1. Centre for Molecular and Biomolecular Electronics, Coventry University, Priory Street, Coventry CV1 5FB, United Kingdom, Institut d'Electronique et de Microelectronique du Nord, Centre National de la Recherche Scientifique, BP69 Avenue Poincaré, Villeneuve d'Ascq F-59652 Cedex, France, and Laboratory for Molecular Electronics, Chemistry Department, The University of Alabama, Tuscaloosa, Alabama 35487-0336