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Dependence of single-molecule junction conductance on molecular conformation


Since it was first suggested1 that a single molecule might function as an active electronic component, a number of techniques have been developed to measure the charge transport properties of single molecules2,3,4,5,6,7,8,9,10,11,12. Although scanning tunnelling microscopy observations under high vacuum conditions can allow stable measurements of electron transport, most measurements of a single molecule bonded in a metal–molecule–metal junction exhibit relatively large variations in conductance. As a result, even simple predictions about how molecules behave in such junctions have still not been rigorously tested. For instance, it is well known13,14 that the tunnelling current passing through a molecule depends on its conformation; but although some experiments have verified this effect15,16,17,18, a comprehensive mapping of how junction conductance changes with molecular conformation is not yet available. In the simple case of a biphenyl—a molecule with two phenyl rings linked by a single C–C bond—conductance is expected to change with the relative twist angle between the two rings, with the planar conformation having the highest conductance. Here we use amine link groups to form single-molecule junctions with more reproducible current–voltage characteristics19. This allows us to extract average conductance values from thousands of individual measurements on a series of seven biphenyl molecules with different ring substitutions that alter the twist angle of the molecules. We find that the conductance for the series decreases with increasing twist angle, consistent with a cosine-squared relation predicted for transport through π-conjugated biphenyl systems13.

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Figure 1: Conductance measurements of 1,4-diaminobenzene and 2,7-diaminofluorene junctions.
Figure 2: Biphenyl junction conductance as a function of molecular twist angle.
Figure 3: Polyphenyl junction conductance as a function of the number of phenyl units.


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We thank H. Stormer, P. Kim and J. Fernandez for discussions. This research was supported by the NSF Nanoscale Science and Engineering Center at Columbia University, New York State Office of Science (NYSTAR). C.N. thanks the Camille Dreyfus Teacher Scholar Program (2004) and the Alfred P. Sloan Fellowship Program (2004); J.E.K. thanks the American Chemical Society Division of Organic Chemistry for the graduate fellowship sponsored by Organic Syntheses; and M.L.S. thanks the Material Research Science and Engineering Center Program of the NSF.

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Correspondence to Latha Venkataraman or Mark S. Hybertsen.

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Venkataraman, L., Klare, J., Nuckolls, C. et al. Dependence of single-molecule junction conductance on molecular conformation. Nature 442, 904–907 (2006).

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