1. Contribution from the Nanoscience Centre, University of Cambridge, 11 J J Thomson Avenue, Cambridge CB3 0FF, U.K., Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K., National Center for NanoScience & Technology, No. 2 First North Street, Zhongguancun, Beijing 100080, China, and Sungkyunkwan Advanced Institute of Nanotechnology and Department of Physics, Sungkyunkwan University, Suwon, 440-746, Korea