Phase transformation behavior of (Pb,La)(Zr,Sn,Ti)O<Subscript>3</Subscript> and Pb (Nb,Zr,Sn,Ti)O<Subscript>3</Subscript> antiferroelectric thin films deposited on LaNiO<Subscript>3</Subscript>-buffered silicon substrates by sol-gel processing

Antiferroelectric (Pb,La)(Zr,Sn,Ti)O₃ (PLZST) and Pb(Nb,Zr,Sn,Ti)O₃ (PNZST) thin films have been fabricated on LaNiO₃/Pt/Ti/SiO₂/Si substrates by a sol-gel processing. These films showed highly preferred (100) orientation due to the grain-on-grain local epitaxial growth. The PLZST films close to the AFE-FE phase boundary showed the electric-field-induced ferroelectric (FE) state, which could return back to its original AFE state only when the thermal activation was high enough. The AFE to FE phase transformation in PNZST films can be adjusted by the dc bias field in temperature. Phase transformation behavior of PNZST and PLZST antiferroelectric thin films were investigated as a function of temperature and dc bias field.     

Preparation and Ferroelectric Properties of YMnO3 Thin Films with c-Axis Preferred Orientation by the Sol-Gel Method

Hexagonal YMnO₃ has a ferroelectric property with an optimal remanent polarization along the c-axis. The c-axis oriented YMnO₃ thin films with a small leakage current were prepared by the sol-gel dipping method. The c-axis orientation of the films was promoted by the addition of diethanolamine to the Mn precursor solution. A heat treatment with multiple steps led to a dense film structure with fine grains. The dense structure resulted in the decrease of the leakage current. Furthermore, when the films were heat-treated in a vacuum, the leakage current became considerably small and the ferroelectricity of the YMnO₃ thin films was observed even at room temperature.     

Evolution of Structure of the Precursor During Sol-Gel Processing of ZnO and Low Temperature Formation of Thin Films

ZnO thin films were prepared on silicon substrate with Pt electrode by the sol-gel processing using Zn alkoxide solution prepared from Zn(NO₃)₂·6H₂O and 2-methoxyethanol. FT-IR spectroscopy showed the presence of Zn species in the alkoxide, with methoxyethoxide and nitrato groups as coordination ligands, indicating formation of Zn(NO₃)(OCH₂CH₂OCH₃). Smooth and homogeneous thin films were obtained by heat treating coating gel films in the temperature range from 250 to 500°C. The ZnO thin films exhibited a preferred growth of crystals with c-axis perpendicular to the Si substrate surface when fired at 250°C. It was discussed that the presence of nitrogen atoms in precursors had affected the phase development of crystals and was the basis of the structural relaxation for crystallization at low temperature.     

Electrochromic properties of WO<Subscript>3</Subscript> and WO<Subscript>3</Subscript>:P thin films

WO₃ and WO₃:P (5 mol% H₃PO₄) thin films were prepared using the sol-gel route and the electrochromic properties of the films were investigated using in situ spectroelectrochemical methods. The measurements were performed in propylene carbonate solution with 0.1 M LiClO₄ as electrolyte. During the cathodic polarization at –0.8 V a blue coloration is observed with a reversible variation between 14% and 84% of the transmittance at λ=633 nm. The kinetics for the bleaching process is faster for the WO₃:P film than for the undoped WO₃ film. Electronic Publication     

Preparation and Characterization of (Ba,Ca)(Ti,Zr)O3 Thin Films Through Sol-Gel Processing

(Ba_0.92,Ca_0.08)(Ti_0.92,Zr_0.08)O₃ thin films were prepared from Ba-Ti and Ca-Zr precursors by sol-gel processing. Polymerizable solutions containing Ba-Ti and Ca-Zr, respectively, were newly synthesized. Decomposition of the starting compounds and crystallization behavior of the film were examined by using TG/DTA and XRD. Microstructure of thin films was observed by using SEM. Polycrystalline (Ba_0.92,Ca_0.08)(Ti_0.92, Zr_0.08)O₃ films obtained by firing at 800°C were dense with fine grains. The thin films showed a dielectric constant of 1200 and dielectric loss of 0.5%.     

Orientation and Surface Properties of Sol-Gel Derived Y2O3 Films

The orientation, surface and optical properties of sol-gel derived Y₂O₃ films have been investigated. Transparent Y₂O₃ films were prepared on quartz glass substrates by sol-gel processes using YCl₃·6H₂O as a starting material. The water droplet contact angles of the films reached constant values between 79° and 90° after the films were left for 8 to 10 days in air at ambient temperature, indicating that the film surface exhibited hydrophobicity. When 2-(2-methoxyethoxy)ethanol (MEE) was added to the sol, yttria in the films crystallized to a strongly oriented cubic phase at firing temperatures between 400°C and 500°C. The intensity of the XRD peaks increased as the firing temperature was increased to 900°C. However, yttria crystallized to a non-oriented cubic phase when MEE was not used. The refractive index and packing density of the Y₂O₃ films increased from 1.55 to 1.68 and from 0.67 to 0.79, respectively, as the firing temperature was raised from 400°C to 900°C, indicating that sol-gel derived Y₂O₃ films are lower in density than evaporated ones.     

