A variety of metal-molecule-metal junctions have been used to elucidate single molecule properties in organic monolayers that are applicable in molecular electronics. For example, pore-based sandwich structures, mechanical break junctions, and Hg drop electrode top contacts have been used to characterize the current-voltage properties of organic monolayers. Studies have also employed scanning tunneling microscopy (STM) and conducting atomic force microscopy (cAFM) to probe the current-voltage properties of molecular junctions with more limited contact areas. In particular, reports of STM studies on redox-active molecular monolayers have described the use of electroactive moieties in molecular junctions to facilitate nonlinear current-voltage behavior. In a recent example, the nonlinear current-voltage phenomenon of negative differential resistance (NDR) was observed in an electroactive, ferrocene-terminated self-assembled monolayer (SAM). The identification of nonlinear current-voltage properties such as NDR for individual molecules expands the potential applicability of molecule-scale components from use as conductive wires to multi-state molecular switches.
Future efforts in molecule-based electronic component design will be aided by a vast library of information gathered by probing the electronic and chemical properties of individual molecules. An initial step to quickly probing a wide variety of molecules is to deposit molecular monolayers on an easy accessible surface, usually a flat metal substrate such as gold. Using a sharp tip as the other electrical contact we can examine how current varies as a function of applied voltage. Here, we show that nonlinear current-voltage properties (such as NDR) observed in electroactive self-assembled monolayers (SAMs) can be modified by controlling the composition of the molecular junction between a substrate (electrode 1, metal or semiconductor) and a probe tip (electrode 2, metal). In addition, we have used non-covalent molecular interactions (hydrogen bonding) to install and remove electroactive functionality in SAM-based mesoscale molecular assemblies. We use a combination of techniques (STM, conducting AFM, electrochemistry and spectroscopy) to characterize our molecular junctions.
Department of Chemistry, North Carolina State University
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