1. School of Electronic and Information Engineering Beijing Jiaotong University 100044 Beijing P. R. China
2. Beijing Institute of Nanoenergy and Nanosystems Chinese Academy of Sciences 100083 Beijing P. R. China
3. School of Nanoscience and Technology University of Chinese Academy of Sciences 100049 Beijing P. R. China
4. College of Mathematics and Physics Beijing University of Chemical Technology 100029 Beijing P. R. China
5. School of Materials Science and Engineering Georgia Institute of Technology Atlanta GA 30332 USA