Sol-gel Thin Films Research Articles

The role of organic groups in ormosil matrices in the photochromism of naphthopyrans in sol–gel thin films

Photochromic 3,3-diphenyl-3H-naphtho[2,1-b]pyran molecules were embedded in sol–gel prepared thin organically modified silica films. The introduction of organic functional groups into a silica matrix allows the surfaces of its pores to be tailored and thus the polarity of the pores to be modified. The photochromic properties of the naphthopyran molecules, such as the spectral properties of the colored forms and the kinetics of thermal bleaching, were found to depend strongly on the polarity of the pores in which the molecules are located and hence, on the nature and loading of the organic groups (Me, Ph or iBu) in the ormosil matrix. The large isobutyl substituents showed the largest reduction in the polarity of the pore, due to the effective screening of the polar OH groups at the surfaces of the pores. Hence, the absorption band of the open colored form of the photochromic dye could be adjusted by as much as 35 nm in the yellow-to-orange range by changing the composition of the embedding matrix. Reducing the polarity of the pore walls in the ormosil matrix resulted in a substantial acceleration of the bleaching kinetics of the photochromic effect.

Synthesis and characterization of Ca1-X SrxCu3Ti4O12 thin films for dielectric applications

ABSTRACTThere is a great deal of interest in CaCu3Ti4O12 system for dielectric applications. We have studied Ca1-xSrxCu3Ti4O12 system for different compositions. The material is synthesized by sol-gel chemical solution route and thin films were deposited by spin coating. Thin films were investigated by x-ray diffraction and Raman spectroscopy for structural properties. These results indicate a solid solution for the compositions x = 0.00 to 0.80. The SEM micrographs shows the uniform films at 800° C, but the dielectric response of Ca1-xSrxCu3Ti4O12 (x = 0.00) shows the dielectric constant value below 200.

Surface conductivity of DR1-functionalized organic-inorganic sol-gel materials

In recent years, in looking for an important photorefractive application, several studies on electro-optic and photoconductive sol-gel responses have been done. A very important effort has been addressed to establish the appropriate induced-orientation procedure, in order to get the highest electro-optic coefficient. In this way a very high coefficient of 48 pm/V at 831 nm in sol-gel has been already found. Similarly, the importance of the non-linear chromophore concentration into the material electro-optic behavior has been studied. However, the influence of the orientation procedure and the chromophore concentration over the photoconductive response has not been performed. In this work we study the vacuum-surface-charge-transport under and without illumination after poling times of 10, 30 and 120 min on DR1-functionalized sol-gel thin films of 1.3 μm in thickness with a suitable concentration of DR1. We include the measures before poling for other chromophore concentrations. We found the largest density of photocurrent at 633 nm for a poling time of 30 min. We also measured the order parameter in order to follow the Corona induced orientation evolution as function of time for each case. The saturation found into this parameter and into the photoconduction show the existence of an optimal poling time.

On Ceramic Thin Film Formation from Gels: Evolution of Stress, Cracks and Radiative Striations

There are still several problems to be scientifically solved for the ``conventional''sol-gel ceramic coating technique to be utilized in industrial scale. Based on the recognition, the author's group studied the stress evolution and crack formation in gel-derived films and the evolution of radiative striations in spin-coating films. Possible modification of the sol-gel technique has also been made for realizing thicker films via non-cycled deposition. First, the residual stress was measured on alkoxide-derived silica and titania films deposited on silica glass substrate. The residual stress increased with increasing heat-treatment temperature due to densification. It was demonstrated that the stress in films is not a simple function of the difference in thermal expansion coefficient between the film and the substrate, but is greatly affected by the processing parameters including the amount of water for hydrolyzing alkoxides and the kinds of the chelating agents. Second, in situ observation was conducted on silica and titania gel films deposited on single crystal silicon substrates. Macroscopic cracking occurred in the heating-up stage, suggesting that the cracks are formed by tensile stress generated in films due to densification. Also, such cracking was found to occur at lower temperatures when the films are thick or heated at lower rates. Third, expecting that organic polymers with amide groups like polyvinylpyrrolidone (PVP) can promote structural relaxation and can suppress stress evolution on heating, PVP-assisted sol-gel thin film deposition was proposed. Incorporating PVP in coating solutions, crack-free ceramic thin films including BaTiO3, PZT and BaBi4Ti4O15 films over submicron or micron in thickness could be prepared via non-cycled deposition. Finally, radiative striations were quantitatively evaluated by surface roughness measurement, and the effects of the processing parameters on striation evolution were studied. The height and spacing of striations were found to be increased with decreasing spinning rate and with increasing sol viscosity. Striations were also found to be formed even without the rotation of the substrate, suggesting that the substrate rotation is not a necessary condition for evolution of striations. On the assumption that the mechanism of the evolution of striations is the same irrespective of the presence or absence of the substrate rotation, in situ observation of the evolution of striations was conducted on a sol layer placed on a stationary substrate. It was observed that striations are formed on solvent evaporation after the sol was spread, not on proceeding of the sol front towards the outer.

