Preparation and optical properties of a Rhodamine 6G-doped sol-gel AlPO4-SiO2 film

AlPO4-SiO2 films doped with Rhodamine 6G （Rh6G） are prepared using the sol-gel dip-coating method. The surface morphology is characterized by atomic force microscopy. The results indicate that the surface morphology of the films is not significantly affected by the amount of dyes loaded. The absorption and excitation spectra indicate low aggregation even at a Rh6G doping concentration of 1.0×10-3 mol/L. Efficient fluorescence with a band centered at 553 nm is observed.     

Fluorescent monodisperse spherical particles based on mesoporous silica containing rhodamine 6G

Fluorescent monodisperse spherical silica (SiO₂) particles with a regular mesoporous structure containing encapsulated Rhodamine 6G (R6G) dye have been synthesized. The as-synthesized particles have been coated with SiO₂ and SiO₂-CTAB (cetyltrimethylammonium bromide, C₁₆H₃₃N(CH₃)₃Br) shells in order to prevent uncontrolled release of the dye from pores. The kinetics of R6G release from the pores of silica particles has been studied. It has been found that the particles synthesized by adding CTAB and R6G to the reaction mixture, as well as the particles coated with the SiO₂-CTAB shell, are characterized by the maximum duration of dye release from the pores, which is probably associated with the formation of chemical bonds between R6G and CTAB molecules.     

Superhydrophobic silica films by sol–gel co-precursor method

Wetting phenomena of water droplets on solid are of crucial concern in our daily life as well as in engineering and science. The present paper describes the room temperature synthesis of superhydrophobic silica films on glass substrates using trimethylethoxysilane (TMES) as a co-precursor. The coating sol was prepared by keeping the molar ratio of tetraethoxysilane (TEOS) precursor, methanol (MeOH) solvent, water (H₂O) constant at 1:38.6:8.68, respectively, with 2 M NH₄OH throughout the experiments and the TMES/TEOS molar ratio (M) was varied from 0 to 1.1. It was found that with an increase in M value, the hydrophobicity of the films increased, however the optical transmission decreased from 88% to 82% in the visible range. The hydrophobic silica films retained their hydrophobicity up to a temperature of 275 °C and above this temperature the films became superhydrophilic. The hydrophobic silica films were characterized by Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), Fourier Transform Infrared (FT-IR) spectroscopy, percentage of optical transmission, humidity test and static and dynamic contact angle measurements.     

Low-n mesoporous silica films: structure and properties

The properties and structure of mesoporous silica films are investigated. Because of their extremely low refractive index (n=1.14), these films are interesting optical waveguide supports. The films have been synthesized by a template-modified sol–gel process using the triblock copolymer Pluronic P123. A significant dependence of the formed structure on the processing conditions has been revealed, allowing an appreciable structure tuning. One set of processing conditions allows the reproducible synthesis of low-n films. They are optically clear, mechanically and chemically resistant, extremely smooth, and sufficiently thick (1 m). Under other processing conditions a novel mesoporous layer structure was synthesized that has very large and well-defined nanoscopic voids. PACS  61.30.Pa; 68.37.Ps; 78.20.Ci     

Optical properties of Fe-doped silica films on Si

Optical properties of Fe-doped silica films on Si were investigated by ellipsometric technique in the region 1-5 eV. Samples were produced by sol-gel method. Precursors were prepared by mixing tetraethoxysilane (TEOS) solution in ethanol and water with aqueous solution of Fe-chloride or Fe-acetate. The coating solution was deposited on Si substrates by spin on technique. The size of Fe-containing nanometric-sized particles depended on technology and varied from 20 to 100 nm. Optical response of complex hybrid samples SiO₂:Fe/Si was interpreted in a multi-layer model. In the inverse problem, the Maxwell equations were solved by transfer matrix technique. Dielectric function of Fe-doped silica layers was calculated in the model of effective media. Analysis of optical data has shown that various Fe-oxides formed. Experimental data for films obtained from precursors with Fe-acetate and annealed in hydrogen were well described by the model calculations taking into account a small contribution 1-5% of metal Fe imbedded in silica. The Fe/Fe-O contribution to optical response increased for samples grown from FeCl₃-precursor. Ellipsometric data for Fe-doped silica films on Si were interpreted taking into account the structural AFM studies as well as the results of magnetic measurements.     

Preparation and optical properties of UV dye DMT-doped silica films

UV dye DMT-doped silica films were prepared by sol-gel process with dioxane as the cosolvent. Luminescence quenching was not observed even if the DMT concentration in the visibly transparent silica film is as high as 1.26×10⁻² M. The emission peaks centered at ∼375 nm for the films show red-shift of 35-45 nm, comparing with that of the cyclohexane solution of DMT. The fluorescence quantum efficiency of the films centered at ∼590 nm are larger than those at ∼375 nm.     

