Control of morphology of silica films
A method of controlling the morphology of silica or silica-like films formed by coating a precursor formulation of hydrolysable silicate oligomer onto a substrate and curing in a vaporous environment that comprises a base, water and a retarder that retards the hydrolysis of the oligomer.
1. A method of forming a silica film coated on a substrate including the steps of:
producing a silica precursor solution by dissolving in a solvent a hydrolysable silicate oligomer of the form:
where each X is independently selected from a hydrolysable or non-hydrolysable group with the proviso that at least three X must be hydrolysable groups and wherein n is an integer between 2 and 10 and wherein the hydrolysable silicate oligomer is not silicic acid tetramethyl ester homopolymer;
forming a coating of the silica precursor solution on the substrate; and
curing the coating of the silica precursor solution by hydrolysing and condensing the coating in a vaporous environment comprising a base, water and a retarder, said retarder retarding hydrolysis of the hydrolysable silicate oligomer to form a film, wherein the concentration of said retarder in the vaporous environment is selected to control the morphology of the film.
2. The method of claim 1 wherein the step of curing includes the steps of:
placing the coated substrate in a chamber with the retarder vapour; and
subsequently introducing water and a base into the chamber.
3. The method of claim 2 further including the step of allowing the coating to equilibrate with the retarder vapour before introducing water and base to the chamber.
4. The method of claim 1 wherein when X is a hydrolysable group it is selected from the group consisting of C 1 -C 10 alkoxy, aryloxy, substituted C 1 -C 10 alkoxy and substituted aryloxy.
5. The method of claim 1 wherein when X is a non-hydrolysable group it is selected from the group consisting of alkyl, alkenyl and, aryl, all of which may be substituted with halogen, nitrogen, oxygen, sulphur or silicon.
6. The method of claim 1 wherein X is substituted with one or more inert substituents selected from the group consisting of halides, heteroaryls, and any nitrogen, oxygen or sulphur containing substituent not interfering with the formation of the film.
7. The method of claim 1 wherein the base is a volatile organic base selected from the group consisting of ammonia, a volatile amino-alkane inclusive of a primary amine, a secondary amine or a tertiary amine, a volatile amino-alkene and an alkaloid.
8. The method of claim 1 wherein the base is ammonia.
9. The method of claim 1 wherein the solvent is an alcohol selected from the group consisting of methanol, ethanol, iso-propyl alcohol, butanol, pentanol and a polyol.
10. The method of claim 1 wherein the hydrolysable silicate oligomer is derivable from a molecular precursor of the form
X m Si(OR) n
where n=3 or 4,
m=4-n,
each R is independently a hydrolysable groups, and
each X is independently a non-hydrolysable group.
11. The method of claim 1 further including the step of controlling morphology of the film by controlling respective partial pressures of the base, the retarder and the water in the vapourous environment.
12. The method of claim 1 further including the step of selecting the solvent to control pore size in the film.
13. The method of claim 1 further including the step of selecting the concentration of solvent to control pore density in the film.
14. The method of claim 1 further including the step of selecting the concentration of retarder in the vapourous environment to control pore density in the film.
15. The method of claim 1 wherein the step of forming a coating is performed by spin coating or dip coating.
16. The method of claim 15 wherein the coating conditions influence the final film thickness.
17. The method of claim 15 wherein the coating conditions influence the refractive index of the film.
18. The method of claim 1 wherein the silica precursor solution is produced at or near neutral pH.
19. The method of claim 1 wherein the silica precursor solution contains less than 5% water by volume.
20. The method of claim 1 wherein the curing step is performed at or near room temperature and ambient pressure.
21. The method of claim 1 wherein the film has a refractive index between 1.1 and 1.56 and a thickness less than 100 microns.