a SEU-FEI Nano Pico Center, Key Laboratory of MEMS of Ministry of Education, School of Electronics Science and Engineering, Southeast University, Nanjing, P. R. China
b Laboratory of Condensed Matter Spectroscopy and Opto-Electronic Physics, and Key Laboratory of Artificial Structures and Quantum Control (Ministry of Education), Department of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai, 200240, P. R. China
c School of Physical Science & Technology, Guangxi Key Laboratory for Relativistic Astrophysics, Guangxi University, Nanning 530004, P. R. China