a
Laboratory for Nanomaterials, National Center for Nanoscience and Technology, 100190 Beijing, China
E-mail: zytang@nanoctr.cn
;
Fax: +86-10-62656765
;
Tel: +86-10-82545580
b
Key Laboratory of Materials Physics, Anhui Key Laboratory of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China
c
Laboratory of Optical Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China