1. Department of Physics, University of California at Berkeley, Berkeley, California 94720, United States
2. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
3. Kavli Energy NanoSciences Institute at the University of California at Berkeley, Berkeley, California 94720, United States
4. Department of Materials, Imperial College London, Prince Consort Rd, London SW7 2BB, U.K.
5. John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, United States
6. NEST, Istituto Nanoscienze-CNR and Scuola Normale Superiore, Piazza San Silvestro 12, Pisa 56127, Italy
7. Tsinghua-Berkeley Shenzhen Institute & Shenzhen International Graduate School, Tsinghua University, Shenzhen 518055, China
8. School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China
9. Research Center for Electronic and Optical Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
10. Research Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
11. Dipartimento di Fisica, Università di Pavia, Pavia 27100, Italy