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PubMed2006

Towards the silicon nanowire-based sensor for intracellular biochemical detection.

A microneedle sensor platform with integrated silicon nanowire tip was developed for intracellular biochemical detection. Because of the virtue of miniaturized size and high sensitivity, this sensor has a great potential for studying individual cell or localized bioenvironment by revealing the pH level and/or enzyme activities. The fabrication of the microneedle sensor was primarily based on conventional silicon processing, where a silicon-on-insulator (SOI) wafer with 50 nm thick (100) p-type Si device layer was used as the substrate. The silicon nanowires of 50 nm height and 50-100 nm width were created by electron beam (E-beam) lithography on the tip of microneedle with good electrical connection to the contact pads for convenient electrical measurement. A three layer structure with base, support cantilever, and needle tip was designed to ensure convenient handling of sensors and minimize the invasive penetration into biological cells. In this paper, we demonstrate a preliminary assessment of this novel intracellular sensor with electrical conductance measurement under different pH levels. It is expected that this sensor with proper chemical modification will enable localized biochemical sensing within biological cells, such as neurotransmitter activities during the synaptic communication between neuron cells.

Biosensing Techniques↗
PubMed2006

Suppression of electron-transfer characteristics of ferrocene by OTS monolayer on a silicon/electrolyte interface.

The passivating behavior of self-assembled monolayers (SAMs) of octadecyltrichlorosilane (OTS) on an n-type Si(100) electrode with and without a redox species like ferrocene in a polar non-aqueous medium has been investigated using techniques like contact angle measurements, Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) to understand the role of the monolayer. The electron-transfer behavior of ferrocene is found to be drastically affected by the presence of monolayer and the reasons for these are analyzed as a function of the change in resistance, dielectric thickness and coverage of the monolayer. Electrochemical impedance analysis in the presence of ferrocene gives the monolayer coverage as 0.998 and the apparent rate constant calculated from this gives 4.85 x 10(-12) cm s(-1) in comparison with 4.4 x 10(-8) cm s(-1) for a similar electrode without any monolayer. A positive shift of 200 mV in the flat-band potential after monolayer formation also suggests the covalent coupling of the silane monolayer offering a protective barrier.

Journal Article↗
PubMedDate not supplied

Quantum size aspects of the piezoresistive effect in ultra thin piezoresistors.

Proximal probe sensors with an ability to detect extremely small forces (10(-15)-10(-18)N) play significant role in scanning probe microscopy applications. The detection of extremely low forces, require producing micromachined cantilevers with as small as possible spring constants, which is considered by the optimization of the sensor design. In the last year many papers describing the fabrication process of producing ultrathin cantilevers (below 100nm) with integrated piezoresistors for deflection read-out have been published. In the case of such cantilevers the required thickness of piezoresistors is in the range of 50nm. From a quantum mechanical point of view, an electrical carrier transport confinement in direction perpendicular to the cantilever surface can be expected and in this manner we have to consider the quantum size effect. The goal of the project described in this paper is to calculate and determine the piezoresistive coefficients in p type Si thin (under 50nm) piezoresistors taking into account the quantum size effect and to compare them with the corresponding coefficients for bulk material. The calculation of the band structure will use the mathematical apparatus of an exact analytical diagonalization six-band k.p model, modified with the envelope function approximation. The behaviour of the thin piezoresistors employed as integrated deflection read-out will be also discussed. Moreover, critical issues in the realization of piezoresistors formed by MOS transistor channel will be presented.

Journal Article↗
PubMed2004

[Postoperative deposits on a second-generation silicone lens].

BACKGROUND: In the literature there have often been reports about deposits (hydroxyapatite) in hydrophilic or hydrophobic intraocular lenses (IOLs). We report, for the first time, about deposits on a silicone IOL of the second generation. MATERIALS AND METHODS: A silicone IOL (Allergan, type SI-40) which was extracted due to gradual visual loss, has been examined. RESULTS: The examination with the scanning electron microscope (SEM) showed crystalline precipitations in the anterior surface of the IOL while the X-ray analysis showed the existence of calcium and carbon, whereas no phosphorus was found. CONCLUSIONS: It could be hypothesised that metabolic disorders in aqueous humor (i. e., in patients with diabetes mellitus), the quality of the production of the lens, and not the IOL material, are the main cause for deposits in the IOL surface.

Aged↗
PubMed2006

Photoluminescence from C60-coupled porous structures formed on Fe+-implanted silicon.

