1. Contribution from the National Nanotechnology Laboratory of CNR-INFM, 73100 Lecce, Italy, Center for Nanoscience, Ludwig Maximilians Universität, München, Germany, TASC-INFM-CNR National Laboratory, Area Science Park - Basovizza, 34012 Trieste, Italy, and CNR-Istituto di Cristallografia (IC), via Amendola 122/O, 70126 Bari, Italy