Tethered to Silicon

Silicon is at the heart of today’s computer microchips. Making faster and cheaper computers means carving vanishingly small transistors into silicon chips-a task that is becoming increasingly difficult and expensive. One potential solution is to use individual organic molecules, which are orders of magnitude smaller than today’s transistors, on a silicon surface to do electronic switching and storage.

Making such silicon-organic hybrids, however, poses a very, very small problem-how do you put the molecules exactly where you want them? Electrical engineers at the University of Illinois at Urbana-Champaign have now found a way to attach individual organic molecules to silicon with atomic precision, using the tip of a scanning tunneling microscope.

First the researchers deposit a layer of hydrogen, one atom thick, on the silicon surface; then they use the microscope’s tip to pluck off individual hydrogen atoms in a desired pattern. The result, says Joe Lyding, professor of electrical and computer engineering at Illinois, is “a dangling silicon bond [where the hydrogen atom was] that is very reactive.” Various organic molecules can then be sprayed on the surface, where they will attach themselves only to the “dangling bonds.”

So far, Lyding and his graduate student Mark Hersam have fabricated simple patterns-columns and a V-shape-by spraying on molecules such as buckyballs (a soccerball-shaped 60-carbon molecule that many researchers believe has promise in electronics). Lyding envisions that the technique could eventually lead to hybrid silicon chips with ultrafast molecular switching and storage arrays. But, he adds: “In a sense this is uncharted territory. Nobody has placed individual molecules into atomically precise arrays on silicon before.”