1. National Center for Nanoscience and Technology (NCNST), Beijing 100190, P.R. China
2. CEA-Saclay, Organic Nanostructures & Semiconductors Laboratory CNRS-CEA-UPMC, SPCSI, 91191 Gif-sur-Yvette, France
3. Institute of Physics, Beijing Natl Laboratory for Condensed Matter Physics, Chinese Academy of Sciences, Beijing 100190, P.R. China