1. CAS Key Laboratory of Standardization and Measurement for Nanotechnology, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, P. R. China
2. Academy for Advanced Interdisciplinary Studies, Peking University, Beijing 100871, P. R. China
3. University of Chinese Academy of Sciences, Beijing 100049, P. R. China
4. College of Engineering, Swansea University, Swansea SA1 8EN, United Kingdom