1. Department of Physics, University of California, Berkeley, CA 94720, USA.
2. Kavli Energy NanoScience Institute, University of California, and Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
3. Institute of Electro-Optical Science and Technology, National Taiwan Normal University, Taipei 11677, Taiwan.
4. School of Physics, Nankai University, Tianjin 300071, China.
5. School of Information Engineering, Nanchang Institute of Technology, Nanchang 330099, China.
6. National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan.
7. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.