Structural and optical properties of vanadium pentoxide sol–gel films

The V₂O₅ films were obtained using sol–gel procedure. The composition and mesostructure of the layers were investigated with the UV and Raman spectroscopy, as well as with electron microscopy. We showed that the changes in the properties of thin layers accompanying the variation of film thickness are connected with the changes in the microstructure of the film rather than with changes in its composition. The thin V₂O₅ layers obtained in the present study are composed of disordered clusters; their mean size is 4–13 nm.     

UV-induced permanent gratings in sol–gel germanosilicate thin films

Germanosilicate thin films have been elaborated by the sol–gel process and the dip-coating technique. Pulsed or continuous wave UV laser (244 nm) was used to write permanent gratings in these films. In the case of exposure to cw laser, the grating diffraction efficiencies were measured using a focused beam from a He–Ne laser at 633 nm and photo-induced changes in refractive index as high as 4×10⁻³ have been obtained. The thermal behaviour of these gratings has been investigated showing a good stability up to 400°C. Exposure to pulsed fringe pattern led to a glass photo-expansion modulated by a strong corrugation which can be due mainly to photo-ablation at the places of the bright fringes. The waveguide surface at the grating places was investigated through Atomic Force Microscopy (AFM) and microscopic profilometry techniques. Preliminary results on the kinetics of the grating growths are also reported.     

Indium-doped ZnO thin films deposited by the sol–gel technique

Conducting and transparent indium-doped ZnO thin films were deposited on sodocalcic glass substrates by the sol–gel technique. Zinc acetate and indium chloride were used as precursor materials. The electrical resistivity, structure, morphology and optical transmittance of the films were analyzed as a function of the film thickness and the post-deposition annealing treatments in vacuum, oxygen or argon. The obtained films exhibited a (002) preferential growth in all the cases. Surface morphology studies showed that an increase in the films' thickness causes an increase in the grain size. Films with 0.18 μm thickness, prepared under optimal deposition conditions followed by an annealing treatment in vacuum showed electrical resistivity of 1.3 × 10^− 2 Ωcm and optical transmittance higher than 85%. These results make ZnO:In thin films an attractive material for transparent electrodes in thin film solar cells.     

Holographic gratings in DR1-doped sol–gel silica and ORMOSILs thin films

Holographic diffraction gratings have been obtained in Disperse Red 1 (DR1)-doped silica and ORganically MOdified SILanes thin films (ORMOSILs), produced by tetraethylorthosilicate (TEOS) and 3-(triethoxysilyl)propyl isocyanate (ICPTEOS) as precursors for the sol–gel process. For the DR1 + TEOS film, the grating is associated to an irreversible bleaching of the dye and may be detected by a Raman micro-spectroscopy mapping. For films based on ICPTEOS, a transient phase diffraction grating is created by the photoinduced birefringence. Two characteristic relaxation times of the birefringence grating have been measured.     

Development of photo-patternable organo-mineral hybrid films from the sol–gel condensation of alkoxysilanes

We describe the synthesis and refractive index characterisation of four organo-siloxanes materials, applicable to the fabrication of photo-patternable channel waveguides. By combining aliphatic, aromatic and vinylic alkoxysilane precursors, organically modified silicates materials have been synthesised using the sol–gel process. Control of the refractive index was achieved through the variation of aliphatic and aromatic alkoxide content within the materials. It is clearly demonstrated that the presence of aromatic groups significantly increases the refractive index to 1.562. Conversely, the addition of vinylic groups decreases the value to 1.490. These findings are attributed to the atypical polarisability exhibited by aromatic and aliphatic precursors, which strongly affect the overall molar refraction.     

Inhomogeneous distribution of organic molecules adsorbed in sol–gel glasses

The effects of the porous matrix upon the radiative characteristics of quinine sulphate doped sol–gel glasses are investigated. The broadenings of the absorption and fluorescence spectra are explained by the attachment of the molecules on distorted sites or in a non-planar fashion, creating an inhomogeneous distribution of adsorbed molecules. For this reason, each emitting center relaxes with its own characteristics. This inhomogeneous distribution is also supported by the non-exponential and the wavelength dependence of the fluorescence decay.     

