Sol-gel Thin Films Research Articles

Roughness and nanoholes in sol–gel thin films

Amorphous thin films have several technological applications. In this work the completeness of a surface probing technique, AFM imaging, and grazing X-ray reflectometry are exploited to inspect surface morphology, roughness and density grading in TiO2 sol–gel derived films.

Optical and electronic characterization of the band structure of blue methylene and rhodamine 6G-doped TiO2 sol–gel nanofilms

The optical properties and the band structure of TiO2 sol–gel thin films prepared by the sol–gel process and doped with methylene blue or rhodamine were studied by absorption, excitation and emission spectroscopy. The absorption spectra show two absorption bands, around 2.3eV and above 3.0eV suggesting that besides the excitation bands observed in rhodamine at 2.3eV an efficient energy transfer occurs between the TiO2 conduction band and the molecular rhodamine excited states, yielding a strong emission band at 600nm, while for methylene blue a strong hypsochromic shift of the absorption bands is observed. The optical absorption and band gap energy for the phase rutile of TiO2 films were calculated with a LDA+U/FPLAPW method, and a comparison of this calculation with experimental data of TiO2 films prepared by undoped sol–gel and sputtering is performed.

Synthesis and characterisation of tantalum-incorporating silica hybrid sol–gel thin films for optical applications

A new acrylate-modified silica incorporating tantalum is synthesised via a sol–gel route. After deposition, the few micron-thick films can be photo-polymerised by UV-irradiation. FTIR spectroscopy gives a first insight into the curing process and the chemical structure of the hybrid material. The refractive index of the layers can be modulated by varying the concentration of tantalum in the material. Coatings exhibit a good transparency in the blue range of the optical spectrum. Finally, the propagation losses are low enough to make the material suitable for applications in integrated optics.

Effect of ethanol variation on the internal environment of sol–gel bulk and thin films with aging

Sol-gel derived bulk and thin films were prepared from different compositions at low pH ( approximately 2.0) containing varying concentrations of ethanol from 15 to 60% at constant water (H(2)O)/tetraethyl-orthosilicate (TEOS) ratio (R=4). The fluorescence microscopic and spectroscopic measurements on fluorescent probe, Hoechst 33258 (H258) entrapped in these compositions were carried out at different days of storage to monitor the effects of concentration of ethanol on the internal environment of sol-gel materials. Fluorescence microscopic observations on sol-gel thin films, prepared by dip coating technique depicted uniform and cracked surface at withdrawal speed 1cm/min (high speed) and 0.1cm/min (low speed) respectively, which did not change during aging. Fluorescence spectral measurements showed emission maximum of H258 at approximately 535 nm in fresh sols at all concentrations of ethanol which depicted slight blue shift to 512 nm during aging in bulk. No such spectral shift has been observed in sol-gel thin films coated at high speed whereas thin films coated at low speed clearly showed an additional band at approximately 404 nm at 45 and 60% concentration of ethanol after about one month of storage. Analysis of the fluorescence lifetime data indicated single exponential decay (1.6-1.8 ns) in fresh sol and from third day onwards, invariably double exponential decay with a short (tau(1)) and a long (tau(2)) component were observed in sol-gel bulk with a dominant tau(1) at approximately 1.2 ns at all concentrations of ethanol. A double exponential decay consisting of a short component (tau(1)) at approximately 0.2 ns and a long component (tau(2)) at approximately 3.5 ns were observed at all ethanol concentrations in both fresh and aged sol-gel thin films. Further, distribution analysis of lifetimes of H258 showed two mean lifetimes with increased width in aged bulk and thin films. These results are likely to have strong implications in designing the internal environment for applications in biosensors.

Flow-through sol–gel optical biosensor for the colorimetric determination of acetazolamide

An optical biosensor based on immobilised carbonic anhydrase and its application to the determination of the anti-glaucoma agent acetazolamide by enzyme inhibition measurements, is described. The enzyme and a pH indicator dye, cresol red, were physically immobilised in overlapped sol-gel thin films, in a dual-layer format. Carbonic anhydrase catalyses the dehydration of HCO3-, which in turn causes a change of pH in the microenvironment of the sensor. By following the colour transition of cresol red, the enzymatic reaction as well as its inhibition by acetazolamide can be monitored. The sensor was integrated in a flow cell and coupled to a continuous flow system operating on a multicommutation and binary sampling approach. Measurements were made at pH 6.0 at the wavelength of 570 nm. Linear response was obtained for acetazolamide concentrations between 1.0 and 10.0 mmol l(-1), with a sampling frequency of 22 samples h(-1) and a detection limit of 0.2 mmol l(-1). The results obtained in the analysis of real samples were in good agreement with those obtained by a reference method, showing no significant differences at a confidence level of 95%.

