1. Department of Materials Science and Engineering, University of California , Irvine, CA , USA
2. Irvine Materials Research Institute (IMRI), University of California , Irvine, CA , USA
3. Institute of Nanotechnology, Karlsruhe Institute of Technology (KIT) , Karlsruhe , Germany
4. Department of Materials Science and Engineering, City University of Hong Kong , Hong Kong SAR , China
5. Department of Mechanical Engineering, The University of Hong Kong , Hong Kong , China
6. Department of Physics and Astronomy, University of California , Irvine, CA , USA