The authors gratefully acknowledge the financial support by the National Natural Science Foundation of China (Nos. 21471160 and 51402362), Huangdao Key Science and Technology Programme (Contract No. 2014-1-50), Shandong Natural Science Foundation (No. ZR2014EMQ012), Qingdao Science and Technology Program for Youth (No. 14-2-4-34-jch), and the Fundamental Research Funds for the Central Universities of Ministry of Education of China.
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