1. Department of Physics; The University of Hong Kong; Pokfulam Road Hong Kong Hong Kong
2. Department of Electronic and Information Engineering; The Hong Kong Polytechnic University, Hung Hom, Kowloon; Hong Kong Hong Kong
3. Department of Chemistry; The University of Hong Kong; Pokfulam Road Hong Kong Hong Kong
4. Department of Physics; South University of Science & Technology of China; Tangchang Road, Nanshan District, Shenzhen Guangdong 518055 China