111-oriented p-type Si wafer with a resistivity of 1-5 Omega cm was implanted with Fe+ and then annealed at 1100 degrees C in N2 for 60 min, followed by anodization in a solution of HF to form porous structure with beta-FeSi2 nanocrystallites. Photoluminescence (PL) spectral measurements show that a strong PL peak appears in the range of 610-670 nm. The position of the PL peak remains unchanged, but its intensity increases with the storage time in air until about three months and then saturates. C60 molecules were chemically coupled on the porous structure through a kind of silane coupling agent to form a nanocomposite. It is revealed that the stable PL peak monotonically shifts to a pinning wavelength at 570 nm. Experimental results from PL, PL excitation, Raman scattering, and x-ray diffraction measurements clearly show that the pinned PL originates from optical transition in C60-related defect states, whereas the photoexcited carriers occur in the beta-FeSi2 nanocrystallites formed during anodization. This work opens a new way to tailor nanometer environment for seeking optimal luminescent properties.

Journal Article↗
PubMed2005

Probing charge transport at the single-molecule level on silicon by using cryogenic ultra-high vacuum scanning tunneling microscopy.

A cryogenic variable-temperature ultra-high vacuum scanning tunneling microscope is used for measuring the electrical properties of isolated cyclopentene molecules adsorbed to the degenerately p-type Si(100)-2x1 surface at a temperature of 80 K. Current-voltage curves taken under these conditions show negative differential resistance at positive sample bias, in agreement with previous observations at room temperature. Because of the enhanced stability of the scanning tunneling microscope at cryogenic temperatures, repeated measurements can be routinely taken over the same molecule. Taking advantage of this improved stability, we show that current-voltage curves on isolated cyclopentene molecules are reproducible and possess negligible hysteresis for a given tip-molecule distance. On the other hand, subsequent measurements with variable tip position show that the negative differential resistance voltage increases with increasing tip-molecule distance. By using a one-dimensional capacitive equivalent circuit and a resonant tunneling model, this behavior can be quantitatively explained, thus providing insight into the electrostatic potential distribution across a semiconductor-molecule-vacuum-metal tunnel junction. This model also provides a quantitative estimate for the alignment of the highest occupied molecular orbital of cyclopentene with respect to the Fermi level of the silicon substrate, thus suggesting that this experimental approach can be used for performing chemical spectroscopy at the single-molecule level on semiconductor surfaces. Overall, these results serve as the basis for a series of design rules that can be applied to silicon-based molecular electronic devices.

Electron Transport↗
PubMed1983

n-Type silicon photoelectrochemistry in methanol: Design of a 10.1% efficient semiconductor/liquid junction solar cell.

n-Type Si electrodes in MeOH solvent with 0.2 M (1-hydroxyethyl)ferrocene, 0.5 mM (1-hydroxyethyl)ferricenium, and 1.0 M LiClO(4) exhibit air mass 2 conversion efficiencies of 10.1% for optical energy into electricity. We observe open-circuit voltages of 0.53 V and short-circuit quantum efficiencies for electron flow of nearly unity. The fill factor of the cell does not decline significantly with increases in light intensity, indicating substantial reduction in efficiency losses in MeOH solvent compared to previous nonaqueous n-Si systems. Matte etch texturing of the Si surface decreases surface reflectivity and increases photocurrent by 50% compared to shiny, polished Si samples. The high values of the open-circuit voltage observed are consistent with the presence of a thin oxide layer, as in a Schottky metal-insulator-semiconductor device, which yields decreased surface recombination and increased values of open-circuit voltage and short-circuit current. The n-Si system was shown to provide sustained photocurrent at air mass 2 levels (20 mA/cm(2)) for charge through the interface of >2,000 C/cm(2). The n-Si/MeOH system represents a liquid junction cell that has exceeded the 10% barrier for conversion of optical energy into electricity.

Journal Article↗
PubMed1995

Slow coevolution of a viral pathogen and its diploid host.

We study a population exposed to a lethal infectious disease. Host response is carried at one locus with two alleles while the pathogen occurs in two variants. Based on an SI-type epidemic model we derive explicit equations for the dynamics of each genotype. By assuming small variations in both host and disease, we obtain a separation in time scales between epidemic and evolutionary processes. This allows us to describe explicitly the changes in host and disease gene frequencies. The resulting model has a rich behaviour including multiple stable states and oscillations. However, in the oscillatory situation the model is degenerate excluding the possibility of limit cycles. We show that the degeneracy can only be removed by frequency dependent selection in the pathogen, for example by including direct interaction of virus in a free-living stage. The qualitative conclusions extend to an SIR-type epidemic model, where recovery with immunity from the disease is possible.

Biological Evolution↗
PubMed2000

Novel behavior of bond-centered muonium in heavily doped n-type silicon: Curie-like spin susceptibility and charge screening.

