Elaboration and characterization of TiO2 nanoparticles incorporated in SiO2 host matrix

Nanometer-scale TiO₂ particles have been synthesized by sol-gel method. It was incorporated in a glass-based silica aerogel. The composite was characterized by various techniques such as particle size analysis, scanning electron microscopy (SEM), atomic force microscopy (AFM), transmission electron microscopy (TEM), X-ray diffraction (XRD), infrared spectroscopy (IR) and photoluminescence (PL). The bulk glass presents a strong luminescence at wavelengths ranging from 750 to 950 nm. This PL was attributed to various non-bridging oxygen hole centers (NBOHCs) defects resulting from thermal treatment and crystallization of TiO₂ at the interface between titania nanoparticles and silica host matrix.     

Synthesis and investigation of nonlinear optical properties of semiconductor ZnS nanoparticles

ZnS nanoparticles were prepared by a simple chemical method and using PVP (poly vinylpyrrolidone) as capping agent. The sample was characterized by UV-vis spectrophotometer, X-ray diffraction (XRD) and Z-scan technique. XRD pattern showed that the ZnS nanoparticles had zinc blende structure with an average size of about 2.18 nm. The value of band gap of these nanoparticles was measured to be 4.20 eV. The nonlinear optical properties of ZnS nanoparticles in aqueous solution were studied by Z-scan technique using CW He-Ne laser at 632.8 nm. The nonlinear absorption coefficient (β) was estimated to be as high as 3.2×10⁻³ cm/W and the nonlinear refractive index (n₂) was in order of 10⁻⁸ cm²/W. The sign of the nonlinear refractive index obtained negative that indicated this material exhibits self-defocusing optical nonlinearity.     

Novel epoxy-silicone thermolytic transparent packaging adhesives chemical modified by ZnO nanowires for HB LEDs

A novel high transparent thermolytic epoxy-silicone for high-brightness light-emitting diode (HB-LED) is introduced, which was synthesized by polymerization using silicone matrix via diglycidyl ether bisphenol-A epoxy resin (DGEBA) as reinforcing agent, and filling ZnO nanowires to modify thermal conductivity and control refractive index of the hybrid material. The interactions of ZnO nanowires with polymers are mediated by the ligands attached to the nanoparticles. Thus, the ligands markedly influence the properties of ZnO nanowires/epoxy-silicone composites. The refractive indices of the prepared hybrid adhesives can be tuned by the ZnO nanowires from 1.4711 to 1.5605. Light transmittance can be increased by 20% from 80 to 95%. The thermal conductivity of the transparent packaging adhesives is 0.89–0.90 W/mK.     

Structural and optical evaluation of WOxNy films deposited by reactive magnetron sputtering

Thin films of tungsten oxynitrides were deposited on substrates preheated at 300 °C from metallic tungsten target using reactive pulsed d.c. magnetron sputtering. The deposition was carried out at different nitrogen to total reactive gas partial pressures ratios. The energy dispersive analysis of X-ray showed that significant incorporation of nitrogen occurred only when the nitrogen partial pressure exceeded 74% of the total reactive gas pressure. X-ray diffraction analysis revealed that the formation of a specific crystalline phase is affected by the composition and the possibility of competitive growth of different phases. The increase of nitrogen content into the films increases the optical absorption and decreases the optical band gap. The refractive index was determined from the transmittance spectra using Swanepoel's method. It was found that the refractive index increases with increasing nitrogen content over the entire spectral range. The values of the tungsten effective coordination number, N_c, was estimated from the analysis of the dispersion of the refractive index, and an increase in N_c with increasing nitrogen content was observed.     

Accurate prediction of refractive index of inorganic oxides by chemical formula

A method is proposed for estimation of mean refractive index of a complex inorganic oxide whose chemical composition is known. This method is based on using mean refractive indices of simple oxides. Applicability of the method has been tested for a representative set of ∼120 known oxide crystals. It has been shown that mean refractive index can be predicted with possible error of 0-15% depending on the chemical nature of the compound. The majority of oxides show acceptable difference <5% between theoretical and experimental refractive index values and the mean error level for a set of 124 oxide crystals is as low as ∼5%. Refractive indices of a set of simple oxides are estimated by the Lorenz-Lorentz relation. Full table of refractive indices of simple oxides for all elements of Periodic Table is proposed.     

