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.     

Structural and nanomechanical properties of sol–gel prepared (K,Na)NbO3 thin films

Perovskite (K, Na)NbO₃ (KNN) thin films (~100 nm) were prepared by sol–gel/spin coating process on Pt/SiO₂/Si substrates and annealed at 650 °C. The structural properties of KNN films were confirmed by X‐ray diffraction analysis (XRD), Raman spectroscopy and scanning electron, transmission electron and atomic force microscopy (SEM, TEM and AFM) analysis. Pure perovskite phase of K_0.65Na_0.35NbO₃ in nonstoichiometric composition with monoclinic symmetry in film was revealed. Uniform homogeneous microstructure of KNN film with the roughness (~6.9 nm) contained spherical particles (~50–90 nm). Nanoindentation technique was used to characterize the mechanical properties of KNN films. Elastic modulus and hardness of Pt, SiO₂ and KNN thin films were calculated from their composite values of KNN/Pt/SiO₂/Si film/substrate system. The modulus and hardness of KNN film (71 and 4.5 GPa) were lower in comparison with SiO₂ (100 and 7.5 GPa). Pt film (~30 nm) did not influence the composite modulus, but had effect on hardness of KNN film. Copyright © 2015 John Wiley & Sons, Ltd.     

Effect of substrate on phase formation and surface morphology of sol-gel lead-free KNbO3, NaNbO3, and K0.5Na0.5NbO3 thin films

Environmentally acceptable lead-free ferroelectric KNbO₃ (KN) or NaNbO₃ (NN) and K_0.5Na_0.5NbO₃ (KNN) thin films were prepared using a modified sol-gel method by mixing potassium acetate or sodium acetate or both with the Nb-tartrate complex, deposited on the Pt/Al₂O₃ and Pt/SiO₂/Si substrates by a spin-coating method and sintered at 650°C. X-ray diffraction (XRD) analysis indicated that the NN and KNN films on the Pt/SiO₂/Si substrate possessed a single perovskite phase, while NN and KNN films on the Pt/Al₂O₃ substrate contained a small amount of secondary pyrochlore phase, as did KN films on both substrates. Scanning electron microscopic (SEM) and atomic force microscopic (AFM) analyses confirmed that roughness R _q of the thin KNN/Pt/SiO₂/Si film (?? 7.4 nm) was significantly lower than that of the KNN/Pt/Al₂O₃ film (?? 15 nm). The heterogeneous microstructure composed of small spherical and larger needle-like or cuboidal particles were observed in the KN and NN films on both substrates. The homogeneous microstructure of the KNN thin film on the Pt/SiO₂/Si substrate was smoother and contained finer spherical particles (?? 50 nm) than on Pt/Al₂O₃ substrates (?? 100 nm). The effect of different substrates on the surface morphology of thin films was confirmed.     

Microstructure and ferroelectric properties of r.f. magnetron sputtering derived PZT thin films deposited on interlayer (PbO/TiO2)

Lead zirconate titanate (PZT) thin films were deposited on Pt/Ti/SiO₂/Si and interlayer/Pt/Ti/SiO₂/Si substrate by radio frequency (r.f.) magnetron sputtering with a Pb_1.1Zr_0.53Ti_0.47O₃ target. The crystallization of the PZT thin films was formed only by substrate temperature. When interlayer (PbO/TiO₂) was inserted between the PZT thin film and the Pt electrode, the grain growth and processing temperature of the PZT thin films were considerably improved. Compared to PZT/Pt structure, the dielectric constant and polarization properties of the PZT/interlayer/Pt structure were fairly improved. In particular, PZT/interlayer/Pt at the substrate temperature of 400 °C showed prevalent ferroelectric properties (_r=475.97, tanδ=0.0591, P_r=23 μC/cm²). As a result of an X-ray photoelectron spectroscopy (XPS) depth-profile analysis, it was found that PZT/interlayer/Pt deposited only by substrate temperature without the post-annealing process via r.f. magnetron sputtering method remained independent of each other regardless of substrate temperatures.     

