1. Donostia International Physics Center (DIPC), Paseo Manuel de Lardizabal 4, San Sebastián 20018, Spain, Unidad de Física de Materiales, CSIC-UPV, San Sebastián, Spain, Institut Catala de Nanotecnologia, Barcelona, Spain, Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720, Surface Science Research Centre, Department of Chemistry, University of Liverpool, Liverpool, U.K., and Departamento de Física de Materiales, UPV/EHU, San Sebastián, Spain