Optical properties and adhesion of air oxidized vacuum evaporated bismuth thin films

Highly adhesive bismuth oxide thin films on glass have been prepared by air oxidation of vacuum evaporated bismuth thin films at various temperatures. The transmittance, optical band gap, refractive index and adhesion show temperature and oxidation time effects. The films show a direct band gap between 2 and 2.5 eV. The refractive indices are in the range 1.854-1.991. The transmittances of the bismuth oxide films are quite high in a large wavelength range. These bismuth oxide films can have potential use in optical waveguides.     

Effect of annealing temperature on the optical constants of zinc oxide films

In this paper, the effect of annealing temperature on optical constants was studied. The ZnO films were deposited on microscopic glass substrates using the sol-gel method for various annealing temperatures. The deposited zinc oxide (ZnO) films were characterized by an X-ray diffractometer (XRD), a spectrophotometer and scanning electron microscopy (SEM). The transmittance spectra recorded through the spectrophotometer exhibits 90% transmittance. The XRD spectra showed polycrystalline nature of ZnO film. Optical constants were determined through transmittance spectra using an envelope method. It was found that there was a significant effect of annealing temperature on the refractive index and extinction coefficient of deposited ZnO films. In this experiment, the optimum refractive index value of 1.97 was obtained at 350 °C annealing temperature at visible (vis) wavelength. The optical energy gap was found to be of ∼3.2 eV for all the samples. The top view of SEM showed the ZnO grain growth on the glass substrates.     

Intersubband optical absorption coefficients and refractive index changes in modulation-doped asymmetric double quantum well

The linear and the third-order nonlinear optical absorption coefficients and refractive index changes in a modulation-doped asymmetric double quantum well are studied theoretically. The electron energy levels and the envelope wave functions in this structure are calculated by the Schrödinger and Poisson equations self-consistently in the effective mass approximation. The analytical expressions of optical properties are obtained by using the compact density-matrix approach. In this regard, the linear, nonlinear and total intersubband absorption coefficients and refractive index changes are investigated as a function of right-well width (Lw₂) of asymmetric double quantum well. Our results show that the total absorption coefficients and refractive index changes shift toward higher energies as the right-well width decreases. In addition, the total optical absorption coefficients and refractive index changes is strongly dependent on the incident optical intensity.     

Optical and electronic polarizability investigation of Nd-doped soda-lime silicate glasses

This paper reports on different physical and optical properties of Nd³⁺-doped soda-lime silicate glass. The glasses containing Nd³⁺ in (65−x)SiO₂:25Na₂O:10CaO:xNd₂O₃ (where x=0.0-5.0 mol%) have been prepared by the melt-quenching method. In order to understand the role of Nd₂O₃ in these glasses the density, molar volume, refractive index and optical absorption were investigated. The results show that the density and molar volume of the glasses increase with an increase in Nd₂O₃ concentration and consequently generate more non-bridging oxygen (NBOs) into glass matrix. The optical absorption spectra were measured in the wavelength range from 300 to 700 nm and the optical band gaps were determined. It was found that the optical band gap decreases with an increase in Nd₂O₃ concentration. On the basis of the measured values of density and refractive index, the Nd³⁺ ion concentration in glasses, the polarizability of oxide ions and optical basicity were theoretically determined.     

Factors affecting optical dispersion in borate glass systems

Series of ternary glass systems namely, Na₂O, B₂O₃, and RO (R=Ba or Mg) doped with TiO₂ are synthesized. The present glasses are dictated by requirement for a small refractive index and a small nonlinear coefficient needed for waveguide and laser fabrication requirements. The effect of MgO and BaO as alkaline earth metals on the optical properties of the glass systems is investigated. The dependence of the refractive index and extinction coefficient dispersion curves on composition is carried out over a wavelength range of 0.3-. Applying a genetic algorithm technique, the parameters of Sellmeier dispersion formula that fit index data to accuracy consistent well with the measurements are given. The zero material dispersion-wavelength (ZMDW) and group velocity are also determined using the refractive index data. The Fermi level is calculated exploiting the extinction coefficient dispersion curves. The absorption coefficient, both direct and indirect optical energy gaps, and Urbach energy are evaluated using the absorption edge calculations. The different factors that play a role for controlling the refractive indices such as coordination number, electronic polarizability, field strength of cations, bridging and nonbridging oxygen, and optical basicity are discussed in accordance with the obtained index data. IR spectroscopy is used as a structural probe of the nearest-neighbor environment in the glass network.     

