1. Institute for Advanced Materials and Guangdong Provincial Key Laboratory of Optical Information Materials and Technology, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, China
2. Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Rd., Pokfulam, Hong Kong
3. Department of Physics, Boston College, Chestnut Hill, Massachusetts 02467, USA
4. 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, China
5. Laboratory of Solid-State Microstructures, Nanjing University, Nanjing 210093, China