1. 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. Center on Nanoenergy Research; School of Physical Science and Technology; Guangxi University; Nanning 530004 P. R. China
4. School of Materials Science and Engineering; Georgia Institute of Technology; Atlanta GA 30332-0245 USA