1. State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
2. Department of Physics and Tsinghua-Foxconn Nanotechnology Research Center, Tsinghua University, Beijing 100084, China
3. Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen, Guangdong 518055, China