1. Department of Materials Science and Engineering, National Cheng Kung University, Tainan 70101, Taiwan
2. Institute of Physics, University of Münster, Wilhelm-Klemm-Strasse 10, Münster 48149, Germany
3. CeNTech−Center for Nanotechnology, University of Münster, Heisenbergstrasse 11, Münster 48149, Germany
4. Physics Department, Technische Universität München, James-Franck-Straße 1, Garching 85748, Germany
5. Laboratory for Biointerfaces, Swiss Federal Laboratories for Materials Science and Technology (EMPA), Lerchenfeldstrasse 5, St. Gallen 9014, Switzerland