1. Department of Electronic Engineering and Materials Science and Technology Research Center; The Chinese University of Hong Kong; Shatin, N. T. Hong Kong SAR 999077 P. R. China
2. Siyuan Laboratory; Department of Physics; Jinan University; Guangzhou Guangdong 510632 P. R. China
3. School of Physics and Engineering; Sun Yat-Sen University; Guangzhou Guangdong 510275 P. R. China