Electrical and Spectroscopic Characterization of Molecular Junctions
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Electrical and
Spectroscopic Characterization of Molecular Junctions
James G. Kushmerick, David L. Allara, Thomas E. Mallouk, and Theresa S. Mayer Abstract The design of future molecular electronic devices requires a firm understanding of the conduction mechanisms that determine their electrical characteristics. Progress toward this goal has been hindered by complications in controlling the exact configuration and makeup of fabricated molecular junctions, thus limiting the availability of quantitative experimental data for developing cohesive theories to model and predict molecular transport. This article summarizes recent research aimed at developing well-controlled systems for comparing molecular conduction and vibrational spectra using crossed-wire and in-wire metal–molecule–metal junctions. Systematic variations in molecular structure and metal–molecule contacts show strong quantitative agreement in device properties, while spectroscopic data provide evidence that the properties are due to the molecular junction. Further investigations using these and other molecular junction test beds will provide the needed experimental data to advance fundamental understanding of molecular transport and facilitate future molecular electronics applications. Keywords: inelastic tunneling spectroscopy, molecular devices, molecular electronics, molecular wires, metal–molecule–metal junctions, self-assembled monolayers.
Introduction The field of molecular electronics is being driven by the potential to reduce the size of electronic, optical, and sensory devices to the scale of a single molecule. Innovations in synthetic chemistry have made it possible to tailor the electronic properties of molecules to serve as insulators and conductors as well as two- and three-terminal nonlinear devices.1 These advances have sparked considerable activity in developing logic and memory architectures based on molecular devices that are scalable in terms of circuit density, power dissipation, and fabrication cost. Proposals range from long-term strategies for the self-assembly of complex logic elements from molecular wires and nonlinear devices2 to nearer-term strategies that incorporate molecular monolayer junctions in logic and memory fabrics.3 Ultimately, their success is predicated upon the ability to relate the termi-
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nal characteristics of single-molecule or monolayer devices to the physical phenomena affecting the underlying transport mechanisms. Many test beds have been developed to characterize the electrical properties of molecular junctions and are the subject of several excellent reviews.4 Briefly, measurements have been taken on single molecules using scanning probe contacts5,6 and controllable break junctions.7 Self-assembled monolayer (SAM) junctions have been characterized with top metal contacts formed using atomic force probes,8 mercury drops,9 low-temperature evaporation,10 electroless deposition,11 and crossed metal wires.12,13 These studies have provided experimental evidence for conduction through aromatic molecular
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