Charge Transport through Molecular Junctions
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Charge Transport
through Molecular Junctions
M.C. Hersam and R.G. Reifenberger Abstract In conventional solid-state electronic devices, junctions and interfaces play a significant if not dominant role in controlling charge transport. Although the emerging field of molecular electronics often focuses on the properties of the molecule in the design and understanding of device behavior, the effects of interfaces and junctions are often of comparable importance. This article explores recent work in the study of metal–molecule–metal and semiconductor–molecule–metal junctions. Specific issues include the mixing of discrete molecular levels with the metal continuum, charge transfer between molecules and semiconductors, electron-stimulated desorption, and resonant tunneling. By acknowledging the consequences of junction/interface effects, realistic prospects and limitations can be identified for molecular electronic devices. Keywords: electrodes, metals, molecular electronics, molecular transport junctions, scanning probe microscopy, semiconductors.
Introduction Acomplete understanding of charge transport at the nanoscale is certainly at the forefront of nanoscience. This topic has received added emphasis, as future electronic devices are predicted to require reliable molecular junctions.1 The performance of such devices will depend critically on the nature of the contact between individual molecules and the supporting electrodes (see Figure 1). In general, the molecules of interest for use in molecular junctions include conjugated organic molecules with end groups that preferentially attach to a substrate of choice. Self-assembly offers great promise, but is often hard to control. Most experiments designed to measure current flow in molecules are difficult to implement and are inherently ill-defined. For a molecular junction to be well characterized, experimentalists need to know (1) the crystallographic nature of the electrodes, (2) the number of molecules being probed, (3) the bonding sites of the molecules to the substrate, and (4) the conformations of the molecules. In particular, the positions of all of the atoms within the dotted box in Figure 1 are critically important for controlling molecular conductance.
MRS BULLETIN/JUNE 2004
Future devices relying on single molecules for their functionality will be extremely sensitive to the details of molecule–substrate and molecule–molecule interactions. The binding of molecules to substrates is a balance among many competing interactions, often causing a restructuring of the substrate underneath the molecular layer. Much effort has gone into characterizing the adsorption mechanisms of molecules on substrates and the resulting surface reconstruction. The added challenge to experimentalists studying current flow through molecules is to sufficiently characterize any molecular junction under study so that a meaningful comparison between theory and experiment can be performed. The delicate nature of this situation was recently underscored by calculations indicating that a s
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