Densification of sol–gel TiO2 very thin films studied by SPR measurements

Very thin TiO₂ films have been prepared by sol–gel and deposited on a silver layer for Surface Plasmon Resonance (SPR) measurements. Densification of the samples has been studied by determining from SPR measurements the optical index (ranging from 1.68 to 1.92) and thickness (ranging from 6 to 2 nm) at each step of the annealing procedure. The structure of the layer (amorphous and/or crystalline) has been checked at the final stage of the thermal treatment by High Resolution Transmission Electron Microscopy (HRTEM).     

Ferroelectric properties of BiFeO3 films grown by sol–gel process

Several methods have been used to prepare ferroelectromagnetic BiFeO₃ films. In this paper, we adopted a sol–gel process to fabricate BiFeO₃ films on indium tin oxide (ITO)/glass substrates. X-ray diffraction pattern indicated that the samples are randomly oriented. Cross section scanning microscopy showed that the thicknesses of both films were about 1.2 μm and no apparent diffusion between the BiFeO₃ films and ITO/glass substrates. Remnant polarization of 2.0 and 1.75 μC/cm² were identified by the measuring of electric hysteresis loops for the films annealed at 500 and 600 °C respectively at an applied field of 108 kV/cm. Dielectric property and loss factor were investigated as a function of frequency. In addition, magnetism was detected at 77 K.     

Dye-doped sol–gel materials for two-photon absorption induced fluorescence

Two-photon absorption (TPA) and subsequent fluorescence properties of laser dyes are retained when doped into solid state sol–gel materials. These properties were demonstrated to be applicable in true 3D displays.     

Stability of second-order nonlinear optical properties in sol–gel matrix bearing silylated chalcone and disperse red 1

We report the syntheses of two triethoxysilanes for thin film fabrication. One silane (SGDR1) contains disperse red 1 (DR1) which is a second-order nonlinear optical (NLO) active chromophore. The other silane (SGCHC) bears a chalcone derivative. In order to improve the poor temporal stability of second-order NLO effects in the resulting poled sol–gel film, photo-crosslink was induced between the double bonds in chalcone units after mixing SGDR1 and SGCHC. Under UV exposure, the photocrosslink was studied with UV–visible absorption and IR vibrational spectroscopy. The decaying behavior of the molecular order parameter (A₂) after corona poling was investigated using polarized absorption spectroscopy (PAS). Second-order non-linear optical properties of the sample were studied using second-harmonic generation (SHG) and linear electro-optic (E/O) signal measurement. Poled and UV-cured film of SGDR1/SGCHC exhibited much better stability than thermally poled sample with respect to the molecular order parameter and linear electro-optic effect.     

Dielectric and piezoelectric properties of sol–gel derived Ca doped PbTiO3

Synthesis of Ca doped PbTiO₃ powder by a chemically derived sol–gel process is described. Crystallization characteristics of different compositions Pb_1−xCa_xTiO₃ (PCT) with varying calcium (Ca) content in the range x = 0–0.45 has been investigated by DTA/TGA, X-ray diffraction and scanning electron microscopy. The crystallization temperature is found to decrease with increasing calcium content. X-ray diffraction reveals a tetragonal structure for PCT compositions with x ≤ 0.35, and a cubic structure for x = 0.45. Dielectric properties on sintered ceramics prepared with fine sol–gel derived powders have been measured. The dielectric constant is found to increase with increasing Ca content, and the dielectric loss decreases continuously. Sol–gel derived Pb_1−xCa_xTiO₃ ceramics with x = 0.45 after poling exhibit infinite electromechanical anisotropy (k_t/k_p) with a high d₃₃ = 80 pC/N, ′ = 298 and low dielectric loss (tan δ = 0.0041).     

