Preparation and luminescent properties of Eu-doped BaTiO3 thin films by sol–gel process

Eu-doped BaTiO₃ thin films with a pseudo-cubic perovskite structure were successfully fabricated on magnesia substrates at low temperature by using a high-concentration sol–gel process, in which the newly developed gel-aging process on substrate was employed. Film microstructure, crystallinity, sintering properties and photoluminescence (PL) were investigated. The xerogel thin films exhibited strong PL associated with Eu³⁺ ions under ultraviolet excitation at room temperature; the PL was visible to naked eyes. The intensity maximum of PL was reached with doping concentration of about 8 mol%. Sintering above 600 °C caused reduction of europium, resulting in a rapid quenching of Eu³⁺ emission and enhancement of Eu²⁺ emission.     

Preparation of nanocrystalline nickel oxide thin films by sol–gel process for hydrogen sensor applications

Preparation of nanocrystalline NiO thin films by sol–gel method and their hydrogen (H₂) sensing properties were investigated. The thin films of NiO were successfully deposited on the glass and SiO₂/Si substrate by a sol–gel coating method. The films were characterized for crystallinity, electrical properties, surface topography and optical properties as a function of calcination temperature and substrate material. It was found that the films produced by this method were polycrystalline and phase pure NiO. The H₂ gas sensitivity of these films was studied as a function of H₂ concentration and calcination temperature. The results indicated that the sol–gel derived NiO films could be used for the fabrication of H₂ gas sensors to monitor low concentration of H₂ in air quantitatively at low temperature range (< 200 °C).     

Preparation and characterisation of lanthanum doped BaTiO3 nanosize polycrystals by sol–gel processing

Abstract

Sol–gel processing has been refined to prepare lanthanum doped barium titanate nanosize polycrystal powders, La_xBa_1-xTiO₃ (x=0.05, 0.15, 0.20, 0.25), in the presence of Ba(OAc)₂, La(OAc)₃, and Ti(OⁿBu)₄ as starting materials with 0.7 mol L^-1 total concentration of precursor. The sol was obtained by cosolubilised hydrolysis of starting materials using HOAc as medium and catalysis with pH～3.8. The wet gel was acquired by concentrating and aging at room temperature. The wet gel to dry gel conversion was carried out at 50°C for ～24 h. First the dry gel was characterised by differential thermal analysis–thermal gravimetry (DTA–TG) and then it was calcined at various temperatures (600, 700, 800, 900°C) to prepare the nanocrystals. The final synthesised polycrystals were characterised by Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction, and scanning electron microscopy.     

Characteristics of CeOx–VO2 composite thin films synthesized by sol–gel process

Pure vanadium dioxide (VO₂) and CeOx–VO₂ (1.5 < x < 2) composite thin films were grown on muscovite substrate by inorganic sol–gel process using vanadium pentaoxide and cerium(III) nitrate hexahydrate powder as precursor. The crystalline structure, morphology and phase transition properties of the thin films were systematically investigated by X-ray diffraction, Raman, X-ray photoelectron spectroscopy, FE-SEM and optical transmission measurements. High quality of the VO₂ and CeOx–VO₂ composite films were obtained, in which the relative fractions of +4 valence state vanadium were above 70 % though the concentrations of cerium reached 9.77 at %. However, much of cerium compounds were formed at the edge of grains and the addition of cerium resulted in more clearly defined grain boundaries as shown in SEM images. Meanwhile, the composite films exhibited excellent phase transition properties and the infrared transmittance decreased from about 70 to 10 % at λ = 4 μm bellow and above the metal–insulator phase transition temperature. The metal–insulator phase transition temperatures were quite similar with about 66 °C of the pure VO₂ and CeOx–VO₂ composite thin films. But hysteresis widths increased with more addition of cerium, due to the limiting effect of grain boundaries on the propagation of the phase transition. Particularly, the CeOx–VO₂ composite film with an addition of 7.82 at % Ce showed a largest hysteresis width with about 20.6 °C. In addition, the thermochromic performance of visible transmittance did not change obviously with more addition of cerium.     

