1. School of Science and Engineering The Chinese University of Hong Kong Shenzhen Guangdong 518172 P. R. China
2. Hefei National Laboratory for Physical Sciences at the Microscale School of Chemistry and Materials Science University of Science and Technology of China Hefei Anhui 230026 P. R. China
3. College of Engineering and Applied Sciences Nanjing University Nanjing Jiangsu 210093 P. R. China
4. Jiangsu Key Laboratory for Nano Technology National Laboratory of Solid State Microstructures Collaborative Innovation Center of Advanced Microstructures School of Physics Nanjing University Nanjing Jiangsu 210093 P. R. China