1. Department of Electronic Engineering, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong SAR, P. R. China
2. School of Electronic Science and Engineering, Nanjing University, Nanjing 210093, Jiangsu, P. R. China
3. Materials Science and Technology Research Center, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong SAR, P. R. China