The diffusion of Pt in BST films on Pt/Ti/SiO2/Si substrate by sol-gel method

Barium strontium titanate (Ba_0.65Sr_0.35TiO₃) ferroelectric thin films have been prepared by sol-gel method on Pt/Ti/SiO₂/Si substrate. The X-ray diffraction (XRD) pattern indicated that the films were a polycrystalline perovskite structure and the atomic force microscope (AFM) image showed that the crystallite size and the root mean square roughness (RMS) were 90 nm and 3.6 nm, respectively. The X-ray photoelectron spectrum (XPS) images showed that Pt consisting in BST thin films was the metallic state, and the Auger electron spectroscopy (AES) analysed the Pt concentration in different depth profiles of BST thin films. The result displayed that the Pt diffusion in BST thin film is divided into two regions: near the BST/Pt interface, the diffusion type was volume diffusion, and far from the interface correspondingly, the diffusion type became grain boundary diffusion. In this paper, the previous researcher’s result was used to verify our conclusion.     

Magnetoelectric Bi3.25Nd0.75Ti3O12–La0.6Ca0.4MnO3 composite thin films derived by SOL–GEL method

Magnetoelectric (ME) Bi_3.25Nd_0.75Ti₃O₁₂–La_0.6Ca_0.4MnO₃ (BNT–LCMO) composite thin films were deposited on Pt/Ti/SiO₂/Si(100) substrates by a simple SOL–GEL method and spin-coating process with two different deposition sequences: BNT/LCMO/Pt/Ti/SiO₂/Si(BLP) and LCMO/BNT/Pt/Ti/SiO₂/Si(LBP). Our results show the composite thin films exhibit both good ferroelectric and magnetic properties, as well as a ME effect. BLP thin films have larger maximum ME voltage coefficient values than LBP structured thin films. The deposition sequence has a notable effect on the ferroelectric and magnetic properties and ME coupling behavior of the bi-layer thin films.     

Enhanced dielectric properties of LSCO-buffered Ba(Sn<Subscript>0.05</Subscript>Ti<Subscript>0.95</Subscript>)O<Subscript>3</Subscript> thin film prepared by sol–gel processing

Ferroelectric Ba(Sn_0.05Ti_0.95)O₃ (BTS) thin films were deposited onto Pt/Ti/SiO₂/Si substrates by sol–gel technique with a 100 nm thick LSCO buffer layer. The influence of buffer layer on the phase and microstructure of the thin films was examined. Dielectric properties of the thin films were investigated as a function of frequency and direct current (DC) electric field. The results show that the LSCO buffer layer had a marked effect on the dielectric properties of the BTS films. The BTS thin films with LSCO buffer layer had enhanced dielectric properties.     

Chemical solution processing and characterization of Ba(Zr,Ti)O<Subscript>3</Subscript>/LaNiO<Subscript>3</Subscript> layered thin films

Ba(Zr,Ti)O₃/LaNiO₃ layered thin films have been synthesized by chemical solution deposition (CSD) using metal-organic precursor solutions. Ba(Zr,Ti)O₃ thin films with smooth surface morphology and excellent dielectric properties were prepared on Pt/TiO _x /SiO₂/Si substrates by controlling the Zr/Ti ratios in Ba(Zr,Ti)O₃. Chemically derived LaNiO₃ thin films crystallized into the perovskite single phase and their conductivity was sufficiently high as a thin-film electrode. Ba(Zr,Ti)O₃/LaNiO₃ layered thin films of single phase perovskite were fabricated on SiO₂/Si and fused silica substrates. The dielectric constant of a Ba(Zr_0.2Ti_0.8)O₃ thin film prepared at 700°C on a LaNiO₃/fused silica substrate was found to be approximately 830 with a dielectric loss of 5% at 1 kHz and room temperature. Although the Ba(Zr_0.2Ti_0.8)O₃ thin film on the LaNiO₃/fused silica substrate showed a smaller dielectric constant than the Ba(Zr_0.2Ti_0.8)O₃ thin film on Pt/TiO _x /SiO₂/Si, small temperature dependence of dielectric constant was achieved over a wide temperature range. Furthermore, the fabrication of the Ba(Zr,Ti)O₃/LaNiO₃ films in alternate thin layers similar to a multilayer capacitor structure was performed by the same solution deposition process.     