Bi4Ti3O12 Thin Films from Mixed Bismuth-Titanium Alkoxides

Bi₄Ti₃O₁₂ thin films were obtained by the sol-gel method. The precursor solution was prepared by allowing the two metallic alkoxides, Bi(OC₂H₄OCH₃)₃ and Ti(OC₂H₄OCH₃)₄, to react in 2-methoxy-ethanol to form the mixed alkoxide. This stable sol was deposited by spin-coating onto platinized silicon substrates. X-Ray diffraction patterns indicate that the perovskite initial crystallization temperature is 460°C for powder samples and it ranges between 400 and 500°C, for thin films, as a function of the number of coating layers. Dense, smooth and crack free thin films with grain sizes ranging from 20 nm to 500 nm are obtained, depending on the number of coating layers and on the post-deposition temperature annealing.     

Sol-gel synthesis of mesoporous CaCu3Ti4O12 thin films and their gas sensing response

A new sol-gel synthesis procedure of stable calcium copper titanate (CaCu₃Ti₄O₁₂—CCTO) precursor sols for the fabrication of porous films was developed. The composition of the sol was selected in order to avoid the precipitation of undesired phases; ethanol was used as solvent, acetic acid as modifier and poly(ethyleneglycol) as a linker agent. Films deposited by spin-coating onto oxidized silicon substrates were annealed at 700 °C. The main phase present in the samples, as detected by X-ray diffraction and Raman spectroscopy, was CaCu₃Ti₄O₁₂. Scanning electron microscopy analysis showed that mesoporous structures, with thicknesses between 200 and 400 nm, were developed as a result of the processing conditions. The films were tested regarding their sensibility towards oxygen and nitrogen at atmospheric pressure using working temperatures from 200 to 290 °C. The samples exhibited n-type conductivity, high sensitivity and short response times. These characteristics indicate that CCTO mesoporous structures obtained by sol-gel are suitable for application in gas sensing.     

Optimization of the photocatalytic activity of CdO + CdTiO3 coupled oxide thin films obtained by sol-gel technique

In the preparation of CdO + CdTiO₃ polycrystalline thin films by the sol-gel method, the optical, structural and crystalline properties, as well as the photocatalytic activity (PA) depends strongly on the sintering temperature (Ts) of the films and of the Ti/Cd ratio used in the precursor solution. In this work, CdO + CdTiO₃ thin films were prepared using a Ti/Cd constant ratio in the precursor solution. The films were sintered at six different Ts in the 450-550 °C range, in an open atmosphere. The structure of the films was characterized by X-ray diffraction and the PA was evaluated by the photobleaching of methylene blue in an aqueous solution using a UV-vis spectrometer. The relative intensity of the diffraction peaks associated with CdO and CdTiO₃, change with the Ts. The better photocatalytic activities were obtained for the films sintered at 490 °C and 550 °C. When the CdO was removed from the films by chemical etching the PA decreased, showing the importance of coupling both oxides.     

The dielectric and leakage current behavior of CoFe2O4-BaTiO3 composite films prepared by combining method of sol-gel and electrophoretic deposition

Composite CoFe₂O₄-BaTiO₃ films were prepared by combining method of sol-gel and electrophoretic deposition. CoFe₂O₄ thick films were prepared by electrophoretic deposition method and then BaTiO₃ precursor solution was spin-coated on these thick films to obtain dense structure. X-ray diffraction indicated that the films consisted of both spinel CoFe₂O₄ and perovskite BaTiO₃ phase. Dielectric properties and leakage current conduction of the films were studied. Leakage current characterization of the films exhibits an Ohmic conduction behavior at lower electric field and space charge limited conduction behavior at higher electric field, respectively.     

