Preparation of Ga<Subscript>2</Subscript>Se<Subscript>3</Subscript> thin films by sol–gel technique

In this study we describe the preparation of Ga₂Se₃ semiconductor compound thin films by sol–gel method for different crystal formation temperatures. The films were characterized by X-ray diffraction analyses (XRD), UV–visible spectrometer, and scanning electron microscope (SEM). The XRD spectrum showed that the formation of Ga₂Se₃ crystals were between 743 and 823 K. The thin film crystals that were formed at 773 K corresponded to the β phase and the preferred crystal structure was monoclinic. The value of band gap from optical absorption spectra for the Ga₂Se₃ thin films was estimated to be about E _g ~ 2.56 eV. The thickness of the one-coat Ga₂Se₃ thin films, which was measured by a Spectroscopic Ellipsometer, was about ~200 nm. The average grain sizes of scattered particles were within the limits between 200 and 500 nm.     

Surface chemical bonding states and ferroelectricity of Ce-doped BiFeO<Subscript>3</Subscript> thin films prepared by sol–gel process

Bi_1−x Ce _x FeO₃ (x = 0, 0.05, 0.1, 0.15 and 0.20) (BCFO) thin films were deposited on Pt/TiN/Si₃N₄/Si substrates by sol–gel technique. Crystal structures, surface chemical compositions and bonding states of BCFO films were investigated by X-ray diffraction and X-ray photoelectron spectroscopy (XPS), respectively. Compared to BiFeO₃ (BFO) counterparts, the fitted XPS narrow-scan spectra of Bi 4f_7/2, Bi 4f_5/2, Fe 2p_3/2, Fe 2p_1/2 and O 1s peaks for Bi_0.8Ce_0.2FeO₃ film shift towards higher binding energy regions by amounts of 0.33, 0.29, 0.43, 0.58 and 0.49 eV, respectively. Dielectric constants and loss tangents of the BCFO (x = 0, 0.1 and 0.2) film capacitors are 159, 131, 116, 0.048, 0.041 and 0.035 at 1 MHz, respectively. Bi_0.8Ce_0.2FeO₃ film has a higher remnant polarization (P _r = 2.04 μC/cm²) than that of the BFO (P _r = 1.08 μC/cm²) at 388 kV/cm. Leakage current density of the Bi_0.8Ce_0.2FeO₃ capacitor is 1.47 × 10⁻⁴ A/cm² at 388 kV/cm, which is about two orders of magnitude lower than that of the BFO counterpart. Furthermore, Ce cations are feasibly substituted for Bi³⁺ in the Bi_0.8Ce_0.2FeO₃ matrix, possibly resulting in the enhanced ferroelectric properties for the decreased grain sizes and the reduced oxygen vacancies.     

Structure and dielectric properties of HfO<Subscript>2</Subscript> films prepared by a sol–gel route

Mono- and multilayer HfO₂ sol–gel thin films have been deposited on silicon wafers by dip-coating technique using a solution based on hafnium ethoxide as precursor. The densification/crystallization process was achieved by classical annealing between 400 and 600 °C for 0.5 h (after drying at 100 °C). Systematic TEM studies were performed to observe the evolution of the thin film structure depending on the annealing temperature. The overall density of the films was determined from RBS spectrometry correlated with cross section (XTEM) thickness measurements. After annealing at 450 °C the films are amorphous with a nanoporous structure showing also some incipient crystallization. After annealing at 550 °C the films are totally crystallized. The HfO₂ grains grow in colonies having the same crystalline orientation with respect to the film plane, including faceted nanopores. During annealing a nanometric SiO₂ layer is formed at the interface with the silicon substrate; the thickness of this layer increases with the annealing temperature. Capacitive measurements allowed determining the value of the dielectric constant as 25 for four layer films, i.e. very close to the value for the bulk material.     

Structure and multiferroic properties of Bi<Subscript>5</Subscript>FeTi<Subscript>3</Subscript>O<Subscript>15</Subscript> thin films prepared by the sol–gel method

The Bi₅FeTi₃O₁₅ (BFTO) films of layered structure have been fabricated on Pt/Ti/SiO₂/Si substrates by the sol–gel method. The thermal decomposition behaviors of precursor powder were examined using thermo-gravimetric and differential scanning calorimeters analysis. The optimal heat treatment process for BFTO films were determined to be low-temperature drying at 200 °C for 4 min and high-temperature drying at 350 °C for 5 min followed by annealing at 740 °C for 60 min, which led to the formation of compact films with uniform grains of ~300 nm. The structural, surface topography, ferroelectric and magnetic properties of the films were investigated. The remnant polarization (2P _r) of BFTO thin films under an applied electric field of ~550 kV/cm are determined to be 67.5 μC/cm² . Meanwhile, the weak ferromagnetic properties of the BFTO films were observed at room temperature.     

