Sol-gel Layer Research Articles

Photogeneration Efficiency Study of New Sol-gel Materials

This paper reports our studies of the preparation and characterization of novel xerographic photoreceptors with sol-gel based silsesquioxane polymeric charge transport layers. Photoreceptors have been prepared where the sol-gel layer is the sole transport layer (CTL) directly contacting the near-IR-absorbing charge generation layer (CGL). The sol-gel layers are novel in that they incorporate charge transport moieties covalently linked in a co-polymer, which is subsequently networked into the silsesquioxane matrix via the sol-gel process. We have investigated the photogeneration efficiencies of these photoreceptors and their dependence on sol-gel composition.

3-D mapping with ellipsometrically determined physical thickness/refractive index of spin coated sol-gel silica layer

Precursor sol for sol-gel silica layer was prepared from the starting material, tetraethylorthosilicate (TEOS). The sol was deposited onto borosilicate crown (BSC) glass by the spinning technique (rpm 2500). The gel layer thus formed transformed to oxide layer on heating to 450°C for ∼ 30 min. The physical thickness and the refractive index of the layer were measured ellipsometrically (Rudolph Auto EL II) at 632.8 nm. About 10 × 10 mm surface area of the silica layer was chosen for evaluation of thickness and refractive index values at different points (121 nos.) with 1 mm gap between two points. Those data were utilized in the Autolisp programme for 3-D mapping. Radial distribution of the evaluated values was also displayed.

Characterization of Metal-Ferroelectric-Metal-Insulator-Semiconductor (MFMIS) Structures Using (Bi, La)4Ti3O12 and HfO2 Buffer Layers

We report the electrical properties of metal-ferroelectric-metal-insulator-semiconductor (MFMIS) structures using (Bi, La)4Ti3O12 (BLT) thin films with HfO2 buffer layers for one-transistor(1T)-type FeRAM applications. Since high-capacitance buffer layers are required to realize the high density of MFMIS gate structures, we used a HfO2 layer as an insulator material. Ferroelectric BLT layers were fabricated by the sol-gel technique and HfO2 buffer layers were deposited by the RF sputtering method. It is demonstrated that Pt/HfO2/n-Si MIS structures have a low leakage current in the 10-9 A/cm2 range with an equivalent oxide thickness (EOT) of 3.5 nm. The capacitance–voltage (C–V) characteristics of the Pt/BLT/Pt/HfO2/n-Si MFMIS diode with an SI/SF area ratio of 6 reveal a large memory window of 3 V due to the ferroelectric properties of the BLT films.

Growth of crystalline SiC film free of cavities by heating PS/silica sol-gel coatings on Si (111) substrate

Single crystalline 6H-SiC films were prepared by spin-coating silica and polystyrene (PS) sol-gel layers subsequently on Si (111) substrate, and then the sample heated in vacuum (10 −3 Pa) to temperature above 1000 °C. The films were investigated by Fourier transform infrared absorption (FTIR), transmission electron diffraction (TED), X-ray diffraction (XRD), and scanning electron microscopy (SEM) measurements. Plan-view SEM observations indicated that the silica layer sandwiched by PS and Si substrate can suppress the interface cavity formation. It was proposed that the silica layer acted as a barrier of out-diffused Si atoms, and took the place of the substrate as the reservoir of Si supply to form SiC during the heat treatment.

PH sensor based on a detection sol–gel layer onto optical fiber

A pH sensor was made via the deposition of detection layer onto an optical fiber. This layer contains a pH-sensitive dye [thymol blue (TB)] trapped within a sol–gel silica matrix. The light was launched directly at the front of the fiber with an angle chosen in order to increase the power transmitted by the evanescent wave. This technique was used to induce the absorption of the dye. TB can be used both as an acid or a base titration indicator. However, the response of the optical device in acid conditions is rather poor and only the alkaline transition in the pH range 8–12 has been exploited. The dye appears to be firmly trapped within the silica matrix and no leaching is observed during repetitive experiments.

