1. Institute
of Physics, Beijing Key Laboratory for Nanomaterials and Nanodevices, Chinese Academy of Sciences, Beijing 100190, P. R. China
2. Institute
of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China
3. Department
of Physics and Astronomy and Department of Electrical Engineering
and Computer Science, Vanderbilt University, Nashville, Tennessee 37235, United State
4. School
of Physical Sciences and CAS Key Laboratory of Vacuum Physics, University of Chinese Academy of Sciences, Beijing 100049, P. R. China