Preparation,dielectric and ferroelectric properties of (Na<Subscript>0.5</Subscript>Bi<Subscript>0.5</Subscript>)<Subscript>0.94</Subscript>Ba<Subscript>0.06</Subscript>TiO<Subscript>3</Subscript> thin films by a sol–gel process

(BiFeO₃)_1−x –(BaTiO₃) _x solid solution thin films are grown on Pt/Ti/SiO₂/Si substrates by chemical solution deposition method. Films with x = 0.00, 0.05 and 0.10 were prepared by annealing at 500°C. X-ray diffraction patterns indicate that the pure BiFeO₃ film adopts random orientation while the solid solution films are highly (100) preferentially oriented. Improved electric property at room temperature was observed in the BaTiO₃ incorporated BiFeO₃ films. The remanent polarization of the film with x = 0.0, 0.05 and 0.10 are 76.6, 51.9 and 19.7 μC/cm² respectively under a measuring electric field of 0.94 MV/cm. The BaTiO₃ incorporated BiFeO₃ films show improved fatigue endurance. By the solid solution with BaTiO₃, the leakage current density is reduced effectively.     

Electrochromic and photoelectrochromic properties of sol–gel derived tungsten trioxide/titania composite thin films

Tungsten trixoide/titania (WO₃-titania) composite thin films with W/Ti molar ratios of 100/0, 98/2, 96/4, 94/6 92/8 and 90/10 were prepared on fluorine-doped tin oxide conducting glass, and their electrochromic (EC) and photoelectrochromic (PEC) performances were investigated in this study. The composite thin films were synthesized by sol–gel process using peroxotungstic acid and titanium (IV) n-butoxide as the precursors. The surface morphology and composition of the composite thin films were characterized using scanning electron microscope with energy dispersive spectrometer. Electrochemical experiments with in situ spectroscopic measurement were employed to study the EC properties of the composite thin films. It was found that the presence of titania in the WO₃ matrix might slightly decreases its EC performance. PEC cells using the composite thin films as the working electrode and a sputtered semitransparent platinum thin film on ITO as the counter electrode were fabricated and their PEC performances were investigated. The device using composite thin film prepared from sol solution with a W/Ti molar ratio of 96/4 exhibited the best PEC performance.     

Preparation and properties of VO2 thin films by a novel sol–gel process

Vanadium dioxide (VO₂) thin films were fabricated using a simple and novel sol–gel process in which V₂O₅ was used as the vanadium source; oxalic acid was used as the reducing agent; and polyvinyl alcohol was used as the film former to control the viscosity of the VO₂ precursor solution and bond vanadium ions. The microstructure and surface morphology of VO₂ films were studied by X-ray diffraction and scanning electron microscopy, respectively. The results showed that using polyvinyl alcohol forms porous nanostructure of VO₂ films with a uniform grain size of ~25 nm. The measured optical reflectance shows well-defined phase transition as observed by an increase of reflectance upon heating above the transition temperature from ~11 to ~30 % at 1,100 nm. Upon cooling, the expected hysteresis is observed.     

Growth and characterization of (K<Subscript>0.5</Subscript> Na<Subscript>0.5</Subscript>)NbO<Subscript>3</Subscript> thin films by a sol–gel method

(K_0.5 Na_0.5)NbO₃ (KNN) perovskite materials have been developed as a promising lead-free piezoelectric material for environmentally benign piezoelectric devices. KNN films with about 320 nm thickness were fabricated on Pt(111)/SiO₂/Si(100) substrates by a sol–gel method from stoichiometric and A-site ion excess precursor solutions. Two different annealing methods were also used to investigate the crystallographic evolution of the films. A layer-by-layer annealing process results in highly (001) oriented KNN from the annealing temperature of 550 °C, while the final annealing method leads to weaker crystalline peaks with a random orientation. The KNN films from the K and Na excess precursor solutions show similar crystallization behavior. However, the ferroelectric hysteresis loops of the films were greatly improved by compensating for an A-site vacancy. In particular, the KNN films from K-excess precursor solutions show better ferroelectric properties compared to the films prepared from Na excess solutions.     

