Preparation and photoluminescence properties of ZnO/amorphous-BaTiO3 thin-films by sol–gel process

ZnO/amorphous-BaTiO₃ thin-films were prepared on glass substrates by a sol–gel process. DSC/TGA, XRD and AFM were used to analyze the thermolysis of the precursor and the crystal growth characteristics of ZnO, and IR spectroscopy was used to determine the presence of OH groups and CO impurities containing in the films. Violet emission centering at 418 and 438 nm was observed in room temperature PL spectra of the films annealed at 400–500 °C. The strongest violet emission was observed in the film annealed at 400 °C. The emission intensity reduces as the annealing temperature increases, and vanishes in the film annealed at 550 °C. All of the films contain OH and CO groups, and the films annealed at the temperature over 450 °C contain CO₂ impurities, indicating the enhancing oxidation of the films. The vanishing of the violet emission is probably due to the improvement of the oxygen deficient of ZnO crystals.     

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.     

Thickness limit of BaTiO3 thin film capacitors grown on SUS substrates using aerosol deposition method

We fabricated BaTiO₃ thin films with 2.2-0.1 μm thickness on hard stainless steel (SUS) substrates by using the ADM to confirm the causes of dielectric thickness limit showing in BaTiO₃ thin films prepared on SUS substrates and suggest key factors which can overcome the limit. Then, from the measurements of thickness dependence of their dielectric properties, the thickness limit of 0.2 μm was confirmed and to confirm the reason why their dielectric properties could not be measured in the thickness below 0.2 μm, the thickness dependence of leakage current mechanisms in BaTiO₃ films were investigated. As a result, by decreasing the thickness of films from 2.2 to 0.2 μm, the mechanism changed from Poole-Frenkel emission to modified-Schottky emission indicating increase of interface effects. Especially, in the case of 0.2 μm thickness, it was confirmed that the dominant mechanism was Fowler-Nordheim tunneling based on electric field concentration at a high electric field. Consequently, from this investigation of leakage current mechanism, it can be expected that the cause of thickness limits was electric field concentration at rough BaTiO₃/SUS interfaces forming in AD process, and to get over the thickness limit and decrease level of leakage currents, the hard substrates are required to reduce the interface roughness and oxygen vacancies acted as donors should be decreased.     

Nano-crystalline LaFeO3 powders synthesized by the citrate–gel method

A novel sol–gel process was developed for preparing nano-sized, perovskite-type LaFeO₃ powder by the thermal decomposition of the gel-complex of LaFe–(C₆H₈O₇·H₂O). The structural evolution has been systematically investigated by X-ray diffraction (XRD), differential thermal analysis (DTA) and thermogravimetric analysis (TGA). Perovskite powder of  25 nm size could be obtained at a temperature of  600 °C without formation of any secondary phases of La₂O₃ and Fe₂O₃ single oxides and no requirements of high temperature/vacuum/pH control etc. Analysis of the X-ray powder diffraction data showed a decrease in the value of lattice strains with increasing decomposition temperature, whereas the particle size increases with increasing decomposition temperature.     

Growth and dielectric properties of solvothermal BaTiO3 polycrystalline thin films

Polycrystalline BaTiO₃ thin films were grown solvothermally on Ti-covered substrates in ethanol, distilled water and 1,4-butanediol at 80 °C, 150 °C and 200 °C. Films grown in organic media are finer grained and required more stringent crystallization conditions due to lower solubility of constituent ions. Solvothermal films exhibit less severe proton incorporation. Solvothermal films grown close to the boundary crystallization conditions tend to have higher frequency dispersion and loss tangents. Dielectric properties of BaTiO₃ films grown in ethanol are comparable to that of hydrothermal films. Butanediol-derived films have lower dielectric constants than the aqueous equivalent.     

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.     

Chemical deposition of Al2O3 thin films on Si substrates

Uniform Al₂O₃ films were deposited on silicon substrates by the sol–gel process from stable coating solutions. The technological procedure includes spin coating deposition and investigating the influence of the annealing temperature on the dielectric properties. The layers were studied by Fourier transform infrared spectroscopy and Scanning Electron Spectroscopy. The electrical measurements have been carried out on metal–insulator–semiconductor (MIS) structures. The C–V curves show a negative fixed charge at the interface and density of the interface state, Dit, 3.7 × 10¹¹ eV^− 1cm^− 2 for annealing temperature at 750 °C.     

