A Nanometer-Scale Etch A Sketch
Scientists use a microscope to write and erase nanowires
PHOTO: Jerome Levy
6 March 2008—Scientists at the University of Pittsburgh say they have found a way to draw and erase tiny nanometer-wide dots and lines that can conduct electricity. The physicists hope that the discovery will lead to a way to draw nanowires to connect devices on a circuit and to change a circuit’s logic by simply redrawing the devices. The technique could also lead to new types of logic devices and ultrahigh-density data storage components, say the scientists.
The process works like a nanoscale Etch A Sketch. The lines are drawn on an insulating surface by dragging the tip of an atomic force microscope (AFM), a device that can see individual atoms by dragging a supersharp tip across them. For their drawing surface, University of Pittsburgh physicist Jeremy Levy and his colleagues chose a 1.2-nanometer layer of lanthanum aluminum oxide deposited on top of strontium titanium oxide. The interface between the two oxides is normally insulating, but a positive voltage from the sharp metal microscope tip creates a conductive spot underneath the tip at the materials’ interface.
Erasing the conducting areas requires flipping the AFM tip’s voltage. ”So if we want to write a wire, we apply a positive voltage and move the tip from point A to point B,” Levy says. ”Then if we want to cut the wire, we apply a negative voltage and move the tip across the wire and snap it in two.”
In a report published online at Nature Materials on 2 March, the researchers showed how they could write conducting lines, or wires, less than 4 nm wide using the technique. They were also able to make an array of 2-nm-wide dots. These areas remained conductive for more than 24 hours.
Why the material interface changes under the AFM tip is speculation at this point. The researchers believe that the microscope’s voltage creates an extremely strong electric field across the lanthanum aluminum oxide layer. A positive field rips away oxygen atoms, freeing electrons that are normally bound to them. The electrons jump down to the interface, where they can move and carry current. A negative voltage adds an oxygen atom to the surface, tying up free electrons.
”I find it pretty remarkable that they can controllably write and rewrite a conducting path,” says Hans Hilgenkamp, who researches oxide interfaces at the University of Twente in the Netherlands.
The technique could find plenty of applications. Wiring devices in a logic circuit is the obvious one. Because the oxides are transparent, the process could also be used in wiring the transistors that control pixels in displays, Hilgenkamp says. ”You would be able to make very complicated high-density patterns of connections,” he says.
Creating nanosize logic components is another possibility. Being able to create patterns smaller than 10 nm to make nanoscale devices has proved to be a ”grand challenge,” says Stephen Streiffer, nanoscale materials researcher at Argonne National Laboratory, in Illinois. The Pittsburgh group has ”been able to demonstrate an extremely clever way to do patterning at 3 nanometers or below.” The technology will not replace conventional silicon-based devices, Streiffer stresses, but it nevertheless paves the way for new types of logic devices and storage devices.
Pittsburgh’s Levy has lots of device ideas for the new technique. The tiny conducting dots could be used to make ultrahigh-density storage devices, for example, where a conducting spot is a ”1” bit and a dot that does not conduct is a ”0,” he suggests. The technique could also be advanced to create a transistor-like logic switch, Levy says. In conventional field-effect transistors—the cornerstone device of logic circuits—voltage at a gate electrode turns a conducting channel on and off. Levy says they can do the same thing using an AFM tip instead of a gate electrode.
Levy and his colleagues also point out that oxide materials could potentially be combined with silicon technology; other research groups have reported growing high-quality strontium titanium oxide films on silicon.
Scientists study complex oxide materials and their interfaces because they have shown a number of surprising properties. ”[Oxide interfaces] can be metals and insulators and have magnetic behavior, ferroelectric [behavior], or superconducting behavior,” Levy says.
”This work shows that you could control properties of devices really down to the atomic scale,” Twente’s Hilgenkamp says. ”It would be very interesting to see if you could tune magnetism on an atomic scale.”