1. Key Laboratory for the Physics and Chemistry of Nanodevices
2. Department of Electronics
3. Peking University
4. Beijing, P. R. China
5. Wuhan National Laboratory for Optoelectronics
6. Huazhong University of Science and Technology
7. Wuhan, P. R. China
8. Department of Physics and Tsinghua-Foxconn Nanotechnology Research Center
9. Tsinghua University