1. Key Laboratory of Materials Physics, Anhui Key Laboratory of Nanomaterials and Nanostructures, CAS Center for Excellence in Nanoscience, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, P. R. China.
2. Institute of Physical Science and Information Technology, Anhui University, Hefei 230601, P. R. China.
3. Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA 15213, USA.