Dielectric,ferroelectric and optical properties of BaZr<Subscript>0.2</Subscript>Ti<Subscript>0.8</Subscript>O<Subscript>3</Subscript> thin films prepared by sol–gel-hydrothermal process

BaZr_0.2Ti_0.8O₃ thin films on Pt/Ti/SiO₂/Si substrates have been fabricated under low temperature conditions by a sol–gel-hydrothermal technique. The dielectric constant is 247–83 in the frequency range of 1 kHz–1 MHz. The corresponding dielectric loss is ~10⁻². The capacitance–voltage curve shows strong non-linear dielectric behavior leading to a high tunability, up to ~30% at 1 kHz. The remanent polarization and coercive field at room temperature are measured to be ~1.5 μC/cm² and ~90 kV/cm. The infrared optical properties of the thin films are investigated using an infrared spectroscopic ellipsometry in the wave number range of 800–4,000 cm⁻¹. Optical constants of the thin films are simultaneously obtained.     

Preparation of LiTaO<Subscript>3</Subscript> nanoparticles by a sol–gel route

A sol–gel route was developed to prepare pure ultrafine LiTaO₃ powders using Ta₂O₅, Li₂CO₃, citric acid (CA) as chelating agent, ethylene glycol (EG) as esterification agent and polyethylene glycol (PEG) as dispersant. The effects of pH value and heat treatment temperature of powder precursor on the synthesis of LiTaO₃ powders were investigated. The phase content and morphology of the final product were evaluated by XRD, SEM and TEM. A transparent gel was produced when heating a mixed-solution of CA, EG, Li and Ta ions with a molar ratio of [CA]:[EG]:[Li]:[Ta] = 3.0:6.0:1.0:1.0 and 2‰ PEG additions with a pH value of 7 at water bath temperature of 80 °C. The results showed that single phase LiTaO₃ powders with average particle sizes of nanometers were produced after heat treatment of the powder precursor at 650, 700, 800, and 900 °C respectively for 2 h.     

Characterization of TiO<Subscript>2</Subscript> powders and thin films prepared by non-aqueous sol–gel techniques

Stable TiO₂ sols were prepared using two non-aqueous sol–gel processes with titanium n-butoxide and titanium isopropoxide, respectively. Crystallization and phase transitions of powders and thin films were studied by ex situ and in situ X-ray diffraction. For both methods, TiO₂ began to crystallize around 320 °C in air. Using helium instead of air during heat treatment slowed down the crystallization and substoichiometric powders were formed. TiO₂ thin films were obtained by spin coating. The morphology of the films was evaluated using scanning electron microscopy. The films were homogeneous and transparent in the visible range. The effect of the heating atmosphere and the type of substrate was investigated.     

Cu-particle-dispersed (K<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>)NbO<Subscript>3</Subscript> composite thin films derived from sol–gel processing

Sol–gel processing of Cu-particle-dispersed (K_0.5Na_0.5)NbO₃ (Cu/KNN) thin films was studied in an attempt to develop a method producing piezoelectric composite films with good mechanical performance. The Cu/KNN films were prepared via crystallization annealing at 650–750 °C for 1 min in air, followed by reduction annealing at 400–500 °C for 1–2 h in a 5% H₂ and 95% Ar gas mixture. The resultant composite films consisted of perovskite KNN, metallic Cu, and Cu₄O₃. This suggests that the decomposition of Cu sources takes two different ways in this study. The Cu/KNN composite films containing Cu₄O₃ phases were produced by the crystallization annealing at 700 °C for 1 min followed by the reduction annealing at 500 °C for 1 h. Surface morphology observations reveal that these films have dense KNN matrix with a grain size of ~200 nm and uniformly dispersed Cu or Cu₄O₃ particles with a size of <500 nm.     

Correlation between structural and optical properties of TiO<Subscript>2</Subscript>:ZnO thin films prepared by sol–gel method

We have studied structural and optical properties of thin films of TiO₂, doped with 5% ZnO and deposited on glass substrate (by the sol–gel method). Dip-coated thin films have been examined at different annealing temperatures (350–450 °C) and for various layer thicknesses (89–289 nm). Refractive index, porosity and energy band gap were calculated from the measured transmittance spectrum. The values of the index of refraction are in the range of 1.97–2.44, the porosity is in the range of 0.07–0.46 and the energy band gap is in the range of 3.32–3.43. The coefficient of transmission varies from 50 to 90%. In the case of the powder of TiO₂, doped with 5% ZnO, and aged for 3 months in ambient temperature, we have noticed the formation of the anatase phase (tetragonal structure with 20.23 nm grains). However, the undoped TiO₂ exhibits an amorphous phase. After heat treatments of thin films, titanium oxide starts to crystallize at the annealing temperature 350 °C. The obtained structures are anatase and brookite. The calculated grain size, depending on the annealing temperature and the layer thickness, is in the range of 8.61–29.48 nm.     

