1. Contribution from the Materials and Process Simulation Center (139-74) and Division of Chemistry and Chemical Engineering (127-72), California Institute of Technology, Pasadena, California 91125, California NanoSystems Institute and Department of Chemistry and Biochemistry, University of California, Los Angeles, 405 Hilgard Avenue, Los Angeles, California 90095-1569, and Nano-Science Center and Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 København Ø, Denmark