Bond-centered muonium ( Mu(0)(BC)) has been observed in very heavily doped n-type Si:P. It exhibits a Curie-like electronic spin susceptibility which leads to a giant negative shift in the muon spin precession frequency. At high dopant levels, the Mu(0)(BC) hyperfine parameters, deduced from a model involving spin exchange with free carriers, are significantly reduced from those in intrinsic Si. This indicates that the spin density distribution for Mu(0)(BC) in metallic Si:P is altered significantly by charge screening effects, likely a general phenomenon for deep impurities in materials with high carrier concentrations.

Journal Article↗
PubMed2005

Intervalley-scattering-induced electron-phonon energy relaxation in many-valley semiconductors at low temperatures.

We report on the effect of elastic intervalley scattering on the energy transport between electrons and phonons in many-valley semiconductors. We derive a general expression for the electron-phonon energy flow rate at the limit where elastic intervalley scattering dominates over diffusion. Electron heating experiments on doped n-type Si samples with electron concentrations (3.5-16.0) x 10(25) m(-3) are performed at sub-Kelvin temperatures. We find a good agreement between the theory and the experiment.

Journal Article↗
PubMed1976

Hypersensitivity to bacteria in eczema. IV. Cytotoxic effect of antibacterial antibody on skin cells acquiring bacterial antigens.

The sera of persons with generalized eczema (Whitfield-type) or with disseminated nummular eczema were examined for complement-activating antibacterial antibodies to test the hypothesis that some eczematous change results from an antibody-mediated cytotoxic reaction. Bacteria dying in the stratum corneum release soluble antigens, some of which diffuse into the stratum Malpighii and become firmly adsorbed to the epidermal cells. Antibacterial antibody and complement diffusing into the epidermis react with the antigens acquired by the cells and may induce vacuolation or lysis. Phenol-extracted and freeze-press-extracted antigens (both containing teichoic acids) from Staphylococcus aureus and a micrococcus (Baird-Parker types SI and MI respectively) are adsorbed by monolayers of human skin, embryo or amnion. Cells acquiring the antigen(s) are severely damaged when treated with sera containing the appropriate antibacterial antibodies and complement. IgM complement-fixing antibody appears to be much more cytotoxic in this test than IgG. The cytotoxic activity of a serum is specific for the acquired bacterial antigen and appears to depend on a sufficient concentration of the effective antibody, and not on the presence of antibodies with special properties. Explants of full thickness skin treated with bacterial antigen extracts were unharmed by the antibodies that were cytotoxic for monolayers of skin cells treated with the same antigens. The in vitro cytotoxic test should represent a potential in vivo cytotoxic phenomenon, because skin cell monolayers from two patients adsorbed bacterial antigen prepared from cultures obtained from the same patients, and were damaged by autologous serum containing anti-staphylococcal antibody and complement. It seems probable that this may be an aggravating but not necessarily an initiating factor in many cases of eczema.

Antibodies, Bacterial↗
PubMed2006

Transmission electron microscopy characterization of the erbium silicide formation process using a Pt/Er stack on a silicon-on-insulator substrate.

Very thin erbium silicide layers have been used as source and drain contacts to n-type Si in low Schottky barrier MOSFETs on silicon-on-insulator substrates. Erbium silicide is formed by a solid-state reaction between the metal and silicon during annealing. The influence of annealing temperature (450 degrees C, 525 degrees C and 600 degrees C) on the formation of an erbium silicide layer in the Pt/Er/Si/SiO(2)/Si structure was analysed by means of cross-sectional transmission electron microscopy. The Si grains/interlayer formed at the interface and the presence of Si grains within the Er-related layer constitute proof that Si reacts with Er in the presence of a Pt top layer in the temperature range 450-600 degrees C. The process of silicide formation in the Pt/Er/Si structure differs from that in the Er/Si structure. At 600 degrees C, the Pt top layer vanishes and a (Pt-Er)Si(x) system is formed.

Journal Article↗
PubMed1995

Dosimetric characterization of radiosurgical beams with a diamond detector.

Dosimetric characteristics of small diameter 6 MV photon beams for a commercial radiosurgery system have been measured in a solid water phantom with a new diamond detector and compared with measurements using a p-type Si photon diode, small volume cylindrical and parallel plate ionization chambers, and radiographic films. Tissue maximum ratios (TMR) and total scatter factors (S c.p) were measured with the three ionometric (diamond, diode, ion chamber) detectors for 12 circular beams ranging in diameter from 12.5 to 40 mm. The TMR values obtained with the three ionometric detectors agreed well (+/- 1%) for all cone sizes and depths investigated when the displacement of the sensitive volume of the detector from its front surface is taken into account. The S c.p factors obtained with the ionometric detectors also agreed well (+/- 1.2%) for field sizes greater than 20 mm in diameter. For smaller field sizes, the cylindrical and parallel plate chambers measure a smaller S c.p factor, as a result of the steep dose gradients across their sensitive volumes. Cross-beam profiles acquired with the diamond detector agree well with the measurements with the diode detector and radiographic film. A distortion in the measured profiles in terms of broadened penumbra is observed with a small volume cylindrical ionization chamber.

