1. Jiangsu Provincial Key Laboratory of Advanced Photonic and Electronic Materials, School of Electronic Science and Engineering, Nanjing National Laboratory of Microstructures, Nanjing University, Nanjing 210093, P. R. China
2. School of Physical and Mathematical Sciences, Nanjing Tech University, Nanjing 211800, P. R. China
3. State Key Laboratory of Bioelectronics, School of Biological Sciences and Medical Engineering, Southeast University, Nanjing 210096, P. R. China
4. School of Electronic Science and Engineering, Nanjing University of Posts and Telecommunications, Nanjing 210023, P. R. China
5. College of Optical, Mechanical and Electrical Engineering, Zhejiang A&F University, Hangzhou 311300, P. R. China