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PubMed2006

Effects of functional groups on surface pressure-area isotherms of hydrophilic silicone polymers.

Organic/inorganic hybrid silicone polymers are increasingly used in cosmetics, inks and paints, and fabric care applications owing to their special Si-O bond characteristics. Because of the presence of organic as well as inorganic groups, they show the properties of both, and the presence of hydrophobic as well as hydrophilic character makes them behave like a hybrid polymer. Though they are widely used, the utilization of hydrophilically modified silicones on a large scale has mainly been empirical due to lack of fundamental knowledge about variation in their properties with systematic change in their structure. The choice of moieties for hydrophilic modification of silicones in most of the earlier works has been nonionic based on ethylene oxide and propylene oxide groups, however, very little is known about their ionic counterparts. The current work focuses on understanding the behavior of functionally grafted silicone polymers with respect to the variation in the hydrophilic part of the grafting chain. Hydrophilically grafted silicone polymers form monolayers at the air-water interface, which are stabilized by interactions of functional groups with water. The present work examined the effects of functional group modifications on the conformational behavior of chains at the interface. It was observed that the shape of the chain depends on the available area at the interface (or surface pressure), and there are conformational changes with an increase in the number of molecules per unit area. While a poly(dimethylsiloxanes) (PDMS) chain may undergo stretched to helix transition as predicted earlier, this may not be the case for hydrophilically grafted chains. On the basis of the shape of the surface pressure-area isotherm and correlation with the scaling theory, a gradation in hydrophilicity of functional groups and hence modified silicone chains at the air-water interface is predicted.

Journal Article↗
PubMed2000

Optical gain in silicon nanocrystals.

Adding optical functionality to a silicon microelectronic chip is one of the most challenging problems of materials research. Silicon is an indirect-bandgap semiconductor and so is an inefficient emitter of light. For this reason, integration of optically functional elements with silicon microelectronic circuitry has largely been achieved through the use of direct-bandgap compound semiconductors. For optoelectronic applications, the key device is the light source--a laser. Compound semiconductor lasers exploit low-dimensional electronic systems, such as quantum wells and quantum dots, as the active optical amplifying medium. Here we demonstrate that light amplification is possible using silicon itself, in the form of quantum dots dispersed in a silicon dioxide matrix. Net optical gain is seen in both waveguide and transmission configurations, with the material gain being of the same order as that of direct-bandgap quantum dots. We explain the observations using a model based on population inversion of radiative states associated with the Si/SiO2 interface. These findings open a route to the fabrication of a silicon laser.

Journal Article↗
PubMed2004

The interface between silicon and a high-k oxide.

The ability of the semiconductor industry to continue scaling microelectronic devices to ever smaller dimensions (a trend known as Moore's Law) is limited by quantum mechanical effects: as the thickness of conventional silicon dioxide (SiO(2)) gate insulators is reduced to just a few atomic layers, electrons can tunnel directly through the films. Continued device scaling will therefore probably require the replacement of the insulator with high-dielectric-constant (high-k) oxides, to increase its thickness, thus preventing tunnelling currents while retaining the electronic properties of an ultrathin SiO(2) film. Ultimately, such insulators will require an atomically defined interface with silicon without an interfacial SiO(2) layer for optimal performance. Following the first reports of epitaxial growth of AO and ABO(3) compounds on silicon, the formation of an atomically abrupt crystalline interface between strontium titanate and silicon was demonstrated. However, the atomic structure proposed for this interface is questionable because it requires silicon atoms that have coordinations rarely found elsewhere in nature. Here we describe first-principles calculations of the formation of the interface between silicon and strontium titanate and its atomic structure. Our study shows that atomic control of the interfacial structure by altering the chemical environment can dramatically improve the electronic properties of the interface to meet technological requirements. The interface structure and its chemistry may provide guidance for the selection process of other high-k gate oxides and for controlling their growth.

Journal Article↗
PubMed2004

All-optical control of light on a silicon chip.

