1. Department of Materials Science and Engineering, University of California , Irvine, CA , USA
2. Department of Physics and Astronomy, University of California , Irvine, CA , USA
3. Insititue of Physics, Chinese Academy of Sciences , Beijing , China
4. Department of Chemistry, Princeton University , Princeton, NJ , USA
5. Irvine Materials Research Institute (IMRI), University of California , Irvine, CA , USA