1. National Key Laboratory of Science and Technology on Micro/Nano Fabrication Shanghai Jiao Tong University Shanghai 200240 P. R. China
2. Department of Micro/Nano Electronics School of Electronic Information and Electrical Engineering Shanghai Jiao Tong University Shanghai 200240 P. R. China
3. Institute of Nano/Micro Energy Shanghai Jiao Tong University Shanghai 200240 P. R. China
4. School of Materials Science and Engineering Henan Polytechnic University Henan 454003 P. R. China
5. School of Physics and Astronomy Shanghai Jiao Tong University Shanghai 200240 P. R. China
6. Department of Chemical and Biological Engineering University at Buffalo The State University of New York Buffalo NY 14260 USA