1. Key Laboratory of Materials Physics; CAS Center for Excellence in Nanoscience; Anhui Key Laboratory of Nanomaterials and Nanotechnology; Institute of Solid State Physics; Chinese Academy of Sciences; Hefei 230031 P. R. China
2. Department of Materials Science & Engineering University of Science and Technology of China; Hefei 230026 P. R. China
3. Department of Mechanical and Aerospace Engineering; West Virginia University; P.O. Box 6106 Morgantown WV 26506 USA
4. Institute of Technical Biology and Agriculture Engineering; Hefei Institutes of Physical Science; Chinese Academy of Sciences; Hefei 230031 P. R. China