Visible Up-Conversion Luminescence in Ho3+: BaTiO3 Nano-Crystals Prepared by Sol Gel Technique

Visible up-conversion emissions at (435, 545, 580, 675 and 690 nm) and (437, 547 575 and 675 nm) have been observed from the sol-gel derived nano-crystalline Ho³⁺: BaTiO₃ powders and thin films respectively, under 808 nm laser diode excitation emissions. Combined with the energy level structure of Ho³⁺ ions and the kinetics of the visible emissions, the up-conversion mechanism has been analyzed and explained. The blue, green and red emissions of both samples has been attributed to the ground state-directed transition from (⁵F₁), (⁵S₂) and (⁵F₅), which are populated through excited state absorption (ESA) for 808 nm excitation. Nano-structure pure barium titanate and doped with different concentrations of Ho³⁺ ions in the from of powder and thin film have been prepared by sol-gel technique, using barium acetate (Ba(Ac)₂), and titanium butoxide (Ti(C₄H₉O)₄), as precursors. The thin films were prepared by sol-gel spin coating method. The as-grown thin films and powders were found to be amorphous, which crystallized to the tetragonal phase after heating at 750°C in air for 30 minutes. The crystallite sizes of the thin film and powder both doped with 4% Ho³⁺ ions was found to be equal to 11 and 16 nm, respectvely.     

Spectroellipsometric Characterization of Multilayer Sol-Gel Fe2O3 Films

Multilayer Fe₂O₃ films were deposited by the sol-gel method on glass substrates using three successive deposition procedures. The films were thermally treated for 1 h at 300°C.The optical and microstructural properties of these films were investigated by spectroscopic ellipsometry (SE) in the 500–1000 nm range. The optical gap was found by fitting the dispersion of the film refractive index (n) with the Wemple-DiDomenico (WDD) formula.The ellipsometric measurements showed also that the Fe₂O₃ films are anisotropic. The birefringence values (n) of the sol-gel films (0.05–0.08) are smaller than the large values of the Fe₂O₃ (which are around 0.28) but increase with the crystalization of the films. AFM mesurements showed that the films treated at 300°C start to crystallize.     

Sol-gel processing of lead-free (Na,K)NbO<Subscript>3</Subscript> ferroelectric films

The sol-gel processing of lead-free (Na,K) NbO₃ ferroelectric films was studied. Sodium ethoxide (NaOC₂H₅) and potassium ethoxide (KOC₂H₅) were prepared by reacting solid Na and K with ethanol (99.7%) in a solvent of 2-methoxyethanol. 0.5-μm-thick (Na,K)NbO₃ thin films with orthorhombic perovskite structure were obtained by pyrolyzing at 400°C and annealing at 800–900°C. The films had relatively dense and uniform microstructure with grain size of about 50 nm, whose ferroelectricity was proved by the P-E hysteresis loop measurement. It was found that excess K was effective to reduce the annealing temperature for the crystallization of sol-gel-derived (Na,K)NbO₃ thin films.     

Structure and Optical Properties of 0.1BiFeO3-0.9SrBi2Nb2O9 Thin Films Using a Modified Sol-Gel Technique

Fe-doped SrBi₂Nb₂O₉ precursor solution was synthesized using bismuth nitrate Bi(NO₃)₃·5H₂O, strontium nitrate Sr(NO₃)₂, iron nitrate Fe(NO₃)₃·9H₂O, and niobium ethoxide Nb(OC₂H₅)₅ as starting materials, ethylene glycol monomethyl ether (C₃H₈O₂) as the solvent. 0.1BiFeO₃-0.9SrBi₂Nb₂O₉ thin films were prepared on fused quartz substrates using sol-gel processing. The surface morphology and crystal structure and optical properties of the thin films were investigated. The thin film annealing at 400°C were found to be amorphous, and the thin films crystallize to a perovskite structure after a post-deposition annealing at 600°C for 1 h in air. The grain of thin film was evenly distributed. The thin films exhibit the designed optical transmission, while the optical transition is indirect in nature. Their optical band gap is about 2.5 eV.     

A Highly Sensitive and Selective Non-enzymatic Hydrogen Peroxide Sensor Based on Nanostructured Co3O4 Thin Films Using the Sol-gel Method

In this work we report an easy and efficient way to fabricate nanostructured cobalt oxide (Co₃O₄) thin films as a non-enzymatic sensor for H₂O₂ detection. Co₃O₄ thin films were grown on ITO glass substrates via the sol-gel method and characterized with several techniques including X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and optical absorbance. The Co₃O₄ thin films’ performance regarding hydrogen peroxide detection was studied in a 0.1 M NaOH solution using two techniques, cyclic voltammetry (CV) and amperometry. The films exhibited a high sensitivity of 1450 μA.mM⁻¹.cm⁻², a wide linear range from 0.05 μM to 1.1 mM, and a very low detection limit of 18 nM. Likewise, the Co₃O₄ thin films produced showed an exceptional stability and a high selectivity.     

