1. State Key Laboratory of Automotive Safety and Energy Tsinghua University Beijing 100084 P. R. China
2. State Key Laboratory for Superlattices and Microstructures Institute of Semiconductors Chinese Academy of Sciences Beijing 100083 China
3. Center of Materials Science and Optoelectronic Engineering University of Chinese Academy of Sciences Beijing 100029 China
4. Institute of Automation Chinese Academy of Sciences Beijing 100190 China