1. College of Electronic and Optical Engineering and College of Microelectronics, Nanjing University of Posts and Telecommunications, Nanjing 210023, People's Republic of China
2. Department of Physics and Interdisciplinary Program of Biomedical, Mechanical and Electrical Engineering, Pukyong National University, Busan 608-737, South Korea
3. School of Material Science and Engineering, Jiangsu University, Zhenjiang 212013, People's Republic of China