1. Key
Laboratory of Artificial Micro- and Nano-structures of Ministry of
Education, School of Physics and Technology, Wuhan University, Wuhan 430072, P. R. China
2. Laboratory
of Functional Nanomaterials and Printing Electronics, School of Printing
and Packaging, Wuhan University, Wuhan 430072, P. R. China
3. Department
of Physics and Materials Science, City University of Hong Kong, Hong Kong SAR, P. R. China