1. Department of Chemistry, Department of Applied Physics and Applied Mathematics, and Materials Research Science & Engineering Center, Columbia University, New York, New York 10027, IBM, T. J. Watson Research Center, Yorktown Heights, New York 10598, Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, and Bell Laboratories, Lucent Technologies, 600 Mountain Avenue, Murray Hill, New Jersey 07974