1. Department of Physics; The Chinese University of Hong Kong; Shatin Hong Kong SAR China
2. School of Materials Science and Engineering; Shaanxi Normal University; Xi'an 710119 China
3. Division of Chemistry and Biological Chemistry; School of Physical and Mathematical Sciences; Nanyang Technological University; 637371 Singapore
4. Testing & Analysis Center; Soochow University; Suzhou 215123 China
5. College of Chemistry; Chemical Engineering and Materials Science; Soochow University; Suzhou 215123 China