Method for optimizing coating properties based on an evolutionary algorithm approach.

In industry as well as many areas of scientific research, data collected often contain a number of responses of interest for a chosen set of exploratory variables. Optimization of such multivariable multiresponse systems is a challenge well suited to genetic algorithms as global optimization tools. One such example is the optimization of coating surfaces with the required absolute and relative sensitivity for detecting analytes using devices such as sensor arrays. High-throughput synthesis and screening methods can be used to accelerate materials discovery and optimization; however, an important practical consideration for successful optimization of materials for arrays and other applications is the ability to generate adequate information from a minimum number of experiments. Here we present a case study to evaluate the efficiency of a novel evolutionary model-based multiresponse approach (EMMA) that enables the optimization of a coating while minimizing the number of experiments. EMMA plans the experiments and simultaneously models the material properties. We illustrate this novel procedure for materials optimization by testing the algorithm on a sol-gel synthetic route for production and optimization of a well studied amino-methyl-silane coating. The response variables of the coating have been optimized based on application criteria for micro- and macro-array surfaces. Spotting performance has been monitored using a fluorescent dye molecule for demonstration purposes and measured using a laser scanner. Optimization is achieved by exploring less than 2% of the possible experiments, resulting in identification of the most influential compositional variables. Use of EMMA to optimize control factors of a product or process is illustrated, and the proposed approach is shown to be a promising tool for simultaneously optimizing and modeling multivariable multiresponse systems.

Shaping mesoporous films using dewetting on X-ray pre-patterned hydrophilic/hydrophobic layers and pinning effects at the pattern edge.

Ordered mesoporous silica micrometer-sized structures have been fabricated via selective dewetting of the coating sol on a hydrophilic/hydrophobic fluorinated silica substrate, which had been pre-patterned using deep X-ray lithography with a synchrotron radiation source. We have observed that deposition of mesoporous films on the pre-patterned areas can be used as a design tool for obtaining regions of specific geometry and dimensions. The evaporation of the solution in constrained conditions because of pinning at the pattern edges gives layers with thicker edges. This edge effect appears dependent upon the dimension of the pre-patterned hydrophilic/hydrophobic layer; in smaller patterns, the evaporation is too fast and thickening of the edges is not observed. We have used infrared imaging, optical profilometry, and atomic force microscopy to characterize the patterned layers and the edge effect, produced by pinning at the border of the microstructures.

Densification of sol-gel silica thin films induced by hard X-rays generated by synchrotron radiation.

In this article the effects induced by exposure of sol-gel thin films to hard X-rays have been studied. Thin films of silica and hybrid organic-inorganic silica have been prepared via dip-coating and the materials were exposed immediately after preparation to an intense source of light of several keV generated by a synchrotron source. The samples were exposed to increasing doses and the effects of the radiation have been evaluated by Fourier transform infrared spectroscopy, spectroscopic ellipsometry and atomic force microscopy. The X-ray beam induces a significant densification on the silica films without producing any degradation such as cracks, flaws or delamination at the interface. The densification is accompanied by a decrease in thickness and an increase in refractive index both in the pure silica and in the hybrid films. The effect on the hybrid material is to induce densification through reaction of silanol groups but also removal of the organic groups, which are covalently bonded to silicon via Si-C bonds. At the highest exposure dose the removal of the organic groups is complete and the film becomes pure silica. Hard X-rays can be used as an efficient and direct writing tool to pattern coating layers of different types of compositions.

Chemical tailoring of hybrid sol-gel thick coatings as hosting matrix for functional patterned microstructures.

A phenyl-based hybrid organic - inorganic coating has been synthesized and processed by hard X-ray lithography. The overall lithography process is performed in a two-step process only (X-rays exposure and chemical etching). The patterns present high aspect ratio, sharp edges, and high homogeneity. The coating has been doped with a variety of polycyclic aromatic hydrocarbon functional molecules, such as anthracene, pentacene, and fullerene. For the first time, hard X-rays have been combined with thick hybrid functional coatings, using the sol-gel thick film directly as resist. A new technique based on a new material combined with hard X-rays is now available to fabricate optical devices. The effect due to the high-energy photon exposure has been investigated using FT-IR and Raman spectroscopy, laser scanner, optical profilometer, and confocal and electron microscope. High-quality thick hybrid fullerene-doped microstructures have been fabricated.

