1. Department
of Electronic Engineering and Materials
Science and Technology Research Center, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong SAR, P. R. China
2. National Laboratory of Microstructures (Nanjing), Key Laboratory on Advanced Photonic & Electronic Materials of Jiangsu Province, School of Electronic Science & Engineering, Nanjing University, Nanjing 210093, P. R. China