Improvement of electrical properties of low dielectric constant nanoporous silica films prepared using sol–gel method with catalyst HF

Microstructures and electrical properties of low dielectric constant (low-k) nanoporous silica films, prepared by sol–gel method using hydrofluoric acid (HF) replacing hydrochloric acid (HCl) as catalyst, have been investigated. It is found that the incorporation of HF effectively adjusts the hydrolysis and condensation of the tetraethoxylane based silica sols and makes the surface modification more sufficiently, leading to the increase of the porosity and the change in chemical bonds, and thus significantly improves the electrical properties of the films. The k value of 1.5 is reached in HF catalyzed films, which is much lower than that in HCl ones. The leakage current density are reduced to the lowest value of 6.12×10^-9 A/cm² in HF catalyzed films. It is determined that the Schottky emission occurs in HF catalyzed films, and both Schottky and Poole–Frenkel emissions occur in HCl ones, which may be due to the lower effective oxide charge density near the nanoporous silica and Si interfaces in HF catalyzed films than in HCl ones. PACS 72.20.Jv; 68.55.Jk; 61.43.Gt     

Structural and energetic heterogeneities of pyrocarbon/silica gel systems and their adsorption properties

Two series of pyrocarbon/silica gel (CS) samples were prepared using pyrolysis of different amounts of glucose adsorbed onto silica gel under dynamic conditions in a rotary reactor and a high-pressure autoclave. The structural characteristics and the morphology of the CS samples studied by means of the AFM, TG, nitrogen and p-nitrophenol (PNP) adsorption and quantum chemical methods depend not only on the amounts of pyrocarbon deposits but also on the synthesis methods, since the autoclave process gives a stronger effect of the pore wall hydrolysis by water formed on the carbonization of glucose. A displacement of the main peak of the pore size distribution of CS samples depends on pyrocarbon content. CS samples prepared in the autoclave possess larger microporosity and adsorb greater amounts of PNP in comparison with the samples synthesized in the rotary reactor.     

Control on wetting properties of spin-deposited silica films by surface silylation method

Control on the wettability of solid materials by liquid is a classical and key issue in surface engineering. Optically transparent water-repellent silica films have been spin-deposited on glass substrates at room temperature (∼27 °C). The wetting behavior of silica films was controlled by surface silylation method using dimethylchlorosilane (DMCS) as a silylating reagent. A coating sol was prepared by keeping the molar ratio of methyltrimethoxysilane (MTMS) precursor, methanol (MeOH) solvent, water (H₂O) constant at 1:8.8:2.64 respectively, with 4 M NH₄OH as a catalyst throughout the experiments and the amount of DMCS in hexane was varied from 0 to 12 vol.%. It was found that with an increase in vol.% of DMCS, the water contact angle values of the films increased from 78° to 136°. At 12 vol.% of DMCS, the film shows static water contact angle as high as 136° and water sliding angle as low as 18°. The hydrophobic silica films retained their water repellency up to a temperature 295 °C and above this temperature the films show superhydrophilic behavior. These results are compared with our earlier research work done on silylation of silica surface using hexamethyldisilazane (HMDZ) and trimethylchlorosilane (TMCS). The hydrophobic silica films were characterized by taking into consideration the Fourier transform infrared (FT-IR) spectroscopy, thermo gravimetric-differential thermal (TG-DT) analyses, scanning electron microscopy (SEM), atomic force microscopy (AFM), % of optical transmission, thermal and chemical aging tests, humidity tests, static and dynamic water contact angle measurements.     

Effect of polyvinylpyrrolidone on the structure and laser damage resistance of sol–gel silica anti-reflective films

The effect of polyvinylpyrrolidone (PVP) on the structure and laser-induced damage threshold (LIDT) of sol–gel silica anti-reflective films is investigated. The results of dynamic light scattering, transmission electron microscopy, and small angle X-ray scattering, show that the PVP molecules surrounded the silica sol particles through the strong hydrogen bonds between Si-OH groups and the PVP. As a result, the growth of silica particles was restricted and thus the interface layer between the silica particles and the solvent become thickened with PVP content. Furthermore, the PVP reduced the porosity of the film, so the anti-reflection properties of the film were weakened. A multi-fractal analysis showed that the appropriate addition of PVP, 1 weight percent (wt%), could improve the surface fractal structure of the film, but that higher PVP content resulted in reduced surface uniformity. The addition of PVP lead to improved LIDT.