1. Department of Materials Science and Engineering, University of California, Berkeley, California 94720, United States
2. Jiangsu Provincial Key Laboratory of Advanced Photonic and Electronic Materials, School of Electronic Science and Engineering, Nanjing University, Nanjing, Jiangsu 210093, China
3. Nanjing National Laboratory of Microstructures, Nanjing, Jiangsu 210093, China
4. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
5. Department of Physics, University of California, Davis, California 95616, United States