1. SKKU Advanced Institute of Nanotechnology, Sungkyunkwan University (SKKU), Suwon 440-746, Korea.
2. School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 440-746, Korea.
3. Research Center for Time-Domain Nano-functional Devices, Samsung Advanced Institute of Technology, Yongin 449-712, Korea.
4. Nano Electronics Laboratory, Samsung Advanced Institute of Technology, Yongin 449-712, Korea.
5. Department of Physics, Sungkyunkwan University, Suwon 440-746, Korea.