1. National Key Lab of Micro/Nano Fabrication Technology 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. University of Michigan–Shanghai Jiao Tong University Joint Institute Shanghai Jiao Tong University Shanghai 200240 P. R. China