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Idealized solubility diagram for film forming species in water. Adapted from B. C. Bunker, P. C. Rieke, B. J. Tarasevich, A. A. Campbell, G. E. Fryxall, G. L. Graff, L. Song, J. Liu, J. W. Virden, and G. L. McVay, Science , 1994, 264 , 48.

Silica can be dissolved in fluorosilicic acid to well above its solubility in water, which is approximately 220 ppm (mg/L). Depending on the concentration of the fluorosilicic acid solution, it can contain up to 20% more silica than is implied by the formula H 2 SiF 6 . After saturation of the solution with SiO 2 , the solvated species is a mixture of fluorosilicates, which reacts as explained earlier. It must be emphasized that addition of water in this reaction is not simply dilution, but is the addition of a reactant, which places the solution in a metastable state (the blue area in [link] ) in preparation for the introduction of a suitable surface to seed the growth of silica.

Another important factor in solution growth methods is interfacial energy. When a substrate with lower interfacial energy than that of a growing homogeneous nucleus is introduced into a growth solution, heterogeneous growth is favored. Thus, a seeded growth mechanism by definition introduces a substrate of lower interfacial energy into a supersaturated solution, facilitating heterogeneous growth. Lower interfacial energies can be a product of surface modification, as well as a property of the materials’ natural state.

Comparing lpd to sol-gel

An alternative method to LPD for forming silica thin films is the sol-gel method. A sol is a colloidal dispersion of particles in a liquid. A gel is a material that contains a continuous solid matrix enclosing a continuous liquid phase. The liquid inhibits the solid from collapsing and the solid impedes release of the liquid. A formal definition of sol-gel processing is the “growth of colloidal particles and their linking together to form a gel.” This method describes both the hydrolysis and condensation of silicon alkoxides and the hydrolysis and condensation of aqueous silicates (the DS process).

In the hydrolysis of silicon alkoxides, an alkoxide group is replaced with a hydroxyl group, [link] . Further condensation reactions between alkoxyl groups or hydroxyl groups produce siloxane bonds, see [link] and [link] .

Tetramethoxysilane [Si(OMe) 4 , TMOS] and tetraethylorthoxysilane [Si(OEt) 4 , TEOS] are the most commonly used precursors in silica sol-gel processing. The alkoxides are hydrolyzed in their parent alcohols, with a mineral acid or base catalyst, producing silicate gels that can be deposited as coatings. The Stober method, which utilizes this chemistry, relies on homogeneous nucleation to produce monodisperse sols.

Iler’s DS method of silica film formation was originally patented as a pigment coating to increase dispersibility of titania particles for use in the paint industry. The DS method is based on the aqueous chemistry of silica and takes advantage of the species present in solution at varying pH. Below pH 7 three-dimensional gel networks are formed. Above pH 7 silica surfaces are quite negatively charged ( [link] ), so that particle growth occurs without aggregation. The isoelectric point of silica is pH 2. Reactions above and below pH 2 are thought to occur through bimolecular nucleophilic condensation mechanisms. Above pH 2 an anionic species attacks a neutral species ( [link] ) and below pH 2 condensation involves a protonated silanol ( [link] ). The DS process has been utilized extensively in sol-gel coating technology and as a growth method for monodisperse and polydisperse sols.

Bibliography

  • B. C. Bunker, P. C. Rieke, B. J. Tarasevich, A. A. Campbell, G. E. Fryxall, G. L. Graff, L. Song, J. Liu, J. W. Virden, and G. L. McVay, Science , 1994, 264 , 48.
  • P.-H. Chang, C.-T. Huang, and J.-S. Shie, J. Electrochem. Soc. , 1997, 144 , 1144.
  • J.-S. Chou and S.-C. Lee, J. Electrochem. Soc. , 1994, 141 , 3214.
  • T. Homma, T. Katoh, Y. Yamada, and Y. Murao, J. Electrochem. Soc. , 1993, 140 , 2410.
  • R. K. Iler, The Chemistry of Silica Solubility, Polymerization, Colloid and Surface Properties, and Biochemistry , John Wiley&Sons (1979).
  • H. R. Jafry, E. A. Whitsitt, and A. R. Barron, J. Mater. Sci ., 2007, 42 , 7381.
  • T. Niesen and M. R. De Guire, J. Electroceramics , 2001, 6 , 169.
  • N. Ozawa, Y. Kumazawa, and T. Yao, Thin Solid Films , 2002, 418 , 102.
  • W. Stober, A. Fink, and E. Bohn, J. Colloid Interface Sci. , 1968, 26 , 62.
  • D. Whitehouse, Glass of the Roman Empire , Corning (1988).
  • E. A. Whitsitt and A. R. Barron, Nano Lett ., 2003, 3 , 775.
  • E. A. Whitsitt and A. R. Barron, Chem. Commun ., 2003, 1042.
  • E. A. Whitsitt and A. R. Barron, J. Colloid Interface Sci ., 2005, 287 , 318.
  • C.-F. Yeh, C.-L. Chen, and G.-H. Lin, J. Electrochem. Soc. , 1994, 141 , 3177.

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Source:  OpenStax, Chemistry of electronic materials. OpenStax CNX. Aug 09, 2011 Download for free at http://cnx.org/content/col10719/1.9
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