AES and FTIR characterization of sol–gel alumina films

In this work we obtained sol–gel alumina coatings on AISI 304 stainless steel substrates. Alumina sols were prepared by using aluminum isopropoxide (AI) as precursor, acetic acid (HOAc) as catalyst, ethanol (C₂H₅OH) or isopropanol (C₃H₈O) as solvent, and water. The as-prepared solutions were deposited on stainless steel substrates by means of the dip-coating technique. The obtained composites were characterized by Fourier transform infrared spectroscopy (FTIR) and Auger electron spectroscopy (AES). We observed that the concentration of AlO type bonds in the obtained alumina coatings depends on the solvent type used, temperature and peptization state of the sol, withdrawal speed, and number of dipping cycles. AES experiments showed that the interface formed between the alumina coating and substrate surface is in general formed by several layers of different chemical compositions.     

XAFS investigations of nitrided NbN–SiO2 sol–gel derived films

This work presents the results of the structural analysis of xNbN–(100-x)SiO₂ (x = 100, 80, 60 mol%) thin films by X-ray absorption spectroscopy (XAS). To prepare the films, thermal nitridation of sol–gel derived coatings have been performed. The resulting films have a granular structure with NbN grains distributed in the SiO₂ matrix. The size of the grains depends on the NbN/SiO₂ molar ratio. A detailed X-ray absorption fine structure (XAFS) data analysis shows that in all the samples both nitrogen and oxygen atoms are present as nearest neighbours of Nb. The intra-granular phase is an ordered NbN phase, whereas the shells around the grains are formed mainly by an oxide phase and, possibly, by other niobium nitride phases (probably with low nitrogen content). Two possible origins of the inter-granular oxide phase were considered: incomplete nitridation of Nb₂O₅ and addition of SiO₂. Both of them are connected with the sample preparation method. The obtained XAS results allowed us to correlate the thickness and stoichiometry of the films under study with the electronic structure of the Nb ions and with the local geometric structure in their environment.     

Structure and electrical properties of nitrided NbN–TiN sol–gel derived films

The results of investigations of electrical conductivity and the structure of NbN–TiN thin films in a different NbN/TiN molar ratio are presented in this work. Sol–gel derived xNb₂O₅?(100?x)TiO₂ coatings (where x = 100, 90, 80, 70, 60, 50, 40, 0 mol%) were nitrided at 1200 °C to obtain NbN–TiN films. The structural transformations occurring in the films as a result of ammonolysis were studied using X-ray diffraction (XRD), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The electrical conductivity was measured with a conventional four-terminal method in the temperature range of 5–280 K. The NbN–TiN samples exhibited a negative temperature coefficient of resistivity. The positive temperature coefficient of resistivity was observed only for the x = 0 sample. The results of conductivity versus temperature may be described on the grounds of a model proposed for a weakly disordered system. The film thickness effect on the superconducting properties was studied for x = 80 and x = 100 samples. The superconducting transition was not observed in all samples, the exception was x = 80 sample, 1050 nm in thickness. It is not clear, why all x = 100 samples do not exhibit superconducting transition in resistivity measurements. It seems to be possible, that the Josephson junction formation between NbN grains could be blocked by non-superconducting phases present in these samples.     

Rare earth centers properties and electron trapping in SnO2 thin films produced by sol–gel route

Some very relevant optical, electrical, and structural properties of SnO₂ doped with rare-earth ions Er³⁺ and Eu³⁺ are presented. Films are produced by the sol–gel-dip coating process, and may be described as a combination of nanoscopic dimension crystallites (about 3–10 nm) with their respective intergrain potential barriers. The Er³⁺ and Eu³⁺ ions are expected to act as acceptors in SnO_2, which is a natural n-type conductor, inducing a high degree of charge compensation. Electron trapping and emission spectra data are presented and are rather distinct, depending on the location of the rare-earth impurity. This behavior allows the identification of two distinct centers: located either in the SnO₂ lattice or segregated at the particles surface. Based on a model for thermally activated cross-section defects, the difference between the capture energy of the photo-excited electron and the intergrain potential barrier is evaluated, leading to distinct values for high and low symmetry sites. A higher distortion in the lattice of undoped SnO₂ and SnO₂:Eu (1 at.%) was evaluated from Rietveld refinements of X-ray diffraction data. This was confirmed by Raman spectra, which are associated with the particles size and disorder. By comparing the samples with the same doping concentration, it was found that this disorder is higher in Eu-doped SnO₂ than in Er-doped SnO₂, which is in agreement with a higher energy for the lattice relaxation in the trapping process by Eu³⁺ centers.     

