1. Department of Materials Science State Key Laboratory of ASIC and Systems Fudan University Shanghai 200433 P. R. China
2. Center of Nanoelectronics and School of Microelectronics Shandong University Jinan 250100 P. R. China
3. Department of Physics Fudan University Shanghai 200433 P. R. China
4. Department of Electrical and Computer Engineering Micro and Nanotechnology Laboratory University of Illinois Urbana−Champaign 208 N. Wright Street Urbana IL 61801 USA