1. Department of Physics, University of California at Berkeley, Berkeley, CA 94720, USA.
2. Material Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA, 94720, USA.
3. Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan.
4. Physical Sciences and Engineering, King Abdullah University of Science and Technology, Thuwal, Saudi Arabia.
5. Kavli Energy NanoSciences Institute, University of California Berkeley and Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.