Photonic circuits, in which beams of light redirect the flow of other beams of light, are a long-standing goal for developing highly integrated optical communication components. Furthermore, it is highly desirable to use silicon--the dominant material in the microelectronic industry--as the platform for such circuits. Photonic structures that bend, split, couple and filter light have recently been demonstrated in silicon, but the flow of light in these structures is predetermined and cannot be readily modulated during operation. All-optical switches and modulators have been demonstrated with III-V compound semiconductors, but achieving the same in silicon is challenging owing to its relatively weak nonlinear optical properties. Indeed, all-optical switching in silicon has only been achieved by using extremely high powers in large or non-planar structures, where the modulated light is propagating out-of-plane. Such high powers, large dimensions and non-planar geometries are inappropriate for effective on-chip integration. Here we present the experimental demonstration of fast all-optical switching on silicon using highly light-confining structures to enhance the sensitivity of light to small changes in refractive index. The transmission of the structure can be modulated by up to 94% in less than 500 ps using light pulses with energies as low as 25 pJ. These results confirm the recent theoretical prediction of efficient optical switching in silicon using resonant structures.

Journal Article↗
PubMed2006

Terahertz all-optical modulation in a silicon-polymer hybrid system.

Although gigahertz-scale free-carrier modulators have been demonstrated in silicon, intensity modulators operating at terahertz speeds have not been reported because of silicon's weak ultrafast nonlinearity. We have demonstrated intensity modulation of light with light in a silicon-polymer waveguide device, based on the all-optical Kerr effect-the ultrafast effect used in four-wave mixing. Direct measurements of time-domain intensity modulation are made at speeds of 10 GHz. We showed experimentally that the mechanism of this modulation is ultrafast through spectral measurements, and that intensity modulation at frequencies in excess of 1 THz can be obtained. By integrating optical polymers through evanescent coupling to silicon waveguides, we greatly increase the effective nonlinearity of the waveguide, allowing operation at continuous-wave power levels compatible with telecommunication systems. These devices are a first step in the development of large-scale integrated ultrafast optical logic in silicon, and are two orders of magnitude faster than previously reported silicon devices.

Journal Article↗
PubMed2001

Microstructure, toughness and flexural strength of self-reinforced silicon nitride ceramics doped with yttrium oxide and ytterbium oxide.

Self-reinforced silicon nitride ceramics with additions of either yttrium oxide or ytterbium oxide have been investigated at room temperature after various processing heat treatments. Devitrification of the intergranular phase in these materials is very sensitive to the heat treatment used during processing and does not necessarily improve their strength and toughness. Hot-pressed ceramics without a subsequent devitrification heat treatment were the strongest. The ytterbium oxide-doped silicon nitride ceramics were consistently tougher, but less strong, than the yttrium oxide-doped silicon nitride ceramics. In all the ceramics examined, the fracture toughness showed evidence for R-curve behaviour. This was most significant in pressureless sintered ytterbium oxide-doped silicon nitride ceramics. A number of toughening mechanisms, including crack deflection, bridging, and fibre-like grain pull-out, were observed during microstructural analysis of the ceramics. In common with other silicon nitride-based ceramics, thin amorphous films were found at the grain boundaries in each of the ceramics examined. Arrays of dislocations left in the elongated silicon nitride grains after processing were found to belong to the {101;0}<0001> primary slip system.

Journal Article↗
PubMed2003

Tissue adhesion to bioactive glass-coated silicone tubing in a rat model of peritoneal dialysis catheters and catheter tunnels.

