Peter William de Oliveira

Organic-inorganic composite materials: optical properties of laser-patterned and protective-coated waveguides

A composite materials based on organically modified Si alkoxides and modified alkoxides from Zr were prepared. The index of refraction of the system was tailored by the variation of the modified Zr component. Channel waveguides were prepared by a laserwriting technique based on the ability to photopolymerize this material. To decrease optical loss caused by effects of the waveguide surface and to be independent of the used substrate, a buffer layer and a protective layer of the same material of lower index of refraction was combined with a waveguiding film and optical losses were determined.

Interference coatings on glass based on photopolymerizable nanomer material

Abstract A new sol-gel method for the preparation of optical SiO 2 and TiO 2 multilayer coatings has been developed. As an example, a 3-layer antireflective (AR-filter) coating and a 5-layer NIR reflective coating on glass are described. For the preparation, a nanoparticulate TiO 2 sol was synthesized by hydrolysis and condensation of tetraisopropylorthotitanate and complex formation with methacrylic acid. A SiO 2 nanoparticulate sol was synthesized analogeously with tetraethoxysilane (TEOS) as the precursor. By mixing both sols in different ratios, layers with tunable refractive index between 1.46 and 2.2 could be obtained on glass after thermal densification at 450°C for 15 min. Three layers (SiO 2 -TiO 2 , TiO 2 , SiO 2 ) were deposited by dip-coating with subsequent UV-curing, and the layer-stack was thermally densified at 450 C. The so obtained AR-filter shows a reflection of ≤2% in the wavelength range between 380 and 610 nm, ≤1% between 450 and 560 nm and 0% at 550 nm. The AR filter shows very good abrasion resistance and scratch hardness, since only 3% haze is obtained after 1000 cycles of Taber test and the AR filter does not show any damage after 300 cycles of rubber test. The high abrasion resistance can be attributed to the surprisingly low surface roughness (about 6 nm) of the filter. The mechanical and optical properties of the AR filter did not change after sun test with 760 W/m 2 for 320 h and after boiling water test for 11 days.

Multilayer NIR reflective coatings on transparent plastic substrates from photopolymerizable nanoparticulate sols

Abstract A new synthesis and processing route for preparing multilayer interference coatings on plastic substrates has been developed. For this purpose, alcoholic sols of surface modified (3-glycidoxypropyltrimethoxysilane, GPTS) SiO 2 and TiO 2 particles with sizes of 10 and 4 nm, respectively, were synthesized. Layers were prepared by dip coating, subsequent UV curing (2.1 J/cm 2 ) and thermal post treatment at 80°C for 15 min. Refractive indices of n D =1.47 for SiO 2 layers and n D =1.93 for TiO 2 were measured. As an example, plastic sheets were coated by angle dependent dip coating with withdrawal speeds from 3 up to 6 mm/s and an inclination angle of 4°. By this way, one side of the substrate was coated with six quarterwave thick layers (peak wavelength at 750 nm), producing a reflective interference filter with a reflectivity of 72% between 650 and 900 nm. On the other side of the substrate, an interference filter with a reflectivity of 66% between 800 and 1100 nm was produced simultaneously. The interference coatings do not show delamination or defects after boiling water test (H 2 O+5 wt.% NaCl, 8 h) and excellent adhesion (GT 0, TT 1) was obtained in the cross cut tape test. Yellowing did not occur (Δ g 2 for 270 h. For mechanical protection a nanocomposite hardcoat can be applied on top of the NIR reflectance filters without changing the optical properties remarkably.

Dynamics of the sol-gel transition in organic-inorganic nanocomposites

Abstract Two different techniques have been used to follow the gelation of photochromic organic—inorganic nanocomposites. The variations of molecular and macromolecular motions in these complex systems have been analyzed. Photo-correlation spectroscopy probes the formation of the gel network. Forced Rayleigh scattering experiences the microstructure of the mixtures via the measurement of the translational diffusion coefficient of entrapped photoreactive targets. In the different mixtures, a drop of the network mobility could be observed around the sol to gel conversion, while the entrapped molecules do not experience the macroscopic transition.

Generation of wet-chemical AR coatings on plastic substrates by the use of polymerizable nanoparticles

Multi-layer interference filters of surface modified SiO2- and TiO2-nanoparticles have been produced on polycarbonat (PC) and on scratch resistant film. AR-coatings (anti-reflex- coatings) were prepared from a dispersion of SiO2 and TiO2 colloidal nanoparticles in a hybrid inorganic organic matrix. The TiO2 particles have been synthesized by addition of HCl and H2O to a solution of titanium tetraisopropanolate in isopropanol. The SiO2 nanoparticles were synthesized by base catalyzed hydrolyses and condensation of tetraethoxysilane (TEOS) mixed with an aqueous solvent. The surface modification of the TiO2 and SiO2 nanoparticles with 3-glycidoxypropyltrimethoxysilane (GPTS) avoids the agglomeration of the nanoparticles and covalent bonds between the matrix and nanoparticles are formed during a photopolymerization step. The nanoparticles were dispersed in a hydrolyzed GPTS matrix and UVI Cygacure 6974 was added as a photoinitiator. The (lambda) /4 ((lambda) equals 550 nm) films of TiO2 and SiO2 sols were prepared on PC and on scratch resistant coated PC by spin-coating techniques. The refractive index of the SiO2/TiO2-coatings can be adjusted from 1.47 up to 1.94 depending on the SiO2 or TiO2 concentration. By the combination of index matching and photopolymerization, a low temperature interference filter can be realized for plastic substrates with reflection of 0.5% at 550 nm. The coatings show excellent adhesion to the substrates in the cross hatch test and promising mechanical properties.

