Controlled switching of single-molecule junctions by mechanical motion of a phenyl ring

CO-tip manipulation using repulsive interactions

Understanding the interactions between a tip apex and a target atom or molecule is crucial for the manipulation of individual molecules with precise control by using scanning tunnelling microscopy (STM) and atomic force microscopy. Herein, we demonstrate the manipulation of target CO molecules on a Cu(111) substrate using a CO-functionalized W tip with atomic-scale accuracy. All experiments were performed in a home-built ultra-high vacuum STM system at 5 K. The CO-tip was fabricated by picking up a single CO molecule from a Cu(111) surface. In contrast to a metal tip, repulsive interactions occur between the CO-tip and the target CO molecule. This repulsive interaction promises perfect lateral hopping without any vertical hopping. Hopping events were directly monitored as sudden current drops in the simultaneously measured I-z curves. A larger barrier height between the CO-tip and the target CO (∼9.5 eV) was found from the slope of the I-z curve, which decreases the electron tunnelling probability between the tip and sample. Therefore, electron-driven manipulation cannot be a major trigger for the CO-CO repulsive manipulation. The CO-tip is able to manipulate only the target CO molecule, even when another CO molecule was located ∼0.5 nm away. Statistical measurements revealed that the nearest neighbour atop site is the energetically stable position after hopping. However, if the CO target has another CO molecule in a neighbouring position (denoted as a 'pair'), the target CO hops more than twice as far. This means that the CO-tip experiences a larger repulsive interaction from the pair. These observations of CO-tip manipulation are useful for the design of two-dimensional artificial molecular networks as well as for developing a better understanding of catalytic oxidation processes.

The influence of nearest-neighbour interactions and assembly dynamics on the transport properties of porphyrin supramolecular assemblies on Au(111)

Here we report on the effect of local molecular organization or "tertiary structure" on the charge transport properties of thiol-tethered tetraphenylporphyrin (ZnTPPF₄-SC₅SH) nanoscale clusters of ca. 5 nm in lateral dimension embedded within a dodecanethiol (C12) monolayer on Au(111). The structure of the clusters in the mixed monolayers and their resulting transport properties were monitored by Atomic Force Microscopy (AFM), Scanning Tunneling Microscopy (STM) and Spectroscopy (STS). The mixed films were deposited on Au(111) for a period of one to five days, during which the lateral dimensions of the ZnTPPF₄-SC₅SH islands that were formed after one day reduced by nearly 35% on average by five days, accompanied by a noticeable depletion of the surrounding C12 monolayer. These subtle changes in mixed monolayer morphology were accompanied by drastic differences in conductance. The ZnTPPF₄-SC₅SH clusters assembled for one day exhibited highly reproducible I-V spectra with simple tunneling behavior. By three days, this evolved into bias-induced switching of conductance, with a ∼100-1000 fold increase. Furthermore, current fluctuations started to become significant, and then dominated transport across the ZnTPPF₄-SC₅SH clusters assembled over five days. Our data suggests that this evolution can be understood by slow surface diffusion, enabling the ZnTPPF₄-SC₅SH molecules to overcome initial steric hindrance in the early stages of island formation in the C12 monolayer (at day one), to reach a more energetically-favored, close-packed organization, as noted by the decrease in island size (by day three). However, when desorption of the supporting matrix of C12 became pronounced (by day five), the ZnTPPF₄-SC₅SH clusters began to lose stabilization, and stochastic switching was then observed to dominate transport in the clusters, illustrating the critical nature of the local organization on these transport properties.

Adsorbed states of chlorophenol on Cu(110) and controlled switching of single-molecule junctions

A molecular junction of substituted benzene (chlorophenol) is fabricated and controlled by using a scanning tunneling microscope (STM). Prior to the junction formation, the bonding geometry of the molecule on the surface is characterized by STM and electron energy loss spectroscopy (EELS). EELS shows that the OH group of chlorophenol is dissociated on Cu(110) and that the molecule is bonded nearly flat to the surface via an O atom, with the Cl group intact. We demonstrate controlled contact of an STM tip to the "available" Cl group and lift-up of the molecule while it is anchored to the surface via an O atom. The asymmetric bonding motifs of the molecule to the electrodes allow for reversible control of the junction.