J. Microscopy 2004, 216, 263-287

 

A Method for Determining Void Arrangements in Inverse Opals

 

Christopher F. Blanford†, C. Barry Carter* and Andreas Stein

 

Department of Chemistry, University of Minnesota,

139 Smith Hall, 207 Pleasant St SE, Minneapolis, MN 55455

 

* Department of Chemical Engineering and Materials Science, University of Minnesota,

421 Washington Ave. SE, 151 Amundson Hall, Minneapolis, MN 55455

 

Abstract

 

The periodic arrangement of voids in ceramic materials templated by colloidal crystal arrays (inverse opals) has been analyzed by transmission electron microscopy (TEM). Individual particles consisting of a approximately spherical array of at least 100 voids were tilted through 90° along a single axis within the TEM. The bright-field images of these particles at high-symmetry points, their diffractograms calculated by fast Fourier transforms (FFTs), and the TEM goniometer angles were compared to model face-centred cubic (fcc), body centred cubic (bcc), hexagonal close-packed (hcp), and simple cubic (sc) lattices in real and reciprocal space. The spatial periodicities were calculated for two-dimensional projections. The systematic absences in these diffractograms differed from those found in diffraction patterns from three-dimensional objects. The experimental data matched only the model fcc lattice, so it could be concluded that the packing of the voids (and, thus, the polymer spheres that composed the original colloidal crystals) was face-centred cubic. In fcc structures, the stacking-fault displacement vector is a6 <211>. No stacking faults were observed when viewing the inverse opal structure along the orthogonal <110>-type directions, eliminating the possibility of a random hexagonally close-packed (rhcp) structure for the particles observed. This technique complements synchrotron X-ray scattering work on colloidal crystals by allowing both real-space and reciprocal-space analysis to be carried out on a smaller cross-sectional area.