Fabrication of GRIN-materials by photopolymerization of diffusion-controlled organic-inorganic nanocomposite materials

New photopolymers have contributed significantly to the recent growth of holographic and lithographic applications. Photopolymerizable organic-inorganic hybrid materials, based on methacrylate functionalized silane and zirconia particles as holographic recording material, are presented. Thick films of this composite system were prepared and volume diffractive gratings were fabricated by a two laser beam interference technique. The formation of the gratings is based on the diffusion of high refractive index components (ZrO 2 -nanoparticles) to areas with high irradiation intensity with subsequent immobilization by full irradiation of the film. The influence of the zirconia particles as the main component for obtaining highly efficient gratings is presented and the correlation between particle concentration and refractive index profile is shown.

Hybrid sols as intermediates to inorganic-organic nanocomposites

For the preparation of inorganic-organic hybrid materials, synthesis processes have been developed to fabricate so-called hybrid sols, which contain an inorganic core (ceramic or glass) with nano-scale dimensions surface modified by organic groupings. These groupings have been reacted to the particle surface either by amino functional silanes (e.g. in case of iron oxide nanoparticles for amino group containing silanes) reacting with aliphatic acids to make surfaces unreactive and reduce the particle-to-particle interaction by acids, or silanes with polymerizable groupings reacted with organic monomers. In the paper a summary of the recent work and the development of structural models is given. Moreover, it could be shown that surface charges can be generated in the particles within already gelled systems and thus, particles can be transported by electric fields to form gradient index materials.

Production of Fresnel lens in sol-gel-derived Ormocers by holography

Micro Fresnel lenses were prepared from photosensitive organic-inorganic nanocomposites of the Ormocer-type by irradiation of films of 10 micrometers in thickness with an interference pattern of two laser beams, mixing a planar reference wavefront and a spherical object wavefront. To monitor the polymerization behavior of the organic groups during irradiation under real time conditions, a characterization method was developed to study changes in optical thickness nd by measuring changes of index of refraction and shrinkage during polymerization. Three-dimensional Fresnel structures were obtained by removing the unpolymerized areas by a solvent. Variation of index of refraction can also be obtained by diffusion of monomers in regions of higher light intensity. A model, similar to the Colburn- Haines model, is presented.

Sol-gel-derived AgCl photochromic coating on glass for holographic application

A sol-gel derived photochromic glass coating of about 1.5 micrometers in thickness is synthesized by infiltration of Ag+ into a predried Na-Al-B-Si gel layer. The formation of small Ag colloids is initiated by a soft heat treatment and the colloids are converted by a HCl vapor treatment into AgCl crystallites of about 40 nm in diameter. The coating darkens by UV irradiation, forming Ag crystallites of about 5 nm in size. This process is completely reversible at 400 degree(s)C and no decay is observed in numerous cycles. An amplitude hologram which acts as a diffraction grating with a grating period of about 10 micrometers can be produced within this layer by a two-wave mixing experiment with laser light of 351 nm wavelength. The diffraction efficiency of this grating is determined to 0.11%. After thermal erasing the diffraction pattern can be rewritten and erased repeatedly without any loss of efficiency.

Organic-Inorganic Hybrid Materials Processing And Applications

Hybrid materials as inorganic-organic nanostructured composites require tailored surface chemistry in order to obtain a homogeneous distribution of the nanoparticles in the matrix. For this reason, nanoparticles with organic functions have been synthesized, first, to provide the desired ae-potential at a given pH value, second, to avoid irreversible agglomeration due to the spacing effect, and third, to provide the appropriate surface chemistry. I could be shown that using this approach, it is possible to switch the ζ-potential of SiO2 nanoparticles from a negative to a positive potential at neutral and to bind DNA fragments to the particles for an effective transfection into cells. Other examples show that nanoparticles (TiO2, SiO2) coated with epoxy and methacryloxy groupings can be used as coating sol for the fabrication of thin films with green densities up to 67% by volume only by photochemical crosslinking of the polymerizable groupings. Using this approach, interference layers have been fabricated on transparent plastics. In soft matrices, these particles permit to establish appropriate ζ-potentials and in electric fields by electrophoresis, it was possible to up-concentrate them to form gradient index optics. The investigations show that surface chemistry-tailored nanoparticles are a useful tool for the fabrication of nanocomposite hybrids.