Silica Λ-shape wedge arrays fabrication and interspace tuning via hot embossing and sol–gel replication

New-type silica Λ-shape wedge arrays (SWAs) were fabricated by a combined process of template fabrication, hot embossing and sol–gel replication. Using silica needle arrays (SNAs) as template, the configuration of Λ-shape was transferred from SNAs to PMMA/PC molds by hot-pressing process. Then they were casted to form silica Λ-shape wedge arrays by sol–gel process. Using this procedure, high quality silica Λ-shape wedge arrays can be obtained with designed sizes and shapes over a large area. The taper height and interspace of SWAs can be tuned by the imprint depth during the hot-embossing process. This technology provides a cost-effective method to fabricate Λ-shape silica wedges. By changing the primal template, other patterned silica substrate can also be fabricated, such as grating, pillar and spherical structures. These silica based microstructure can find many applications in semiconductor devices, solar cells and optics etc.     

Optical characterization of 316L stainless steel coated with sol–gel titania

In this work 316L stainless steel substrates were coated with sol–gel derived films by means of the dip-coating technique. Titanium isopropoxide and ethanol were used as chemical precursor and solvent, respectively. The dip-coating step was performed using withdrawal speeds of 6 mm/min, 30 mm/min, and 60 mm/min. Next, the samples were heat treated in air for 30 min at 100 °C, 300 °C, and 400 °C. The processed composites were examined by FTIR and UV–vis spectroscopies. We observed that the materials prepared in this work can exhibit a variety of colors depending on the heat treatment temperature, withdrawal speed, and precursor:solvent molar ratio used in their processing. It is an important finding since this behavior could lead to architectural application of these materials. We believe that the changes observed in the UV–vis spectra and the colors of these samples could be related to the variation of the coating thickness as the processing conditions were modified. FTIR tests revealed that the ratio between the intensities of features ascribed to hydroxyl groups and TiO bonds decreased as the heat treatment temperature was increased. On the other hand, the ratio between bands related to TiOTi and TiO bonds decreased when the heating temperature was raised from 100 °C to 300 °C.     

Preparation and properties of europium-doped phosphosilicate glasses obtained by the sol–gel method

In this work, we report the synthesis of europium-doped phosphosilicate glasses from tetraethylorthosilicate (TEOS), phenyltrietoxysilane (PTES) and ammonium phosphate (NH₄H₂PO₄) prepared by the sol–gel process. The matrix was synthesized by modified Stöber methodology. The alkoxide precursors PTES and TEOS were mixed with NH₄H₂PO₄, in the presence of europium III chloride, using ethanol as solvent in basic catalysis. These materials were studied by photoluminescence spectroscopy (PL), thermal analysis (TGA/DTA), transmission electron microscopy (TEM) and energy-dispersive spectroscopy (EDS). The results obtained for the materials show the formation of conchoidal-fractures, which are characteristics of glass materials. The thermal analysis showed the thermal stability of materials up to 300 °C. Eu III has been used as structural probe due to its photophysical properties. The PL spectra displays the lines characteristics of the Eu (III) ion ⁵D₀ → ⁷F_J (J = 0, 1, 2, 3 and 4). Wide bands were observed, indicating non-homogeneous sites that are characteristic of amorphous systems.     

EXAFS study of the Er3+ ion coordination in SiO2–TiO2–HfO2 sol–gel films

The local order around ion-implanted Er³⁺ ions in SiO₂–TiO₂–HfO₂ thin films prepared by sol–gel, was studied by extended X-ray absorption fine structure at the Er-L_III edge. Both the first and second coordination shells of Er³⁺ were analyzed for different heat-treatments. While the first coordination shell always consisted of ～6–7 oxygen atoms at distances varying between 2.23 and 2.27 Å, the structure of the second shell was found to vary with the film composition and heat-treatment. Namely, whereas Si was found to be the only second neighbor of erbium in binary SiO₂–TiO₂ films, the addition of HfO₂ caused a preferential replacement of Si by Hf. The post-implantation thermal treatments also played a fundamental role in determining the final environment of the erbium ions.     

