1. College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, P. R. China
2. Department of Physical Science, Hiroshima University, Higashihiroshima, Hiroshima 739-8526, Japan
3. Department of Physics, Beihang University, Beijing 100191, P. R. China
4. State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry, Sun Yat-sen University, Guangzhou 510275, P. R. China