1. Department of Electrical and Computer Engineering, University of Nebraska-Lincoln, Lincoln, Nebraska 68588-0511, USA
2. School of Mechanical Engineering, Beijing Institute of Technology, 100081 Beijing, People's Republic of China
3. Institut de Chimie de la Matière Condensée de Bordeaux–ICMCB-CNRS 87, Avenue du Docteur Albert Schweitzer, F-33608 Pessac Cedex, France