* Corresponding authors

^a CAS Key Laboratory of Standardization and Measurement for Nanotechnology, National Center for Nanoscience and Technology, Beijing 100190, P. R. China
E-mail: xielm@nanoctr.cn
Tel: +86-010 8254 5692

^b Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan, Republic of China
E-mail: ysh@mail.ntust.edu.tw

^c State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China

^d Center for Nanochemistry, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China