1. School of Nano-Tech and Nano-Bionics University of Science and Technology of China Hefei 230026 China
2. Nanchang Nano-Devices and Technologies Division Suzhou Institute of Nano-Tech and Nano-Bionics Chinese Academy of Sciences Nanchang 330200 China
3. Key Laboratory of Multifunctional Nanomaterials and Smart Systems Suzhou Institute of Nano-Tech and Nano-Bionics CAS Suzhou 215123 China
4. Department of Electrical and Computer Engineering Iowa State University Ames IA 50011 USA