1. School of Materials Science and EngineeringGeorgia Institute of Technology Atlanta GA 30332‐0245 USA
2. Department of Applied PhysicsState Key Laboratory of Power Transmission Equipment and System Security and New TechnologyChongqing University Chongqing 400044 P. R. China
3. College of Nanoscience and TechnologyUniversity of Chinese Academy of Sciences Beijing 100049 P. R. China