For the rational design of single-molecular electronic devices, it is essential to understand environmental effects on the electronic properties of a working molecule. Here we investigate the impact of molecular interactions on the single-molecule conductance by accurately positioning individual molecules on the electrode. To achieve reproducible and precise conductivity measurements, we utilize relatively weak π-bonding between a phenoxy molecule and a STM-tip to form and cleave one contact to the molecule. The anchoring to the other electrode is kept stable using a chalcogen atom with strong bonding to a Cu(110) substrate. These non-destructive measurements permit us to investigate the variation in single-molecule conductance under different but controlled environmental conditions. Combined with density functional theory calculations, we clarify the role of the electrostatic field in the environmental effect that influences the molecular level alignment.

Adsorption of phenol and thiophenol (benzenethiol) on Cu(110) is investigated by a scanning tunneling microscope and electron energy loss spectroscopy. Phenol adsorbs intact and forms a cyclic trimer at 78 K. It is dehydrogenated to yield a phenoxy (C6H5O) group at 300 K. On the other hand, thiophenol is dehydrogenated to a thiophenoxy (C6H5S) group even at 78 K. Both products are bonded via chalcogen atom to the short-bridge site with the phenyl ring oriented nearly parallel to the surface. The C6H5O and C6H5S groups are preferentially assembled into the chains along the [001] and [112] directions, respectively. Dipole-dipole interaction is responsible for the chain growth, while the chain direction is ruled by the steric repulsion between chalcogen atoms and adjacent phenyl ring. This work demonstrates a crucial role of chalcogen atom of phenol species in their overlayer growth on the surface.

SummaryMechanical methods for single-molecule control have potential for wide application in nanodevices and machines. Here we demonstrate the operation of a single-molecule switch made functional by the motion of a phenyl ring, analogous to the lever in a conventional toggle switch. The switch can be actuated by dual triggers, either by a voltage pulse or by displacement of the electrode, and electronic manipulation of the ring by chemical substitution enables rational control of the on-state conductance. Owing to its simple mechanics, structural robustness, and chemical accessibility, we propose that phenyl rings are promising components in mechanical molecular devices.