Structural and pyroelectric properties of sol–gel derived multiferroic BFO thin films

Multiferroic BFO/PZT multilayer films were fabricated by spin-coating method on the (1 1 1)Pt/Ti/SiO₂/Si substrate alternately using PZT(30/70), PZT(70/30) and BFO alkoxide solutions. The structural and ferroelectric properties were investigated for uncooled infrared detector applications. The coating and heating procedure was repeated six times to form BFO/PZT multilayer films. All films showed the typical XRD patterns of the perovskite polycrystalline structure without presence of the second phase such as Bi₂Fe₄O₃. The thickness of BFO/PZT multilayer film was about 200–220 nm. The ferroelectric properties such as dielectric constant, remnant polarization and pyroelectric coefficient were superior to those of single composition BFO film, and those values for BFO/PZT(70/30) multilayer film were 288, 15.7 μC/cm² and 9.1 × 10^?9 C/cm² K at room temperature, respectively. Leakage current density of the BFO/PZT(30/70) multilayer film was 3.3 × 10^?9 A/cm² at 150 kV/cm. The figures of merit, F_V for the voltage responsivity and F_D for the specific detectivity, of the BFO/PZT(70/30) multilayer film were 6.17 × 10^?11 Ccm/J and 6.45 × 10^?9 Ccm/J, respectively.     

Structural, optical, and electric properties of BNT–BT0.08 thin films processed by sol–gel technique

Thin films with the composition [(Bi_0.5Na_0.5)TiO₃]_0.92–[BaTiO₃]_0.08 (hereafter BNT–BT_0.08) were deposited on Pt–Si by spin-coating from a stable sol precursor. The BNT–BT_0.08 film, crystallized on the Bi_0.5Na_0.5TiO₃ rhombohedral lattice, was obtained after annealing the film-gel at 700 °C. The films have a smooth surface (Rms = 2.76 nm) and grains with ferroelectric domains. The film showed a bandgap of 3.25 eV and a refractive index of 2.20 at a wavelength of 630 nm. The dielectric characteristics of BNT–BT_0.08 thin films were measured at room temperature and 10 kHz the dielectric constant (ε _r) was 243 and the loss tangent (tanδ) was 0.38. The remnant polarization (P _r) was 0.87 μC/cm² and the coercive field (E _c) was 220 kV/cm at 10 kHz and room temperature. The current density was approximately 2.7 × 10⁻⁵ A/cm² at low electric fields (100 kV/cm). BNT–BT_0.08 thin films shown piezoelectric properties (d _33eff = 100 pm/V) comparable to those of PZT thin films.     

Composition control in Cu2ZnSnS4 thin films by a sol–gel technique without sulfurization

Kesterite Cu₂ZnSnS₄ (CZTS) thin films with a smooth, compact and crack-free morphology are obtained via a sol–gel method without sulfurization process. Non-toxic ethylene glycol is selected as solvent, while Cu(CH₃COO)₂, Zn(CH₃COO)₂, SnCl₂·2H₂O and thiourea are used as raw materials. Chemical composition dependence of CZTS films on pre-annealing and post-annealing process is comprehensively investigated. The analysis of energy dispersive X-ray indicates that composition control of CZTS films can be easily realized by the preparation of precursor solution and varying the annealing conditions.     

Effect of stabilizer on optical and structural properties of MgO thin films prepared by sol–gel method

The effects of monoethanolamine (MEA) and acetylacetone (ACAC) addition as stabilizer on the crystallization behaviour, morphology and optical properties of magnesium oxide were investigated using thermogravimetry (TG/DTG), X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-Visible, photoluminescence (PL) and Fourier transform infrared (FTIR) spectroscopy. Stabilizer addition reduces transparency of the films. MgO films prepared at 500 °C showed weak orientation of (200). However, the films prepared by addition of stabilizer are amorphous. MgO powders were prepared for exhibiting the structural properties. The patterns of MgO powders showed a preferred orientation of (200). The addition of stabilizer causes a reduction in grain size. SEM micrographs show that a homogenous and crack-free film can be prepared at 500 °C and addition of stabilizer causes an increase in packing density.     

Fabrication and characteristics of La,Cd and Sn doped BST thin films by sol–gel method

Ba_1−x Sr _x TiO₃ (BST) thin films doped with La, Cd and Sn were prepared by sol–gel method on the Pt/Ti/SiO₂/Si substrate. X-ray diffraction analysis and atomic force microscopy showed that dopant La and Cd causes decreased grain size of BST thin films obviously, and Sn-doped BST thin films was similar to BST films in grain size. La and Cd doped decreased the tunability of BST thin films and Sn doped increased it, which may be explained by stress, electronegativity and oxygen vacancies factors. All the doped BST thin films improved the leakage current characteristic.     

