1. Graphene Research Center & Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542, Singapore
2. II. Institute of Physics, RWTH Aachen University, 52074 Aachen, Germany
3. JARA: Fundamentals of Future Information Technology, Jülich-Aachen Research Alliance, Aachen, Germany
4. SKKU Advanced Institute of Nanotechnology (SAINT) and Center for Human Interface Nanotechnology (HINT), Department of Chemistry, Sungkyunkwan University, Suwon, 440-746, Korea
5. Nanocore, National University of Singapore, Faculty of Engineering, Engineering Block A, EA, Level 4, Room No.27, Singapore 117576, Singapore
6. NUS Graduate School for Integrative Sciences and Engineering (NGS), 05-01, 28 Medical Drive, Singapore 117456, Singapore