1. National Laboratory of Solid State Microstructures, Department of Materials Science and Engineering, College of Engineering and Applied Science, and Collaborative Center of Advanced Materials, Nanjing University, Nanjing 210093, China
2. Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, USA
3. Helmholtz-Zentrum Dresden-Rossendorf, Institute of Ion Beam Physics and Materials Research, Bautzner Landstr, 400, 01328 Dresden, Germany