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. College of Nanoscience and Technology; University of Chinese Academy of Sciences; Beijing 100049 P. R. China
3. State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources; School of Renewable Energy; North China Electric Power University; Beijing 102206 China
4. School of Materials Science and Engineering; Georgia Institute of Technology; Atlanta GA 30332-0245 USA