1. Beijing Institute
of Nanoenergy and Nanosystems, Chinese Academy of Sciences, National
Center for Nanoscience and Nanotechnology (NCNST), Beijing 100083, P. R. China
2. Department
of Organic Materials and Fiber Engineering, Soongsil University, Seoul 156-743, Korea
3. School
of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332−0245, United States