1. Department of Materials Science and Engineering University of Washington Seattle WA 98195‐2120 USA
2. Guangdong Provincial Key Laboratory of Optical Information Materials and Technology and Institute of Electronic Paper Displays South China Academy of Advanced Optoelectronics South China Normal University Guangzhou 510006 P. R. China
3. Department of Physics and Astronomy Shanghai Jiao Tong University Shanghai 200240 China
4. State Key Laboratory of Luminescent Materials and Devices South China University of Technology Guangzhou 510640 P. R. China
5. Department of Chemistry City University of Hong Kong Kowloon 999077 Hong Kong
6. Department of Materials Science and Engineering City University of Hong Kong Kowloon 999077 Hong Kong