1. Department of Physics; Harbin Institute of Technology; Harbin 150001 P. R. China
2. College of Chemistry and Environment Science; Key Laboratory of Chemical Biology of Hebei Province; Hebei University; Baoding 071002 P. R. China
3. Key Laboratory of Organic Synthesis of Jiangsu Province; Soochow University; Suzhou 215123 P. R. China
4. School of Physical Science and Technology; Soochow University; Suzhou 215006 P. R. China