1. School of Physics State Key Laboratory of Crystal Materials Shandong University Jinan Shandong 250100 P. R. China
2. State Key Laboratory of Physical Chemistry of Solid Surfaces College of Chemistry and Chemical Engineering Xiamen University Xiamen Fujian 361005 P. R. China
3. ARC Centre of Excellence in Exciton Science School of Chemistry The University of Melbourne Parkville Victoria 3010 Australia