1. School of Physics and Electronics, Shandong Normal University, Jinan 250358, China
2. Single Molecule Study Laboratory, College of Engineering and Nanoscale Science and Engineering Center, University of Georgia, Athens, Georgia 30602, USA
3. Biodesign Center for Bioelectronics and Biosensors, School of Electrical, Computer and Energy Engineering, Arizona State University, Tempe, Arizona 85287, USA