1. Department of Physics, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan 430074, P. R. China
2. Department of Chemical Engineering, The Ohio State University, Columbus, Ohio 43210, USA
3. Department of Electronics and Information Engineering, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan 430074, P. R. China