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.     

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.     

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.     

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.     

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.     

Optical properties of sol–gel prepared Cerium doped Lutetium and Yttrium oxyorthosilicates

The optical properties of sol–gel prepared Cerium doped Lutetium and Yttrium oxyorthosilicates are investigated in the vacuum ultraviolet energy range by means of synchrotron radiation. The excitation and emission properties are compared to commercial samples grown by Czochralski method. The sol gel polycrystals do show emission features comparable to the ones of the monocrystals but with a slightly smaller decay time. Preliminary radioluminescence measurements indicate the possibility to apply the sol gel synthesized polycrystals as scintillating materials in the low X-ray energy range.     

Electrical conductivity and photoconductivity studies of TiO2 sol–gel thin films and the effect of N-doping

Several sol–gel TiO₂ thin films, N-doped and undoped, were prepared using the dip-coating technique. Dark conductivity in vacuum and in air gives information on the role of the adsorbed oxygen or water and the influence of nitrogen. Transient photoconductivity in vacuum and in air was studied with illumination time for various light intensities and found to be very sensitive to the environment. The rise and decay are much faster in air than in vacuum, suggesting the predominant mechanism of conduction in each case. The role of the presence of nitrogen is also discussed.     

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.     

Synthesis and characterization of sol–gel derived highly condensed fluorinated methacryl silica and silica–zirconia hybrid materials

Sol–gel reaction of 3-trimethoxysilylpropyl methacrylate (MPTS), heptadecafluorodecyltrimethoxysilane (PFAS), and zirconium n-propoxide (ZPO) chelated with methacrylic acid (MAA) was investigated depending on the catalyst concentration, the reaction time and the zirconium alkoxide addition for the fabrication of highly condensed fluorinated methacryl silica and silica–zirconia hybrid material (MF and MFZ hybrimer). Controlling the catalyst concentration and the reaction time has been found to be important parameters for the strengthening of the silica networks in MF hybrimer. The addition of zirconium alkoxide enhanced the condensation degree in short reaction time, which could efficiently fabricate the highly condensed MFZ hybrimer. In particular, their molecular sizes have been found to be highly dependent on the extent of sol–gel reaction and were experimentally calculated.     

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.     

Thio sol–gel synthesis of titanium disulfide thin films and powders using titanium alkoxide precursors

Titanium alkoxides react at room temperature with H₂S to form an amorphous titanium alkoxy-sulfide precursor which can be converted to TiS₂ by heat-treatment in a flowing H₂S gas stream. The reaction of titanium isopropoxide (Ti(OPrⁱ)₄) with H₂S in n-butylamine solvent has been studied using infrared spectroscopy (FTIR), gas chromatography/mass spectroscopy (GC/MS), XRD and EDAX measurements. Based on these studies, it is shown that a partially sulfidized alkoxide precursor forms through the partial replacement of some alkoxy groups by hydrosulfide moieties. The alkoxy-hydrosulfide is believed to form following a thiolysis–condensation mechanism similar to the hydrolysis–condensation process that occurs during the oxide sol–gel reaction. The alkoxy-hydrosulfide species then undergoes complete sulfidization at 800 °C in a stream of H₂S to yield pure, hexagonal TiS₂ in either film or powder forms.     

Effects of post-annealing on electrical properties of amorphous Ga-doped Zn–Sn–O semiconductor films

We investigated the effect of post-annealing on the electrical properties of amorphous gallium-zinc-tin oxide (a-GZTO) films with different Ga contents. The films were deposited at room temperature by sputtering and annealed in air for 1 h. It was found that the doping with Ga, which acts as the carrier suppressor, contributes to the thermal stability of characteristic properties of a-GZTO thin films. The film with a small amount of Ga showed significant variations in carrier concentration according to the annealing temperature. Increases in carrier concentration and mobility can be ascribed to the reduction of subgap density of states by annealing. After annealing at 400 °C, however, the enrichment of Zn cations in surface region resulted in considerable changes in chemical bonding states and consequently, the carrier concentration decreased by two orders of magnitude for the low Ga-doped ZTO film.     

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.     