Optical properties and electrical resistivity of boron-doped ZnO thin films grown by sol–gel dip-coating method

Sol–gel dip-coating was used to grow ZnO thin films doped with various concentrations of B ranging from 0 to 2.5 at.% on quartz substrates. The effects of B doping on the absorption coefficient (α), optical band gap (E_g), Urbach energy (E_U), refractive index (n), refractive index at infinite wavelength (n_∞), extinction coefficient (k), single-oscillator energy (E_o), dispersion energy (E_d), average oscillator strength (S_o), average oscillator wavelength (λ_o), moments M₋₁ and M₋₃, dielectric constant (ε), optical conductivity (σ), and electrical resistivity (ρ) of the BZO thin films were investigated. The transmittance spectra of the ZnO and BZO thin films show that the transmittance of the BZO thin films was significantly higher than that of the ZnO thin films in the visible region of the spectrum and that the absorption edge of the BZO thin films was blue-shifted. The BZO thin films exhibited higher E_g, E_U, and E_o and lower E_d, λ_o, M₋₁ and M₋₃ moments, S_o, n_∞, and ρ than the ZnO thin films.     

Improvement of physical properties of IGZO thin films prepared by excimer laser annealing of sol–gel derived precursor films

Indium gallium zinc oxide (IGZO) transparent semiconductor thin films were prepared by KrF excimer laser annealing of sol–gel derived precursor films. Each as-coated film was dried at 150 °C in air and then annealed using excimer laser irradiation. The influence of laser irradiation energy density on surface conditions, optical transmittances, and electrical properties of laser annealed IGZO thin films were investigated, and the physical properties of the excimer laser annealed (ELA) and the thermally annealed (TA) thin films were compared. Experimental results showed that two kinds of surface morphology resulted from excimer laser annealing. Irradiation with a lower energy density (≤250 mJ cm⁻²) produced wavy and irregular surfaces, while irradiation with a higher energy density (≥350 mJ cm⁻²) produced flat and dense surfaces consisting of uniform nano-sized amorphous particles. The explanation for the differences in surface features and film quality is that using laser irradiation energy to form IGZO thin films improves the film density and removes organic constituents. The dried IGZO sol–gel films irradiated with a laser energy density of 350 mJ/cm² had the best physical properties of all the ELA IGZO thin films. The mean resistivity of the ELA 350 thin films (4.48 × 10³ Ω cm) was lower than that of TA thin films (1.39 × 10⁴ Ω cm), and the average optical transmittance in the visible range (90.2%) of the ELA 350 thin films was slightly higher than that of TA thin films (89.7%).     

Preparation of continuous alumina gel fibres by aqueous sol–gel process

Continuous alumina gel fibres were prepared by sol–gel method. The spinning sol was prepared by mixing aluminum nitrate, lactic acid and polyvinylpyrrolidone with a mass ratio of 10:3:1· 5. Thermogravimetry–differential scanning calorimetry (TG–DSC), Fourier transform infrared (FTIR) spectra, X-ray diffraction (XRD), and scanning electron microscopy (SEM) were used to characterize the properties of the gel and ceramic fibres. The Al₂O₃ fibres with a uniform diameter can be obtained by sintering gel fibres at 1200 °C.     

Piezoelectric BNT-BT<Subscript>0.11</Subscript> thin films processed by sol–gel technique

0.89(Na_0.5Bi_0.5)TiO₃–0.11BaTiO₃, (BNT-BT_0.11) thin film was fabricated by sol–gel/spin coating process, on platinized silicon wafer. Perovskite structure with random orientation of crystallites has been obtained at 700 °C. Piezoelectric activity of BNT-BT_0.11 thin film was detected using piezoresponse force microscopy (PFM). Effective piezoelectric coefficient d _33eff of such film, recorded at 5 V applied dc voltage, was ~29 pm/V, which is similar to other BNT-BT _x thin films. The complex refractive index and dielectric function of BNT-BT_0.11 thin films were also investigated. The high leakage current density significantly influences the dielectric, ferroelectric, and piezoelectric properties of the BNT-BT_0.11 films.     

