Fourier Transform Infrared Spectroscopy Exposes Self-Assembly
of Mesoporous Thin Films
The low index of refraction and ultralow dielectric constant of mesoporous ceramic thin films make the materials of interest for a variety of applications in optics and electronics. Now a spectroscopic technique has demonstrated the ability to monitor the self-assembly processes involved in the formation of these thin films.
Plinio Innocenzi of Università di Sassari in Alghero, Italy, in collaboration with the laboratory of infrared synchrotron light at the Instituto Nazionale di Fisica Nucleare in Frascati, Italy, has led the development of a time-resolved Fourier transform infrared (FTIR) spectroscopy method that can identify the chemical stages of thin-film self-assembly. The setup incorporates a standard broadband IR globar, a Bruker Optics Inc. Equinox 55 spectrometer that has been modified to operate in vacuum and a Bruker IR microscope. The output is detected using a cooled MCT detector. With the resolution set to 8 cm–1, the range from 500 to 6000 cm–1 can be covered in a 133-ms scan.
Mesoporous thin films self-assemble with voids 2 to 50 nm in diameter. A precursor solution that contains a supramolecular template collects into micelles. Simultaneously, sol-gel polycondensation creates “nanobricks” that interact with the surface of the micelles, producing a porous structure that remains after the removal of the organic template. These processes must be balanced to create stable mesoporosity.
In a demonstration of the technique, the scientists placed 2 to 5 μl of solution on a diamond disk and collected spectra for about 50 seconds. The spectra were organized into a 3-D map, highlighting spectral changes as a function of time. Several of the molecules involved in the self-assembly process share similar spectral features, but the different molecular constituents could be identified by observing the time-dependency of the spectra.
The work exposed a four-stage mechanism in the self-assembly of silica and titania films. A rapid initial evaporation step is followed by an intermediate stage and by a water-enrichment stage. For silica, the greater presence of water is a byproduct of polycondensation, while the presumed mechanism in titania is water captured from vapor in the atmosphere. After a second evaporation stage, the assembly is complete.
Innocenzi said that knowledge of the exact kinetics of the chemical processes involved in self-assembly will enable scientists to engineer highly organized materials such as porous structures with cubic or hexagonal pores. The pore surfaces are covered with OH groups, which enable functional molecules or nanoparticles to be incorporated easily.
The researchers believe that this is the first time that time-resolved infrared spectroscopy has been applied to in situ liquid film deposition. They have performed simultaneous small-angle x-ray scattering and FTIR experiments to provide insight into the structural and chemical evolution of the self-assembly process.
“We believe that [a] multitechnique approach for the in situ analysis of time-resolved experiments is a really new frontier in materials science,” Innocenzi said.
Journal of Physical Chemistry B, June 8, 2006, pp. 10837-10841.
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