Braun Group People
Matthew George
Graduate Student in Materials Science and Engineering
Bachelor of Science from U.C. Berkeley, Department of Materials Science and Mineral Engineering
Secondary School: McKinleyville High
Research Interests: Inorganic Photoresists, Phase Mask Lithography, Multiphoton Laser Direct Writing, Photo-responsive Colloidal Systems, Colloidal Laser Gel Writing
Inorganic Photoresists for 3D Microfabrication
Photoresists are used as templates for photonic crystal based devices, but organic photo-resists are not compatible with Silicon LPCVD (550°C) and other high temperature processing steps, thus limiting their utility. My primary efforts are in developing an inorganic photoresist for 3D microfabrication applications, including photonic crystal fabrication. I generally utilize Proximity Field Nano-Patterning (PFBP)[1] to characterize the resists’ ability to photo-pattern 3D microstructures. In PFNP, a single beam of radiation interacts with a phase mask, giving multiple diffracted orders. The diffracted beams from the phase mask interfere within the photo resist, producing a periodic three dimensional intensity distribution. When the sample is baked and developed, only the areas that were exposed to high intensities remain, giving the desired three-dimensional periodic structure .
SEM micrographs of 3D photonic crystals generated via PFNP
Careful choice of developer is necessary to prevent swelling and delamination or collapse of the patterned regions.
SEM micrographs of swollen, distorted PFNP structures
Photo-responsive Colloidal Systems
Colloidal Systems provide another route to photonic crystals. Self assembly of isotropic, spherical colloids can only form FCC and HCP structures, without precise control over defect placement. We are working on photoresponsive colloidal systems in which the assembly is controlled by light in some fashion. We showed the ability to use light to control the adsorption of colloids onto polymer brush coated cover slips.[ii]
Photo-patterned colloidal adsorption schematic
Confocal image of photo-patterned colloidal adsorption
Colloidal Laser Gel Writing
More recently, we have had success in locally controlling the gelation behavior of colloidal sediments using multiphoton absorption at the focal point of a Ti:Sapphire Laser coupled to a confocal microscope .
S. Jeon, J.-U. Park, R. Cirelli, S. Yang, C.E. Heitzman, P.V. Braun, P.J.A. Kenis, and J.A. Rogers, “Fabricating Complex Three Dimensional Nanostructures With High Resolution Conformable Phase Masks”, Proc. Nat. Acad. Sci. USA, 101 (34), 12428-12433 (2004).
M. Piech, M. C. George, N. S. Bell and P. V. Braun: “Patterned colloid assembly by grafted photochromic polymer layers”,Langmuir, 22, 1379-1382 (2006).
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