1. Key Laboratory of Excited-State Processes, Changchun Institute of Optics Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130033, P. R. China, Quantum Dot Corporation, 26118 Research Road, Hayward, California 94545, and Van't Hoff Institute for Molecular Science, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WS Amsterdam, The Netherlands