1. Department of Chemical Engineering, University of Florida, Gainesville, Florida 32611, USA
2. Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611, USA
3. Department of Physics, Arizona State University, Tempe, Arizona 85287, USA
4. Department of Chemical and Biological Engineering, Korea University, Seoul 136-713, South Korea
5. Department of Electronic Science and Technology, Hebei University of Technology, Tianjin 300401, China