1. School of Physics, Shandong University, Jinan 250100, P.R. China
2. Nano-organic Photoelectronic Laboratory, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China
3. Institute I—Theoretical Physics, University of Regensburg, D-93040 Regensburg, Germany
4. Bremen Center for Computational Material Science, University Bremen, 28334 Bremen, Germany
5. Center of Super-Diamond and Advanced Films and Department of Physics and Materials Science, City University of Hong Kong, Hong Kong Special Administrative Region, China