1. Institute of Microelectronics Tsinghua National Laboratory for Information Science and Technology (TNList) Tsinghua University Beijing 100084 China
2. Department of Electrical Engineering and Computer Science & Microsystems Technology Laboratories (MTL) Massachusetts Institute of Technology 77 Massachusetts Avenue Cambridge MA 02139 USA
3. Center for Excellence in Nanoscience Key Laboratory of Nanosystem and Hierarchical Fabrication (CAS) National Center for Nanoscience and Technology Beijing 100190 China