1. Department of Materials Science and EngineeringCollege of EngineeringPeking University Beijing 100871 P. R. China
2. CAS Key Laboratory of Standardization and Measurement for NanotechnologyCAS Center for Excellence in NanoscienceNational Center for Nanoscience and Technology Beijing 100190 P. R. China
3. The State Key Laboratory of BioelectronicsSoutheast University Nanjing 210096 P. R. China