Synthesis and characterization of amorphous aluminosilicates prepared by sol–gel to encapsulate organic dyes

We synthesized via the sol–gel process three amorphous aluminosilicates having the stoichiometry of zeolite types A, X, and Y, respectively. The aluminosilicate, so-called SX, having the chemical composition of faujasite X, proved to be amorphous in the range 298 K up to 1273 K. At the higher temperature the sample begins to crystallize as nepheline. The anionic organic dye 1,2-dihydroxyanthraquinone (Alizarine, AL) was successfully encapsulated in the aluminosilicate matrix SX. Still, 7-α-D-glucopyranosil-9,10-dihydro-3,5,6,8-tetrahydroxy-7-methyl-9, 10 dioxanthracene carboxylic acid (carminic acid, CA) could only be deposited on the matrix surface. The adsorbents and the dye impregnated matrices were characterized using several techniques: Fourier Transform Infrared (FTIR) and Diffuse Reflectance Spectroscopies used to determine the hydroxylation as well as the adsorbed species, Small Angle X-ray Scattering (SAXS) to know the shape of the heterogeneities and the fractal dimension of each sample, nitrogen physisorption to measure the specific surface area and X-ray Diffraction (XRD) to identify the crystalline compounds present in the samples. Therefore, with these complementary techniques, the structure and the morphology of the samples were obtained.     

Correlation between the method of preparation and the properties of the sol–gel HfO2 thin films

This work describes the preparation of HfO₂ thin films by the sol–gel method, starting with different precursors such as hafnium ethoxide, hafnium 2,4-pentadionate and hafnium chloride. From the solution prepared as mentioned above, thin films on silicon wafer substrates have been realized by ‘dip-coating’ with a pulling out speed of 5 cm min^?1. The films densification was achieved by thermal treatment for 10 min at 100 °C and 30 min at 450 °C or 600 °C, with a heating rate of 1 °C min^?1. The structural and optical properties of the films are determined employing spectroellipsometric (SE) measurements in the visible range (0.4–0.7 μm), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The main objective of this paper was to establish a correlation between the method of preparation (precursor, annealing temperature) and the properties of the obtained films. The samples prepared from pentadionate and ethoxide precursors are homogenous and uniform in thickness. The samples prepared starting from chloride precursor are thicker and proved to be less uniform in thickness. Higher non-uniformity develops in multi-deposition films or in crystallized films. A nano-porosity is present in the quasi-amorphous films as well in the crystallized one. For the samples deposited on silicon wafer, the thermal treatment induced the formation of a SiO₂ layer at the coating–substrate interface.     

Electrochromic properties of nickel oxide and mixed Co/Ni oxide films prepared via sol–gel route

The Ni oxide and mixed Co/Ni oxide films were prepared by sol–gel dip coating method at optimum conditions. The XRD analysis reveals the pure and Co mixed nickel oxide films to be in amorphous state. The field emission SEM images reveal nanopore like structure for Ni oxide film and well defined grains with pores for Ni oxide films containing 5 wt.% of Co. Electrochromic properties have been studied using cyclic voltammetric (CV) and in situ spectro-electrochemical techniques. The pure and cobalt mixed (5 wt.%) Ni oxide films exhibit anodic/cathodic diffusion coefficient of 4.93 ± 0.14/3.74 ± 0.10 × 10^?10 cm²/s and 10.00 ± 0.24/7.60 ± 0.20 × 10^?10 cm²/s respectively after 300 cycles. The cobalt mixed (5 wt.%) Ni oxide films exhibit the bleached/coloured state transmission of 90.42/7.21% with a photopic constrast ratio of 12.54 and the colouration and bleaching time were 5.9 and 2.4 s respectively. The addition of cobalt beyond 5% leads to poor transparency and inhibited electrochromic switching character.     

Structural investigations of nitrided Nb2O5 and Nb2O5–SiO2 sol–gel derived films

Sol–gel derived xNb₂O₅–(100 ? x)SiO₂ films (where x = 100, 80, 60, 50, 40, 20, 0 mol%) were nitrided at various temperatures (800 °C, 900 °C, 1000 °C, 1100 °C and 1200 °C). The structural transformations occurring in the films as a result of ammonolysis were studied using X-ray diffraction (XRD), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The XRD results have shown that the temperatures below 1100 °C were too low to obtain a pure NbN phase in the samples. The AFM observations indicate that the formation of the NbN phase and the size of NbN grains are related to the silica content in the layer. NbN grains become more regular and larger as the niobium content increases. The maximum grain size of about 100 nm was observed for x = 100. Preparation of the Nb₂O₅–SiO₂ sol–gel derived layers and the subsequent nitridation is a promising method of inducing crystalline NbN in amorphous matrices. It follows from the XPS results that a small amount of Nb₂O₅ remains in the films after nitridation at 1200 °C and that nitrogen reacted not only with Nb₂O₅ but also with SiO₂.     

