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.     

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.     

Synthesis of nanocrystalline photocatalytic TiO2 thin films and particles using sol–gel method modified with nonionic surfactants

A simple sol–gel route has been developed for the preparation of nanocrystalline photocatalytic TiO₂ thin films and particles at 500 °C. The synthesis involved a novel chemistry method employing nonionic surfactant molecules as a pore directing agent along with acetic acid-based sol–gel route without direct addition of water molecules. This study investigated the effect of surfactant type and concentration on the homogeneity, morphology, light absorption, dye adsorption and degradation, and hydrophilicity of TiO₂ films as well as on the structural properties of the corresponding TiO₂ particles. The method resulted in the synthesis of mesoporous TiO₂ material with enhanced structural and catalytic properties including high surface area, large pore volume, pore size controllability, small crystallite size, enhanced crystallinity, and active anatase crystal phase. The prepared TiO₂ thin films were super-hydrophilic and possessed thermally stable spherical bicontinuous mesopore structure with highly interconnected network. Highly porous TiO₂ films prepared with polyethylene glycol sorbitan monooleate surfactant exhibited four times higher photocatalytic activity for the decoloration of methylene blue dye than the nonporous control TiO₂ films prepared without the surfactant. This sol–gel method modified with surfactant templates is useful in the preparation of nanostructured anatase TiO₂ thin films with high photocatalytic activity and desired pore structure.     

Electrochromic properties of nanocrystalline MoO3 thin films

Electrochromic MoO₃ thin films were prepared by a sol–gel spin-coating technique. The spin-coated films were initially amorphous; they were calcined, producing nanocrystalline MoO₃ thin films. The effects of annealing temperatures ranging from 100 °C to 500 °C were investigated. The electrochemical and electrochromic properties of the films were measured by cyclic voltammetry and by in-situ optical transmittance techniques in 1 M LiClO₄/propylene carbonate electrolyte. Experimental results showed that the transmittance of MoO₃ thin films heat-treated at 350 °C varied from 80% to 35% at λ = 550 nm (ΔT =  45%) and from 86% to 21% at λ ≥ 700 nm (ΔT =  65%) after coloration. Films heat-treated at 350 °C exhibited the best electrochromic properties in the present study.     

Thermal annealing effect on Y2O3:Eu phosphor films prepared by yttrium 2-methoxyethoxide sol–gel precursor

In this work, we demonstrate that yttrium 2-methoxyethoxide is a convenient sol–gel precursor to synthesize the Y₂O₃:Eu³⁺ phosphor films. The crystallization of Y₂O₃:Eu³⁺ phosphor films prepared from the yttrium 2-methoxyethoxide occurs at about 550 °C. We have also observed that our Y₂O₃:Eu³⁺ phosphor films undergo crystal structure change above annealing temperature of 750 °C which is not previously observed in the sol–gel fabrication method. The change of photoluminescent (PL) spectra is related to the evolution of Y₂O₃ crystal structure. It is shown in this investigation that the post-annealing treatment will help to produce phosphor films of improved brightness. The reasons assigned are the effective elimination of OH impurities and the grain growth of phosphor films.     

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).     

Optical properties of Er-doped SiO2-GeO2-Al2O3 planar waveguide fabricated by sol–gel processes

In this paper, we report silica based planar waveguides doped with Er³⁺, and co-doped with GeO₂ and Al₂O₃. These sol–gel derived planar waveguides were fabricated on SOS (silica on silicon) using multiple spin-coating and rapid thermal processing (RTP). Investigation has been made on their characteristics in terms of their application in optical amplification and lasing, including photoluminescence (PL), fluorescence lifetime, refractive index, propagation loss, surface roughness, Fourier transform infrared (FTIR) spectrum and X-ray diffraction (XRD) analysis. The propagation loss of a 20-layer planar waveguide was measured to be about 1.6 dB/cm for TE₀ and 2.2 dB/cm for TM₀ mode. A strong emission transition (⁴I_13/2→⁴I_15/2) at 1.536 μm with a lifetime of 3.6 ms has been obtained for an optimized molar composition of 90SiO₂: 10GeO₂: 20AlO_1.5: 1ErO_1.5.     

