1. Institute for Advanced Materials and Guangdong Provincial Key Laboratory of Optical Information Materials and TechnologyAcademy of Advanced OptoelectronicsSouth China Normal University Guangzhou 510006 China
2. Guangdong Provincial Key Laboratory of Optical Information Materials and Technology & Institute of Electronic Paper DisplaysSouth China Academy of Advanced OptoelectronicsSouth China Normal University Guangzhou 510006 China
3. Laboratory of Solid State MicrostructuresNanjing University Nanjing 210093 China
4. Department of PhysicsBoston College Chestnut Hill MA 02467 USA