1. Institute for Advanced Materials and Laboratory of Quantum Engineering and Quantum Materials; South China Normal University; Guangzhou 510006 China
2. Electronic Paper Displays Institute; South China Normal University; Guangzhou 510006 China
3. Department of Physics; Boston College; Chestnut Hill Massachusetts 02467 USA
4. Department of Physics and TcSUH; University of Houston; Houston Texas 77204 USA
5. Institute of Experimental Physics; Freie University of Berlin; Berlin 14195 Germany
6. Laboratory of Solid State Microstructures; Nanjing University; Nanjing 210093 China