Optical properties of methane to 288 GPa at 300 K

Optical properties of solid methane (CH₄) were studied at high pressure and room temperature using a diamond anvil cell. Reflectivity and transmission measurements were used to measure the refractive index to 288 GPa. Fabry-Perot interferometery was used to measure the sample thickness to 172 GPa. This data was fitted to the derived expression of thickness vs. pressure that was then used to calculate the thickness to 288 GPa. This in turn was combined with optical absorption experiments to obtain the absorption coefficient and hence the extinction coefficient k^*. From combined reflection and absorption experiments the refractive index n=n_s+ik^* was obtained. The index of refraction and the ratio of molar refraction to molar volume showed a large increase between 208 and 288 GPa. This behavior indicated that a phase transformation of insulator-semiconductor might have occurred in solid CH₄ by 288 GPa.     

Nonlinear optical properties of an off-center donor in a quantum dot under applied magnetic field

The nonlinear optical properties of an off-center hydrogenic donor in a two-dimensional quantum dot under applied magnetic field are investigated in detail by using the matrix diagonalization method. Based on the computed energies and wave functions, the linear, third-order and total optical absorption coefficients as well as the refractive index changes have been examined between the ground state (L=0) and the first excited state (L=1). The results show that the ion position, the applied magnetic field, the confinement frequency, and the incident optical intensity have an important influence on the nonlinear optical properties of off-center donors.     

Structure and optical properties of chemically synthesized titanium oxide deposited by evaporation technique

Titanium oxide inorganic ion exchange material was synthesized by hydrolysis with water and ammonia solution. Structural feature of the synthesized titanium oxide was analyzed using X-ray diffraction, X-ray fluorescence and infrared spectrometer technique. Tentative formula of titanium oxide was determined and written as TiO₂·0.58H₂O. Titanium oxide films were deposited on glass substrates by means of an electron beam evaporation technique at room temperature from bulk sample. The films were annealed at 250, 350, 450, and 550 °C temperatures. Transmittance, reflectance, optical energy gap, refractive index and extinction coefficient were investigated. The transmittance values of 85% in the visible region and 88% in the near infrared region have been obtained for titanium oxide film annealed at 550 °C. Kubelka-Munk function was used to evaluate the absorption coefficient which was used to determine the optical band gap. It was found that the optical band gap increases with increasing annealing temperature whereas the refractive index and extinction coefficient decreases.     

Nonlinear diffraction in gratings based on polymer–dispersed TiO 2 nanoparticles

In this letter we report a simple technique to produce volume holographic gratings based on photopolymerizable composites containing TiO₂ nanoparticles. Diffraction gratings with high refractive index modulation amplitude (up to 1.25 × 10⁻²) have been formed due to the periodic distribution of high refractive index nanoparticles in a low refractive index polymer matrix. The diffraction efficiency increases strongly on increasing the nanoparticle concentration. Taking the mixture with 10 wt.% TiO₂ nanoparticles, gratings with high diffraction efficiency, low level of scattering and high transparency in the visible-wavelength range have been obtained. This will ultimately lead to different applications of diffractive optical elements based on nanocomposites. The dependence of the gratings’ diffraction efficiency on the intensity of probe laser pulses at 1064 nm has been explored. It is shown that the nonlinear response of the gratings is attributed mainly to the nonlinear properties of the TiO₂ nanoparticles embedded in the polymer matrix. The mechanism of the grating formation and the reasons for the nonlinear behavior of the diffraction efficiency are discussed.     

Metal nanoparticle-doped coloured films on glass and polycarbonate substrates

In a program on the development of metal (e.g. Au, Ag, Cu and their alloy) nanoparticles in sol-gel derived films, attempts were made to synthesize different coloured coatings on glasses and plastics. The absorption position of surface plasmon resonance (SPR) band arising from the embedded metal nanoparticles was tailored by controlling the refractive index of the matrix for the development of different colours. Thus different coloured (pink to blue) coatings on ordinary sheet glasses were prepared by generating Au nanoparticles in mixed SiO₂-TiO₂ matrices having refractive index values ranging from about 1.41 to 1.93. In another development,in situ generation of Ag nanoparticles in the inorganic-organic hybrid host leads to the formation of different abrasion resistant coloured coatings (yellow to pink) on polycarbonate substrates after curing. As expected, the SPR peak of Ag or Au is gradually red-shifted due to the increase of refractive index of the coating matrices causing a systematic change of colour     

Facile synthesis of titania/hyperbranched polyglycidol nanohybrids with controllable morphologies: from solid spheres,capsules to tubes

