1. Graduate School of
Advanced Integration Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
2. Advanced Materials Laboratory, National Institute for Materials Science, 1-1 Namiki, Tsukuba, 305-0044, Japan
3. Department
of Material Science and Engineering, Yonsei University, Seoul 120-749, Korea
4. Creative Research Center for Graphene Electronics and Telecommunications Research Institute (ETRI), Daejeon 305-700, Korea
5. Department of Physics, Harvard University, Cambridge, Massachusetts 02138, United States