Peptide Arrays for Highly Sensitive and Specific Antibody-Binding Fluorescence Assays

We report a novel generation of peptide arrays fabricated by site-specific ligation of glyoxylyl peptides onto glass slides covered by a semicarbazide sol-gel layer. These arrays allowed the highly sensitive and specific detection of antibodies in very small blood samples from infected individuals using three model peptidic epitopes (HCV Core and NS4, EBV Capsid) in an immunofluorescence assay. Comparison with standard enzyme-linked immunosorbent assays (ELISAs) demonstrated a large gain in sensitivity and specificity. These unique properties, combined with the possibility to immobilize glycoproteins such as antibodies, offer the possibility to perform sandwich immunofluorescent assays in a highly parallel format.

An Amperometric Sensor for Hydrogen Peroxide Based on a (3‐Mercaptopropyl)trimethoxysilane Self‐Assembled Layer Containing Hydrazine

A new modified electrode constructed by self-assembly of (3-mercaptopropyl)trimethoxysilane on a gold electrode followed by immobilization with hydrazine exhibits excellent electrocatalytic activity for the reduction of hydrogen peroxide at a reduced overpotential of −0.15 V. Immobilization of hydrazine in the sol-gel layer formed on the electrode was confirmed by ESCA and FT-IR experiments. The determination of hydrogen peroxide with the hydrazine modified silane electrode shows the advantages of ease of fabrication, high sensitivity, good selectivity, rapid response, and remarkable stability.

Incorporation and optical behaviour of 4-dimethylaminazobenzene in sol-gel silica thin coatings

Various percentages of 4-dimethylaminazobenzene (dimethyl yellow, DY) have been incorporated into silica sol-gel coatings prepared from tetraethylorthosilicate and methyltriethoxysilane. The dye was entraped in the sol-gel layer maintaining its acid-basic properties. Sensitivity of coatings against pH in aqueous media was evaluated by absorption spectrophotometry for each type of samples, including one pre-treated at low temperature. The response time of coatings from acid pH to neutral conditions and vice versa allows to propose the use of this system as a pH optical sensor. On the other hand, several tests oriented to point out the behaviour of coatings during chemical attack were performed: firstly by dipping the samples in distilled water for different lengths of time and, secondly, submitting them to reiterated cycles at 60 °C in a washing machine using conventional abrasive detergent. Results obtained in both cases indicated good resistance, especially for those hybrid films prepared from tetraethylorthosilicate and methyltriethoxysilane doped with 5 wt.% DY.

Novel fiber-optic biosensor based on immobilized glutathione S-transferase and sol–gel entrapped bromcresol green for the determination of atrazine

The aim of this work was to investigate, for the first time, the potential of the enzyme glutathione S-transferase I (isoenzyme GST-I) for uses in analytical chemistry. A novel fiber-optic biosensor for the detection and determination of the triazine herbicide atrazine was developed based on maize GST-I expressed in E. coli. The sensing bioactive material was a three-layer mini-sandwich. The enzyme was immobilized on the outer layer that consisted of a hydrophilic polyvinylidenefluoride membrane. This membrane was supported on an inner glass disk by means of an intermediate binder sol–gel layer that incorporated bromcresol green (BCG). The biosensor operated in a static mode at 25 °C and the rate of the enzymatic reaction, using atrazine as a substrate, served as an analytical signal. A calibration curve was obtained for atrazine, with analytically useful concentration range 2.52–125 μM. The sensor detection limit was 0.84 μM. The reproducibility of atrazine sensing was in the order of ±3–5%. The method was successfully applied to the determination of this herbicide in real water samples, without sample preparation steps. Atrazine recovery ranged between 85 and 110%. No interference from other pesticides, such as alachlor and carbaryl was observed in the absence of atrazine. The immobilized enzyme retained about 75% of its original activity after 1 month use. Simply unscrewing the terminal holding ring of the probe and placing a new bioactive sandwich could easily replace a deteriorated mini-sandwich.

