1. State Key Laboratory of Crystal Materials; Shandong University; Jinan 250100 P.R. China
2. Jiangsu-Singapore Joint Research Center for Organic/Bio- Electronics & Information; Displays and Institute of Advanced Materials; Nanjing Tech University; Nanjing 211816 P.R. China
3. Department of Physics and Astronomy; Green State University; Bowling Green OH 43403 USA
4. Beijing Institute of Nanoenergy and Nanosystems; Chinese Academy of Science; Beijing 100864 P.R. China