1. Department of Applied Physics, The Hong Kong Polytechnic University 1 , Hung Hom, Hong Kong, China
2. Songshan Lake Materials Laboratory 2 , Dongguan, Guangdong 523808, China
3. College of Electronic Information and Mechatronic Engineering, Zhaoqing University 3 , Zhaoqing, Guangdong 526061, China
4. School of Science and Engineering, Chinese University of Hong Kong 4 , Shenzhen, Guangdong 518172, China