1. CAS Key Laboratory of Nanosystem and Hierarchical FabricationCAS Centre for Excellence in NanoscienceNational Centre for Nanoscience and Technology Beijing 100190 China
2. School of Nanoscience and TechnologyUniversity of Chinese Academy of Sciences Beijing 100049 China
3. CAS Key Laboratory of Standardization and Measurement for NanotechnologyCAS Center for Excellence in NanoscienceNational Centre for Nanoscience and Technology Beijing 100190 China
4. State Key Laboratory of Superlattice and MicrostructureInstitute of SemiconductorsChinese Academy of Science Beijing 100083 China