1. Institute for Advanced Materials; South China Academy of Advanced Optoelectronics and Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials; South China Normal University; Guangzhou 510006 P. R. China
2. School of Chemistry and Environment; South China Normal University; Guangzhou 510006 P. R. China
3. Electronic Paper Displays Institute; South China Academy of Advanced Optoelectronics; South China Normal University; Guangzhou 510006 P. R. China
4. Laboratory of Solid State Microstructures; Nanjing University; Nanjing 210093 P. R. China
5. Department of Physics; Boston College; Chestnut Hill MA 02467 USA