BACKGROUND: Silicone peritoneal dialysis catheters do not develop tissue ingrowth, lack a mechanical barrier to periluminal bacterial migration and need cuffs for anchorage. We hypothesized that a bioactive glass coating composed of silicon, calcium, sodium and phosphorous oxides would cause a beneficial tissue reaction causing catheter adhesion, and tested this in a rat model. METHODS: A hexane solvent-based method of coating silicone tubes with Bioglass powder was used, which maintained flexibility, and then the ultrastructure was confirmed with scanning electron microscopy (EM). Segments 2.5 cm were implanted subcutaneously in 8 Sprague-Dawley rats, with uncoated tubes as a contralateral control, and histology was done at 2, 4 and 6 weeks, including special stains and EM. RESULTS: The uncoated segments grossly had no adherence to surrounding tissue, and were physically separate from a thin fibrous capsule of approximately 50 micro width. Trichrome stains demonstrated the capsule was rich in collagen. There was minimal adjacent tissue reaction. In contrast, the coated tubes were palpably fixed to the soft tissues, and sections demonstrated an adjacent prominent layer of macrophages and multinucleated giant cells. Small numbers of lymphocytes were noted. This cellular reaction increased over the 6-week implant duration, and was also associated with neovascularization of the tissue adjacent to the segments (33 vessels in coated vs. 20 in controls per x 200 field, P < 0.0001). Many refractile silicone particles and prominent multinucleated giant cells were present, with small numbers of lymphocytes and macrophages. Stains showed scattered discontinuous calcific deposits. These findings are consistent with reports that the Bioglass(R) silicon oxide leads to the formation of a layer of hydroxyapatite, which binds to collagen and induces a tissue cellular reaction. CONCLUSIONS: In summary, bioactive glass coatings can improve the tissue retention of silicone tubing by promoting adhesion by collagen and cell proliferation, and are promising for future studies of peritoneal dialysis catheters.

Animals↗
PubMed2005

Efficient silicon surface and cluster modeling using quantum capping potentials.

A one-electron, silicon quantum capping potential for use in capping the dangling bonds formed by artificially limiting silicon clusters or surfaces is developed. The quantum capping potentials are general and can be used directly in any computational package that can handle effective core potentials. For silicon clusters and silicon surface models, we compared the results of traditional hydrogen atom capping with those obtained from capping with quantum capping potentials. The results clearly show that cluster and surface models capped with quantum capping potentials have ionization potentials, electron affinities, and highest occupied molecular orbital-lowest unoccupied molecular orbital gaps that are in very good agreement with those of larger systems. The silicon quantum capping potentials should be applied in cases where one wishes to model processes involving charges or low-energy excitations in silicon clusters and surfaces consisting of more than ca. 150 atoms.

Journal Article↗
PubMed2003

Increasing the efficiency of erbium-based sources using silicon quantum dots.

Silicon nanoclusters exhibit novel and interesting optical and electrical properties that are not observed in bulk silicon. Moreover, it has been discovered that there exists a strong coupling between nanoclusters and rare-earth ions that results in efficient energy exchange between the two species. This paper presents a review of recent work at University College London in this area, in which we have studied the optical properties of silicon nanoclusters in silica with rare-earth co-dopants and have developed a model for the excitation of erbium ions in erbium-doped silicon nanocrystals via coupling from optically generated excitons confined within the silicon nanoclusters. The model provides a phenomenological description of the exchange mechanism and allows us to evaluate an effective absorption cross-section for erbium that is up to four orders of magnitude higher than the corresponding value in stoichiometric silica. This paper also discusses the origin of the 1.6 eV emission band associated with the silicon nanoclusters and determines absorption cross-sections and excitonic lifetimes for nanoclusters in silica.

Journal Article↗
PubMed2004

Theory of hydrogen-related metastability in disordered silicon.

Density functional electronic structure calculations are employed to examine hydrogen for a variety of configurations in silicon. A novel complex is found for hydrogen in amorphous silicon. The complex involves the breaking of a weak silicon bond to form two Si-H bonds with both hydrogens in between the original silicon atoms. This complex provides a microscopic model for new metastable complexes observed in amorphous silicon. Mechanisms for hydrogen-related metastability are discussed for amorphous and polycrystalline silicon.

Journal Article↗
PubMed2006

An analog silicon retina with multichip configuration.

