Establishing the molecular basis for molecular electronics.

Performing logic and memory operations with one or a very small collection of molecules would be the ultimate in electronic-device miniaturization. For this reason, the field of molecular electronics has received attention that ranges from scientific curiosity to the generation of intellectual property and venture capital. While new paradigms and financial rewards in nanotechnology will probably come (although perhaps not as fast as an investor would like), answers to several key questions are a necessary first step in this evolution. In this regard, chemists (who might be regarded as molecular engineers) have an exciting task ahead of them—sorting out the fundamental precepts that will govern this field. A number of central questions have emerged. Some loom large and will probably require substantial shifts in our approaches for working with molecules. For example, what mix of lithography (top-down engineering) and self-assembly (bottom-up manufacturing) will be required to achieve the dense integration of components that allow us to truly exploit the size scale of single-molecule devices? How will nanotubes be used in these regards? To date, no realistic approach has addressed this issue. Other questions have proven to be more manageable and are equally important. They require us to question fundamentally how molecular science will work in nanometer-scale collections. For example, how does one make contact with a molecule? What is the electronic structure of a molecule when it is in contact with “wires”? Can molecular structure–property relationships be derived that relate the structure of a molecule to nonlinear current–voltage behaviors, switching, and, ultimately, gating? These latter questions have been addressed with some recent, plausible approaches. Such work is highlighted herein. In performing nanoscale electronic measurements, the issue at hand, first and foremost, is how to make electrical contact to these elements. In doing so, one must confront the issue that this contact is going to perturb the molecules under study. The first strategies for contact to small collections of molecules began with the mechanical break junction. A break junction is formed by attaching a metal wire onto a flexible substrate and then bending the substrate just until the wire has broken. The gap produced is then exposed to molecules designed to bind across it. Resistances are measured that are determined to be consistent with the resistance of a single molecule. A second top contact can be made to a collection of molecules (e.g., a selfassembled monolayer (SAM) or Langmuir–Blodgett (LB) film) by metal evaporation. In a nanopore configuration the area of the nanopore is designed to be smaller than the domain size of the SAM and the evaporated metal accumulates only on the top of the SAM. By using nanopores, Reed, Tour, et al. showed current–voltage measurements in molecules containing a nitroamine redox center that exhibited negative differential resistance. As these metal–molecule–metal assemblies must be made one at a time, it can be difficult to get a sense of how variable their behavior is. Furthermore, although evaporating a top contact makes a metal–molecule–metal sandwich that most naturally resembles a device, metals are strong reducing agents. Reduced molecules are typically quite chemically reactive. Thus, the molecule that is placed into the sandwich may not be the structure that is ultimately measured. This concern is exacerbated by the fact that the geometry of the sandwich precludes any spectroscopic characterization of the molecules in that device. To address this issue, a number of investigators have employed the tip of a scanning tunneling microscope (STM, or conducting atomic force microscope, AFM) as a second contact to a molecule (often organized into a self-assembled monolayer). Several examples are noted. Hipps and co-workers reported orbital-mediated tunneling through phthalocyanins and porphyrins that contain metal centers. Tour, Bard, and co-workers displayed peak shaped I–V curves in phenylene ethynylene oligomers (OPEs) by using a tuning-fork STM. We have studied negative differential resistance in patterned, electroactive SAMs by using STM. Weiss and co-workers inserted individual OPEs into an insulating n-alkanethiolate SAM background and determined that these molecules were more conducting than the background. By visualizing individual molecules over time, they observed changes in conductance. These variations in conductance (stochastic switching) were attributed to [*] R. A. Wassel, Prof. C. B. Gorman Department of Chemistry North Carolina State University Raleigh, NC 27695-8204 (USA) Fax: (+1)919-515-8920 E-mail: chris_gorman@ncsu.edu Highlights