1. Center for Biomolecular Nanotechnologies, Istituto Italiano di Tecnologia, Via Barsanti 14, Arnesano (LE) 73010, Italy
2. Department of Mathematics and Physics “E. De Giorgi”, University of Salento, Via Arnesano, Lecce 73100, Italy
3. Institute of Physics, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudzia̧dzka 5, Toruń 87-100, Poland
4. Institute of Nanotechnology, National Research Council (CNR-NANOTEC), c/o Campus Ecotekne, via Monteroni, Lecce 73100, Italy
5. Institute for Microelectronics and Microsystems (CNR-IMM), Via Monteroni, Campus Unisalento, Lecce 73100, Italy