1. Department of Physics and Laboratoryof Materials Physic; Zhengzhou University; Zhengzhou 450001 P. R. China
2. Beijing Institute of Nanoenergy and Nanosystems; Chinese Academy of Sciences; National Center for Nanoscience and Technology (NCNST); Beijing 100083 P. R. China
3. School of Materials Science and Engineering; Georgia Institute of Technology; Atlanta Georgia 30332-0245 USA
4. Institute of Microelectronics; Peking University; Beijing 100871 P. R. China