1. Department of Materials Science and Engineering, University of California, Berkeley, California 94720, USA
2. Department of Physics, University of California, Berkeley, California 94720, USA
3. Department of Materials Science and Engineering, Zheijian University, Hangzhou, China
4. Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
5. School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853, USA
6. Kavli Institute at Cornell for Nanoscale Science, Ithaca, New York 14853, USA
7. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA