1. Institute of Physics & University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100190, P. R. China
2. Physikalisches Institut, Westfälische Wilhelms-Universität & Center for Nanotechnology (CeNTech), Wilhelm-Klemm-Straße 10, 48149 Münster, Germany
3. Nanoscale Materials Science, Empa, Swiss Federal Laboratories for Materials Testing and Research, Überlandstrasse 129, CH-8600 Dübendorf, Switzerland