Structure and properties of self-assembled fluorocarbon–silica nanocomposites

The formation of fluorocarbon–silica nanocomposites by the self-assembly of a fluorinated surfactant and aminoalkoxysilane coupling agents was studied by X-ray diffraction, electron microscopy, NMR spectroscopy and thermal analysis. The prepared materials posses a lamellar nanostructure consisting of non-crystalline fluorinated and condensed silica layers, the latter being very thin. The prepared materials show interesting properties for applications, such as hydrophobicity, thermal stability, high content of aminopropyl groups and low dielectric constant (≈2.8), which is almost independent on frequency. Moreover, the dielectric response can be interpreted in the framework of the Maxwell–Wagner model.     

Periodic alignment of sol–gel derived,monodisperse phenylsilsesquioxane particles on a pregrooved substrate

Preparation and alignment of monodisperse inorganic–organic hybrid particles have become one of the most attractive research topics in recent years. In this study, phenylsilsesquioxane (PhSiO_3/2) particles were synthesized from phenyltriethoxysilane (PhSi(OEt)₃), ethanol (EtOH), hydrochloric acid for hydrolysis and ammonia water for polycondensation via sol–gel process. Spherical PhSiO_3/2 particles of 0.1–5.0 μm in diameter were obtained by varying the composition of the sols. The particle size distribution decreased with increasing the amounts of catalyst-containing H₂O and EtOH, and increasing the concentrations of catalysts. Monodisperse particles obtained were aligned on a pregrooved glass substrate using an ascending liquid flow and an attractive capillary force between the particles.     

Morphological and mechanical properties of hybrid matrices of polysiloxane–polyvinyl alcohol prepared by sol–gel technique and their potential for immobilizing enzyme

Hybrid matrices of polysiloxane–polyvinyl alcohol (POS–PVA) were prepared by sol–gel technique using different concentrations of the organic component (polyvinyl alcohol, PVA) in the synthesis medium. The goal was to prepare carriers for immobilizing enzyme by taking into consideration properties as hardness, mean pore diameter, specific surface area and pore size distribution. The matrices were activated with sodium metaperiodate to render functional groups for binding the lipase from Candida rugosa, used here as a study model. Results showed that low proportion of PVA gave POS–PVA with low surface area and pore volume, although with higher hardness. The chemical activation decreased the pore volume and increased the pore size with a decrease on the surface area of about 60–75%. The matrices for enzyme immobilization were chosen considering the best combination of high surface area and hardness. Thus, the POS–PVA prepared with 5.56 × 10^?5 M of PVA with a surface area of 123 m²/g and hardness of 71 HV (50 gf 30 s) was shown to be suitable to immobilize the lipase, with an immobilization yield of about 40%.     

Silica xerogel films hybridized with carbon nanotubes by single step sol–gel processing

Synthesis of multi-walled carbon nanotubes (MWCNTs) doped silica xerogel films was reported in this work. A crucial step of introducing MWCNTs was achieved by functionalizing them by acid treatment to form stable and homogenous SiO₂/MWCNTs sol. Scanning electron microscopy showed spherical particles in honeycomb network structure for undoped xerogel films whereas dispersion and wrapping of MWCNTs in silica matrix was observed for MWCNTs doped films. Various bond formations during the sol–gel process and surface modification were confirmed using Fourier transform infra-red and detailed study on the chemical bonding state of the films was carried out using X-ray photoelectron spectroscopy. Nanoindentation studies showed that the mechanical properties of MWCNTs doped xerogel film increase dramatically: higher modulus (E = 2.127 ± 0.095 GPa) and hardness (H = 0.035 ± 0.017 GPa) values than those of pristine xerogel film (E = 0.234 ± 0.058 GPa, H = 0.01 ± 0.003 GPa).     

Influence of Fe/Ba ratio,basic agent and calcination temperature on the physical properties of nanocrystalline barium hexaferrite thin films prepared by a sol–gel method

In this work nanocrystalline BaFe₁₂O₁₉ thin films have been prepared on the Si (1 1 0) substrates by a sol–gel method using the aqueous solution of metal nitrates. The efforts have been done to decrease the calcination temperature and to reduce the crystallite size of the single-phase barium ferrite thin films. The precursor solutions were primed with the various Fe/Ba ratios and two kinds of the basic agents, and the coated films were heat treated at the different temperatures. The thin films were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM) techniques. The effects of calcination temperature, molar ratio of Fe/Ba and basic agent on composition, crystallites size and morphology were also investigated.