Ferroelectric thin and thick films for microsystems

Ferroelectric oxide thin and thick films are interesting materials for microsystem devices because of their wide range of useful properties, particularly the pyroelectric and piezoelectric effects. The ability to grow these films at relatively low temperatures onto a wide range of substrates, including silicon, is especially important. This paper discusses the use of CSD processes to grow high-quality ferroelectric PZT30/70 thin films onto platinised silicon substrates at low temperatures (from 400°C to 575°C), with particular reference to their use in pyroelectric infra-red detector arrays and other MEMS devices. The various factors important to the use of sol–gel processes are discussed, including mechanisms for sol ageing and for perovskite nucleation and growth. The latter is interesting, involving the formation of a transient Pt 3Pb phase that acts as a nucleation layer for the templated growth of the PZT layer. The effects of Mn doping on the resulting materials properties are discussed. It leads to a strong asymmetry in the ferroelectric hysteresis behaviour as well as improved pyroelectric performance. Techniques for increasing the thickness of sol–gel layers, and the reasons for the appearance of nanoporosity, are reviewed. Finally, the use of sol–gel techniques for the fabrication of composite piezoelectric ceramic thick films (10–20 μm thick) at low temperatures (710°C) are discussed.

A Simplified Broadband Coupling Approach Applied to Chemically Robust Sol−Gel, Planar Integrated Optical Waveguides

A new generation waveguide spectrometer with broadband coupling capabilities has been developed. As opposed to previous devices, this attenuated total reflection (ATR) spectrometer is much simpler in design, is more chemically robust, and transmits light down to at least 400 nm. The attenuated total reflection element consists of a single-mode, planar integrated optical waveguide fabricated by dip-coating a approximately 300 nm thick, sol-gel composite layer on a glass substrate. A commercially available prism is used as the incoupler with an integral holographic diffraction grating acting as the dispersive outcoupling element. The transmission of narrow band-pass filters was used to compare the response of the waveguide spectrometer to that of a conventional transmission instrument. Spectral resolution was assessed by measuring the fwhm of various laser lines, which were found to range from 0.5 to 1.3 nm. The measured limits of detection for the waveguide spectrometer from 400 to 600 nm are 8.0 and 10.1 milliabsorbance units for TE and TM polarizations, respectively. Finally, to demonstrate the application of this technology to a molecular film confined to a solid-liquid interface, visible ATR spectra of an adsorbed submonolayer of horse heart cytochrome c were acquired. A procedure to correct the waveguide spectra for the wavelength dependence in ATR path length is described.

Structural and electrical characteristics of zirconium oxide layers derived by photo-assisted sol-gel processing

This paper reports the photo-assisted formation of ZrO2 layers derived by sol–gel processing at low temperatures using intense radiation from ultraviolet (UV) excimer lamps. Excellent layer properties can be readily obtained for these sol–gel layers after 5-min exposure to the UV irradiation at around 300 °C. Analyses of the as-deposited sol–gel layers by UV/VIS spectrophotometry show that the organic species contained in the layers have been removed to a large extent after 5-min irradiation. This is further confirmed by X-ray photoelectron spectroscopy analyses of the same irradiated layers, which indicate the formation of ZrO2 with little carbon contamination contributed by the organic species and less oxidation of Si at the interface. Electrical measurements of these layers are also reported.

Stability of new optical pH sensing material based on cross-linked poly(vinyl alcohol) copolymer

Cross-linked poly(vinyl alcohol) (PVA)–silica gel copolymer has been employed as a optical pH sensor substrate for immobilisation of fluorescein. Cross-linking was carried out by the sol–gel process incorporating PVA in initial sol–gel solution of tetra-methoxysilane (TMOS) under acidic conditions. Three dimensional network formation could be achieved using compositions of PVA/TMOS=80–90/20–10vol.% to result in crack-free films. The fluorescent sensor layers were prepared by dip-coating of gel solution onto glass slides. The dynamic fluorescence response towards different pH values was investigated in terms of the influence of sample ionic strength, membrane composition as well as age of sol–gel layers. Depending on the composition of the matrix pKa values of 6.50, 6.68 and 7.06 were found 18 days after continues storage in buffer.

Novel Sol-gel Materials with Charge Transporting Properties

A series of novel copolymers having components with hole transport and silane functionality have been synthesized. These copolymers were prepared by radical polymerization. Through a sol-gel process with methyltrimethoxysilane, solutions of the copolymers with were coated to form the hole transport layer of an organic photoreceptor. These hole transport active silsesquioxane layers were coated as either a charge transport layer (CTL) on a charge generation layer (CGL) or a protective overcoat on a CTL. The electrophotographic properties of photoreceptors prepared with these sol-gel layers were described.

