1. Department of Chemistry and Laboratory for Surface Studies, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin 53211, United States
2. Center for Integrated Nanotechnologies, Sandia National Laboratories, P.O. Box 5800,
MS-1304, Albuquerque, New Mexico 87185, United States
3. Institute of Tribology, School of Mechanical Engineering, Hefei University of Technology, Hefei 230009, China
4. Laboratoire de Tribologie et Dynamique des Systèmes, CNRS UMR5513, Ecole centrale de Lyon, 36 avenue Guy de Collongue, F-69134 Ecully cedex, France