教育部科学技术研究重点项目212177;陕西省自然科学基金2011JQ1003;陕西省教育厅科研基金批准号:12JK0973资助的课题.*Project supported by the Key Program of Science and Technology Research Foundation of Ministry of Education, ChinaGrant 212177;the Natural Science Foundation of Shaanxi, ChinaGrant 2011JQ1003;the Scientific Research Fundation of the Education Department of Shaanxi