Photo-induced swelling of hybrid sol-gel thin films: Application to surface micro-patterning

A single-step process was used for patterning the surface of an acrylate-rich silica-based hybrid sol-gel with UV light. A spontaneous swelling of the illuminated areas was observed when the sample was irradiated through a photo-mask prior to any drying step. Relief height up to 2.5 μm was induced in a few minutes when imprinting structures with feature size in the range 5 to 40 μm. High resolution was also demonstrated by imprinting 1 μm-period gratings with a 150 nm groove depth in 0.2 sec. It was found that the process was strongly activated by temperature. The formation of the relief was attributed to mass transport occurring in the material during the gel phase. The very short processing time required for creating significant surface swelling with a good resolution, makes the process potentially attractive for micro-patterning large surfaces.

Molybdenum-based nanostructured mixed oxides for sensing applications: Effect of the Mo oxide composition on the structure of sol–gel thin films

In this work, we report on the structural investigation by transmission electron microscopy of molybdenum-based mixed oxides for use as gas sensitive films. In particular, sol–gel thin films of molybdenum–titanium and molybdenum–tungsten oxides, deposited by spin coating on silicon substrates, have been analyzed by selected area electron diffraction, conventional, and high-resolution transmission electron microscopy. Our results show that the relative percentage of the molybdenum precursors used during the sol–gel deposition controls the morphology of the films, by modifying the size and the arrangements of the polycrystalline grains on the substrate.

Absorption and fluorescence properties of bifluorene crystal and microcrystals

Optical properties of 2,2'-(9,9-dihexyl) bifluorene bulk crystal and microcrystals embedded in thin sol–gel films are reported. Reflection spectroscopy and Kramers–Kronig analysis are used to derive the absorption spectrum of the bulk crystal. The absorption and emission spectra obtained for the bulk crystal, microcrystals and molecules in dilute solution are very similar. Due to the perpendicular arrangement of molecules in the crystal structure, intermolecular interactions in the crystal phase are weak and have no significant effect on the electronic transitions of luminescent dye. Thus, the crystal phase has a high blue fluorescence efficiency and appears to provide the basis for a convenient approach to overcoming the aggregation tendency that degrades the electroluminescence properties of fluorene-based oligomers and polymers.

On the study of feature extraction methods for an electronic nose

In this study, we analyzed the transient of microsensors based on tin oxide sol–gel thin film. A novel method to this research field for data analysis and discrimination among different volatile organic compounds is presented. Moreover; several feature extraction methods have been considered, both steady-state (fractional change, relative, difference and log) and transient (Fourier and wavelet descriptors, integral and derivatives) information. Feature extraction methods have been validated qualitatively (by using principal component analysis) and quantitatively on the classification rate (by using a radial basis function neural network).

Structural characterization of sol-gel lanthanum cobaltite thin films

The present paper is focused on structural, microstructural and compositional studies on nanophasic LaCoO 3 thin films obtained by the sol-gel route. The sample structure and microstructure were investigated by Glancing Incidence X-Ray Diffraction (GIXRD) and X-Ray microdiffraction (MicroXRD), whereas the surface and in-depth chemical composition was studied by X-Ray Photoelectron Spectroscopy (XPS). All the films are structurally homogeneous and not textured. A cubic-to-rhomboedral phase transition was detected after the thermal treatment at 800 °C. Evidence of residual stress was found by GIXRD patterns collected at different incidence angles. After thermal annealing at 1000 °C, only the crystalline La 2O 3 phase was detected.

Preparation and isolation of gold nanoparticles coated with a stabilizer and sol-gel compatible agent

In this work an attractive technique is presented that brings together the advantage of the micelle reverse technique to control the particle growth and the efficiency of the amine silanes as sol-gel-compatible surface modifiers. The diamino silane is far from being a passive agent in the formation process of the gold particle; it strongly modifies the growth of the gold particle in the reverse micelle. The diamino silane allows the gold particles to keep their individual properties unaltered throughout the process, which ends with their incorporation into a SiO2–TiO2 sol-gel thin film.

