1. Nanofabrication Laboratory, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China
2. Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100039, China
3. Department of Physics, University of the Punjab, Quaid-e-Azam Campus, Lahore 54000, Pakistan
4. Key Laboratory of Photochemical Conversion & Optoelectronic Materials, Technical Institute of Physics & Chemistry, Beijing 100190, P. R. China