The neuromorphic silicon retina is a novel analog very large scale integrated circuit that emulates the structure and the function of the retinal neuronal circuit. We fabricated a neuromorphic silicon retina, in which sample/hold circuits were embedded to generate fluctuation-suppressed outputs in the previous study [1]. The applications of this silicon retina, however, are limited because of a low spatial resolution and computational variability. In this paper, we have fabricated a multichip silicon retina in which the functional network circuits are divided into two chips: the photoreceptor network chip (P chip) and the horizontal cell network chip (H chip). The output images of the P chip are transferred to the H chip with analog voltages through the line-parallel transfer bus. The sample/hold circuits embedded in the P and H chips compensate for the pattern noise generated on the circuits, including the analog communication pathway. Using the multichip silicon retina together with an off-chip differential amplifier, spatial filtering of the image with an odd- and an even-symmetric orientation selective receptive fields was carried out in real time. The analog data transfer method in the present multichip silicon retina is useful to design analog neuromorphic multichip systems that mimic the hierarchical structure of neuronal networks in the visual system.

Algorithms↗
PubMed1999

SILICON.

Silicon is present in plants in amounts equivalent to those of such macronutrient elements as calcium, magnesium, and phosphorus, and in grasses often at higher levels than any other inorganic constituent. Yet except for certain algae, including prominently the diatoms, and the Equisetaceae (horsetails or scouring rushes), it is not considered an essential element for plants. As a result it is routinely omitted from formulations of culture solutions and considered a nonentity in much of plant physiological research. But silicon-deprived plants grown in conventional nutrient solutions to which silicon has not been added are in many ways experimental artifacts. They are often structurally weaker than silicon-replete plants, abnormal in growth, development, viability, and reproduction, more susceptible to such abiotic stresses as metal toxicities, and easier prey to disease organisms and to herbivores ranging from phytophagous insects to mammals. Many of these same conditions afflict plants in silicon-poor soils-and there are such. Taken together, the evidence is overwhelming that silicon should be included among the elements having a major bearing on plant life.

Journal Article↗
PubMed1996

Peripheral nerve regeneration in a silicone tube: effect of collagen sponge prosthesis, laminin, and pyrimidine compound administration.

Regeneration of transected peripheral nerve with a 10-mm gap encased in a silicone tube was evaluated in the presence of collagen sponge with or without laminin, or with systemic administration of a pyrimidine compound, MS-818. The sciatic nerve of 20 adult rats was transected and the proximal and distal nerve stumps were fixed in a silicone tube. The lumen of the silicone tube was empty, or filled with a collagen sponge alone or with a laminin-soaked collagen sponge. Also, a pyrimidine compound was injected intraperitoneally after implantation of the empty silicone tube. Three weeks later, the contents of the silicone tubes were processed for histological examination of regenerated nerve fibers. Other animals were observed 6, 12, and 18 months after surgery to examine the long-term effects of the collagen sponge on nerve regeneration. All animals had regenerated tissue within the tube 3 weeks after nerve transection. The diameter of the tissue decreased toward the distal stump in the empty tube, but was the same throughout the full length in the collagen sponge-containing tube. Immunohistochemical studies revealed that the nerve fibers extended beyond the midline of the regenerated tissue in animals treated with a laminin-containing collagen sponge or receiving a pyrimidine compound. Long-term observation showed the regenerated nerve was thick as the proximal stump and many neurofilament- and peripheral myelin-positive fibers were observed around the collagen sponge. Collagen sponge assists the progress of regenerated tissues in silicone tubes, and laminin-containing prostheses and administration of a pyrimidine compound enhance peripheral nerve regeneration.

Animals↗
PubMed1999

Milestones in the Biochemistry of Silicon: From Basic Research to Biotechnological Applications.

6.7 Gigatonnes of silicon are processed each year by marine organisms. Since it was known that silicon is an essential element for many biological systems, significant advances in the biochemistry of this element have been achieved from the classical viewpoint of silicon being a purely inorganic element. This article describes the proteins, genes, and molecular mechanisms of silicon metabolism in diatoms and sponges. These studies may help to reveal the role of silicon for optimal development and growth in many plants and animals as well as initiate the development of new technological methods for the shape-controlled production of new patterned silicone-based materials.

Journal Article↗
PubMed2006

[Determination of polyether silicon oil by IR].