Biophysical Phenomena↗
PubMed1996

Rate Constants for Charge Transfer Across Semiconductor-Liquid Interfaces

Interfacial charge-transfer rate constants have been measured for n-type Si electrodes in contact with a series of viologen-based redox couples in methanol through analyses of the behavior of these junctions with respect to their current density versus potential and differential capacitance versus potential properties. The data allow evaluation of the maximum rate constant (and therefore the electronic coupling) for majority carriers in the solid as well as of the dependence of the rate constant on the driving force for transfer of delocalized electrons from the n-Si semiconducting electrode into the localized molecular redox species in the solution phase. The data are in good agreement with existing models of this interfacial electron transfer process and provide insight into the fundamental kinetic events underlying the use of semiconducting photoelectrodes in applications such as solar energy conversion.

Journal Article↗
PubMed1982

Characterization of ATP citrate lyase from Chlorobium limicola.

ATP citrate lyase (EC 4.1.3.8) from Chlorobium limicola was partially purified. It was established that the consumption of substrates and the formation of products proceeded stoichiometrically and that citrate cleavage was of the si-type. ADP and oxaloacetate inhibited enzyme activity. Oxaloacetate also inhibited the growth of C. limicola.

ATP Citrate (pro-S)-Lyase↗
PubMed2006

Surface modification by electric discharge implemented with electrodes composed of carbon nanotubes.

In this work the electric discharge machining (EDM) implementing with multi-wall carbon nanotubes (MWCNT) as a miniscule electrode for pursuing precise surface modification was studied. The excellent upright growth of carbon nanotubes on copper based alloy substrates by a radio frequency (RF) assisted hot filament chemical vapor deposition (HFCVD) method suggests us to exploit MWCNTs as the miniature electrodes for discharge machining. The results reveal that the electrodes are much endurable to be distorted even when the spoiling rates for the polishing of n-type Si wafer (of 10 to approximately 100 omega-cm) are up to 30 nm/min with. It is expected that MWCNTs can be applied to non-conventional material processing especially in miniature discharge machining.

Alloys↗
PubMed2003

Electrical characterization of metal-molecule-silicon junctions.

Direct assembly of molecules onto silicon surfaces is of particular interest for potential employment in hybrid organic-semiconductor devices. In the study we report here, aryl diazonium salts were used to assemble covalently bound molecular groups on a hydride-passivated, oxide-free n-type Si(111) surface. The reaction of 4-(trimethylsilylethynyl)benzenediazonium tetrafluoroborate generates a molecular layer of 4-(trimethylsilylethynyl)phenylene (TMS-EP) on the n++-Si(111) surface. The monolayer modifies the electrical properties of the interface and exhibits nonlinear current-voltage characteristics, as compared with the ohmic behavior observed from metal-n++-Si(111) junctions. The result of current-voltage measurements at variable temperatures (from 300 to 10 K) on samples made with the TMS-EP molecule does not show significant thermally-activated transport, indicating that tunneling is the dominant transport mechanism. The measured data is compared to a tunneling model.

Electric Conductivity↗
PubMed1990

[The surface degradation of various light-cured composite resins by thermal cycling].

The durability of four commercially available light-cured composite resins was investigated by thermal cycling, GR containing inorganic fillers treated with the graft polymerization of acryl ester, LF inorganic fillers treated with a silane coupling agent, PC silanized inorganic fillers and organic composite fillers, and the MFR-type SI containing the organic composite fillers. These materials were given 10,000, 30,000 and 50,000 thermal cycles (4 degrees C-60 degrees C) and the deterioration of materials by thermal cycling was evaluated by the measurement of the mechanical properties and the SEM observations of the surface of the thermocycled materials. Compressive strength and bending elastic moduli for all materials did not change greatly by thermal cycling. However, bending strength, toothbrush abrasion resistance and surface hardness decreased with increasing number of thermal cycles between 0 and 30,000, and changed little after 30,000 cycles. The percentage of bending strength after 50,000 thermal cycles to that of the non-thermocycled sample was 75% for GR, 60% for LF, 50% for PC and 65% for SI, respectively. Deterioration of materials was observed as cracks on the surface, which generated at the interface of the filler and matrix. The cracks generated relatively earlier during thermal cycling for SI and PC which contained the organic composite filler, later for LF which contained the silanized inorganic fillers, and the number of cracks on LF were fewer than SI and PC. On the other hand, for GR, no cracks were observed even after 50,000 thermal cycles. From these results, it can be presumed that the pre-treatment of filler by the graft polymerization is more effective to improve the durability of composite resin.

Composite Resins↗