Smart Thin Films via the Sol-Gel Route

Abstract

The sol-gel route appears to be very convenient for the deposition of thin films by such techniques as spin-coating or dip-coating.¹ The sol-gel process is based on the hydrolysis and condensation of molecular precursors.² The molecular engineering of these precursors provides a chemical control of condensation reactions giving nanostructured the oxide materials. In sol-gel chemistry two chemical routes are currently followed depending on the nature of the molecular precursors; the inorganic route with metal salts in aqueous solutions and the metal-organic route with metal alkoxides in organic solvents. In both cases the reaction is initiated via hydrolysis in order to get reactive M-OH groups. This reaction can be simply performed by adding water to an alkoxide or by changing the pH of an aqueous solution.³ Condensation then occurs leading to the formation of metal-oxygen-metal bonds. More and more condensed species are formed from the solution leading to oligomers, oxopolymers, colloids, gels or precipitates. Oxopolymers and colloidal particles give rise to sols which can be shaped, gelled, dried and densified in order to get powders, films, fibres or monolithic glasses.^1,2