Influence of temperature on the photocatalytic activity of sol-gel TiO2 films.

The photocatalytic activity of TiO(2) films synthesized via the sol-gel process has been measured as a function of UV irradiation time and substrate temperature. Fourier-transform infrared spectroscopy has been used to address the chemical changes in stearic acid and block copolymer Pluronic F127 films deposited on the photocatalytic surface. When the temperature of the photocatalytic substrate was raised above 50 degrees C, the removal of stearic acid from the surface was strongly affected by a process involving evaporation, whereas Pluronic F127 revealed a superior stability. Our study shows that heat enhances the photocatalytic activity, suggesting the importance of an accurate temperature control in photocatalytic efficiency measurements.

Evaporation of ethanol and ethanol-water mixtures studied by time-resolved infrared spectroscopy.

The knowledge of the physics and the chemistry behind the evaporation of solvents is very important for the development of several technologies, especially in the fabrication of thin films from liquid phase and the organization of nanostructures by evaporation-induced self-assembly. Ethanol, in particular, is one of the most common solvents in sol-gel and evaporation-induced self-assembly processing of thin films, and a detailed understanding of its role during these processes is of fundamental importance. Rapid scan time-resolved infrared spectroscopy has been applied to study in situ the evaporation of ethanol and ethanol-water droplets on a ZnSe substrate. Whereas the evaporation rate of ethanol remains constant during the process, water is adsorbed by the ethanol droplet from the external environment and evaporates in three stages that are characterized by different evaporation rates. The adsorption and evaporation process of water in an ethanol droplet has been observed to follow a complex behavior: due to this reason, it has been analyzed by two-dimensional infrared correlation. Three different components in the water bending band have been resolved.

Thermal-induced phase transitions in self-assembled mesostructured films studied by small-angle X-ray scattering.

Two examples of phase transition in self-assembled mesostructured hybrid thin films are reported. The materials have been synthesized using tetraethoxysilane as the silica source hydrolyzed with or without the addition of methyltriethoxysilane. The combined use of transmission electron microscopy, small-angle X-ray scattering and computer simulation has been introduced to achieve a clear identification of the organized phases. A structural study of the self-assembled mesophases as a function of thermal treatment has allowed the overall phase transition to be followed. The initial symmetries of mesophases in as-deposited films have been linked to those observed in samples after thermal treatment. The monodimensional shrinkage of silica films during calcination has induced a phase transition from face-centered orthorhombic to body-centered cubic. In hybrid films, instead, the phase transition has not involved a change in the unit cell but a contraction of the cell parameter normal to the substrate.

Highly ordered "defect-free" self-assembled hybrid films with a tetragonal mesostructure.

One-pot self-assembled hybrid films were synthesized by the cohydrolysis of methyltriethoxysilane and tetraethoxysilane and deposited via dip-coating. The films show a high "defect-free" mesophase organization that extends throughout the film thickness and for domains of a micrometer scale, as shown by scanning transmission electron microscopy. We have defined these films defect-free to describe the high degree of order that is achieved without defects in the pore organization, such as dislocations of pores or stacking faults. A novel mesophase, which is tetragonal I4/mmm (space group), is observed in the films. This phase evolves but retains the same symmetry throughout a wide range of temperatures of calcination. The thermal stability and the structural changes as a function of the calcination temperature have been studied by small-angle X-ray scattering, scanning transmission electron microscopy, and Fourier transform infrared spectroscopy. In situ Fourier transform infrared spectroscopy employing synchrotron radiation has been used to study the kinetics of film formation during the deposition. The experiments have shown that the slower kinetics of silica species can explain the high degree of organization of the mesostructure.