1. Department of Electrical and Computer Engineering, Ajou University, Suwon, South Korea
2. Department of Physics and Department of Energy Systems Research, Ajou University, Suwon, South Korea
3. Center for Integrated Nanostructure Physics, Sungkyunkwan University (SKKU), Suwon, Republic of Korea
4. Department of Nano Engineering, Department of Nano Science and Technology, SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University (SKKU), Suwon, Republic of Korea