Planar Integrated Optical Waveguide Sensor for Isopropyl Alcohol in Aqueous Media

An attenuated total reflection (ATR) sensor for water-miscible organic solvents was constructed using a combination of sol-gel processing and integrated optical waveguide (IOW) technologies. The sensor consisted of single-mode, sol-gel based planar waveguide coated with a 40 nm thick, porous sol-gel indicator layer prepared from methyltriethoxysilane and doped with methyl red. The response of the senor to aqueous isopropyl alcohol (IPA) was investigated. Solvation of the indicator dye by IPA causes the absorbance spectrum to undergo a blue shift coupled with an increase in molar absorptivity. IPA was detected by measuring changes in ATR of the guided mode at 488 nm. A response curve extending from 1 to 100% (v/v) IPA in water was constructed for the sensor, from which a detection limit of 0.7% (v/v) IPA/water was estimated. Response and reversal times were typically less than one minute, making this sensor potentially attractive for on-line monitoring applications. The rapid response characteristics are attributable to relatively weak, reversible interactions between the indicator and analyte.

Photoluminescence properties of sol–gel derived SiO 2 layers doped with porous silicon

We proposed and studied a new light emitting material based on silicon nanocrystals (Si-nc). The new material was fabricated using a low cost way by incorporating Si-nc into a sol–gel derived SiO 2 matrix. The Si-nc were prepared (i) by electrochemical etching of monocrystalline Si wafers and (ii) pulverizing the obtained porous silicon film. The porous silicon powder was then dispersed in the SiO 2 sol. After solidification, we obtained transparent and self-supporting SiO 2 layers of about 1 mm thickness containing Si nanocrystals. The sol–gel layers exhibit bright red photoluminescence (PL) under UV lamp excitation at room temperature. This novel method circumvents the usual Si +-implantation step. We present first basic experimental studies of the PL properties of these sol–gel derived SiO 2 layers and compare them with that of as-prepared porous silicon. We mention also observation of a non-linear behavior in the PL spectra. We emphasize potential advantages of this technology compared to the standard Si +-implanted SiO 2 layers.

A portable fiber-optic pesticide biosensor based on immobilized cholinesterase and sol–gel entrapped bromcresol purple for in-field use

A fiber-optic biosensor for the detection and determination of the pesticides carbaryl and dichlorvos was developed. The sensing bioactive material was a three-layer sandwich. The enzyme cholinesterase was immobilized on the outer layer, consisting of hydrophilic modified polyvinylidenefluoride membrane. The membrane was in contact with an intermediate sol–gel layer that incorporated bromcresol purple, deposited on an inner glass disk. The sensor operated in a static mode at room temperature and the rate of the inhibited reaction served as an analytical signal. Calibration curves were obtained for carbaryl and dichlorvos, with useful concentration ranges 0.11–8.0 mg l −1 for carbaryl and 5.0–30 μg l −1 for dichlorvos. The respective detection limits were 108 μg l −1 and 5.2 μg l −1. The method reproducibility was in the order of ±3–5%. The method was successfully applied to the detection and determination of these pesticides in real water samples, without sample preparation steps. Recovery experiments were made and the accuracy of the method was 94.9%. No enzyme regeneration steps were applied and the sensor lifetime was 3 weeks (30% activity reduction). The bioactive mini sandwich can easily be replaced by simply unscrewing the terminal holding ring of the probe and placing a new sandwich, before the sensor is ready for use.

