1. Center for Integrated Nanostructure Physics; Institute for Basic Science; Sungkyunkwan University; Suwon 440-746 Republic of Korea
2. Department of Energy Science; Department of Physics; Sungkyunkwan University; Suwon 440-746 Republic of Korea
3. School of Advanced Materials Science and Engineering; Sungkyunkwan University; Suwon 440-746 Republic of Korea
4. Department of Energy and Materials Engineering; Dongguk University-Seoul; Seoul 100-715 Republic of Korea