1. National Laboratory of Solid State Microstructures and Jiangsu Provincial Key Laboratory of Advanced Photonic and Electronic Materials; School of Electronic Science and Engineering; Nanjing University; Nanjing 210093 China
2. Department of Energy Conversion and Storage; Technical University of Denmark; Risø Campus Roskilde 4000 Denmark
3. Department of Electronic Engineering; Royal Holloway; University of London; Egham Surrey TW20 0EX UK
4. College of Engineering and Applied Sciences; Nanjing University; Nanjing 210093 China
5. School of Physics; Nanjing University; Nanjing 210093 China
6. Collaborative Innovation Center of Advanced Microstructures; Nanjing University; Nanjing 210093 China
7. Department of Chemical Engineering and Materials Science; University of California Irvine; Irvine CA 92697 USA