Molecular rulers may help build nanoscale optoelectronics structures

March 13, 2001
Scientists at Penn State (University Park, PA) have discovered a precision technique to build ultraminiature metal wires in very close proximity to each other.

Scientists at Penn State (University Park, PA) have discovered a precision technique to build ultraminiature metal wires in very close proximity to each other. Their work--which is important because nanoscale construction methods have been limited to structures with larger less-controlled spacings--is expected to be useful in the effort to further miniaturize electronic and optoelectronic devices used for circuits, high-density data storage, and sensors.

The scientists use organic molecules as molecular rulers that permit the fabrication of useful wires dozens of times smaller than the period at the end of this sentence. So far, the researchers have produced wires from 15 to 70 nm wide and a few micrometers long that are spaced 10 to 40 nm apart.

�We have known for some time how to make smaller and smaller structures using techniques developed for the fabrication of computer chips, and we also have known how to make molecules bigger and bigger,� says Paul Weiss, associate professor of chemistry at Penn State. �The intermediate region between the two approaches has been essentially inaccessible, though, and our technique of using 'molecular rulers' represents a step toward bridging that gap.�

The molecular-ruler construction process requires some existing nanoscale structures to �grow� in order to produce the even smaller structures. The growth process begins with two parallel gold nanostructures on a silicate substrate that have been formed by electron-beam lithography. Layers of organic molecules then are applied atop the initial structures to make them bigger and wider, while at the same time reducing the gap between the structures.

Because the scientists knew the size and spacing of the initial structures and the thickness of the layers of films created by the molecules atop the structures, they could calculate the size of the narrowing space between the structures. As a result, the organic molecules, which selectively bind to each other and to the substrate materials, provide molecular rulers that precisely determine the size of the resulting space between the initial structures. Filling the space with gold can then produce even smaller wires.

During their experiments, the Penn State researchers used silicate as the substrate, gold for the prefabricated initial structures, and mercaptoalkanoic acid as the organic molecule. Those organic molecules, also are referred to as resists because they resist attack and protect the material underneath them in various lithographic processing steps, were used by Weiss and his team to improve the construction process for nanoscale structures.

�We know how to make the ends of organic molecules so that they bind selectively, both to each other and to the substrate,� says Weiss. �We also know they form films when they interact, and from that we can determine a precise thickness of the film. That's what makes the whole thing work. If they did not do that, the process would be just as crude as the standard polymer resists.�

Along with precision and increased miniaturization, the construction process outlined by Weiss and Amat Hatzor, a post-doctoral fellow at Penn State, includes a method to selectively remove the molecular resists after the wires are cast, thereby improving upon the efficiency and flexibility of existing methods. Whereas other fabrication methods require scientists to build structures individually, the molecular ruler method allows an entire cookie sheet of structures or wires to be completed at once.

�It is a single fabrication process,� adds Weiss. �You do not have to draw every single line one at a time. You simply do the overall design, and then in one set of steps, you can complete the whole surface. We can make a number of shapes and sizes that we cannot make by other means.� For more details on this research project, contact Steve Sampsell at [email protected].

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