1. National Laboratory of Solid State MicrostructuresJiangsu Key Laboratory of Artificial Functional MaterialsCollege of Engineering and Applied SciencesNanjing University Nanjing 210093 P. R. China
2. Collaborative Innovation Center of Advanced MicrostructuresNanjing University Nanjing 210093 P. R. China
3. Department of Materials Science and EngineeringUniversity of California Irvine CA 92697 USA
4. Department of Physics and AstronomyUniversity of California Irvine CA 92697 USA