1. Thomas Young Centre, London Centre for Nanotechnology and Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, United Kingdom
2. Univ. Lyon, Université Claude Bernard Lyon 1, CNRS, Institut Lumière Matière, F-69622 Villeurbanne,
France
3. Department of Mechanical Engineering, State Key Laboratory of Tribology and Center for Nano and Micro Mechanics, Tsinghua University, Beijing 100084, China
4. Department of Chemistry and Centre for Scientific Computing, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL,
United Kingdom