1. Department of Chemistry; Columbia University; New York NY 10027 USA
2. Department of Materials Science and Engineering; Yonsei University; Seoul 120-749 Korea
3. Department of Physics; Columbia University; New York NY 10027 USA
4. Creative Research Center for Graphene Electronics; Electronics and Telecommunications Research Institute(ETRI); Daejeon 305-700 Korea
5. National Institute for Materials Science; 1-1 Namiki Tsukuba 305-0044 Japan
6. Department of Mechanical Engineering; Columbia University; New York NY 10027 USA