1. Department of Chemistry, Columbia University, New York, New York 10027, Nanoscale Science and Engineering Center, Columbia University, New York, New York 10027, Department of Physics, Columbia University, New York, New York 10027, Department of Applied Physics and Applied Mathematics, and Materials Research Science and Engineering Center, Columbia University, New York, New York 10027, Department of Chemical and Biological Engineering, Tufts University, Medford, Massachusetts 02155