Sol–gel deposition and characterization of Mn-doped silicate phosphor films

Mn²⁺-doped Zn₂SiO₄ and Mg₂Gd₈(SiO₄)₆O₂ phosphor films were deposited on silicon and quartz glass substrates by sol–gel process (dip-coating). The variations of sol viscosity with time and film thickness with the number of layers were investigated in Zn₂SiO₄: Mn system. The results of XRD and IR showed that the Zn₂SiO₄: Mn films remained amorphous below 700°C and crystallized completely around 1000°C. From AFM studies, it was observed that the grains with 0.5–0.8 μm size packed closely in Zn₂SiO₄: Mn films, which were uniform and crack free. The luminescence properties of Zn₂SiO₄: Mn films were characterized by absorption, excitation and emission spectra as well as luminescence decay. These properties were discussed in detail by a comparison with those of Mn²⁺ (and Pb²⁺)-doped Mg₂Gd₈(SiO₄)₆O₂ phosphor films.     

Photoluminescence properties of an oligo-phenylkene vinylene derivative doped in polymer/silica hybrid sol–gel matrix

An oligo-phenykene vinylene derivative 4,4′-(1,4-phenylene dithenylene)-bis-(N-methyl pyridinium iodide) (OPVD) was introduced into poly(hydroxyethyl methacrylate) (PHEMA)-impregnated silica composite film by the sol–gel process. The X-ray diffraction and UV–VIS reveal that this method can restrict the crystallization of OPVD and OPVD molecules tend to form J-aggregates in the composite film. The results of photoluminescence of OPVD in the composite film indicate that as compared with OPVD itself, the emission peak of OPVD in the composite film undergoes a blue shift about 80 nm, fluorescence intensity enhances and the photostability is much better, which make such composite film of more practical significance.     

Effects of rare-earth concentration and heat-treatment on the structural and luminescence properties of europium-doped zirconia sol–gel planar waveguides

Sol–gel zirconia films doped with Eu³⁺ concentrations ranging from 0.2% to 10%, were prepared by dip-coating a solution of the starting precursor, zirconium n-propoxide, ethanol, methanol, water, acetic acid and europium nitrate on glass and SiO₂/Si wafer substrates. The ZrO₂sol thus synthesized remains stable for several months. Structural characterization of the zirconia films was performed using Waveguide Raman Spectroscopy. These films present an amorphous phase up to an annealing temperature of 400 °C. Above 400 °C the matrix evolves towards a metastable tetragonal phase. This transformation was found to depend on the concentration of Eu³⁺ ions. Indeed, while for samples doped with 0.2% Eu³⁺ this transformation occurs around 450 °C, in the case of 10% of Eu³⁺ ions, the transition is pushed off to 500 °C. The optical losses of these waveguides were found to be about 0.3 dB cm⁻¹ for samples annealed at 400 °C. The surfaces of the films were characterized using Atomic Force Microscopy and the roughness was measured. The Eu-doped films were investigated using Waveguide Photoluminescence Spectroscopy. The dynamical behaviour of the Eu³⁺ emissions indicated that concentration quenching effect is not observed even when the matrix is doped up to 10%.     

Correlation of sol–gel processing parameters with microstructure and properties of a ceramic product

A very high purity MgAl₂O₄ spinel precursor can be prepared via a low temperature process called “Oxide One Pot Synthesis (OOPS)”. Sol–gel processing of such precursors offers the opportunity to prepare spinels with controlled microstructures, which is key to optimizing their properties for application as humidity sensors. Sol–gel processing of a double alkoxide precursor was carried out in buffer solutions in the pH range 8–12, to investigate the effect on the physical properties of the calcined ceramic products. The structure and morphology of the latter were characterized using FTIR, XRD, SEM and BET surface area measurements. Sol–gel processing results in a calcined product with a narrow pore size distribution, which contains a spinel phase of high crystallinity. At higher pH values, increasing amounts of an -Al₂O₃ phase are formed. At all pH values, the calcined product exhibits high water adsorption, up to 0.312 g H₂O/g sample.     

