Braun Group People
Robert Shimmin ,
Former Graduate Student in Materials Science and Engineering
B.S. degree in Chem Engr, UIUC, 2000, Ph.D. received 7/07
Under conditions as simple as being dried, suspensions of monodisperse polymer microspheres can self-assemble into ordered arrays of surprising regularity. The resulting colloidal crystals, and the inverse opals that can be templated from them, have promise in a variety of applications, including filters, chemical sensors, and optical components. The twofold goal of my research is to better understand the mechanisms behind colloidal self-assembly, and to expand the range of processing techniques that exist for preparing useful materials from colloidal crystal templates.
To better understand the mechanisms behind colloidal assembly, one focus of my research has been the study of suspended particles near an evaporating liquid interface. I have predicted and observed that when an evaporating liquid interface falls faster than the suspended particles sediment, the particles accumulate beneath the interface, and under some conditions accumulate in an ordered fashion. The relationship between such wet interfacial colloidal crystals, and the dry colloidal crystals that can be deposited on a substrate vertically immersed in an evaporating colloidal suspension, remains a topic of my research.
Colloidal crystals, while having diffraction properties worth study in their own right, become more interesting from an applications point of view when thought of as templates for other three-dimensionally ordered microporous structures. When infiltrated with appropriate functional materials, they can become chemical sensors, size-selective filters, catalytic supports, and photonic bandgap materials. Unfortunately, many of the materials one would like to implant in colloidal crystals can be prepared only at elevated temperatures, where most polymers become soft and microstructure sinters away. Another goal of my research is to resolve this problem, both by infiltrating polymer colloidal crystals with pre-formed nanoparticles of functional materials, and by preparing colloidal crystal templates from heat-resistant polymers.
A colloidal crystal of 1-micron polystyrene spheres, infiltrated with germanium nanoparticles. Germanium has a very high refractive index, and composites such as these may represent a step towards photonic bandgap materials.
Publications:
Shimmin, R. G.; DiMauro, A. J.; Braun, P. V. "Slow vertical deposition of colloidal crystals: A Langmuir-Blodgett process?" Langmuir 2006, 22(15), 6507-6513.
Shimmin, R. G.; Schoch, A. B.; Braun, P. V. "Polymer size and concentration effects on the size of gold nanoparticles capped by polymeric thiols." Langmuir 2004, 20(13), 5613-5620.
Professor Paul Braun • Phone: +1.217.244.7293 • Fax: +1.217.333.2736 • Email: pbraun@illinois.edu
Department of Materials Science and Engineering • University of Illinois at Urbana-Champaign