1. Laboratory for Nanomaterials, National Center for Nanoscience and Technology, Beijing 100190, China
2. Laboratory of Optical Physics,
Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
3. Key Laboratory of Materials Physics,
Anhui Key Laboratory of Nanomaterials and Nanotechnology, Institute
of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, Anhui, China