1. Contribution from the IBM T.J. Watson Research Center, Nanoscale Materials and Devices Group, 1101 Kitchawan Road, Yorktown Heights, New York 10598, Department of Chemistry, Michigan State University, East Lansing, Michigan 48823, and Department of Applied Physics & Applied Mathematics, Columbia University, 200 SW Mudd Building, 500 West 120th Street, New York, New York 10027