国家自然科学基金项目(50747026);广西科学基金项目(0728027);广西研究生教育创新计划资助项目(GXU11T32516;GXU11T32598);Project Supported by National Natural Science Foundation of China(50747026);Guangxi Natural Science Foundation(0728027);Innovation Project of Guangxi Graduate Education(GXU11T32516;GXU11T32598)