1. Department of Physics, Chemistry and Biology, Linköping University, 58183 Linköping, Sweden
2. Physikalisches Institut, Justus-Liebig-University Giessen, Heinrich-Buff-Ring 16, 35392 Giessen, Germany
3. Department of Chemical and Biological Engineering, Korea University, Seoul 136-713, South Korea
4. Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611, USA