1. College of Physics; Optoelectronics and Energy & Collaborative Innovation Center of Suzhou Nano Science and Technology; Soochow University; 215006 Suzhou P. R. China
2. International Iberian Nanotechnology Laboratory (INL); 4715-330 Braga Portugal
3. State Key Laboratory of Electronic Thin Film and Integrated Devices; University of Electronic Science and Technology of China; Chengdu 610054 P. R. China
4. State Key Laboratory of Coal Conversion; Institute of Coal Chemistry; Chinese Academy of Sciences; Taiyuan 030001 P. R. China