1. Department of Applied Physics, Aalto University, P.O. Box 14100, FIN-00076 Aalto Espoo, Finland
2. Department of Microtechnology and Nanoscience, Chalmers University of Technology, 41296 Göteborg, Sweden
3. State Key Laboratory of Functional Materials for Informatics Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences 865 Changning Road, Shanghai 200050, China