Novel Routes to Photonic Crystals
Wendy Chan, Former Graduate student in Materials Science and Engineering
One method of achieving self-assembled colloidal crystals is by settling the colloidal particles into a closely packed arrangement by gravitational forces and Brownian motion. Under the appropriate conditions, a reasonable degree of crystallinity is achieved. However, when the solvent is allowed to evaporate, the drying forces act to randomize the arrangement of the particles and the crystallinity of the dry sample is significantly lower than that of the wet state.
The Lewis group has developed a novel technique for resolving this issue called nanoparticle haloing, which involves the addition of charged zirconia nanoparticles to a colloidal suspension near its isoelectric point.1,2 The nanoparticles halo the negligibly charged colloidal spheres and facilitate close packing by inhibiting the long range van der Waals forces. Another function of this system is that when the pH is elevated, the zirconia nanoparticles undergo a gelling transition and the dynamic system is immobilized. There is very little difference in the packing arrangement between the wet and gelled state.
Currently, I am investigating the crystallinity change of the binary system as it transitions from the wet, gelled, and finally the dried state. A qualitative and quantitative study is underway which consists of taking spectroscopic data and IR images of the sample over the three phases of interest.
Another area of interest in our group is colloidal epitaxy, or the sedimentation of colloidal spheres onto a 2-D array of holes. This bottom-up approach was first introduced by van Blaaderen and his group.3 The template directs the assembly of the first layer and each subsequent layer uses the former as a template. This technique places a great deal of control in the hands of the experimentalist, which allows for the minimization of defects in the colloidal crystal. Our group is working on settling the binary suspension described earlier onto these patterned substrates and monitoring the changes in crystallinity through the different phases.
Image: Upper left corner of an FIB pattern. The nominal pitch size of the holes is 342nm. The pattern is milled into the substrate by the dual-beam focused ion beam (DB-FIB) microscope.
References
1Tohver, V; Smay, J.E.; Braem, A.; Braun, P.V.; and Lewis, J.A. Nanoparticle halos: A new colloid stabilization mechanism. Proc. Nat. Acad. Sci. 98, 8950-8954, (2001).
2Tohver, V.; Chan, A.; Sakurada, O.; and Lewis, J.A. Nanoparticle Engineering of Complex Fluid Behavior. Langmuir, 17, 8414-8421 (2001).
3van Blaaderen, A.; Ruel, R; Wiltzius, P. Nature, 385, 321-323 (1997).
Professor Paul Braun • Phone: +1.217.244.7293 • Fax: +1.217.333.2736 • Email: pbraun@uiuc.edu
Department of Materials Science and Engineering • University of Illinois at Urbana-Champaign