A. Brunet-Bruneau

Highly porous TiO2 anatase optical thin films with cubic mesostructure stabilized at 700 °C

TiO2 optical thin films stable to 700 °C, exhibiting 35% volume porosity, more than 100 m2·g-1 in surface area, fully nanocrystalline anatase framework, and organized mesostructure (cubic Im3m derived), have been stabilized by careful delayed rapid crystallization (DRC) thermal treatments. In-situ time-resolved SAXS and WAXS investigations were simultaneously performed during such treatments. They revealed that a slow and progressive heating to a temperature just below that of the formation of anatase (Tc ≈ 400 °C), followed by a long pretreatment at this temperature, stabilizes the amorphous network. A following rapid increase of temperature up to temperatures as high as typically 700 °C, followed by a short residence time at this high temperature, provokes the homogeneous formation of crystalline small nanoparticles and the total elimination of organic residues. The crystallization is accompanied by matter migration through diffusing sintering and pore merging along the [111] directions of the cubic str...

A first insight in the mechanisms involved in the self-assembly of 2D-hexagonal templated SiO2 and TiO2 mesostructured films during dip-coating

Since mesoporous materials can be prepared by combining the sol-gel chemistry and the structuring effect of surfactants, they have attracted attention for application in various high technology fields. The present work deals with the analyses of the mechanisms involved in the formation of SiO2 and TiO2 highly organised 2D-hexagonal meso-structured films using Brij 58 as surfactant. The preparation of such films by dip-coating involves rapid evaporation which makes the different steps difficult to control. Simultaneous in-situ SAXS (synchrotron) and interferometry analyses have been performed to get a first understanding of the self-assembly process. SiO2 and TiO2 materials have a different chemical reactivity (kinetics and coordination aspects). However, we show that the mechanisms involved during dip-coating are quite similar : the self-assembly leading to the organised phase takes place at a final stage of the drying process, involves the formation of a disorganised intermediate phase and depends also on the presence of micellar interfaces in addition to film/air and film/substrate interfaces.

Making an omnidirectional reflector

The effect of having a finite number of layers on the design of omnidirectional reflectors was investigated. It was shown that the structure should be finished with a low-index layer having a thickness larger than a quarter-wave to increase reflectivity, whereas layers below may remain of quarter-wave optical thickness at normal incidence angle. This general trend has been used for designing and realizing two a-Si-SiO(2) (amorphous silicon and silicon dioxide) omnidirectional reflectors in the near-infrared range on a silicon and a silica substrate, respectively. Owing to the decrease of absorption of recrystallized silicon as compared with a-Si in the visible range, the transmissivity of the structure realized on silica substrate was dramatically increased in the visible range upon annealing, whereas the high reflectivity and the omnidirectional effect were maintained in the near-infrared range.

Highly ordered CTAB-templated organosilicate films

Well-ordered organic–inorganic mesoporous films are prepared using an original two-step synthesis and compositions up to 50 mol% methyltriethoxysilane (MTES). These materials exhibit the three different mesostructures (2D-hexagonal, 3D-hexagonal and cubic) previously observed in the pure silica system with significant modifications of their domain of stability. During deposition and drying of organosilicate films, MTES monomers progressively condense at the internal surface of the silica clusters in agreement with the monomer–cluster growth model, leading to a significant reduction of the microporosity in the silica walls of the mesoporous structure.

Infrared ellipsometric study of SiO2 films: relationship between LO mode frequency and porosity

Abstract A method for evaluating the pore volume fraction of silica films for different pore connectivities is presented. A series of SiO2 films, evaporated by electron gun with or without ion bombardment, have been studied using visible and infrared ellipsometry. The TO and LO mode frequencies are deduced from the dielectric function calculated from infrared ellipsometric data. The study of both TO and LO mode frequencies is shown to bring independent information on the film microstructure. The TO mode frequency (near 1075 cm−1) varies mainly with the density of the silica matrix. On the other hand, variation in the LO mode frequency (near 1245 cm−1) is mainly due to changes in porosity. In the case of films with pores largely connected, the evaluation of the pore volume fraction from the LO frequency is compared to that obtained from the analysis of the large water absorption band near 3300 cm−1 (H–OH and Si–OH stretching absorption band).

