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C.K.C. acknowledges support from a National Science Foundation Graduate Fellowship and Stanford Graduate Fellowship. Correspondence and requests for materials should be addressed to Y.C. Supplementary information accompanies this paper on www.nature.com/naturenanotechnology. Author contributions C.K.C. conceived and carried out the experiment and data analysis. H.P., G.L., K.M. and X.F.Z. assisted in experimental work. R.A.H. carried out data analysis. Y.C. conceived the experiment and carried out data analysis. C.K.C., R.A.H. and Y.C. wrote the paper. Reprints and permission information is available online at http://npg.nature.com/reprintsandpermissions/ LETTERS nature nanotechnology |VOL 3 | JANUARY 2008 |www.nature.com/naturenanotechnology 35