Optimization of Graphene Layers Grown on Pt/Ti/SiO2 by Hot Filament Chemical Vapor Deposition

Large area single and bilayer graphene are grown on Pt/Ti/SiO₂ substrates by hot filament chemical vapor deposition (HFCVD) with and without the assistance of Cu foil. The quality and number of graphene layers deposited on the substrate are assessed by Raman Spectroscopy. Atomic Force Microscopy (AFM) is used for assessing the surface topography of the graphene films grown on the Pt/Ti/SiO₂ substrates. The microstructure and elemental analyses are performed by Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). The results show that bilayer graphene growth is facilitated by a copper foil placed nearby Pt/Ti/SiO2 substrate and by a high filament temperature in the HFCVD reactor. Monolayer graphene grows only when no copper foil is placed near the Pt/Ti/SiO₂ substrate at a low filament temperature. The approach paves a novel pathway towards the layer-controlled growth of graphene on Pt/Ti/SiO₂ substrates by HFCVD for frontier applications.     

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.     

Fluorescence biosensing micropatterned surfaces based on immobilized human acetylcholinesterase

Human acetylcholinesterase (AChE) is a widely studied target enzyme in drug discovery for Alzheimer’s disease (AD). In this paper we report evaluation of the optimum structure and chemistry of the supporting material for a new AChE-based fluorescence sensing surface. To achieve this objective, multilayered silicon wafers with spatially controlled geometry and chemical diversity were fabricated. Specifically, silicon wafers with silicon oxide patterns (SiO₂/Si wafers), platinum-coated silicon wafers with SiO₂ patterns (SiO₂/Pt/Ti/Si wafers), and Pt-coated wafers coated with different thicknesses of TiO₂ and SiO₂ (SiO₂/TiO₂/Pt/Ti/Si wafers) were labelled with the fluorescent conjugation agent HiLyte Fluor 555. Selection of a suitable material and the optimum pattern thickness required to maximize the fluorescence signal and maintain chemical stability was performed by confocal laser-scanning microscopy (CLSM). Results showed that the highest signal-to-background ratio was always obtained on wafers with 100 nm thick SiO₂ features. Hence, these wafers were selected for covalent binding of human AChE. Batch-wise kinetic studies revealed that enzyme activity was retained after immobilization. Combined use of atomic-force microscopy and CLSM revealed that AChE was homogeneously and selectively distributed on the SiO₂ microstructures at a suitable distance from the reflective surface. In the optimum design, efficient fluorescence emission was obtained from the AChE-based biosensing surface after labelling with propidium, a selective fluorescent probe of the peripheral binding site of AChE.

The (100) orientation evolution and temperature-dependent electrical properties of Bi(Zn1/2Ti1/2)O3–PbTiO3 ferroelectric films

The 0.2Bi(Zn_1/2Ti_1/2)O₃–0.8PbTiO₃ (0.2BZT–0.8PT) ferroelectric thin film was successfully fabricated on Pt(111)/Ti/SiO₂/Si substrates by a sol–gel method. The result indicates that the film exhibits the (100) preferred orientation and has a relatively dense and uniform microstructure with a thickness of ~230 nm. The formation mechanism of the oriented films was ascribed to the growth of the (100) oriented PbO layer at ~450 °C during a layer-by-layer crystallization process. Temperature-dependent electrical properties of the 0.2BZT–0.8PT films were investigated, showing that the film has a potential for high temperature applications.     

