Electron transport properties of a carotene molecule in a metal-(single molecule)-metal junction

Using a recently developed method for tethering gold contacts to single molecules, we have measured current versus voltage (I-V) data for single carotenedithiol molecules with a total length of 28 carbons (18 in the conjugated alkene chain). The molecules are inserted into a docosanethiol monolayer (C₂₂H₄₅SH) on Au(111), and a Au nanoparticle is tethered to each molecule via the protruding thiol group. Electrical contact is made by placing a gold-coated AFM probe in contact with the nanoparticle. The I-V curves are relatively insensitive to contact probe force and cluster around integer multiples of a fundamental curve, suggesting that members of the smallest set correspond to data obtained from a single molecule. First principles calculations based on tunnel transport yield results that are remarkably close (within a factor of 4) to the measured data. The remaining small discrepancy can be well accounted for by taking account of the effects of charging caused by a probe-to-gold particle contact resistance. Thus, it appears that electron tunneling dominates transport even in this 3-nm-long molecule. The carotenoid is a better conductor than a saturated n-alkane of equivalent length by a very large factor and is significantly more conductive than 2,5-di(phenylethynyl-4′-thioacetyl)benzene, another candidate "molecular wire".