1. State Key Laboratory of New Ceramics & Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, P. R. China
2. State Key Laboratory of Electronic Thin Film and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, P. R. China
3. State Key Laboratory of Low-Dimensional Quantum Physics, Tsinghua-Foxconn Nanotechnology Research Center, Department of Physics, Tsinghua University, Beijing 100084, P. R. China
4. Key Laboratory of Ministry of Education for Advanced Materials in Tropical Island Resources, Materials and Chemical Engineering Institute, Hainan University, Haikou 570228, P. R. China