1. INFM and Università di Brescia, Via Valotti 9, 25133 Brescia, Italy, INFM, INFN Sezione di Ferrara, Università di Ferrara, Via Paradiso, 44100 Ferrara, Italy, and Center for Nanoscience and Nanotechnology, School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245