1. Department of Electrical and Computer Engineering and National University of Singapore Nanoscience and Nanotechnology Initiative, National University of Singapore, Singapore 117576, Singapore.
2. National Institute for Materials Science, Tsukuba 305-0047, Japan.
3. Department of Materials Science and Engineering, Korea University, Seoul 02841, Korea.
4. KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul 02841, Korea.
5. Department of Physics, The University of Tokyo, Bunkyo, Tokyo 113-0033, Japan.