Synthesis, phase transformation and dielectric properties of sol–gel derived Bi2Ti2O7 ceramics

Sol–gel derived Bi₂Ti₂O₇ ceramic powders have been prepared from methoxyethoxides of bismuth and titanium (molar ratio of Ti/Bi = 1.23 and water/alkoxides = 1.31). The Bi₂Ti₂O₇ phase was stable at a low temperature (700 °C), but it then transformed into mixed phases of Bi₄Ti₃O₁₂ and Bi₂Ti₄O₁₁ at 850–1150 °C. The single phase of Bi₂Ti₂O₇ reoccurred at 1200 °C. Dielectric properties and ferroelectric behavior of samples sintered at 1150 and 1200 °C were examined. Under frequency of 1 MHz, samples sintered at 1150 and 1200 °C had a dielectric constant of 101.3 and 104.2, and a loss tangent of 0.0193 and 0.0145, respectively. Only the sample sintered at 1150 °C showed ferroelectric behavior, where remanent polarization is 3.77 μC cm⁻² and coercive field is 24 kV cm⁻¹. Thus, the Bi₂Ti₂O₇ did not exhibit ferroelectricity, but the mixed phase of Bi₄Ti₃O₁₂ and Bi₂Ti₄O₁₁ did.     

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.     

Sol–gel synthesis and luminescence of unexpected microrod crystalline Ca5La5(SiO4)3(PO4)3O2:Dy phosphors employing different silicate sources

Ca₅La₅(SiO₄)₃(PO₄)₃O₂ doped with Dy³⁺ were synthesized by sol–gel technology with hybrid precursor employed four different silicate sources, 3-aminopropyl-trimethoxysilane (APMS), 3-aminopropyl-triethoxysilane (APES), 3-aminopropyl-methyl-diethoxysilane (APMES) and tetraethoxysilane (TEOS), respectively. The SEM diagraphs show that there exist some novel unexpected morphological structures of microrod owing to the crosslinking reagents than TEOS as silicate source for their amphipathy template effect. X-ray pictures confirm that Ca₅La₅(SiO₄)₃(PO₄)₃O₂:Dy³⁺ compound is formed by a pure apatitic phase. The Dy³⁺ ions could emit white light in Ca₅La₅(SiO₄)₃(PO₄)₃O₂ compound, and the ratio of Y/B is 1.1, when the Dy³⁺ doped concentration is 1.0 mol%.     

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.     

Sol–gel thin film deposition and characterization of a new optically active compound: Er2Ti2O7

This paper reports on the first sol–gel thin film preparation of a new optically active compound: Er₂Ti₂O₇ (ETO). Optical, microstructural and spectroscopic properties of ETO films annealed in a temperature range 300–1000°C are studied. This work shows that the porosity and microstructure of ETO films depend closely on the heat-treatment temperature. Photoluminescence (PL) has been observed for films heat-treated at 600°C or more. The PL decay appears strongly influenced by quenching effects. For thin films treated at 600°C, quenching is essentially due to the presence of hydroxyl groups. After heat-treatment at 800°C or more, quenching can be explained by the high concentration of erbium atoms and by their distribution in the ETO lattice.     

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).     

Synthetic process of the sol–gel derived Pb(Zr0.53Ti0.47)O3 films using poly vinyl butyral binder

Synthetic process of sol–gel grown lead-zirconate-titanate (PZT) films using poly vinyl butyral (PVB) binder has been studied. By the present method, thick PZT films with about 7 μm in thickness were successfully grown, whereas thick films were not achieved without the use of the binder. In the synthetic process, amorphous PZT particles with 0.6–1.0 μm in diameter were formed at a relatively low temperature of 50°C and Pb projections were observed at 300°C. Furthermore, PbO was observed at the surface of amorphous PZT particles at 400°C. At 500°C, PZT perovskite phase started to be observed together with residual PbO phase. Finally, single PZT perovskite phase, which shows the dielectric constant and dissipation factor of 145.5 and 0.032, respectively, at a frequency of 1 kHz, was obtained at 800°C. We suggest that the use of PVB binder plays an important role in the fabrication of thick PZT film.     

Preparation and luminescence properties of nanocrystalline La2O3:Eu phosphor

Powders La₂O₃ doped with 1 mol% Eu were prepared via a combustion route using different reducers (urea, glycine and citric acid). The structure and morphology were determined with XRD and HRTEM measurement. The main emission positions centered at 626 nm for ⁵D₀ → ⁷F₂ transition are observed. The variation of CT band with different reducers is observed. The intensity of ⁵D₀ → ⁷F₂ transition centered at 626 nm with respect to that of ⁵D₀ → ⁷F₁ transition is a function of the energy difference ΔE between the two CT band positions.     