Large electric polarization in BiFeO3 film prepared via a simple sol–gel process

BiFeO₃ film has been deposited on Pt/Ti/SiO₂/Si substrate by a simple sol–gel process annealed at 500 °C. X-ray diffraction analysis revealed that the film was fully crystallized and no impure phase was observed. Cross-section scanning electric microscopy results indicated that the thickness was about 600 nm. Large remanent polarization was observed. The double remanent polarization is 95.8 μC/cm² at an applied field of 800 kV/cm. Intense dielectric dispersion was observed above 100 kHz. At a biased electric field of 167 kV/cm, the leakage current densities were identified as ∼10⁻⁵ to 10⁻⁴ A/cm².     

Influence of YSZ buffer layer on the electric properties of compositionally graded (Ba,Sr)TiO<Subscript>3</Subscript> thin film

Compositionally graded Ba_1−x Sr _x TiO₃ (BST) (0 ≤ x ≤ 0.4) thin films were fabricated on Pt/Ti/SiO₂/Si and YSZ/Pt/Ti/SiO₂/Si substrates by a modified sol–gel technique. The YSZ buffer layer was prepared by RF magnetron sputtering. The microstructure of the graded BST films was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM). The results showed that all the films have uniform and crack-free surface with a perovskite structure. The graded BST film with an YSZ buffer layer has larger dielectric constant and lower dielectric loss. The leakage current density of the graded BST film with an YSZ buffer layer lowers two orders than the film without buffer layer. The improved electric properties of the graded films with an YSZ buffer layer was attributed to the YSZ buffer layer act as an excellent seeding layer to enhance the graded BST film growth.     

Stoichiography in the study of spatial heterogeneity of the chemical composition and structure of thin films and nanostructured systems

We present the principles of stoichiography and a reference-free stoichiographic differential (separating) dissolution method used to study the composition and structure of thin films and nanostructured systems: HTS films with 123 different compositions, Al–Au–Sn–Co–Mn, Si/SiO₂/Ni(Cr)–Cu–Cu2S, Cr–Cu–S, and Cu–S multilayer films, Bi–Ti–O films on Ru/SiO₂/Si, Mn_1–xZn _x S, and ZnS–EuS supports, and also nanostructured manganese ferrite in borate glass matrices, nanodisperse composite sorbents and the Co–Si–Pt–O/Al₂O₃ catalyst modified by Pt nanoparticles, and oxide catalyst precursor Fe₂Co/Al₂O₃ for the synthesis of carbon nanotubes.     

Textured PLCT Thin Films Prepared on Pt/Ti/SiO2/Si Substrate by Metal-Organic Decomposition

Textured calcium modified (Pb,La)TiO₃ (PLCT) films were deposited on Pt/Ti/SiO₂/Si substrates by using a metal-organic decomposition (MOD) process. The PLCT films exhibit good ferroelectric properties, a very low leakage current and a sharp PLCT/Pt interface. The (100) texture of the PLCT film is growth-controlled; the (100) oriented grains grow preferentially so as to minimize the surface energy. Particularly, the (100) preferred orientation is easy to form in the PLCT film with a layered structure for which the substrate almost does not affect the nucleation and growth of the film.     

Effect of BiFeO3 doping on ferroelectric properties of Na0.5Bi0.5TiO3–BaTiO3 based thin film derived by sol–gel method

0.94Na_0.5Bi_0.5TiO₃–0.06BaTiO₃ (NBT–6BT) and (0.94 ? x)Na_0.5Bi_0.5TiO₃–0.06BaTiO₃–xBiFeO₃ (NBT–6BT–xBFO, x = 0.03, 0.05 and 0.08) thin films were deposited on Pt/Ti/SiO₂/Si substrates by a sol–gel process. Relative permittivity and remnant polarization were maximized at 5 % BFO substitution. Compared with 0.94NBT–0.06BT, the leakage current density of 0.89NBT–0.06BT–0.05BFO at 600 kV/cm is reduced by one order of magnitude. Enhanced ferroelectricity was also achieved in 0.89NBT–0.06BT–0.05BFO, the remnant polarization (2P _r) values of 0.89NBT–0.06BT–0.05BFO and 0.94NBT–0.06BT are 46 and 24 µC/cm².     