Optical properties of mixed TiO2 - SiO2 films, from infrared to ultraviolet

Mixed oxides are useful for obtaining the intermediate refractive indices needed in the realization of graded-index thin films. Co-evaporated TiO2-SiO2 mixtures are studied for a large range of concentrations via UV-VIS, IR ellipsometry and XPS. An understanding of the nature of these mixtures and their air exposure stability is important for further applications. At low TiO2 concentrations, Ti4+ ions are inserted into the silica tetrahedral network, as shown by the IR peak at 945 cm-1. At higher concentrations, an evolution from TiO4 tetrahedra to TiO6 octahedra is presumed. The behavior of the O1s core level peak indicates that a least two phases coexist. Comparison between concentration determined using XPS and RBS shows a deficit in TiO2 at the surface of the films, especially at high TiO2 concentrations. The evolution of the mixtures optical constants will be presented in a large wavelength range, going from IR to UV. Particular attention will be paid to the variations with respect to the frequency of the vibration modes in the IR range, to the refractive index in the transparency region, and to the extinction coefficient at he absorption threshold. In addition, AFM measurements show the variation of the grain size as a function of the TiO2 concentration.

Ellipsometric investigation of the Si/SiO2 interface formation for application to highly reflective dielectric mirrors

We present a study of the Si/SiO 2 interface formation during the growth of Si on SiO 2 using in situ ellipsometry measurements performed at one wavelength (X = 0.4 μm). Variable angle X-ray photoelectron spectroscopy (XPS) is also used to confirm the ellipsometric measurements. In situ ellipsometry and XPS provide evidence for the formation of a SiO x (0 <x < 2) interlayer between the SiO 2 and Si layers. The variation of x with depth is analysed from (cos Δ, tan ψ) trajectories and the results obtained allow reproduction of the spectroscopic ellipsometry spectra too. The effects of the substrate temperature on the growth are investigated and show that the higher the substrate temperature the larger the interface. The utilisation of a SiO 2 /Si multilayer stack for elaborating an omnidirectional reflector, angle-insensitive in the near infrared range, is also presented.

Latex-templated porous silica films for antireflective applications

Thin films with low-refractive index are of great interest to adjust the optical properties of optical components, e.g. reflectivity. In this respect, sol-gel coatings are very efficient for flat glass functionalisation because of ease of application, low cost and versatility. Mesoporous silica films prepared by surfactant self-assembly have been extensively studied but show poor refractive index stability as capillary condensation of atmospheric water can occur in the pores. One way to tackle this issue is to prepare films with larger pores for which capillary condensation is impossible at ambient humidity. Starting from custom made latex nanoparticles, we successfully prepared sol-gel porous silica film with pore size above 30nm and no microporosity in the silica walls. We are then able to independently and accurately tailor pore size, pore volume fraction and pore surface chemistry, leading to a stable refractive index that can be tuned in a large range (from 1.15 to 1.40 at 600 nm). Pore accessibility as a function of pore size and porous fraction was investigated through ellipsometry-porosimetry for ethanol adsorption, and a transition between open and closed pore structure at decreasing volume fraction was shown. Below the threshold, the films showed a closed porosity structure with a low stable refractive index (down to 1.29 at 600 nm), opening the way to their use for antireflective applications.

Ordered mesoporous organosilica films

Publisher Summary This chapter demonstrates that the optical and mechanical properties of mesoporous silica films provide the possibility to use them as low-index or low dielectric constant coatings. However, the surface of the pores must be carefully controlled to avoid any introduction of contaminations. This implies a chemical modification of the surface of the pores, which must be performed without ignoring the structure of the porosity (proportion and accessibility of micropores and mesopores). A first possibility consists in a post-synthesis gaffing with a reactive silane (chlorosilane, silazane, or alkoxysilane), but some more work is required to optimize this passivation strategy. Other functionalization processes such as the direct co-condensation of organic/inorganic silica precursors are also investigated. In any case, the functional organic group must be chosen to efficiently block the accessibility of the pores, either because of its bulky nature or because of its hydrophobic and oleophobic nature.

Darwinian evolution of homogeneous multilayer systems: a new method for optical coatings design

An original way of synthesizing optical multilayer systems is presented. Based on a genetic algorithm (GA), this method searches for the optimal sequence of materials and the thicknesses of each layer. The basic theory of GAs as well as their specific application to the design of optical coatings is described. Different examples are shown and the interest of the method is discussed.