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Single-molecule contacts exposed Using a scanning tunnelling microscopy-based method it is now possible to get an atomistic-level description of the most probable binding and contact configuration for single-molecule electrical junctions.

Richard J. Nichols and Simon J. Higgins

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n recent years, new techniques have emerged for studying the electrical properties of nanoscale junctions, including junctions containing a single molecule. In contrast with measurements based on large-area planar junctions, in which molecular monolayers are sandwiched between two contacts, techniques able to electrically address just one molecule facilitate detailed mechanistic studies that have given many insights into charge transport across molecular junctions. Popular approaches have used either a scanning tunnelling microscope (STM) tip1,2 or mechanically controlled break junctions to trap single molecules between metallic contacts, typically made from gold2–5. As the two electrodes are pulled apart, a molecule (or molecules) can become anchored in place by chemical groups at either end of the molecular bridge, and the conductance is measured as the junction is first formed, and subsequently stretched to breaking. By statistically analysing many of these pulling traces, the conductance of the junction can be obtained1. A surprise, which came to light in later measurements, is that a given molecular target can give rise to several most probable conductance values6, which relate in some way to different junction configurations7. This junction configuration is dependent on the arrangement of gold contact atoms and also on the binding between the molecular contacting groups and the gold. Although a combination of theory and experiment has given insight into possible junction configurations8, a precise configuration has been difficult to either experimentally control or define. Habid Rascón-Ramos and colleagues now report in Nature Materials a significant advance in this respect, showing an experimental method that is capable of determining the most probable binding and contact configuration for molecular junctions9. The researchers used an STM in their determination of single-molecule junction conductance. For charge transport through a homologous series of molecular bridges

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b

STM tip vibration

On-top configuration

Bridging configuration

Figure 1 | Characterization of a single-molecule junction. a, A schematic illustration of an electrical junction with a hexanedithiol molecular bridge between a gold STM tip and gold substrate. A smallamplitude vibration is applied to the STM tip as it is withdrawn and the a.c. current response of the electrical junction is measured. Gold, sulphur, carbon and hydrogen atoms are shown as yellow, purple, dark grey and light grey spheres, respectively. This represents the most probable junction configuration with the contacting gold atom in an ‘on-top’ site at the upper contact and in a ‘bridging’ site at the lower contact. b, A mechanical ‘spring junction’ model is applied to analyse the most probable junction configuration. Panel a adapted from ref. 9, Nature Publishing Group.

by tunnelling, the conductance decays exponentially with molecular length, with this decay being quantified by the decay parameter β. Typically, well-conjugated moieties give low β values, while saturated molecular bridges give higher values. In the current study, as well as withdrawing the STM tip from the substrate surface, Rascón-Ramos and colleagues superimposed a small-amplitude, high-frequency distance modulation to the tip position as the molecule was pulled-up in the junction10 (Fig. 1a). In addition to determining the β value as usual, they also measured the degree to which the tip vibration affects the conductance by normalizing the a.c. current response of the junction. The resulting parameter is named α. When the junction is devoid of a molecular bridge, α and β

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are simply equivalent. However, when a molecule bridges the contact gap, α values fall to a small fraction of the β values. For a series of alkanedithiols, α is found to be ~15% of the β value. Moreover, unlike β, α increases with the length of the alkane chain in the series of alkanedithiols and diamines used. These differences between α and β are attributed to the mechanical properties of the junction, with much of the applied distance modulation being absorbed by the relatively soft gold contacts. This means that the effective distance modulation in the molecular bridge region is much smaller than the applied modulation, hence α 

Molecular junctions: Single-molecule contacts exposed.

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