1. Key Laboratory of Materials Physics and Anhui Key Laboratory of Nanomaterials and Nanostructures, Institute of Solid State Physics; Chinese Academy of Sciences; P. O. Box 1129; Hefei; 230031; P. R.; China
2. Key Laboratory of Ion Beam Bioengineering, Hefei Institutes of Physical Science; Chinese Academy of Sciences; Hefei; 230031; P. R.; China