Up-conversion emission in rare earth-doped Y 2Ti 2O 7 sol–gel thin films

Yttrium titanate (YTO) thin films doped with rare earth elements have been deposited using the aerosol–gel process. The up-conversion emission (UPE) of films doped with erbium and/or thulium has been studied with respect to the film crystallization degree and rare earth concentration. The effects of ytterbium codoping have been considered as well. Mechanisms that control UPE of YTO films are discussed with respect to the experimental conditions. It is shown that films, which selectively emit in the blue ( 1G 4→ 3H 6 transition of thulium ions), green ( 2H 11/2→ 4I 15/2 and 4S 3/2→ 4I 15/2 transitions of erbium ions), or red ( 4F 9/2→ 4I 15/2 transition of erbium ions), can readily be achieved.

Sol?Gel Coating of Thin Display Glasses - Problems and Remedy

Evaporation cooling is an inevitable but often neglected concomitant effect of sol–gel dip coating and related techniques, which is responsible for occasional coating problems. The phenomenon, however, emerges mostly for thin substrates of less than 1 mm thickness as the heat capacity of the substrate - the main source of heat for the evaporation - is reduced. The cooling of the substrate results in a reduced evaporation rate and an increased humidity in the surroundings of the film, leading to demixing of liquid components and an uncontrolled hydrolysis. The resulting coatings exhibit a hazy and cloudy appearance with crater-like structures in the micron range and consequently, functional properties such as the electrical conductivity of ATO coatings (antimony-doped tin oxide, SnO2:Sb) are unsatisfactory. Among a variety of measures to overcome this cooling problem, a moderate heating of the coating liquid to only 25_°C was found to be the method of choice to obtain high quality sol–gel thin films on substrates with a thickness down to 0.4 mm as required for display applications. This slight modification of the dip coating processing helps to reduce the susceptibility of the wet film to humidity and to improve the coating quality and the reproducibility in general - both on thin and thick substrates.

Modeling of silica sol–gel thin films with ballistic aggregation

Abstract An algorithm based on ballistic aggregation is proposed for modeling of atomic structure of sol–gel thin films. In the model, growth of structures changes smoothly from three-dimensional to two-dimensional as a limiting parameter starts to influence the results. Thus, this algorithm enables modeling of flat structures such as, practically important, thin films of various glass-like materials. Various sets of aggregation parameters are investigated and limitations of the model are determined.

Resistance of thin films to acid solutions

The dynamics of interaction between thin sol-gel films and acid solutions is described. It is established that two opposite processes occur simultaneously. Whereas swelling increases, dissolution decreases the coating thickness. The effect of acid concentration and pickling duration on these phenomena is described. The change of the refractive index of films after their reaction with the aggressive medium is estimated and explained.

Nanocrystalline ZnO thin films on porous silicon/silicon substrates obtained by sol–gel technique

In this study, nanocrystalline ZnO thin films deposited onto silicon substrates by sol–gel technique were studied. The starting material for ZnO sol–gel thin films was Zinc acetate. Single crystalline silicon wafers with different orientations and the porous silicon substrates were used as the substrates. XRD was employed to investigate the evolution of the crystalline orientation during the thermal treatment, and SEM was used to observe the surface morphology of the ZnO films. The effects of the anneal temperature and the substrate doping type on the deposited ZnO thin films were studied in detail. The results indicated that the highly oriented ZnO could be generated on Si substrates by controlling the process conditions. It was found that the substrates types had less influence on the obtained ZnO thin films compared with the anneal temperatures. Thin porous silicon substrates introduced is beneficial to enhance the bonding strength between the films and the substrates. These results are helpful to deposit the highly orientation-controlled ZnO thin films for different kinds of research and application.

