Superconducting Nanowires Use DNA as Base

By using DNA molecules as scaffolds, scientists have created superconducting nanodevices that demonstrate a new type of quantum interference, and could be used to measure magnetic fields and map regions of superconductivity. In the future, the technology could be generalized to produce semiconducting or other types of electronic devices.

Researchers at the University of Illinois at Urbana-Champaign have fabricated and studied nanostructures consisting of pairs of suspended superconducting wires as tiny as three to four molecular diameters (typically 5-15 nm) in width. The team consisted of physics professors Alexey Bezryadin and Paul Goldbart, and graduate students David Hopkins and David Pekker.

"Our measurements on these two nanowire devices revealed a strange class of periodic oscillations in resistance with applied magnetic field," Bezryadin said. "Through experimentation and theory, we found both an explanation for this odd behavior and a way to put it to work."

To make their nanodevices, the researchers began by placing molecules of DNA across a narrow trench (~100 nm wide) etched in a silicon wafer. The molecules and trench banks were then coated with a thin film of superconducting material (molybdenum germanium). The result was a device containing a pair of homogeneous, superconducting nanowires with extremely fine features.

This conceptual drawing shows how researchers stretched DNA molecules across a bridge, and sputter-coated them with molybdenum to produce a sputter-conducting nanowire only 10 nm in diameter. (Source: University of Illinois)

"By creating thin films on the surface of DNA, we can create nanowires that are below 10 nm in diameter, which is hard to do by other methods...probably impossible by traditional methods," Bezryadin said. "To do that, we make a substrate with holes, and it turns out that if you drop a DNA and it crosses one of those holes, it stretches itself. So you get these straight wires and it's quite a convenient substrate. Then we can just sputter metal on them and produce this superconducting nanowire."

"In the absence of a magnetic field, these ultranarrow wires exhibited a non-zero resistance over a broad temperature range," Bezryadin added. "At temperatures where thicker wires would already be superconducting, these DNA-templated wires remained resistive."

Tuning the strength of a magnetic field applied to the device, however, caused highly pronounced and periodic oscillations in resistance, at any temperature in the transition region.

"The applied magnetic field causes a small current to flow along the trench banks, and this current then causes a large change in resistance," Goldbart said. "The strength of the current is controlled only by the magnetic field and the width of the banks supporting the wires."

Goldbart noted that the resulting periodic oscillation is a reflection of the wave nature of matter that goes to the very heart of quantum mechanics. "Unlike ordinary matter, the electrons in these wires are behaving as though they are one quantum mechanical object in one great quantum mechanical wave function," he said.

Metallic nanodevices based on DNA scaffolds could be used in applications such as local magnetometry and the imaging of phase profiles created by supercurrents — in essence, a superconducting phase gradiometer, the researchers report.

"By taking advantage of DNA self-assembly processes, complex scaffolds could be created for electronic devices with features having molecular-scale dimensions," Bezryadin said. "You can synthesize DNA with a particular sequence and if you have different types, they will assemble in a system in which you envision. People use DNA to make networks and some geometrical objects — some constructs like cubes — and they can program DNA and it self-assembles in complicated networks."

In related work, to appear in the August issue of the journal Nanotechnology, Bezryadin and undergraduate student Mikas Remeika improved the nanofabrication process by using a focused electron beam to locally alter the shape and structure of metallized nanowires.

Performed in a transmission electron microscope, electron-beam sculpting and crystallization can modify small segments of the nanowires, with a spatial resolution of ~3 nm, Bezryadin noted. The technique could be used to fabricate novel electronic nanodevices, such as single-electron transistors, with dimensions <10 nm.

Bezryadin stated that the method could be used to make other types of electronic devices. "You can put semiconductor materials or some magnetic metals on these DNAs and thus produce semiconducting or magnetic wires. It's a method that can be generalized for other applications," he said.

For more information on emerging technologies, go to www.semiconductor.net/emerging.