1. Laboratory of Photonics and Interfaces, Institute of Chemical Sciences and Engineering, and Institute of Electron Microscopy, Swiss Federal Institute of Technology, 1015 Lausanne, Switzerland, Laboratory LEM/UMR 7569, CNRS INPL-ENSG, 5401 Vandoeuvre-les-Nancy, Cedex, France, Degussa AG, Industriepark Wolfgang, Rodenbacher Chaussee. 4 D-63457 Hanau/Main, Germany, and LEPMI-ENSEEG, Domaine Universitaire, BP75, F-38402, St Martin d'Heres, Cedex, France