supported by the Fundamental Research Funds for the Central Universities (XDJK2010B006);Ministry of Science and Technology International Cooperation Project (2008GR1256);the National Natural Science Foundation of China (41072192);the Doctoral Program of Higher Education Research Fund (20100182120029 and 2008- 06350008);the Foundation Southwest University Ph.D. (SWUB2008013);the Graduate Innovation Fund of Southwest University (KY2010002 and KB2009004)