Microstructural, photocatalysis and electrochemical investigations on CeTi2O6 thin films

The properties of sol–gel derived CeTi₂O₆ thin films deposited using a solution of cerium chloride heptahydrate and titanium propoxide in ethanol are discussed. The effect of annealing temperature on structural, optical, photoluminescence, photocatalysis and electrochemical characteristics has been examined. Lowest annealing temperature for the formation of crystalline CeTi₂O₆ phase in these samples is identified as 580 °C. The optical transmittance of the films is observed to be independent of the annealing temperature. The optical energy bandgap of the 600 °C annealed film for indirect transition is influenced by the presence of anatase phase of TiO₂ in its structure. Fourier transform infrared spectroscopy investigations have evidenced increased bond strength of the Ti–O–Ti network in the films as a function of annealing temperature. The photoluminescence intensity of the films has shown dependence on the annealing temperature with the films fired at 450 °C exhibiting the maximum photoluminescence activity. The decomposition of methyl orange and eosin (yellow) under UV–visible light irradiation in the presence of crystalline CeTi₂O₆ films shows the presence of photoactivity in these films. The photocatalytic response of CeTi₂O₆ films is found to be superior to the TiO₂ films. In comparison to crystalline films, the amorphous films have shown superior electrochemical characteristics. The 500 °C annealed amorphous films have exhibited the most appropriate properties for incorporation in electrochromic devices comprising tungsten oxide as the primary electrochromic electrode.     

Sol–gel preparation of boron-containing cordierite Mg2(Al4−xBx)Si5O18 and its crystallization

Five cordierite-based powders were investigated regarding their thermal and crystallization behaviors. The powders were obtained from amorphous gels having nominal compositions of 2Mg : xAl : (4 − x)B : 5Si where x = 4 down to 0. Thermal gravimetry analysis of the dry gels showed some absorbed water and decomposition of organic ligands in addition to network condensation. Gradual substitution of B for Al in the dried gel powders showed a new band in their infrared spectra corresponding to triangular BO₃, whereas the bands corresponding to Al vanished. This also showed a noticeable effect on the crystallization trends, type and stability of cordierite. Cordierite crystallized in samples of B/Al ratio up to 1 while protoenstatite predominated in samples of higher B/Al ratios. In addition, some silica minerals, with little amorphous phase, were formed. Incorporation of boron and increase in temperature enhanced the transformation of γ cordierite to its form.     

Preparation of TeO2 based thin films by nonhydrolytic sol–gel process

To solve the problem of the extremely high hydrolytic reactivity of tellurium alkoxides in hydrolytic sol–gel method, the nonhydrolytic sol–gel process has been applied as a novel route for producing TeO₂ based thin films. The transition of nonhydrolytic sol–gel was monitored by means of ¹H NMR, FT-IR and Raman techniques. These results show that the formation of Te–O–Te bonds in gel networks mainly resulted from the nonhydrolytic cross-condensation reaction between different Te–OR groups. The decomposition process and structure evolution of the nonhydrolytic gel products were investigated and managed. Results from DTA and XRD analyses show that metallic tellurium, β-TeO₂ and α-TeO₂ phase appeared in the film during heat-treatment process at around 300, 350 and 400 °C, respectively. The formation of metallic tellurium can be alleviated through preheating the gel films under O₂ atmosphere or by additions of the second component. Crystallization of α-TeO₂ could be retarded by additions of TiO₂ or Al₂O₃, and the transparent, homogeneous amorphous TeO₂ based thin films were obtained by the methods above. The nonhydrolytic sol–gel process developed in this study offers a simple and practical method for fabricating TeO₂ based thin film devices.     

Luminescent properties of Gd2Ti2O7: Eu phosphor films prepared by sol-gel process

Gd₂Ti₂O₇: Eu³⁺ thin film phosphors were fabricated by a sol-gel process. X-ray diffraction (XRD), atomic force microscopy (AFM) and photoluminescence (PL) spectra as well as lifetimes were used to characterize the resulting films. The results of XRD indicated that the films began to crystallize at 800 °C and the crystallinity increased with the elevation of annealing temperatures. Uniform and crack free phosphor films were obtained, which mainly consisted of grains with an average size of 70 nm. The doped Eu³⁺ showed orange-red emission in crystalline Gd₂Ti₂O₇ phosphor films due to an energy transfer from Gd₂Ti₂O₇ host to them. Both the lifetimes and PL intensity of the Eu³⁺ increased with increasing the annealing temperature from 800 to 1000 °C, and the optimum concentrations for Eu³⁺ were determined to be 9 at.%. of Gd³⁺ in Gd₂Ti₂O₇ film host.     

Rietveld refinement of amorphous SiO2 prepared via sol–gel method

Using the Rietveld refinement, we analyze the structural evolution under thermal treatment of silica xerogel samples prepared by the sol–gel method with molar ratio (R) of water to TEOS of R = 5 and R = 11.66. We refine the structure of compounds using the Maud program and we found the unit cell parameters and atomic positions of the refined silica amorphous atoms for whole samples. The results show us that the amorphous structure is quartz-like or low-cristobalite-like, depending on the molar ratio values and the heat-treatment of the samples. For R = 5 different quartz structures are obtained, whereas for R = 11.66 we obtain close structures in which a transformation of quartz-like amorphous to low-cristobalite amorphous phase occurs at about 600 °C. These results give some scopes to explain the partial crystallization of the silica at relatively low temperatures obtained when metallic species are present.