1. School of Nano Technology and Nano Bionics University of Science and Technology of China Hefei 230026 P. R. China
2. Nanofabrication Facility of Suzhou Institute of Nano-Tech and Nano-Bionics Chinese Academy of Sciences (CAS) Suzhou 215123 P. R. China
3. Department of Electrical and Computer Engineering Iowa State University Ames IA 50011 USA
4. School of Electrical, Computer and Energy Engineering Arizona State University Tempe AZ 85287 USA
5. Division of Nano-Devices and Technologies & Nanchang Key Laboratory of Advanced Packaging Jiangxi Institute of Nanotechnology Nanchang 330200 China