Titania/Hyperbranched polyglycidol (HBP) nanohybrids with tunable morphologies have been synthesized via a sol–gel process at ambient temperature. One-shot addition of varied amounts of titanium precursor tetraisopropoxide (TTIP) yields spherical titania/HBP solid particles with tunable size, while a controlled addition of TTIP results in spherical titania/HBP capsules. The average outer and inner diameters of the resultant capsules are also controllable according to the amount of TTIP via an Oswald ripening process. In addition, the modality of additional water supplied in the reaction systems can tune the morphologies of the resulting titania/HBP particles from nanocapsules to nanotubes owing to the accelerated hydrolysis rate of TTIP. The tunability in morphologies of the titania/HBP nanostructures ranging from solid spheres, capsules to tubes could be attributed to the self-assembly of a large amount of titania/HBP aggregates in a rapid, controlled and anisotropic manner, respectively. Surprisingly, by means of HBP contained in the resulting titania/HBP nanostructures, the gold nanoparticles are in situ generated and encapsulated into titania/HBP matrix in the absence of additional reducing agent. The as-prepared gold nanoparticles functionalized titania/HBP hybrids exhibit excellent catalytic function toward the reduction of 4-nitrophenol. This strategy demonstrates a typical example for functionalizing the titania/HBP hybrids targeted to specific applications.     

Spatially controlled dissolution of Ag nanoparticles in irradiated SiO2 sol-gel film

In this paper, we report the spatially controlled dissolution of silver nanoparticles in irradiated SiO₂ sol-gel films. The Ag nanoparticles have been formed in the sol-gel solution before the film deposition by adding Triton and ascorbic acid and also after the film deposition using a heat treatment at 700 °C for few minutes or at 550 °C for 6 h in reducing atmosphere. Using a spectrometer, a new view white light interferometer and a micro-thermal analyzer, we demonstrate that the silver nanoparticles can be dissolved using a continuous black ray UV lamp or with a near-infrared (NIR) femtosecond laser, due to a significantly increase in the local temperature. We confirm that the micro-thermal analyzer can be used as a new tool to study the dissolution of metallic nanoparticles in thin film if located at the surface of the films.     

Fullerene–epoxy nanocomposites-enhanced mechanical properties at low nanofiller loading

In this study, we characterized the mechanical properties of fullerence (C₆₀) epoxy nanocomposites at various weight fractions of fullerene additives in the epoxy matrix. The mechanical properties measured were the Young’s modulus, ultimate tensile strength, fracture toughness, fracture energy, and the material’s resistance to fatigue crack propagation. All of the above properties of the epoxy polymer were significantly enhanced by the fullerene additives at relatively low nanofiller loading fractions (~0.1 to 1% of the epoxy matrix weight). By contrast, other forms of nanoparticle fillers such as silica, alumina, and titania nanoparticles require up to an order of magnitude higher weight fraction to achieve comparable enhancement in properties.     

Tailoring optical properties of TiO2 nanowires coated with Ag nanoparticles by plasmon coupling of Ag nanoparticles

TiO₂ nanowires were grown on titanium foil by an alkali hydrothermal growth method. The as-synthesized nanowires are structurally uniform with diameters of 50-100 nm and lengths of up to a few micrometers. The as-prepared TiO₂ nanowires were coated with Ag nanoparticles by reducing AgNO₃ in solution. The experimental results indicate that the Ag nanoparticles can aggregate together on the surfaces of TiO₂ nanowires by interconnection between nanoparticles. The degree of aggregation of Ag nanostructures can be controlled by changing the concentrations of Ag nanoparticles. The as-prepared nanostructures exhibit a wide optical absorption from 387 to 580 nm that can be easily tuned by controlling the degree of aggregation of Ag nanostructures. The results reveal that optical properties of the Ag-coated TiO₂ nanowires can be enhanced by plasmon coupling of Ag nanoparticles. The as-prepared nanostructures may find potential applications in the field of solar cells.     

Polystyrene-co-maleic acid/CdS nanocomposites: Preparation and properties

Composites of CdS nanoparticles confined in a polystyrene-co-maleic acid (PS-co-MAc) matrix have been prepared and characterized. It was shown that the acid groups of the co-polymer could be successfully used to control the aggregation of the nanoparticles, because they act as coordinate sites for Cd ions. UV-VIS measurements showed a blue shift of the absorption threshold, proving the presence of nanoparticles. An average size of the nanoparticles of about 4 nm is estimated from the change in band gap energy. Although the FTIR spectrum of the nanocomposite showed the presence of C-S bonds, a broad emission originating from surface recombination sites are noticed. DSC and TGA measurements revealed changes in thermal properties upon incorporation of nanoparticles. No thermal transition was observed in the nanocomposite, while the pure co-polymer exhibits a glass transition at 190 °C. In the presence of nanoparticles the onset of the thermal decomposition of the matrix is also shifted by 50 °C towards a higher temperature.