Sound Waves Open the Door to Advanced Placement of Nanowires

Through the use of sound waves, researchers at Penn State announced Wednesday they have devised a way to place nanowires in repeatable patterns - a discovery that could have significant ramifications in the manufacturing of the world's increasingly small devices.

Among the researchers' perceived uses of the innovation are a variety of sensors, optoelectronics and nanoscale circuits, all of which, the scientists say, face an increase in scalability through the method.

"There are ways to create these devices with lithography, but it is very hard to create patterns below 50 nanometers using lithography," said Tony Jun Huang, associate professor of engineering science and mechanics.

In contrast, Huang argues, their technique allows pattern transfer of the alignment of nanomaterials easily created through synthetic chemistry onto substrates incompatible with conventional lithography.

"For example," he explained, "we could make networks of wires and then pattern them to arrays of living cells."

In coming to their discovery, the scientists looked at the placement of metallic nanowires in solution on piezoelectric substrate, which refers to materials that move when an electric voltage is applied to them and create an electric voltage when compressed.

In this instance, they chose an alternating current to apply to the substrate so that the material's movement crated a standing surface acoustic wave in the solution. Because a standing wave has node locations that do not move, the nanowires arrive at these nodes and stay there.

Had the scientists only applied one current, the nanowires would have formed a one-dimensional array with nanowires lined up head-to-tail in parallel rows; however, if perpendicular currents were used, a two-dimensional grid of standing waves formed and the nanowires moved to those grid-point nodes and formed a three-dimensional spark-like pattern.

"Because the pitch of both the one-dimensional and two-dimensional structures is sensitive to the frequency of the standing surface acoustic wave field, this technique allows for the patterning of nanowires with tunable spacing and density," the researchers report in a recent issue of ACS Nano.

After the nanowires are settled, the solution evaporates, preserving the pattern.

Moving the resulting patterned nanowires to organic polymer substrates was then done by transferring them by placing the polymer on top of the nanowires with slight pressure, effectively transferring them over.

Finally, the researchers suggest that the nanowires could then be transferred to rigid or flexible substrates from the organic polymer through the use of properly developed microcontact-printing techniques.

Furthermore, unlike lithography, the spacing of the nodes where nanowires deposit can be adjusted on the go simply by changing the frequency of the interaction between two electric fields.

"We really think our technique can be extremely powerful," Huang said. "We can tune the pattern to the configuration we want and then transfer the nanowires using a polymer stamp."

Going forward, the group plans on investigating more complex designs.