1. Beijing Institute of Nanoenergy and Nanosystems; Chinese Academy of Sciences; Beijing 100083 China
2. College of Materials Science and Opto-Electronic Technology; University of Chinese Academy of Science; Beijing 100049 China
3. School of Nanoscience and Technology; University of Chinese Academy of Science; Beijing 100049 China
4. Department of Electrical and Computer Engineering; Johns Hopkins University; Baltimore MD 21093 USA