Silica-based sol-gel films doped with active elements

The preparation of glassy films doped with rare earth ions is important for planar waveguide fabrication in active integrated optic devices. The sol-gel process is a promising route for the preparation of such films and the eventual overcoming of the problem of rare earth clustering. In the present work, both SiO₂ and 90SiO₂-10TiO₂ films were prepared by spin coating on silica glass or single crystal silicon substrates and they were doped with neodymium in a Nd/Si atomic ratio up to 15%, using NdCl₃ as precursor. These films were subjected to selected heat treatments and their thickness and infrared spectra were measured. The refractive index and porosity were also determined for some of the samples. Visible absorption spectra were then measured in bulk gels, due to the insufficient optical path of the films.     

Processing and Characterization of Sol-Gel Derived Modified PbTiO3 for Ferroelectric Composite Applications

Powders of (Pb_0.8Ca_0.2)(Ti_0.99Mn_0.01)O₃ have been prepared by sol-gel processing. A tetragonal phase is formed after heat treatment at as low as 800°C. The tetragonality was found to be 1.053±0.005 and Curie temperature 315°C. Composite films with 0–3 connectivity were prepared from 800°C heat treated powders and P(VDF-TrFE) by the solvent casting technique. Composites poled at 20 MV/m, exhibited a pyroelectric coefficient of 17.4 μC/m²K and a pyroelectric figure of merit (FOM_p=p/ε) of 0.51 μC/m²K.     

Low-temperature processing of sol-gel derived Pb(Zr,Ti)O<Subscript>3</Subscript> thick films using CO<Subscript>2</Subscript> laser annealing

Fabrication of ferroelectric Pb(Zr_0.52Ti_0.48)O₃ (PZT) thick films on a Pt/Ti/SiO₂/Si substrate using powder-mixing sol-gel spin coating and continuous wave CO₂ laser annealing technique to treat the specimens with at a relatively low temperature was investigated in the present work. PZT fine powders were prepared by drying and pyrolysis of sol-gel solutions and calcined at temperatures from 400 to 750°C. After fine powder-containing sol-gel solutions were spin-coated on a substrate and pyrolyzed, CO₂ laser annealing was carried out to heat treat the specimens. The results show that laser annealing provides an extremely efficient way to crystallize the materials, but an amorphous phase may also form in the case of overheating. Thicker films absorb laser energy more effectively and therefore melt at shorter periods, implying a significant volume effect. A film with thickness of 1 μm shows cracks and rough surface morphology and it was difficult to obtain acceptable electrical properties, indicating importance of controlling interfacial stress and choosing appropriate size of the mixing powders. On the other hand, a thick film of 5 μm annealed at 100 W/cm² for 15 s exhibits excellent properties (P _r = 36.1 μC/cm², E _c = 19.66 kV/cm). Films of 10 μm form a melting zone at the surface and a non-crystallized bottom layer easily at an energy density of 100 W/cm², showing poor electrical properties. Besides, porosity and electrical properties of thick films can be controlled using appropriate processing parameters, suggesting that CO₂ laser annealing of modified sol-gel films is suitable for fabricating films of low dielectric constants and high crystallinity.     

Water Permeation of SiO2-CH3SiO3/2 Thin Films Modified with Trimethylsilyl Groups on Nylon-6 Substrates

SiO₂-CH₃SiO_3/2 thin films coated on nylon-6 substrates by the sol-gel method were modified with trimethylsilyl (TMS) groups and their water permeability was evaluated. The water permeability coefficient of the nylon-6 substrates coated with TMS-modified SiO₂-CH₃SiO_3/2 thin films was smaller than that of unmodified ones. The wettability for water of SiO₂-CH₃SiO_3/2 thin films modified with TMS was smaller than that of unmodified ones. The decrease in the wettability by the modification with TMS resulted in the decrease in water permeability. The measurements of pore size distribution and the water permeability coefficient of coating films with different pressures of upstream side suggested that the mechanism of permeation was governed by the capillary condensation flow. This mechanism was also supported by the result that the water permeability coefficient was decreased with a decrease in wettability.     

Dielectric and optical properties of BaTiO<Subscript>3</Subscript> thin films prepared by low-temperature process

Barium titanate (BaTiO₃) thin films have been prepared by low temperature processing on Pt/Ti/SiO₂/Si substrates using sol-gel-hydrothermal (SGHT) technique, which combined the conventional sol-gel process and hydrothermal method. X-ray diffraction analysis showed that the barium titanate thin films are polycrystalline. As-reacted barium titanate films grown on Pt(111)/Ti/SiO₂/Si(100) substrates had a dielectric constant (ε) and loss tangent (tanδ) of 80 and 0.05 at 1 MHz, respectively. The optical constants including refractive index n, extinction coefficient k, and absorption coefficient α of the barium titanate thin films in the wavelength range of 2.5–12.6 μm were obtained by infrared spectroscopic ellipsometry.