1. Department of Energy Science, Sungkyunkwan University, Suwon 16419, Republic of Korea
2. Center for Integrated Nanostructure Physics (CINAP), Institute for Basic Science (IBS), Suwon 16419, Republic of Korea
3. Department of Physics, Sungkyunkwan University, Suwon 16419, Republic of Korea
4. Korea Research Institute of Standards and Science (KRISS), Daejeon 34113, Republic of Korea