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 Electron microscopy studies were performed at Center for Functional Nanomaterials, Brookhaven National Laboratory supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under contract no. DEAC02-98CH10886. X. S. is grateful for the financial support of the China Scholarship Council and Brookhaven National Laboratory for his exchange program. K. A. -M. and J. H. N. acknowledge the support of The University of Texas at Arlington. D. W. thanks National Key Basic Research Program of China (2015CB921203) for support. The authors thank Dr. Woodhead for the proof reading.