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Posted: Dec 12, 2006
Precision control of single-molecule electrical junctions
(Nanowerk Spotlight) There is much discussion of molecules as components for future electronic devices and in recent years it has been possible to position single molecules in electrical junctions. Molecular and nanoscale structures have been shown to be capable of basic electronic functions such as rectification, negative differential resistance and single-electron transistor behavior. These observations show that molecular-electronic functions can be controlled through chemical manipulation. However, the contacts, the local environment and the temperature can all affect molecules' electrical properties. This sensitivity, particularly at the single-molecule level, may limit the use of molecules as active electrical components, and therefore it is important to design and evaluate molecular junctions with a robust and stable electrical response over a wide range of junction configurations and temperatures. A step in this direction, researchers in the UK now report an approach to monitor the electrical properties of single-molecule junctions, which involves precise control of the contact spacing and tilt angle of the molecule.
"During recent years we have developed experimental methods to investigate electron transfer through single molecules attached to metal contacts" Dr. Wolfgang Haiss explains to Nanowerk. "In our recent Nature Materials publication we present novel results, which demonstrate the dependence of single-molecule conductance on the absorption geometry. These findings are corroborated by state of the art calculations. We also have shown that the conductance of some molecules may depend on temperature. These results contribute to an improved understanding of electron transfer and may help to develop novel electronic devices, which facilitate electronic functions on the nanometer scale."
Furthermore, Haiss and his colleagues also show a novel route to synthesize fully conjugated, long axially rigid molecules, which may find applications in molecular electronics.
Haiss is a member of the Nichols Group at the University of Liverpool. Single molecule electrical measurements is a major focus of the group.
The incorporation of molecules into molecular and nanoscale electronic devices requires new methods for measuring and controlling the electronic properties of molecules, as well as robust protocols for wiring them into addressable arrays. Several approaches have been used for measuring transport through either single or small groups of molecules, including mechanically formed break junctions, break junctions formed by electromigration methods and a variety of scanning probe microscopy techniques based on either STM or conducting AFM.
Representation of the experimental procedures used to measure the electrical conductance of a single molecule at a well-defined contact separation. (Left) Schematic representation of molecule A before formation of a contact to the STM tip. The tip–sample separation s is determined by the set-point current I0. (Right) After formation of a contact to the STM tip, molecule A may show a maximum tilt angle (θ) (Reprinted with permission from the Nature Publishing Group)
"At Liverpool we have developed two new methods based on the STM for measuring single molecule conductance" says Haiss. "These methods do not require the fabrication of break junctions or the assembly of molecules into micro-fabricated devices and reliable measurements of electrical behavior can be readily made down to the single molecule level in a wide variety of environments. The techniques rely on the formation of molecular bridges directly between the STM tip and a substrate surface. Importantly, this method provides chemical attachment to the contacts at both ends of the molecule, a prerequisite for the reliable measurement of molecular electrical properties (in the absence of a chemical contact, contact resistances have been shown to be significant, with alkanethiols exhibiting two orders of magnitude higher resistance than alkanedithiols)."
In their recent work, the British scientists demonstrated that contact geometry and thermal fluctuations can be systematically controlled, through precise control of the nanoelectrode gap spacing, allowing molecules to be tilted or stretched, and by engineering the intrinsic rigidity of the molecules.
Future work in this field includes the design of improved contact groups to reduce conductance fluctuations at room temperature and the realization of electronic functions in a single molecule.