1. School of Nano-Tech and Nano-Bionics, University of Science and Technology of China, Hefei, Anhui 230026, People's Republic of China
2. Nano Fabrication Facility, Suzhou Institute of Nano-Tech and Nano-Bionics, CAS, Suzhou, Jiangsu 215123, People's Republic of China
3. School of Physics and Electronics, Central South University, Changsha 410083, People's Republic of China