Microstructural evolution during pyrolysis of triol-based sol-gel single-layer Pb(Zr0.53Ti0.47)O3 thin films

Advanced analytical transmission electron microscopy has been used to investigate microstructural evolution during pyrolysis in triol-based sol-gel thin films. At pyrolysis temperatures up to 300 °C, the films remained amorphous; however, nanometer-sized precipitates were observed in films heat-treated up to 400 °C for 10 min. Analytical transmission electron microscopy indicated that the precipitates were Pb-rich, as well as deficient in O, Ti, and Zr. Films pyrolyzed up to 500 °C for 10 min were composed of a nanocrystalline pyrochlore phase; however, pores could be observed, situated in the same position as the nanometer-sized precipitates at 400 °C. Face-centered cubic Pb-rich crystallites were also present on the surface of pyrolyzed films but absent in the fully crystallized films annealed at 650 °C. A tentative mechanism is proposed to explain these observations.

Preparation of porous titania sol-gel matrix for immobilization of horseradish peroxidase by a vapor deposition method.

A new and facile vapor deposition method has been developed for the preparation of sol-gel matrix. This method was used to form a titania sol-gel thin film and to immobilize horseradish peroxidase (HRP) on a glassy carbon electrode surface for the production of an amperometric hydrogen peroxide biosensor. This process prevented the cracking of conventional sol-gel-derived glasses. The morphologies of both titania sol-gel and the enzyme membranes were characterized using scanning electron microscopy and proved to be chemically clean, porous, and homogeneous and to have a very narrow particle size distribution. The sol-gel-derived titania-modified electrode retained the enzyme bioactivity and provided for long-term stability of the enzyme in storage. In the presence of catechol as a mediator, the sensor exhibited a rapid electrocatalytic response (less than 5 s), a linear calibration range from 0.08 to 0.56 mM with a detection limit of 1.5 microM and a high sensitivity (61.5 microA mM(-1)) for monitoring of H2O2. Effects of pH and operating potential were also explored for optimum analytical performance by using the amperometric method. The apparent Michaelis-Menten constant of the encapsulated HRP was 1.89 +/- 0.21 mM.

Single-step fabrication of continuous surface relief micro-optical elements in hybrid sol-gel glass by laser direct writing

A negative tone hybrid sol-gel material was applied to the fabrication of continuous surface relief micro-optical elements by laser direct writing lithography. The hybrid sol-gel glass was synthesized as UV photosensitive material and used for the fabrication of micro-optical elements with continuous surface relief profile. The surface profile was directly controlled by the UV exposure dosage and the exposed areas were crosslinked and converted into sol-gel glass with relief structures. The UV exposure dosage was realized by an acousto-optic modulator (AOM) in the laser direct writing system. Characterization results indicated that the thickness of the sol-gel glass has a linear response to the AOM values ranging from 3.25 to 5.5. The sol-gel thin film was measured to have a refractive index of 1.52 and the maximum thickness of 2 microm. For various designs of diffractive optical elements within the visible wavelength, the sol-gel film enables us to achieve an arbitrary phase change between 0 and 2 pi with the linear AOM exposure range. As an example, a blazed grating with a period of 45 microm and height of 1.17 microm was fabricated in the sol-gel glass using direct laser writing method.

Sol–gel-immobilised terbium complexes for luminescent sensing of dissolved oxygen by analysis of emission decayElectronic supplementary information (ESI) available: examples of the analysis of the emission decay data for calibration purposes and examples of data sets and their treatment at different delay times. See http://www.rsc.org/suppdata/nj/b1/b110743g/

Cationic terbium complexes bearing an N-methylphenanthridinium chromophore have been incorporated in sol–gel thin films and their response characteristics analysed with respect to variations in dissolved oxygen concentration. Analysis of the emission lifetime decay curves (547 nm) has allowed a linear calibration model to be defined over an oxygen concentration range from zero to 0.5 mM (H2O, 295 K).

Effects of erbium oxalate content and temperature on green up-conversion luminescence of erbium oxalate nanoparticles/titania/ormosil composite thin films

Erbium(III) oxalate nanoparticles prepared by a microemulsion technique was dispersed into titania/organically modified silane (ormosil) composite sol–gel thin films derived at low temperature. An up-conversion processes of the composite thin film was studied and a relatively strong room temperature green up-conversion emission at 543 nm ( 4 S 3/2→ 4 I 15/2) was observed for the composite thin films with different erbium oxalate content and heat treatment temperature upon near infrared excitation wavelength of 993 nm. The lifetime of the green up-conversion emission was measured and the up-conversion emission mechanism was explained by means of an energy level diagram.

Preparation and tribological properties of sol-gel zirconia thin films stabilized with ceria

High oriented ceria-stabilized zirconia (CSZ) thin films were prepared by the sol-gel method using zirconium oxychloride octahydrate and cerium chloride hexahydrate as starting materials. The morphology and microstructure of the films were investigated via X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and scanning electron microscopy (SEM). The films contained only tetragonal phase and showed nanoscale uniformity. Tribological properties were investigated by using a one-way reciprocating tribometer. Under low load (0.05 N), the films displayed excellent tribological properties in sliding against Si 3N 4 balls. The friction coefficient was as low as 0.15–0.19 and the wear life was more than 5000 sliding cycles. The tribological properties of CSZ/AISI52100 steel system was inferior to CSZ/Si 3N 4 system. The wear mechanism of CSZ films in sliding against Si 3N 4 under low load was fatigue wear, slight abrasion and scuffing, whereas that of in sliding against AISI52100 steel was abrasive wear and plastic deformation.

