1. CAS Center for Excellence in Nanoscience; Beijing Key Laboratory of Micro-Nano Energy and Sensor; Beijing Institute of Nanoenergy and Nanosystems; Chinese Academy of Sciences; Beijing 100083 P. R. China
2. School of Nanoscience and Technology; University of Chinese Academy of Sciences; Beijing 100049 P. R. China
3. National Center for Electron Microscopy in Beijing; School of Materials Science and Engineering; The State Key Laboratory of New Ceramics and Fine Processing; Key Laboratory of Advanced Materials (MOE) and Center for Nano and Micro Mechanics; Tsinghua University; Beijing 100084 China
4. Center on Nanoenergy Research; School of Physical Science and Technology; Guangxi University; Nanning Guangxi 530004 P. R. China
5. School of Materials Science and Engineering; Georgia Institute of Technology; Atlanta GA 30332-0245 USA