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Frontiers of Engineering: Reports on Leading-Edge Engineering from the 2008 Symposium
tational state. Ironically, the use of heavy carriers is impractical in larger devices because of their limited mobility. However, in a nanometer-sized device, the ion/atom transport can be fast enough for practical applications.
Short molecules and macromolecules can be used as active materials for heavier switching devices. Devices built with short molecules have long been considered promising candidates for the post-CMOS era for a number of reasons. First, organic molecules can be extremely small and at the same time exactly reproducible as stand-alone units. In addition, numerous synthetic techniques have been developed, and the variety of organic compounds is enormous. Some well-known approaches to molecular electronics already rely on molecular conformations or oxidation to achieve electronic functionality (e.g., Chen et al., 1999; Collier at al., 2000).
However, the reliable fabrication of devices and the assembly of molecules into circuits turn out to be extremely challenging. In this presentation, I will describe some examples from our research that illustrate some of the challenges of fabricating and characterizing molecular devices. Before we began designing a molecular switch or transistor, we tested simpler building blocks in the molecular “tool box,” such as molecular “wires” and molecular “barriers.”
The investigation of the electronic properties of molecular devices is intimately related to research on alternative fabrication routes that can be compatible with the new materials. First, the required feature size is often beyond the limits of the best lithography machines. Second, the properties of pristine material can be substantially altered by, for example, exposure to a high-energy electron beam encountered in the e-beam lithography step, etching, or contact deposition.
NEW FABRICATION TECHNIQUES
In our research, we focused on the noninvasive fabrication of nano- and mesoscale molecular devices and the effects of fabrication on their structural and electronic properties (Zhitenev et al., 2006). We fabricated metal-molecular monolayer-metal junctions using three complementary original techniques that target different fabrication issues. After screening many possible candidates for molecular “wires” and molecular “barriers,” we selected representative molecules capable of forming a dense self-assembled monolayer (SAM) with the most robust structural and electrical properties.
The first technique targeted nearly single-molecule devices. The junctions were formed on the surface of the tips to exploit the evaporation of contacts from different angles with an assembly of SAMs in the middle. Device conductance was monitored during the formation of the junction. Devices were studied at multiple stages, from minimally detectable conductance below the conductance level of a single-molecule junction to an approximately single-molecule device to a multimolecule device. The shortcoming of this technique was that it relied