Substrate effects on domain structures of PZT 30/70 sol-gel films via PiezoAFM

Abstract Using an atomic force microscope (AFM) modified to perform PiezoAFM we have investigated the piezoelectric response of sol-gel thin film lead zirconate titanate (PZT 30/70, PbZr0.3Ti0.7O3) on Pt–Ti/SiO2/Si, indium tin oxide (ITO)/glass and Pt/MgO. The films were produced by spin coating a PZT 30/70 sol and firing at 520 °C for Pt electrode systems and 600 °C for the ITO system. By conducting PiezoAFM hysteresis loops we have shown that the localised piezoelectric response varies for PZT on differing substrates. The degree of asymmetry in the hysteresis loops varies for each substrate, as do the coercive fields. The coercive fields have been found to be ±18 V/μm for PZT/ITO/glass, +22 and −18 V/μm for PZT/Ti-Pt/SiO2/Si and +35 and −20 V/μm for PZT/Pt/MgO. The PZT grown on Pt/MgO, Pt-Ti/SiO2 and ITO/glass shows an offset or asymmetric hysteresis loop, which was confirmed by the differing fields required for poling during domain modification experiments performed on PZT/Pt/MgO. δ33 values obtained for the PZT thin films investigated range from 50pm/V for PZT/Pt–Ti/SiO2/Si to 40 pm/V for PZT/ITO/glass.

Preparation and characterization of nanostructured materials for an artificial olfactory sensing system

Pure and Ni-, Pt-, Pd-, Os-doped tin oxide and pure and Fe-doped indium oxide thin films have been prepared by the sol–gel process. The microstructural properties of the as-prepared films have been investigated by means of X-ray diffraction (XRD). All the samples showed a polycrystalline structure that resulted to be dependent on the presence of the particular doping element. Integrated gas microsensor devices based on both pure and doped sol–gel thin films of these two different metal oxides (SnO 2 and In 2O 3) were realized. They were linked in an array configuration and exposed to the volatile compounds of different commercial olive oils samples in order to test the application of the array in an electronic nose for the discrimination among different commercial olive oils. The differently modified sensitive materials showed different sensing characteristics to the headspace of the olive oil, hence modulating the selectivity of the array. PCA analysis was performed on the array responses and the results showed the capability to discriminate olive oils of different quality.

Preparation and characterization of novel luminescent sol-gel films containing a RE3+ carboxylic acid complex

RE3+ (Eu3+, Tb3+) complexes with carboxylic acid (salicylic acid and benzoic acid) were introduced into the sol, which was prepared by the hydrolysis of tetraethoxysilane (TEOS). A sol-gel luminescent thin film (SG-LTF) was then prepared by dispersing the sol onto a silica substrate by a spin coating method. Multi-layer luminescent thin films were prepared by repeating the same process. The luminescent spectra, fluorescence lifetime and thermal stability of the SG-LTFs were investigated. For the reason of comparison polyvinylbutyral (PVB) was added into a N,N-dimethylformamide (DMF) solution in which the comparative RE3+ carboxylic acid complexes were previously dissolved to form the DMF/PVB solution and the PVB luminescent thin film (PVB-LTF) was prepared. The results show that a broad excitation band indicates the formation of RE complexes in the solid SG-LTFs. RE ions, which are restrained in the silica matrix, present longer lifetimes and higher thermal stability than that in the PVB-LTF containing the corresponding pure complexes. The different doping concentration of RE (III) complexes in the SG-LTFs and the different change of the emission intensities with the heat treatment temperature in the sol-gel thin film and the sol-gel bulk gel were also discussed in this paper.

Organic nanocrystals grown in sol–gel matrices: a new type of hybrid material for optics

We have engineered new hybrid organic–inorganic materials through a simple and generic preparation of stable organic nanocrystals grown in gel–glass matrices. This process is based on the confined nucleation and growth of dyes in the pores of dense gels. For bulk samples, narrow size distributions of particles are obtained between 10 and 20 nm in diameter. We have extended this method to the preparation of organic nanocrystals embedded in sol–gel thin films by spin-coating. For all these nanocomposite samples, we have significantly increased the stability of the dye and obtained promising linear and nonlinear optical properties. To cite this article: J. Zaccaro et al., C. R. Physique 3 (2002) 463–478.