1. National Laboratory of Solid State Microstructures and Physics School, Nanjing University, Nanjing 210093, People's Republic of China
2. Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, USA
3. Collaborative Innovation Center of Advanced Microstructures, Nanjing 210093, People's Republic of China
4. Department of Engineering, Aarhus University, 8000 Aarhus C, Denmark