1. Department of Chemistry and Biochemistry, Ohio State University, Columbus, Ohio 43210, USA
2. MEMSIC, One Tech Drive, Suite 325, Andover, Massachusetts 01810, USA
3. School of Materials Science and Engineering-Low dimensional Carbon Materials Center, Ulsan National Institute of Science and Technology, Ulsan 688-798, South Korea
4. Department of Physics, Ohio State University, Columbus, Ohio 43210, USA