Structural,morphological and optical properties of undoped and Co-doped ZnO thin films prepared by sol–gel process

Undoped and Co-doped ZnO thin films with different amounts of Co have been deposited onto glass substrates by sol–gel spin coating method. Zinc acetate dihydrate, cobalt acetate tetrahydrate, isopropanol and monoethanolamine (MEA) were used as a precursor, doping source, solvent and stabilizer, respectively. The molar ratio of MEA to metal ions was maintained at 1.0 and a concentration of metal ions is 0.6 mol L^?1. The Co dopant level was defined by the Co/(Co + Zn) ratio it varied from 0 to 7 % mol. The structure, morphology and optical properties of the thin films thus obtained were characterized by X-ray diffraction (XRD), energy dispersive X-ray spectrometer (EDX), scanning electron microscopy (SEM), ultraviolet–visible (UV–Vis), photoluminescence (PL) and Raman. The XRD results showed that all films crystallized under hexagonal wurtzite structure and presented a preferential orientation along the c-axis with the maximum crystallite size was found is 23.5 nm for undoped film. The results of SEM indicate that the undoped ZnO thin film has smooth and uniform surface with small ZnO grains, and the doped ZnO films shows irregular fiber-like stripes and wrinkle network structure. The average transmittance of all films is about 72–97 % in the visible range and the band gap energy decreased from 3.28 to 3.02 eV with increase of Co concentration. DRX, EDX and optical transmission confirm the substitution of Co²⁺ for Zn²⁺ at the tetrahedral sites of ZnO. In addition to the vibrational modes from ZnO, the Raman spectra show prominent mode representative of Zn_yCo_3?yO₄ secondary phase at larger values of Co concentration. PL of the films showed a UV and defect related visible emissions like violet, blue and green, and indicated that cobalt doping resulted in red shifting of UV emission and the reduction in the UV and visible emissions intensity.     

Effect of Zn doping on structure and ferroelectric properties of PST thin films prepared by sol–gel method

(Pb _y Sr_1−y )Zn _x Ti_1−x O_3−x thin films were prepared on ITO/glass substrate by sol–gel process using dip-coating method. The phase structure, morphology and ferroelectric property of the thin film were studied. All the thin films show the typical perovskite phase structure. Both the crystallinity and c/a ratio of the perovskite phase increases initially and then decreases gradually with doping Zn in the thin film. Ferroelectric properties of the Zn-doped PST thin films, including ferroelectric hysteresis-loop, remnant polarization and coercive force, decrease gradually with increasing Zn. And the effect of Zn on ferroelectric properties is more obvious in PST thin film with high content of Pb than that with low Pb although the high lead thin film exhibits high intrinsic ferroelectric properties.     

Post annealing effect on structural and optical properties of ZnO thin films derived by sol–gel route

Zinc oxide thin films have been spun coated on p-Si (100) substrates by sol–gel route. These films were annealed at different annealing temperatures from 300 to 1,000 °C in the oxygen ambient. In this way a suitable annealing temperature window for the sol–gel derived ZnO films exhibiting minimum defects (points and dislocations) and better quality (crystal and optical) was investigated. The structural and optical features of ZnO thin films have been examined by X-ray diffraction, atomic force microscopy, UV–Vis spectroscopy, and photoluminescence spectra. The results revealed that the crystallization in the films initiated at 300 °C, improved further with annealing. All the deposited films exhibited wurtzite phase with c-axis orientations. The variations in the position of characteristic (002) peak, stress, strain and lattice parameters are investigated as a function of annealing temperature. The optical band gap is not significantly affected with annealing as observed by UV–Vis transmission spectroscopy. The Photoluminescence spectra exhibited three luminescence centers. The near band edge esmission was observed in UV region which enhanced with the heat treatment, is an indication of improvement in the optical quality of films. The other two visible emissions are related to native defects in ZnO lattice were appeared only for higher annealing (≥700 °C).     

Spectroscopic,microscopic, and electrical characterization of nanostructured SnO2:Co thin films prepared by sol–gel spin coating technique

Cobalt doped SnO₂ thin films were prepared by sol–gel spin coating technique and influence of dopant concentration on structural, morphological and optical properties of thin films were investigated by XRD, XPS, FTIR, SEM, AFM, PL, UV–vis, and Hall effect measurement. All samples have a tetragonal rutile structure and the grain size decreases with increasing the doping concentration. XPS results clearly showed the presence of Co²⁺ ions into the SnO₂. The SEM and AFM images reveal that the morphology of samples was affected by dopant. Conductivity type of the films changes from n-type to p-type with increasing Co-dopant above 3 mol% and electrical resistivity increases with increasing Co content. The optical band gap gradually decreases with improved cobalt concentration from 3.91 eV to 3.70 eV. The PL measurements revealed the decrease in intensity of blue emission lines and increase in green emission when content of Co is enhanced in the thin films.