and poly(4,4′-dipentoxy-2,2′-bithiophene). An electrochromic device using these polymer blends and a liquid electrolyte was assembled and presented Δ%T 620 nm =56%. This contrast decreased after 300 double potential steps to 33%.

Identifying conducting phase from the insulating matrix in percolating metal-insulator nanocomposites by conducting atomic force microscopy

(SiO2)1-x and Fex(SiO2)1-x by conducting atomic force microscopy (C-AFM). The conducting networks (CNs) were mapped via current images using the constant bias mode of C-AFM. By combining the topographic and electric current images simultaneously, the conducting phase can be identified from the insulator matrix. A rapid increase of the conducting phase was observed when x crossed the percolation threshold xc. Therefore the percolation was directly “seen” on a nanometer scale. With this technique, C-AFM is not only a “topographic profiler” but also a “chemical profiler” if the volume fraction of metal phase exceeds the percolation threshold. The factors limiting the lateral resolution will also be discussed.

Semiconducting property of a wide-band-gap oxide crystal: Impact ionization and avalanche breakdown

It is shown that a wide band-gap crystal, appropriately doped, can exhibit semiconducting properties. When a dc voltage was applied to a lithium-doped MgO single crystal containing [Li${]}^{0}$ centers, negative differential resistance, self-excited current oscillations, and filament-driven current were observed. The conductivity increase and the nonlinear behavior of the I-V characteristics are described by an impact ionization mechanism. Avalanche breakdown can be produced with low electric fields at moderate temperatures (\ensuremath{\sim}373 K), which eventually leads to electrical breakdown. We postulate that filamentary semiconducting inclusions imbedded in the insulator matrix are responsible for the electrical degradation.

Poly(benzimidazole) as insulator for superconductors

We have developed a poly(benzimidazole) (PBI) insulator for superconductors for the Superconducting Supercollider. The PBI compounds used for the insulator matrix are expected to be a superior material with high radiation resistance, high flexural modulus and high thermal resistance. On superconductor wire, PBI coating is superior as an insulator to Kapton-film wrapping. In particular, after electron-beam irradiation of 12 000 Mrad of a Cu PBI system, the radiation-dose resistance of the PBI-coated insulator was evaluated as 4.4 kV of the test voltage (initial 4.8 kV), whereas that of a Kapton-wrapped conductor was 2.5 kV after 6000 Mrad of irradiation.

Computer Simulation of Percolation Structure in Composites

Two-dimensional computer simulations were conducted on percolative structure in which second phases of various aspect ratios were arranged in a matrix phase. The second phases were randomly arranged in an array with two different computational programs; one prohibiting an overlap among second phases and the other allowing the overlap. From the simulation prohibiting the overlap, it was predicted that a continuous path was formed at less amounts of the second phase with higher aspect ratios. In the simulation allowing the overlap, a continuous path throughout the array was formed by arranging the second phase of an aspect ratio of 100 with less than 4% in the array. These results agreed well with the conduction path formation in hotpressed ceramic composites consisting of an insulator matrix and electrically conductive whisker additives.

Hopping conductivity of granular metals; test network method

We present a new approach to study the conductivity of granular metals in a hopping conductivity regime. It is shown that in a bicomponent mixture of metallic spheres embedded in an insulator matrix the current-carrying routes are a sparse network of grains with given correlations between grain separations and grain sizes. The type of correlations depends on the parameters of the system and temperature. As temperature changes the current switches to the networks with a different type of correlation and composition, which provide the minimal bulk resistance at a given temperature.

Theoretical analysis of the conditions for a thermal explosion and of a photoionization instability of transparent insulators containing absorbing inclusions

An analysis is made of the conditions favoring thermal explosion of an absorbing inclusion under the influence of laser radiation. The concept of a threshold function is introduced in such a way that the extrema of this function determine the threshold temperature and the intensity for a thermal explosion of an inclusion. A two-threshold model of damage to a transparent insulator is discussed. It is shown that photoionization of the insulator matrix by the thermal radiation emitted from a heated inclusion results in a thermal explosion in the vicinity of the inclusion. The explosion induction time is found and an experimental method for the determination of this time is discussed. Numerical estimates are given of the threshold intensity of a photoionization thermal explosion and of its induction time.

Dielectric response of small metal particle composites

The authors report the first measurements of the dielectric function for well characterised metal-insulator composite materials. These composite materials were prepared by a novel sputtering method and consist of uniformly sized (nanometre diameter) metal spheres embedded in the insulator matrix. The dielectric functions were measured by electron energy loss spectroscopy (EELS) and optical methods over the energy range 0.4 to 40 eV. No existing theory satisfactorily explains the results.