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Conduction mechanisms in biphenyl-dithiol single-molecule junctions

Based on density-functional theory calculations, we report a detailed study of the single-molecule charge-transport properties for a series of recently synthesized biphenyl-dithiol molecules [D. Vonlanthen et al., Angew. Chem., Int. Ed. 48, 8886 (2009); A. Mishchenko et al., Nano Lett. 10, 156 (2010)]. The torsion angle ϕ between the two phenyl rings, and hence the degree of π conjugation, is controlled by alkyl chains and methyl side groups. We consider three different coordination geometries, namely top-top, bridge-bridge, and hollow-hollow with the terminal sulfur atoms bound to one, two, and three gold surface atoms, respectively. Our calculations show that different coordination geometries give rise to conductances which vary by one order of magnitude for the same molecule. Irrespective of the coordination geometries, the charge transport calculations predict a cos^{2}ϕ dependence of the conductance, which is confirmed by our experimental measurements. We observe that the calculated transmission through biphenyl dithiols is typically dominated by a single transmission eigenchannel formed from π electrons. Only for a single molecule with a completely broken conjugation we find a perfect channel degeneracy for the hollow-hollow-type contact in our theory.