1. School of Nano-Tech and Nano-Bionics, University of Science and Technology of China, 230026 Hefei, China
2. Nanofabrication facility, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou, Jiangsu 215123, China
3. Department of Electrical and Computer Engineering, Iowa State University, Ames, Iowa 50011, USA