In situ formation of highly conducting covalent Au–C contacts for single-molecule junctions

Charge transport across metal–molecule interfaces has an important role in organic electronics1. Typically, chemical link groups such as thiols2 or amines3 are used to bind organic molecules to metal electrodes in single-molecule circuits, with these groups controlling both the physical structure and the electronic coupling at the interface. Direct metal–carbon coupling has been shown through C60, benzene and π-stacked benzene4,5,6,7, but ideally the carbon backbone of the molecule should be covalently bonded to the electrode without intervening link groups. Here, we demonstrate a method to create junctions with such contacts. Trimethyl tin (SnMe3)-terminated polymethylene chains are used to form single-molecule junctions with a break-junction technique2,3. Gold atoms at the electrode displace the SnMe3 linkers, leading to the formation of direct Au–C bonded single-molecule junctions with a conductance that is ∼100 times larger than analogous alkanes with most other terminations. The conductance of these Au–C bonded alkanes decreases exponentially with molecular length, with a decay constant of 0.97 per methylene, consistent with a non-resonant transport mechanism. Control experiments and ab initio calculations show that high conductances are achieved because a covalent Au–C sigma (σ) bond is formed. This offers a new method for making reproducible and highly conducting metal–organic contacts.