Most people date modem interest in using molecules as electronic devices to the publication of a classic paper by Aviram and Ratner. Their paper outlined a scheme for a molecular rectifier and drew attention to the concept of using molecules as incredibly small electronic components. Despite thirty years of subsequent research, a central challenge remains: How can molecules be connected between wires to make useful molecular electronic devices? One macroscopic approach, subject to failures at a microscopic level, is to lay a metal film on top of a self-assembled monolayer that sits on a metal electrode. A microscopic approach uses a scanning tunneling microscope (STM) to address one or a few molecules. In this case, the contact between the tip and the molecule is characterized only through measurement of the current itself, making independent characterization of the contact geometry difficult. Somewhat better control of the contact is obtained by using an atomic force microscope with a conducting probe. Despite this work, many questions remain: Is just one molecule contacted? What is the atomic arrangement of the top contact? Does contamination on the probe affect the current? Reed and co-workers used a breakjunction and molecules with two "sticky ends". The junction was made of a gold film that was cracked open to a remarkable degree of precision with a mechanical lever. The molecule, benzenedithiol, had opposing ends of the benzene ring functionalized with thiol groups. These thiol groups react with gold, so that careful breaking of the junction in the presence of the molecules should result in two macroscopic metal contacts bridged by one, or a few, molecules. This system is much better defined than the scanning probe experiments, but even so, the interpretation is open to question because the microscopic nature of the junction was unknown. The results of theoretical modeling underpin this concern. Currents calculated for this system with modern density-functional methods are some 500 times larger than the experimental values, assuming that, in the best case, the experiment really measures the conductance of a single molecule. This discrepancy is hard to understand, because modern calculations are generally reliable. Therefore, the problem lies with uncertainties about the atomic structure of the junction. Complex electronic responses can be obtained from metallic nanojunctions in the absence of molecules spanning the gap.
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