1. Department of Electrical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States
2. State Key Laboratory of Nuclear Physics and Technology, School of Physics, Peking University, Beijing 100871, P. R. China
3. Department of Chemistry, Division of Materials Science and Engineering, and The Photonics Center, Boston University, Boston, Massachusetts 02215, United States
4. School of Advanced Materials, Shenzhen Graduate School, Peking University, Shenzhen 518055, P. R. China