1. Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, National CRI Center for Semiconductor Nanorods and Department of Materials Science and Engineering, POSTECH, Pohang 790-784, Korea, Department of Physics, Harvard University, Cambridge, Massachusetts 02138, The Rowland Institute at Harvard, Cambridge, Massachusetts 02142, and Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853