1. Department of Physics, Informatics and Mathematics, University of Modena and Reggio Emilia, Via Campi, 213/A, Modena 41125, Italy
2. CNR-Institute of Nanoscience, S3 Center, Via Campi 213/A, Modena 41125, Italy
3. Tribology Group, Department of Mechanical Engineering, Imperial College London, London SW7 2AZ, U.K.