Effect of Ni-modified α-Al2O3 prepared by sol–gel and solvothermal methods on the characteristics and catalytic properties of Pd/α-Al2O3 catalysts

In the present study, Ni-modified α-Al₂O₃ with Ni/Al ratios of 0.3 and 0.5 were prepared by sol–gel and solvothermal method and then were impregnated with 0.3 wt.% Pd. Due to different crystallization mechanism of the two preparation methods used, addition of nickel during preparation of α-Al₂O₃ resulted in various species such as NiAl₂O₄, mixed phases between NiAl₂O₄ and α-Al₂O₃, and mixed phases between NiAl₂O₄ and NiO. As revealed by NH₃-temperature programmed desorption, formation of NiAl₂O₄ drastically reduced acidity of alumina, hence lower amounts of coke deposited during acetylene hydrogenation was found for the Ni-modified α-Al₂O₃ supported catalysts. For any given method, ethylene selectivity was improved in the order of Pd/Ni–Al₂O₃-0.5 > Pd/Ni–Al₂O₃-0.3 > Pd/Ni–Al₂O₃-0  Pd/α–Al₂O₃-commercial. When comparing the samples prepared by different techniques, the sol–gel-made samples showed better performances than the solvothermal-derived ones.     

Atomic force microscopy study of TiO2 sol–gel films thermally treated under NH3 atmosphere

Multilayered TiO₂ films were obtained by sol–gel and dipping deposition on quartz substrate followed by thermal treatment under NH₃ atmosphere. In an attempt to understand the close relationship between microstructural characteristics and the synthesis parameters, a systematic research of the structure and the morphology of NH₃ modified TiO₂ sol–gel films by XRD and Atomic Force Microscopy is reported. The surface morphology has been evaluated in terms of grains size, fractal dimension and surface roughness. For each surface, it was found a self-similar behavior (with mean fractal dimension in the range of 2.67–3.00) related to an optimum morphology favorable to maintain a nano-size distribution of the grains. The root mean square (RMS) roughness of the samples was found to be in the range of 0.72–6.02 nm.     

Growth and dielectric properties of BaTiO3 thin films prepared by the microwave-hydrothermal method

Polycrystalline BaTiO₃ thin films were grown on Ti-covered polymer substrates at 80 °C using the microwave-hydrothermal technique. Onset of BaTiO₃ formation occurred almost instantaneously at 80 °C and complete film coverage was achieved within 2 min. Longer reaction time was necessary for extensive grain growth to achieve dense films. Good quality capacitor films were only achieved at 4 h reaction time but loss tangents were high. Film dielectric constant and dielectric loss values of as-grown M-H films decreased with longer reaction time. Oxygen plasma treatment improved loss tangents to 4% by removal of both absorbed moisture and lattice hydroxyls.     

Microstructure and ferroelectric properties of epitaxial BaTiO3/SrRuO3/SrTiO3 (001) films grown by pulsed laser deposition under high oxygen pressure

BaTiO₃ films were epitaxially grown on SrTiO₃ (001) substrates buffered with SrRuO₃ films as bottom electrode by pulsed laser deposition under high oxygen pressure of 30 Pa. The quality of the BaTiO₃/SrRuO₃/SrTiO₃ multilayer films was analyzed by means of X-ray diffraction, atomic force microscopy and transmission electron microscopy. BaTiO₃ films were found to be highly c-axis-oriented tetragonal phase with c/a = 1.002. The dielectric constant first increased with increasing temperature, and showed a peak at the Curie temperature of about 356 K. The films had well-saturated hysteresis loops with a remnant polarization of 7.3 μC/cm² and a coercive field of 29.5 kV/cm at room temperature.     

Molecular beam epitaxy growth of BaTiO3 thin films and crucial impact of oxygen content conditions on the electrical characteristics

In this study, high quality crystalline BaTiO₃ films were fabricated in different oxygen content conditions on Nb-doped SrTiO₃ (001) substrates by molecular beam epitaxy (MBE). BaTiO₃ epitaxial film presents a critical thickness of 6.4 nm and is almost entirely relaxed on SrTiO₃ with a thickness of ~ 50 nm. The electrical properties of 50 nm strain-free BaTiO₃ films were investigated by both macro- and microscopic measurements. Their electrical characteristics were found to be strongly influenced by different oxygen content conditions used during the preparation or annealing of the samples. Limited molecular oxygen partial pressure in a MBE chamber probably leads to a great amount of oxygen vacancies in the oxide film and results in rectification behavior of the oxygen-deficient BaTiO_3-x/SrTiO₃:Nb interface. Several approaches such as using atomic oxygen ambiance during the growth, annealing under elevated oxygen pressure were employed in order to decrease the oxygen vacancy density and these approaches eventually permit obtaining BaTiO₃ films with good ferroelectric characteristics.     

