1. Key Laboratory of Nanosystem and Hierarchical FabricationCAS Center for Excellence in NanoscienceNational Center for Nanoscience and Technology Beijing 100190 P. R. China
2. Center of Materials Science and Optoelectronics EngineeringUniversity of Chinese Academy of Sciences Beijing 100049 P. R. China
3. Department of Materials Science and EngineeringCollege of New Energy and MaterialsChina University of Petroleum Beijing Beijing 102249 P. R. China