Preparation and optical properties of sol–gel derived Er-doped Al2O3–Ta2O5 films

Thin films of the system xAl₂O₃–(100 − x)Ta₂O₅–1Er₂O₃ were prepared by a sol–gel method and a dip-coating technique. The influences of the composition and the crystallization of the films on Er³⁺ optical properties were investigated. Results of X-ray diffraction indicated that the crystallization temperature of Ta₂O₅ increased from 800 to 1000 °C with increased values of x. In crystallized films, the intensities of the visible fluorescence and upconversion fluorescence tend to decrease with an increase in x values, due to the high phonon energy of Al₂O₃; the strongest fluorescence is observed in a crystallized film for x = 4 heat treated at 1000 °C. In amorphous films obtained by heat treatment at relatively low temperatures the Er³⁺ fluorescence could not be observed because strong fluorescence from organic residues remaining in the films thoroughly covered the Er³⁺ fluorescence. On the other hand, the Er³⁺ upconversion fluorescence in the amorphous films was observed to be stronger than that in the crystallized films. The strongest upconversion fluorescence is observed in an amorphous film for x = 75 heat treated at 800 °C.     

Deposition and photoluminescence of sol–gel derived Tb:Zn2SiO4 films on SiO2/Si

Tb³⁺ doped Zn₂SiO₄ films have been deposited on SiO₂ buffered Si wafers by sol–gel method. The structures of these films have been investigated with X-ray diffraction (XRD), atomic force microscopy (AFM), and scanning electron microscopy (SEM). The results revealed that these films were composed of nanometer-size grains with a Willemite structure and had smooth surfaces. Photoluminescence measurements of the films showed a strong emission from ⁵D₄ to ⁷F₅ at 544 nm. The blue emission from ⁵D₃–⁷F_j was depressed because of cross-relaxation effect. The decay kinetics of the ⁵D₄–⁷F₅ green emission was studied and a best fitting was obtained by a double exponential function. The lifetime of the excited ⁵D₄ state is estimated to be 5.2 ms.     

Photoluminescence characteristics of Ti2S3 nanoparticles embedded in sol–gel derived silica xerogel

Ti₂S₃ nanocrystallites embedded in sol–gel derived silica xerogel have been prepared. Their photoluminescence (PL) characteristics have been evaluated and compared with those of pure silica xerogel. UV–vis absorption spectra, transmission electron micrograph, excitation spectra and PL spectra of the doped and undoped samples have all been investigated. Two emission peaks have been observed from the doped samples, one at 440 nm (λ_ex=380 nm) while the other at 600 nm (λ_ex=550 nm). The latter has been assigned to the Ti₂S₃ nanocrystallites in the silica xerogel. Therefore, a novel luminescence property can be observed by introducing the semiconductor nanoparticles into the silica xerogel.     

Preparation of uniform titania nanocoating on ZnS-based phosphors by a sol–gel process

A sol–gel process has been developed to coat micron-sized ZnS:Cu,Au,Al phosphors with a smooth and uniform layer of amorphous titania having nanosize thickness. The titania nanocoating is based on the hydrolysis and condensation of titanium tetrabutoxide Ti(OBu)₄. Acetylacetone was used to decrease the reactivity of Ti(OBu)₄. The experimental variables such as water concentration, the amount of ZnS particles, and reaction time were investigated. The thickness of the titania nanocoating was homogeneous and can easily be controlled from 20 to 54 nm by adjusting the experimental variables. The as-prepared titania nanocoating was amorphous phase and could be crystallized to anatase phase upon heating at 500 °C in Ar atmosphere.     

Photorefractive gratings in DR1-doped hybrid sol–gel films

Photorefractive gratings have been obtained with 632.8 nm writing beams in organic–inorganic SiO₂-based films. The hybrid glass is prepared by a sol–gel technique, starting from organic Si precursors, and contains Disperse Red 1 (DR1), carbazole units and 2,4,7-trinitro-9-fluorenone (TNF). The photorefractive gain, which has been found unexpectedly even without poling field, has been determined through an asymmetric energy exchange by two-beam coupling measurements. The effects of the polarization of the writing beams and of a circularly polarized photoisomerizing radiation during the erasure of the grating have been interpreted in terms of an orientational contribution to the grating formation.