1. Department of Electronics Peking University Center for Nanoscale Science and Technology Key Laboratory for the Physics and Chemistry of Nanodevices Beijing 100871 China
2. Chinese Academy of Sciences CAS Key Laboratory of Molecular Nanostructure and Nanotechnology CAS Research/Education Center for Excellence in Molecular Sciences Beijing National Laboratory for Molecular Science (BNLMS) Beijing 100190 China
3. University of Chinese Academy of Sciences Beijing 100049 China
4. School of Physics Trinity College AMBER and CRANN Institute Dublin 2 Ireland