1. Nanoelectronics Lab., Samsung Advanced Institute of Technology, Suwon 16678, South Korea
2. Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
3. Department of Physics and Astronomy, Seoul National University, Seoul 08826, South Korea
4. Daegu Gyeongbuk Institute of Science and Technology, Daegu 42988, South Korea