1. Materials Research Center, Samsung Advanced Institute of Technology, Samsung Electronics, Suwon 443-803, South Korea.
2. Department of Nano Applied Engineering, Kangwon National University, Chuncheon 200-701, South Korea.
3. Department of Energy Science, Sungkyunkwan University, 300 Cheoncheon-dong, Jangan-gu, Suwon 440-746, South Korea.
4. IBS Center for Integrated Nanostructure Physics, Institute for Basic Science, Sungkyunkwan University, Suwon 440-746, South Korea.
5. Materials Science, California Institute of Technology, Pasadena, California 91125, USA.