Hexagonal to lamellar mesostructural changes in silicate films caused by organic additives

The presence of dopant molecules and cosolvents in the sols from which mesostructured silicate thin films are pulled can have profound influences on the resulting structure and long range order. Dopants are frequently desired in order to induce a function such as luminescence. An important luminescent molecule, carbazole, is very effective at converting the film mesophase from hexagonal to lamellar. A very small change in the mole ratio of carbazole to the structure-directing surfactant, from 0.41 to 0.44, causes a very abrupt change of the structure of CTAB-templated mesostructured silica from two-dimensional hexagonal to lamellar. A molecule with a very similar structure, fluorene, also affects the structure in a similar manner, but much higher concentrations are necessary and the final order is not as high. Molecules having similar functional groups (especially amine groups) but nonplanar structures show that both the shape and the presence of the amine group play a role in causing the structural change. Many luminescent molecules require cosolvents in order to incorporate them into the sol from which the films are pulled. The effects of cosolvents, especially THF and benzene, on the structures of the films are also studied. The reasons for the structural change from 2-d hexagonal to lamellar as the concentrations of these molecules are increased are discussed in terms of modifications in the effective shapes of the micelles, and subtle local effects on the hydrolysis and condensation reaction rates caused by the basic aromatic amine functional group.