Sandia researcher brings Bell Labs technique to wiring molecules
Attaching electrical wires to a chain of atoms is a lot harder than wiring a light bulb.
That may explain why Sandia researcher Julia Hsu’s (1114) invited talk on creating electrical contacts for molecules attracted approximately 250 researchers at the American Physical Society’s March Meeting in Montreal. The overflow crowd filled the seats and jammed the aisles of the too-small double room.
Single molecules could function as inexpensive, reliable chem/bio detectors, if an electrical signal could be made to flow through a molecule in a controlled manner. Such molecules can be designed to chemically bind to specific substances. Attachment of a target substance onto the molecule would then result in easily detectable variations in electrical output.
The American Institute of Physics’ The Industrial Physicist magazine had briefly highlighted Julia in its December issue. The magazine noted that while "large changes in conductivity can be achieved with a single molecule," still "conductivity is profoundly affected by the way in which molecules are contacted." The article helped emphasize the need to return to pursuing more fundamental problems in molecular electronics after the journal Science had attacked the fledgling field in an article last year for claims of technological successes that the journal found dubious.
One problem was that earlier attempts to pass a current through a molecule foundered on the difficulty of attaching electrical nanocontact points to molecules. Electrical short-circuiting between the positive and negative contacts generally plagued device yields, and spurious electrical paths have often been misinterpreted as currents passing through the length of the signal molecule.
Julia’s solution, brought with her from Bell Labs, uses a technique that resembles a child’s printing set. It is called nanotransfer printing. The molecules she uses have a chain of carbon atoms anchored by sulfur atoms at each end. These molecules form a monolayer on a gallium-arsenide substrate with one of the sulfur ends binding to the gallium arsenide. The carbon chains and the other sulfur end groups elevate above the substrate like a field of wheat rising above the ground. To the top of this raised field, Julia applies a soft silicone stamp covered by a gold film. The gold film is able to chemically bond to the raised sulfur atoms while remaining strong enough to form an otherwise unsupported canopy (something like the way an umbrella’s fabric does not need to be supported at every point to form a surface), rather than dripping down to the substrate and shorting out the device.
This gold canopy and the gallium arsenide substrate form positive and negative contacts for the attached field of molecules.
Her method boasts a 97 percent success rate, she says, compared to less than five percent successful yield in devices that involved merely evaporating metals onto the stand-up molecules.
"This is still basic science, not a useful device yet," Julia says. "It does not yet have features such as integration with silicon or a separate gate to make a transistor. There are many technical challenges to realize these goals and many researchers are tackling these questions. What I am doing is laying the basic scientific foundation for this very promising but still emerging technology."
More work remains to be done, she says, to establish parameters for different metals and varying temperatures and pressures. She is building collaborations with researchers in 1100, 1700, and 8700 to establish a research program in this area within Sandia.
Mark J. Cardillo, executive director at the Camille and Henry Dreyfus Foundation in New York, exited Julia’s talk smiling. Asked why, he said it was an excellent talk and then volunteered that "I’ve never had a better postdoc for being unafraid to get to the core of an issue, and I have had some very good postdocs." Cardillo was a former executive at Bell Labs, from which Julia recently emigrated to Sandia.
Julia will co-chair the fall meeting of the Materials Research Society in 2004.