1. State
Key Lab of Optoelectronic Materials and Technologies, School of Electronics
and Information Technology, Sun Yat-sen University, Guangzhou 510275, P.R. China
2. Department
of Physics, Hong Kong University of Science and Technology, Clear
Water Bay, Kowloon, Hong
Kong, China
3. Institute
of Applied Physics and Materials Engineering, University of Macau, Avenida da Universidade, Taipa, Macau, China