The authors gratefully acknowledge financial support by the National Basic Research Program of China (No. 2012CB932800), National Natural Science Foundation of China (Nos. 51371106 and 51222202), and Young Tip-top Talent Support Project (the Organization Department of the Central Committee of the CPC). The Institute of Materials of Ruhr University Bochum (Germany) is acknowledged for the support of SEM and TEM characterization. This work also made use of the resources of the Center of Electron Microscopy of Zhejiang University.
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