The research was partially funded by the Key Program of National Natural Science Foundation of China (Grant Nos. 61133005, 61432005), the National Natural Science Foundation of China (Grant Nos. 61370095, 61472124, 61572175), International Science & Technology Cooperation Program of China (2015DFA11240). The work was supported by Research Project of the Education Department of Hunan Province (Grant No. 14C0210).