1. Guangdong Provincial Key Laboratory of Optical Information Materials and Technology & Institute of Electronic Paper Displays South China Academy of Advanced Optoelectronics South China Normal University Guangzhou 510006 P. R. China
2. National Center for International Research on Green Optoelectronics South China Normal University Guangzhou 510006 P. R. China
3. Department of Chemistry City University of Hong Kong 83 Tat Chee Avenue Kowloon Hong Kong 999077 P. R. China