1. State Key Lab of Optoelectronic Materials and Technologies; School of Physics and Engineering; Sun Yat-sen University; Guangzhou 510275 P. R. China
2. Department of Materials Science and Engineering; College of Engineering; Peking University; Beijing 100871 P. R. China
3. Institute of Applied Physics and Materials Engineering; University of Macau; Avenida da Universidade; Taipa Macau China