1. School of Chemistry and Environment, Beijing University of Aeronautics and Astronautics, Beijing 100083, P. R. China, Department of Physics, Peking University, Beijing 100871, China, Laboratary of Optoelectronic Functional Materials and Molecular Engineering and Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100080, China, and Institute of High Energy Physics, Chinese Academy of Science, Beijing 100039, China