1. Department of Applied Physics The Hong Kong Polytechnic University Hung Hom Hong Kong P. R. China
2. Yangtze Delta Region Institute (Huzhou) & Institute of Fundamental and Frontier Science University of Electronic Science and Technology of China Huzhou 313001 P. R. China
3. School of Chemistry and Chemical Engineering Key Laboratory of Functional Inorganic Material Chemistry of Anhui Province Anhui University Hefei 230601 P. R. China
4. School of Physical and Mathematical Sciences Nanyang Technological University Singapore 637371 Singapore