Organic materials (multi-carbon compounds) are amenable to almost Lego©-like tinkering with their structure and composition, and can exhibit a wide range of properties that might make possible electronic devices far more capable than today's state of the art. A few to tens of nanometers across, polymers and other organic molecules are easy to make as stand-alone units, but the next step--fabricating organic molecules into devices and circuits—is extremely challenging. A major goal of this project is to develop device-fabrication and circuit-assembly techniques optimized for organic materials, yet compatible with silicon-based electronics manufacturing. We are investigating three fabrication strategies and measuring how variations in the properties of the materials associated with each strategy affects device performance.
After five decades, the unabated miniaturization of silicon-based integrated circuits is approaching fundamental physical limits. One promising technology that offers a way forward is hybrid electronics, which combines organic chemistry approaches with established device fabrication and patterning techniques. This strategy entails integrating molecular switches, typically made from polymers as well as other organic molecules or chemically engineered inorganic clusters, with technologies made from inorganic materials, including silicon-based devices optimized for certain tasks.
Eventually, it may even be possible—and practical—to turn nanoscale switches on or off by using changes in the shape or spatial location of heavy particles (ions, atoms or molecules) or other non-electronic features rather than the movement of electrons. In addition, the huge variety of organic compounds offers the possibility that the nanodevices made with these materials will diversify into applications well beyond computing, from environmental monitoring to smart packaging.
A key step toward realizing this hybrid technology is to develop affordable and reliable processing methods that, in effect, combine the new with the old. Now, for example, chemical synthesis of organic thin films and nanoscale objects such as quantum dots is incompatible with photolithography or other standard patterning techniques used to make today's computer chips. Conversely, those patterning processes can significantly alter the electronic properties of devices incorporating organic materials.
In this project, we are measuring the material transformations that occur during fabrication, using three alternative strategies for making molecular wires, barriers, and other basic nanoscale components. Developed as part of this research, the fabrication methods are relatively non-invasive, and do not involve etching, direct electron beam exposure, or other material-changing steps. Two of the techniques are variations on self-assembly and nanoscale stenciling, while the third uses flexible polymer stamps to transfer nanoscale metal patterns to organic molecules or clusters.
With this suite of techniques, we are examining fabrication-related issues that influence the electrical properties and behaviors of the resulting molecular devices. Results indicate that the atomic precision in the placement of all relevant device constituents is hard to achieve. At the junction between organic molecule and metal electrode, defects are common, the type and number varying with the particular combinations of metal and organic compound involved. Electronic and structural material transformations at the interfaces often have a greater impact on the movement of electrons than do the electronic properties of molecules that form the device.
However, imperfections in the positions of individual molecules or defects created during fabrication may not matter if a large number of functional sites also are present in the devices. For example, in a switch controlled by the movement of ions, we have observed that the total number of ions in the nanoscale device was large enough to give reproducible switching performance, despite variability in position or motion among individual ions. Moreover, the process of systematically modifying ions attached to polymers that make up a molecular switch is rather straightforward and, often, reversible. This process enables us to examine how switching behavior is modified by chemical variations within a given device.
For devices built from the combinations of dissimilar materials that we have studied thus far, switching behavior depends on the movement of heavy particles over distances ranging from elementary cells of the crystal structure to entire devices. The relative slowness of heavier particles makes their usage completely impractical in larger, previous-generation devices. However, the very same property would enable the building of switches at the smallest of scales, where electrons cannot be reliably confined and retained inside a device. The ability to reliably create nanoscale switching devices in this way could lead to new hybrid logic circuits with novel architectures, perhaps even mimicking the connectivity of the human brain.