1. State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University Qianjin Street 2699, Changchun, Jilin 130012, P. R. China, Fax: +86‐431‐85168624
2. Center for Optoelectronics Materials and Devices, Department of Physics, Zhejiang Sci‐Tech University, Hangzhou 310018, P. R. China