Photooxidation studies with perylenediimides in solution, PVC and sol–gel thin films under concentrated sun light

The singlet oxygen quantum yields ( ϕ Δ), LUMO energy levels (eV), photo stabilities under Xe-lamp irradiation of N, N′-di-dodecyl-3,4,9,10-bis(dicarboximide)perylene and N, N′-di-(1-dehydroabiethyl)-3,4,9,10-bis(dicarboximide)perylene, N-DODEPER and ABIPER, respectively, in different host matrices (solution, PVC and sol–gel matrix) are determined. Quenching of their emission by KI is found to give k q values of 3.5–37.4 × 10 9 M −1 s −1. Quenching is more efficient in solution phase than it is in immobilized phases. ϕ Δ’s of ABIPER and N-DODEPER are in negligible range, 0.090 and 0.095, respectively. The LUMO energy levels determined by cyclic voltammetric studies are 3.65 and 3.49 eV for ABIPER and N-DODEPER, respectively. Neither ABIPER nor N-DODEPER showed degradation tendency in sol–gel matrix. Photooxidation of α-terpinene is performed with ABIPER and N-DODEPER as photosensitizers, both in acetonitrile solutions and immobilized phases (PVC and sol–gel host matrices) by the use of two different irradiation sources, Na lamp and concentrated sun light. The photosensitizers exhibited different activities on the product profile, which were strongly dependent on the irradiation source, period of irradiation and the phase that the sensitizers were placed (solution or immobilized phases). Product analysis results point that in immobilized phases of PVC films 3,6-dioxoheptanal is the major product. In solution and immobilized phase of sol–gel ascaridol is the most abundant product. Superoxide anion radical, forms as reactive intermediate at photoinduced electron transfer process, directs the primary mechanistic step of photooxidations. A secondary mechanistic step is being proposed for the clarification of product formations.

Desorption/ionization mass spectrometry on nanocrystalline titania sol-gel-deposited films.

This paper describes a matrix-free method for performing desorption/ionization directly from mesoporous nanocrystalline titania sol-gel thin films, which have good absorption capacity in the ultraviolet (UV) range and can act as assisting materials during UV matrix-assisted laser desorption/ionization mass spectrometric (MALDI-MS) analysis. A high concentration of citrate buffer was added into this system to provide the proton source and to reduce the presence of alkali cation adducts of the analytes. The analyte signals appear uniformly over the whole sample deposition area. Protonated molecules (MH(+) ions) of analytes dominate the titania MALDI mass spectra. Surfactants, peptides, tryptic digest products, and small proteins with molecular weights below ca. 24 000 Da, are observed in the titania MALDI mass spectra. Detection limits for insulin are as low as ca. 2 fmol with mass resolution of ca. 660.

Mechanical properties and adhesion characteristics of hybrid sol–gel thin films

The hardness and Young's modulus of organic–inorganic hybrid coatings, synthesised using sol–gel technology, deposited on silicon and copper were determined using indentations at low forces with a spherical tipped indenter and found to depend strongly on the size of the organic substituent. The indentation creep response of the coating systems was compared based on fast loading rates and for different times at maximum load. The adhesion characteristics of the coatings on copper were examined to ascertain the influence of the organic substituents on the film cracking behaviour and debond tendencies. For this purpose, coated tensile test specimens were strained uniaxially in a universal testing machine while the surface was examined using an optical microscope. The mechanical response was analysed from the multiple cracking patterns observed and the extent of film delamination from the underlying substrate. The results indicate that the interfacial adhesion and film toughness are dramatically affected by the nature of the organic substituent.

Luminescent sol–gel thin films based on europium-substituted heteropolytungstates

Polyester thin films containing europium-substituted heteropolytungstate were obtained on quartz plate by the sol–gel method. The films exhibited the characteristic emission bands of the europium ion. The red to orange intensity ratio (R:O) of Eu3+ in the films increased as compared to the corresponding heteropolytungstate solids. The fluorescence lifetime of europium is shorter in the thin film than in the heteropolytungstate solid. The results indicated that the formation of europium-substituted heteropolytungstate/polyester thin film has great effect on the luminescence of europium-substituted heteropolytungstate.

Semiconductor TiO2Gas Sensor for Controlling Nitrocarburizing Processes

TiO2 films can serve as oxygen sensors for controlling the nitrogen potential in the process of nitrocarburizing. In contrast to conventional semiconductor-base oxygen sensors that lose stability under extreme thermal and chemical conditions in nitriding and carburizing atmospheres, TiO2 films obtained by the sol-gel method offer a promising alternative. In the present work TiO2 films with a density of about 80% and a grain size of 30 – 50 nm are deposited by the sol-gel technology. Steel substrates and commercial substrates from Al2O3 aluminum oxide with platinum electrodes are coated and calcinated. In order to preserve the sensitive element of the sensor a special casing is constructed for operation under conditions very close to the atmosphere of nitrocarburizing. The sensitive element consists of an Al2O3 substrate with built-in Pt-electrodes, which is coated by a thin sol-gel TiO2 film. The direct current in the furnace is measured for nitrocarburizing in various mixtures (N2, O2, H2, and NH3) at a temperature of 500 – 600°C. A linear dependence log[R] – log[p O2], where R is the electrical resistivity, is obtained for the films in the studied range of partial pressure of oxygen independently p O2 of the presence of NH3 or H2 in the atmosphere. The dependence log[R] – log[p O2] for a nitriding furnace with a capacity of 90 liters is shown to be stable with 3% scattering per month.

