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Solar & Alternative Energy
Sol-gel technology for optoelectronic systems
Advances in a versatile approach to making glass have produced a range of innovative optical materials.
14 December 2006, SPIE Newsroom. DOI: 10.1117/2.1200610.0465
The sol-gel process
The sol-gel method is a low-temperature technique for synthesizing solid glass bulks or thin films that can be used as coatings on glass, ceramic, metal, and other solid substrates. It is based on subjecting organometallic compounds, such as silicon alkoxides, to hydrolysis and polycondensation reactions. Mild synthetic conditions also allow for addition of dopants such as organic dyes, inorganic ions, and nanoparticles. The resulting doped materials are finding increased use in optical and sensing applications.
The precursor compounds usually consist of alkoxides or inorganic solutions that can be applied on various substrates by dip coating, spin coating, or laminar coating. The most common precursors are used to produce silicates, titania, germanates, alumina, zirconia, tungstate vanadates, and ORMOSILs (organically modified silicates).1–3
The process can be explained using the formation of a silica glass as an example. The first stage subjects a starting mixture of tetra alkoxysilane and water to hydrolysis reactions of the form
Specifically, the complete hydrolysis of a tetraalkoxysilane molecule proceeds as follows:
This reaction yields a solution of silanol groups, Si(OH)4, in alcohol. The hydrolysis can be catalyzed by acid (most commonly by HCl or HNO3), which increases the reaction rate. Another option is nucleophilic base catalysis.
The hydrolysis reaction rate is also influenced by steric considerations. For instance, it decreases with the size of the alkoxy group, as shown by the faster hydrolysis of tetramethoxysilane over that of tetraethoxysilane.
The second stage of the process is polycondensation of the hydrolysis products, which yields a 3D cross-linked network. Each condensation reaction has the following form:
Composite glass/polymer hosts
Composite glass refers to materials in which organic polymers fill the pores of porous glass. The resulting material contains two different phases, an inorganic skeleton of porous glass and an organic phase.
A large number of sophisticated materials have been prepared and studied using the sol-gel process, and significant experimental and theoretical insights have been gained. Recent publications report luminescent solar concentrators; photochromic, electrochromic, and gasochromic plates for smart windows; sensors for environmental and biological impurities; solid lasers tunable in the visible spectral range; active waveguides; materials for linear and nonlinear optics; semiconductor quantum dots; and complexes of rare earth ions that can be used as diagnostic and biological markers.2–7
ORMOSILs usually exhibit lower porosity and enhanced mechanical properties that facilitate cutting, grinding, and polishing prior to heat treatment. A typical ORMOSIL gel network also displays several organic functionalities that provide great flexibility with respect to the chemical compatibility of the gel with the dye selected for incorporation. For example, various organic laser dyes have been successfully incorporated into ORMOSILs derived from poly(dimethylsiloxane) and tetraethoxysilane, demonstrating their usefulness as effective host matrices for organic laser dyes.
Using sol-gel techniques, our laboratory has prepared a large number of new optical materials.2–7 For example, we fabricated tunable solid-state lasers incorporating stable perylene (BASF 241) and pyrromethene dyes into composite and ORMOSIL glasses. Table 1 lists the efficiencies and threshold energies obtained with transverse pumping. We have also prepared and characterized europium sulfide nanoparticles that can be used as light-conversion molecular devices.4
Table 1. Laser performance with sol-gel glasses.
Inorganic Chemistry, The Hebrew University
Renata Reisfeld is a professor of chemistry and holds a chair in solar energy. She has published some 500 publications in sol-gel glass chemistry and the optical properties of glasses and related compounds. She is the recipient of three honorary doctorates.