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. School of Physics and Electronics; Hunan University; Changsha 410082 China
4. Tsinghua National Laboratory for Information Science and Technology; Institute of Microelectronics; Tsinghua University; Beijing 100084 China
5. Center on Nanoenergy Research; School of Physical Science and Technology; Guangxi University; Nanning Guangxi 530004 P. R. China
6. Center for Electron Microscopy; TUT-FEI Joint Laboratory; Tianjin Key Laboratory of Advanced Functional Porous Materials; Institute for New Energy Materials & Low-Carbon Technologies; School of Materials Science and Engineering; Tianjin University of Technology; Tianjin 300384 China
7. School of Materials Science and Engineering; Georgia Institute of Technology; Atlanta GA 30332-0245 USA