Preparation and characterization of ZnO particles embedded in organic–inorganic planar waveguide by sol–gel route

Hybrid organic–inorganic waveguides based on ZnO-(3-glycidoxypropil)trimethoxisilane (GPTS) have been fabricated by sol–gel route. A transparent sol of ZnO was added to the GPTS host and the resulting sol was deposited on silica substrates by spin coating technique. Waveguides with different molar composition (100?x)GPTS?xZnO (x = 10, 20, 30) were investigated by different diagnostic techniques. Morphological measurements were carried out by means of an AFM apparatus, and a roughness of few nanometers was estimated for all the waveguides. Optical properties such as refractive index, thickness, number of propagating modes and attenuation coefficient were measured at 632.8, 543.5, 1319 and 1542 nm by the prism coupling technique as a function of the ZnO content. Photoluminescence measurements, upon excitation at 325 nm, showed a large luminescence band in the region between 350 and 600 nm with a main peak centered at about 380 nm, due to the presence of ZnO nanoparticles.     

Effects of tantalum doping on microstructure and ferroelectric properties of Bi4Ti3O12 thin films prepared by a sol–gel method

Tantalum-substituted Bi₄Ti₃O₁₂ (Bi₄Ti_3-x/5Ta_x/5O₁₂, BTTO) thin films were fabricated on Pt(111)/Ti/SiO₂/Si(100) substrates by sol–gel technology. The effects of various processing parameters, including Ta content (x=0～0.08) and annealing temperature (500～800 °C), on the growth and properties of thin films were investigated. X-ray diffraction analysis shows that the BTTO thin films have a bismuth-layered perovskite structure with preferred (117) orientation. With the increase of Ta content, the grain size of film decreased slightly, and highly (117)-oriented BTTO films were obtained in the composition of x=0.06. Ta doping on the B-site of Bi₄Ti₃O₁₂ could induce the distortion of oxygen octahedral and decrease the oxygen vacancy concentration by a compensating effect. The highly (117)-oriented BTTO thin films with x=0.06 exhibits the maximum remanent polarization (2P_r) of 50 μC/cm² and a low coercive field (2E_c) of 104 kV/cm, fatigue free characteristics up to ≧ 10⁸ switching cycles.     

Transition from amorphous semiconductor to amorphous insulator in hydrogenated carbon–germanium films investigated by IR spectroscopy

Thin a-Ge_XC_1?X:H plasma polymerized films, depending on deposition conditions, can be produced in two very different structures, namely amorphous semiconductor and amorphous insulator. The transition from amorphous insulator to amorphous semiconductor is related to the formation of germanium nanoclusters due to ions bombarding the surface of the growing material. This paper concentrates on investigations of the transition by means of IR spectroscopy. To this end a quantitative analysis of IR spectra obtained for thin films deposited on silicon substrate has been described and used for estimation of hydrogen atom concentration and bonding in the investigated material. It was found that the probability that a given H atom is bonded to a germanium or to a carbon atom is almost the same. This conclusion is true both for a-S and a-I films. The average concentration of hydrogen in the investigated material was found to be about 2.4–3.4 × 10²² cm^?2 which means that there are two times more atoms of the carbon family than hydrogen atoms in the film structure.     

Superconducting properties of an NbN–SiO2 disordered system obtained by ammonolysis of NbN–SiO2 sol–gel derived coatings

Studies in superconducting properties of NbN–SiO₂ films are reported. The films were obtained through nitridation of sol–gel derived Nb₂O₅–SiO₂ coatings at 1200 °C, a process leading to the formation of disordered structures with NbN metallic grains dispersed in the insulating SiO₂ matrix. Electrical resistivity was measured with the conventional four-terminal method in the temperature range from 5 to 280 K. The samples’ superconducting properties, examined with magnetically modulated microwave absorption (MMMA), depend on the NbN/SiO₂ molar ratio and the film’s thickness.     

Microstructural characteristics and crystallization of CaO–P2O5–Na2O–ZnO glass ceramics prepared by sol–gel method

Porous phosphate-based glass ceramics prepared by the sol–gel method were characterized using Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and differential thermal analysis (DSC). The 48CaO–45P₂O₅–2ZnO–5Na₂O glassy system can remain fully amorphous up to 550 °C. After heat treated at 650 °C, the obtained porous bodies consisted of dense struts and macropores where β-Ca₂P₂O₇ and Na₂CaP₂O₇ phases crystallized from the glass matrix. When treated at 750 °C, Ca₄P₆O₁₉ and NaZn(PO₃)₃ precipitated homogeneously as new phases among the residual glass matrix. The material was assessed by soaking samples in phosphate-based buffer solution (PBS) solution to determine the solubility and observe apatite formation.