1. Contribution from the Laboratory for Photonics and Interfaces, Institute of Physical Chemistry, Swiss Federal, Institute of Technology, CH-1015 Lausanne, Switzerland, Laboratoire d'Optoélectronique des Matériaux Moléculaires, Institut des Matériaux, Ecole Polytechnique Fédérale, CH-1015 Lausanne, Switzerland, and CFG Microelectronic, CH-1110 Morges, Switzerland