1. Korea Photonics Technology Institute, Chumdan 4-ro
5, Buk-gu, Gwangju 500-779, Republic of Korea
2. Department of Physics and
Graphene Research Institute, Sejong University, 98 Gunja-dong, Gwangjin-gu, Seoul 143-747, Republic of Korea
3. Creative Future Research Laboratory, Electronics and Telecommunications Research Institute, 218 Gajeong-ro, Yuseong, Daejeon 305-700, Republic of Korea