1. School of Physics State Key Laboratory of Crystal Materials Shandong University Jinan Shandong 250100 P. R. China
2. State Key Laboratory of Supramolecular Structure and Materials Institute of Theoretical Chemistry Jilin University Changchun 130012 P. R. China
3. ARC Centre of Excellence in Exciton Science School of Chemistry University of Melbourne Parkville Victoria 3010 Australia