1. Department of Energy Science (DOES) and Center for Artificial Atoms, Sungkyunkwan University (SKKU), Suwon, Gyeonggi-do 16419, South Korea
2. Sungkyunkwan Institute of Energy Science and Technology (SIEST), Sungkyunkwan University (SKKU), Suwon, Gyeonggi-do 16419, South Korea
3. Graduate School of Nanoscience and Technology, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, South Korea
4. Korea Research Institute of Standards and Science (KRISS), 267 Gajeong-ro, Daejeon 34113, South Korea
5. University of Science and Technology (UST), 217 Gajeong-ro, Daejeon 34113, South Korea
6. Department of Physics, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, South Korea