1. School of Electronic Science and Engineering, Nanjing University, Nanjing 210093, P. R. China
2. National Laboratory of Solid State Microstructures, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, P. R. China
3. School of Materials and Energy, University of Electronic Science and Technology of China, Chengdu 611731, P. R. China
4. College of Engineering and Applied Sciences, Nanjing University, Nanjing 210093, P. R. China
5. Department of Materials Science and Engineering, University of California, Berkeley, California 94720, United States
6. School of Geography and Biological Information, Nanjing University of Posts and Telecommunications, Nanjing 210023, P. R. China