Comparison of the influence of titanium and chromium adhesion layers on the properties of sol–gel derived NKN thin films

Lead-free (Na_0.5K_0.5)NbO₃ (NKN) thin films were prepared on Pt/X/SiO₂/Si substrates (with the adhesion promoters X = Ti, Cr) by a sol–gel process with and without post-annealing treatment. The effect of the diffusion of the adhesion layer elements Ti and Cr into the NKN film was analysed by secondary ion mass spectrometry, scanning electron microscopy pictures, X-ray diffraction (XRD), and leakage current measurements. It turned out that Cr diffuses into the films to a higher extent than Ti. The high amount of Cr diffusion led to the formation of a secondary phase, as seen in the XRD pattern, and to pore formation on the surface of the NKN films. In contrast, the films with Ti adhesion layer were single phase NKN without pore formation. Also, the leakage current measurements showed a strong influence of the Cr diffusion. The leakage current of the films with Cr adhesion layer was about four orders of magnitude higher than that of the films with Ti adhesion layer. The study shows the strong influence of the adhesion layer of the substrate on the properties of NKN films.     

Synthesis of Free-Standing Crystalline Barium Titanate Films at Vapor/Liquid or Liquid/Liquid Interface

Synthesis of free-standing crystalline barium titanate (BaTiO₃) films at vapor/liquid or liquid/liquid interface at room temperature has been investigated. High concentration Ba, Ti alkoxides precursor solution (1.2 mol/l) or pre-hydrolyzed precursor solutions by water vapor in a H₂O/Ba molar ratio of 0 to 6 were used as dropping solutions at the interfaces. Transparent gel films were formed when partially hydrolyzed precursors (H₂O/Ba = 2 to 3) were spread out on a N₂/liquid paraffin interface. The films shrank from syneresis and vaporization of the solvent during aging at room temperature. As a result, free-standing transparent films with a thickness of around 1 m and little stoichiometric deviation were obtained by separation from the liquid surface and rinse by hexane. The films consisted of crystalline BaTiO₃ particles of less than a few nanometers. Nanostructured dense BaTiO₃ free-standing films with a grain size less than 100 nm were obtained at 1030°C.     

Silica veils-conductive diamond powder composite as a new propitious substrate for platinum electrocatalysts

An innovative composite was obtained by a straightforward sol-gel procedure, involving boron-doped diamond powder (BDDP) incorporation into a SiO₂ veil (SiO₂V) matrix. Composite-coated glassy carbon plates were used as substrate for Pt electrochemical deposition, and the electrodes thus obtained (Pt/BDDP–SiO₂V) were compared on a relative basis with those prepared in the absence of the silica matrix (Pt/BDDP). SEM measurements have shown that a BDDP substrate promotes Pt cluster formation, whereas on BDDP–SiO₂V, particles are much smaller (ca. 45 nm to ca. 140 nm). The activity for CH₃OH oxidation was checked by cyclic voltammetry, and it was found that at Pt/BDDP–SiO₂V, the main anodic peak is shifted with ca. 0.35 V toward lower potentials, indicating a considerable improvement in the overall process kinetics. Stripping experiments together with long-term polarization measurements demonstrated that when deposited on the BDDP–SiO₂V support, Pt particles are less susceptible to CO poisoning and this behavior was tentatively ascribed to the presence of a higher relative surface concentration of more stable, oxidized platinum species, as evidenced by XPS.