In the present paper, with polyoxyethylene(10) octylphenol ether and hydrogenous silicon oil used as raw materials, and stannous octoate used as catalyst, polyoxyethylene(10) octylphenolether-hydrogenous silicon oil polymer was synthesized in toluene solvent. Residual hydrogenous silicon oil in the polymer was determined by IR. There is a characteristic peak at 2 167 cm(-1) for Si-H bond in hydrogenous silicon oil. Using span-60 as internal standard, depending Lambert-Beer law and some calculation process, the linear relation between the absorbance ratio(y) and the mass ratio(x) of sample was obtained. The linear regression equation is y=2. 07 2x+0.2963 with a correlation coefficient (r2) of 0.9989. The experimental results of the precisions and recoveries are satisfactory. An IR method to determine the content of hydrogenous silicon oil in polyether was established. The method is very necessary for confirming the reaction extent and controlling product quality and adjusting polyether silicon oil characters. The best process parameter was chosen in experiments. The polyether has good charactesistic for extracting alcohols and benzene homologues with bright future of application.

English Abstract↗
PubMed2006

Critical oxide thickness for efficient single-walled carbon nanotube growth on silicon using thin SiO2 diffusion barriers.

The ability to integrate carbon nanotubes, especially single-walled carbon nanotubes, seamlessly onto silicon would expand their range of applications considerably. Though direct integration using chemical vapor deposition is the simplest method, the growth of single-walled carbon nanotubes on bare silicon and on ultrathin oxides is greatly inhibited due to the formation of a noncatalytic silicide. Using X-ray photoelectron spectroscopy, we show that silicide formation occurs on ultrathin oxides due to thermally activated metal diffusion through the oxide. Silicides affect the growth of single-walled nanotubes more than multi-walled nanotubes due to the increased kinetics at the higher single-walled nanotube growth temperature. We demonstrate that nickel and iron catalysts, when deposited on clean silicon or ultrathin silicon dioxide layers, begin to form silicides at relatively low temperatures, and that by 900 degrees C, all of the catalyst has been incorporated into the silicide, rendering it inactive for subsequent single-walled nanotube growth. We further show that a 4-nm silicon dioxide layer is the minimum diffusion barrier thickness that allows for efficient single-walled nanotube growth.

Catalysis↗
PubMed2002

New silicones for the evaluation of sudomotor function with the impression mold technique.

Three new silicone-based impression materials manufactured for use in dentistry (Silasoft, CutterSil and Xantopren) have been evaluated for the silicone mold sweat test in humans and mice and compared with the well-known silicone material Elasticon. The new materials produced more translucent molds and the sweat impressions show less contrast than Elasticon. Molds made of Xantopren and CutterSil retained air bubbles that make counting of sweat impressions more difficult than with Elasticon. The density of sweat droplets from human skin varied depending on the silicone used, but differences were not significant. In the mouse hindpaw, total counts of sweat droplets were similar in Elasticon, Silasoft and CutterSil molds but slightly higher in Xantopren molds. There was good correlation between the number of sweat droplet impressions recorded with Elasticon and that recorded with the new materials, with the exception of CutterSil, both in humans and mice. From these results we recommend Silasoft as a valid substitute of Elasticon for the evaluation of sudomotor function with the silicone impression technique.

Adult↗
PubMed1990

Silicone gel including antimicrobial agent.

Silicone gel sheets containing Ofloxacin (OFLX), that provide a continual drug delivery system from a wound dressing to the wound so as to prevent infection and to promote healing, are described. It was found that silicone gel sheets without added medication did not inhibit microbial growth but that gel sheets containing 0.02% and 0.2% of OFLX had a positive antimicrobial effect against Staphylococcus aureus and Pseudomonas aeruginosa in a dose-dependent fashion in vitro. Further, this antimicrobial efficacy was greatly increased in a silicone gel sheet that contained 0.02% of OFLX and an additional 10% of silicone oil. In animal experiments, a silicone gel sheet containing OFLX prevented microbiol growth and promoted rapid epithelialisation in wounds to which Staphylococcus aureus had been applied, whereas wounds covered only with OpSite all resulted in continued infection.

Administration, Topical↗