1. Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China
2. Department of Applied Physics, Hong Kong Polytechnic University, Kowloon, Hong Kong, China
3. Nano and Advanced Materials Institute Limited, No. 8 Science Park West Avenue, Hong Kong Science Park, New Territories, Hong Kong, China
4. Department of Chemistry, William Mong Institute of Nano Science and Technology, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China