1. Contribution from the Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel, CNR-IMIP, Sezione di Bari, Via Orabona, 4 I-70126 Bari, Italy, Dipartimento di Fisica Universita’ degli Studi di Bari and INFM, Via Orabona, 4 I-70126 Bari, Italy, and Department of Physics, Brooklyn College, City University of New York, Brooklyn, New York 11210