1. School of Mathematics and Physics; Shanghai Key Laboratory of Materials Protection and Advanced Materials in Electric Power; Shanghai University of Electric Power; Shanghai 200090 China
2. Beijing Institute of Nanoenergy and Nanosystems; Chinese Academy of Sciences; National Center for Nanoscience and Technology (NCNST); Beijing 100083 China
3. School of Material Science and Engineering; Georgia Institute of Technology; Atlanta GA 30332-0245 USA