基金This work is supported by the National Natural Science Foundation of China (NSFC, nos. 61340046), the National High Technology Research and Development Programme of China (863 Programme, no. 2006AA04Z247), the Scientific and Technical Innovation Commission of Shenzhen Munici-pality (nos. JCYJ20130331144631730), and the Specialized Research Fund for the Doctoral Programme of Higher Edu- cation (SRFDP, no. 20130001110011).