BaTiO3-SrTiO3 layered dielectrics for energy storage

BaTiO₃-SrTiO₃ (BST) thick films (~ 250-390 μm) with layered structures were fabricated by tape-casting and lamination process. Layered composites with various Ba/Sr ratios were obtained by lamination of BaTiO₃ (BT) and SrTiO₃ (ST) tapes in different spatial configurations (2-2). As-prepared BST ceramics showed much improved sinterability over the laminates of pure BT or pure ST tapes. Dielectric properties of materials were measured in the temperature range of 25 °C to 200 °C. The method of utilizing of layered structures offered flexibility to maximize the energy storage capability at specific operating conditions: (temperature and electric field) by tailoring the dielectric properties through varying the spatial configurations of BT and ST films.     

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.     

Mechanical and dielectric properties of porous Si3N4–SiO2 composite ceramics

In order to prepare a structural/functional material with not only higher mechanical properties but also lower dielectric constant and dielectric loss, a novel process combining oxidation-bonding with sol–gel infiltration-sintering was developed to fabricate a porous Si₃N₄–SiO₂ composite ceramic. By choosing 1250 °C as the oxidation-bonding temperature, the crystallization of oxidation-derived silica was prevented. Sol–gel infiltration and sintering process resulted in an increase of density and the formation of well-distributed micro-pores with both uniform pore size and smooth pore wall, which made the porous Si₃N₄–SiO₂ composite ceramic show both good mechanical and dielectric properties. The ceramic with a porosity of 23.9% attained a flexural strength of 120 MPa, a Vickers hardness of 4.1 GPa, a fracture toughness of 1.4 MPa m^1/2, and a dielectric constant of 3.80 with a dielectric loss of 3.11 × 10⁻³ at a resonant frequency of 14 GHz.     

Fabrication of YbAl3 film via annealing amorphous Yb-Al film deposited by RF magnetron sputtering

YbAl₃ single-phase bulk alloy was synthesized by melting at 1258 K using raw Yb and Al metals in the ratio of 1:2.2. A sputtering YbAl₃ target was prepared using the precursor YbAl₃ bulk alloy with the spark plasma sintering process. An amorphous Yb-Al film was fabricated by RF magnetron sputtering using the YbAl₃ target, and a single-phase crystalline YbAl₃ film was fabricated by annealing the amorphous Yb-Al film at temperatures higher than 923 K in Ar atmosphere.     

Preparation and electrical properties of nanoporous BaTiO3

Nanoporous barium titanate with high specific surface area was prepared from co-gel precursors through solvothermal method followed by supercritical drying. The samples were accumulated by BaTiO₃ nanoparticles with excellent crystallinity. The BaTiO₃ obtained at 60 °C exhibited a high BET surface area of 117 m²/g. The porosity reduced with the increasing solvothermal temperature. Raman spectra indicated that the solvothermal-synthesized BaTiO₃ was composed by both cubic phase and tetragonal phase. The relations between dielectric properties and the porosity of the samples were also investigated. The introduction of pores reduced the dielectric constant obviously. The dielectric constant of the obtained sample increased with the decrease of the porosity.     

Structural evolution and its influence on luminescence of SiO2–SrF2–ErF3 glass ceramics prepared by sol–gel method

Er³⁺ doped SrF₂–SiO₂ transparent glass ceramics were prepared by sol–gel method and heat treatment. The decomposition of Sr²⁺–CF₃COO⁻ and the formation of SrF₂ nano-crystals were found to proceed synchronously in the xerogel. After crystallization of the xerogel, SrF₂ nano-crystals with 8–10 nm in size distributed homogenously among the glassy matrix, and the microstructure of the glass ceramic was stable under and at the temperature of 800 °C probably due to interfacial interaction between nano-crystals and glassy matrix. When heat-treated at 800 °C, the chemically bonded water in the sample was eliminated, resulting in the appearance of the visible luminescence bands of ²H_11/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions.     

Effect of working pressure on the properties of BaTiO3-CoFe2O4 composite films deposited on STO (100) by PLD

The BaTiO₃-CoFe₂O₄ (BTO-CFO) composite films were grown on SrTiO₃ (STO) (100) substrates at 750 °C under various working pressures by pulsed laser deposition. The composite film grew into a supersaturated single phase at the working pressure of 10 mTorr, BTO and CFO (00 l) oriented hetero-epitaxial films on STO (100) at 100 mTorr, and a polycrystalline composite film at 500 mTorr. The slow growth rate at high working pressure led to the phase separation in the composite film. The CFO was compressively strained along out-of-plane due to the lattice mismatch with the BTO matrix phase. The BTO-CFO composite film grown at 100 mTorr showed reversible switching of ferroelectric polarization and magnetic hysteresis with strong magnetic anisotropy.