1. Nanoscale Science and Engineering Center, University of California, Berkeley, CA 94720, USA.
2. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
3. Physical Sciences and Engineering Division, King Abdullah University of Science and Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia.
4. Department of Physics, Nanjing Normal University, Nanjing, Jiangsu 210023, China.