* Corresponding authors

a Key Laboratory of Materials Physics, and Anhui Key Laboratory of Nanomaterials and Nanostructures, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei, China

E-mail: gwmeng@issp.ac.cn

Fax: (86)551-5591434

b Graduate School of Chinese Academy of Sciences, Beijing, People's Republic of China

c Structure Research Lab, University of Science and Technology of China, Hefei, China