Novel sol–gel synthesis of transparent and electrically bistable LiNbO<Subscript>3</Subscript>–SiO<Subscript>2</Subscript> nanocomposites thin films

The morphological and electrical characterization of transparent nanostructured LiNbO₃–SiO₂ thin films synthesized by a novel sol–gel route is reported. Films annealed at different temperatures exhibit different size of the nanocrystals, as demonstrated by Atomic Force Microscopy and Glancing Incidence X-ray diffraction. The dc electrical measurements performed on planar devices reveal electrical bistability. A clear relationship between the electrical bistability and the size of LiNbO₃ nanocrystals embedded in the matrix is observed.     

Preparation of nanostructured thin films of yttrium iron garnet (Y<Subscript>3</Subscript>Fe<Subscript>5</Subscript>O<Subscript>12</Subscript>) by sol–gel technology

The synthesis of hydrolytically active heteroligand coordination compounds [M(C₅H₇O₂)_3?x(C₅H₁₁Oⁱ)_x] (M = Fe³⁺ and Y³⁺) using iron and yttrium acetylacetonates has been studied. The gel formation kinetics in solutions of these compounds upon hydrolysis and polycondensation has also been studied. Thin films of a solution of these precursors have been applied to polished sapphire substrates by dip coating technology. The crystallization of nanostructured yttrium iron garnet (Y₃Fe₅O₁₂) films during heat treatment of xerogel coatings under various conditions has been studied. How the phase composition, microstructure, and particle size depend on the synthesis parameters has been recognized.     

Influence of PEG additive and annealing temperature on structural and electrochromic properties of sol–gel derived WO<Subscript>3</Subscript> films

Sol–gel derived tungsten oxide (WO₃) films have been deposited by spin coating route using acetylated peroxotungstic acid (APTA) or a mixture of APTA and polyethylene glycol (PEG) dissolved in ethanol as the precursor solution, followed by thermal treatment in air. The influence of PEG additive and annealing temperature on the structural and electrochromic (EC) behavior of the films have been investigated. For films annealed at 300 °C, a porous nanocrystalline/amorphous microstructure was obtained in the WO₃-PEG film, while monoclinic microstructure was formed in the pure WO₃ film. Moreover, for the WO₃-PEG films, the film microstructure was found to depend on the annealing temperature. Electrochemical studies indicate that the WO₃-PEG film annealed at 300 °C (WP-300) exhibits superior EC properties, which produces faster switching speed (t _c = 19 s, t _b = 3 s),better reversibility (K = 0.97) as well as higher optical modulation (ΔT = 32% at 550 nm) and coloration efficiency (η = 22 cm²/C at 550 nm). Our results suggest that PEG addition in combination with an appropriate annealing treatment can benefit the EC properties, arising from the ease of ion diffusion within the EC material, as evident from the nanocrystallines embedded into the amorphous matrix with a porous character.     

Structural and optical characterization of sol–gel derived boron doped Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> nanostructured films

Pure and boron (B) doped iron oxide (Fe₂O₃) nanostructured thin films were prepared by sol–gel spin coating method. The effects of B (0.1, 0.2, 0.5 and 1 %) content on the crystallinity and morphological properties of Fe₂O₃ films were investigated by X-ray diffractometer and atomic force microscopy. X-ray diffraction patterns revealed that the Fe₂O₃ films have a rhombohedral crystalline phase of α-Fe₂O₃ phase (hematite) with nanostructure and their crystallite size (D) is changed from 27 ± 2 to 45 ± 5 nm with B dopant content. The minimum crystallite size value of 27 ± 2 nm was obtained for 0.2 % B doped Fe₂O₃ film. Carrying out UV–VIS absorption study for both doped and undoped films at room temperature, it was realized that allowed optical transitions may be direct or indirect transitions. The direct and indirect energy gap values for pure Fe₂O₃ were obtained to be 2.07 and 1.95 eV, respectively. The optical band gap value of the films was changed with 0.1 % B doping to reach 1.86 eV for direct band gap and 1.66 eV in case of indirect band gap.