Optical fiber probes for fluorescence based oxygen sensing

An optical fiber sensing system, for monitoring oxygen aiming in vivo nuclear magnetic resonance (NMR) applications is presented. Oxygen detection is based on the dynamic quenching of the fluorescence of a ruthenium complex trapped in the porous structure of a sol–gel silica film. Oxygen concentration is determined by phase-modulation fluorometry. Preliminary results concerning the characterization of doped sol–gel thin films deposited by dip coating in glass slides and in optical fiber probes are presented. Four different probe configurations are tested and compared. Best results are obtained with a fiber taper configuration which shows reproducibility and best excitation efficiency. This structure is fully characterized and some considerations regarding optimal fiber optical sensing probes for O2 detection are addressed.

Interfacial structure dependence of layered TiO2/WO3 thin films on the photoinduced hydrophilic property

Layered-type TiO2/WO3 thin films were fabricated by sol–gel and sputtering methods and the effects of those different fabricating methods on the interfacial structures and the photoinduced hydrophilic properties were investigated. A third amorphous phase was observed by cross-sectional transmission electron micrograph at the interface of the sol–gel derived TiO2/WO3 thin film, but was not at that of the sputtered TiO2/WO3 thin film. Under weak ultra-violet light irradiation with intensity of 0.05 mW/cm2, the photoinduced hydrophilic property on the sputtered thin film was superior to that on the sol–gel thin film. These results suggest that the interfacial third phase acts as a barrier against holes and electrons’ transfer and degrades the photoinduced hydrophilic property.

Development of a sol–gel optical sensor for analysis of zinc in pharmaceuticals

A Zn(II) optical sensor was developed by incorporating 4-(2-pyridylazo)resorcinol (PAR) in a sol–gel thin film. Acid- and base-catalyzed methods to prepare the sol–gel layers have been studied, as well as different types of precursors and different PAR concentrations. Sensors based on co-polymerization of tetraethoxysilane (TEOS) with 3-amynopropyltriethoxysilane (3-APTES), basic catalysis, water:alkoxyde ratio of 4 and PAR concentration of 1.0gl−1 showed optimum performance in the proposed working conditions. The sensor was coupled to a multicommutated flow system and applied to the direct determination of Zn(II) in injectable insulins. Optical transduction was based on the use of a dual-color LED and a photodiode. The sensor showed optimum response at pH 5.5 with maximum absorbance at 500nm. Its regeneration was accomplished with a solution of KSCN acidified to pH 3.0 with HCl. Linear response was obtained for Zn(II) concentration range of 5.0–25.0μgl−1, with detection limit of 2.0μgl−1 and sampling frequency of 16 samplesh−1. Interference from foreign ions was studied at 10:1 (w/w) ratio added ion:Zn. The results obtained on real samples analyzed were in good agreement with those obtained by a standard method, with relative deviation errors inferior to 1.2%.

Densification and porosity evaluation of ZrO 2–3 mol.% Y 2O 3 sol–gel thin films

Densification of ZrO 2–3 mol.% Y 2O 3 sol–gel thin films after heating in air at 100 °C≤ T≤1100 °C is investigated, with particular emphasis on porosity evaluation. For this purpose, the refractive index and thickness of the films are determined from transmission spectrometry. Three densification regimes were clearly identified: T<300 °C, 300 °C ≤ T≤ 800 °C and T>800 °C. For porosity evaluation, a simple strategy based only on film thickness measurements is used. These porosity values are compared with those obtained from refractive index measurements, applying commonly used analytical expressions. Our results clearly show that the porosity of thin films is underestimated when the Lorentz–Lorenz expression is used. Implications for preparation of ZrO 2 sol–gel thin films are also discussed.

Influence of the heat-treatment on thickness and porosity of thin-films obtained by sol-gel route

The present work is aimed to investigate the thermal densification mechanisms of sol-gel zirconia thin films deposited by dip coating, with particular emphasis on thickness and porosity evolution at different firing treatments (100-1100°C). For this purpose, we have only used data from transmission spectrum. Implications for controlled design of sol-gel zirconia films will be considered.