EXAFS studies on erbium-doped TiO 2 and ZrO 2 sol–gel thin films

High concentration (15% in a molar ratio) Er-doped TiO 2 and ZrO 2 sol–gel layers, annealed under an oxygen flow, at 300 and 650 °C, respectively, were studied at the erbium L III edge (8358 eV) by X-ray near edge structure spectroscopy (XANES) and extended X-ray absorption fine structure spectroscopy (EXAFS) in order to obtain some useful information about the local atomic arrangement around the incorporated rare earths. Transmission electronic microscopy (TEM) experiments were also carried out in order to support the X-ray absorption spectroscopy (XAS) studies by a knowledge of the global structure of the sol–gel layer. The analysis of the XAS data reveals that, in the case of an annealing treatment at 300 °C, the local atomic structure surrounding the erbium ion appears amorphous for both samples, while after an annealing treatment at 650 °C, a crystalline structure around the dopant is strongly dependent on the nature of the lattice cation (Ti or Zr). Indeed, for such an annealing temperature, the local atomic environment around the erbium ion appears to be amorphous in the case of an anatase matrix while it seems to have crystallized around the dopant in the case of zirconia. Furthermore, for both these compounds the XAS data reveal that a pairing effect between the erbium ions probably occurs. The TEM studies performed on the samples annealed at 650 °C show the appearance of other phases different from the matrix, confirming this fact. For both compounds, the analysis of these sets of data reveals that the erbium ion has not formed metallic erbium in the matrix. For the Er-doped TiO 2 sol–gel layers, it can be concluded that the rare earth has not precipitated in the form of erbium oxide clusters. Whereas, for the Er-doped ZrO 2 sol–gel layers, nothing can be deduced about the possible appearance of Er 2O 3. It can be then concluded that the appearance of another phase, with the formula near Zr 3Er 4O 12, likely occurs in the case of the Er-doped ZrO 2 sol–gel layer annealed at 650 °C, while a solid solution of Er x Ti y O z was formed in the case of the Er-doped TiO 2 sol–gel layer (annealed at 650 °C). Nevertheless it should be noted that the high concentration of erbium doping used on this study (15%) is not the one which presents the most useful interest for applications as optical devices. Such a high concentration of erbium, selected for the XAS measurement, provides a clue to the chemical mechanisms which could explain the optical properties of the rare earth shown in former studies.

Optical and structural properties of sol-gel SiO2 layers containing cobalt

SiO 2 coatings containing cobalt were prepared using the sol-gel method and the Si to Co nominal atomic ratio in the coatings was varied from 1.3 to 7.1. The structure and optical properties of the coatings, heat-treated in air at 300 and 500 °C, were characterized using optical, x-ray diffraction, and Auger depth profile measurements. The optical transmission data in the UV-visible range, in samples with low cobalt concentrations, show only the absorption bands associated with the tetragonal Co2+, regardless of the heat treatment temperature. Coatings with a larger amount of Co treated at 300 °C show absorption bands associated with both tetragonal and octahedral Co2+. When these coatings are heat treated at 500 °C, most of the cobalt migrates to the free surface of the coatings, where it is oxidized by the atmospheric oxygen and forms a top layer of Co3O4. The thickness of the cobalt oxide layer depends on the heat treatment temperature and on the Co concentration. To describe the absorption bands of both tetragonal and octahedral Co, the Lorentz oscillator model was used to represent the complex effective dielectric function of the SiO2 cobalt doped layers. The frequency dependence of the optical constants, in the UV-visible range for the top cobalt oxide layer, was obtained with a Jellison–Modine model.

Investigation of mass transfer through inorganic membranes with several layers

Suitable porous membranes for application in membrane reactors usually consist of several layers. The selective separation layer is in general deposited on one or several supporting layers having larger pores. For the sake of simplicity the mass transfer through such composites is frequently modelled using integral parameters. However, this simplified description has serious limitations, e.g. it is not capable to quantify the often observed effects of direction dependencies of flow and selectivity. In this paper the mass transfer is studied simultaneously to the production process of asymmetric membranes. Membranes made of different α- and γ-alumina layers were prepared. In a final preparation step a further silica sol–gel layer was deposited. To characterise the membranes SEM, EDX, 29 Si -NMR and permeation measurements were applied. After each deposition of a new layer, permeation was studied and analysed in order to determine characteristic parameters of this layer assuming the parameters of the previous layers to be known. The dusty gas model could be applied successfully for the quantification of the mass transfer through the multi-layer alumina membranes. Using the determined model parameters for all alumina layers, internal pressure profiles were simulated for the two possible flow directions. The differences of fluxes for the same total pressure gradients were quantified. The mass transfer through the finally deposited sol–gel layer was found to be more complex. A qualitative description based on the concept of configurational diffusion was performed. All results obtained emphasise the necessity of taking membrane asymmetries properly into account during the design and analysis of membrane reactors.