Researchers at North Carolina State Univ. have developed a cheap and easy
method for assembling nanowires, controlling their alignment and density. The
researchers hope the findings will foster additional research into a range of
device applications using nanowires, from nanoelectronics to nanosensors,
especially on unconventional substrates such as rubber, plastic, and paper.
“Alignment is a critical first step for developing devices that use
nanowires,” says Dr. Yong Zhu, an assistant professor of mechanical and
aerospace engineering at NC State and co-author of a paper describing the
research. “Hopefully our simple and cost-effective method will facilitate
research in this field.”
Aligning nanowires is challenging because, when they are created, the user
is faced with a profusion of randomly oriented nanoscale wires that are, by
definition, incredibly small. For example, the nanowires are between 10 and 100
nm in dia., whereas a white blood cell is approximately 10,000 nm in dia.
Before any practical applications can be pursued, the user must assemble the
nanowires in an orderly way. Specifically, users need to align the nanowires in
a common direction and define their density—meaning the number of nanowires in
a given area. Controlling both alignment and density is commonly called
“assembling” the nanowires.
In the new method, Zhu’s team deposited the nanowires on a stretched rubber
substrate, and then released the tension on the substrate. When the nanowires
settled, they aligned at a right angle to where the tension was coming from.
Picture a rubber band being stretched to the east and west. If nanowires were
placed on the rubber band, and the band was allowed to snap back to its
original shape, the nanowires would be oriented to the north and south. The
more the rubber substrate is stretched, the more aligned the nanowires will be,
and the greater the nanowire density will be.
Previous research has presented a number of other methods for assembling
nanowires. But the new method offers a number of distinct advantages. “Our
method is cost-effective,” says Feng Xu, a Ph.D. student working on this
project, “because it is so simple. It can also be used for nanowires
synthesized by different methods or processed in different conditions, for
instance, silver nanowires synthesized in solution and silicon nanowires
synthesized by the vapor-liquid-solid method, as demonstrated in our work.” In
addition, the new method can be used in conjunction with previous methods to
achieve even better nanowire assembly.
The use of a rubber substrate in this method facilitates broad research and
manufacturing sectors. For example, a key element of research into stretchable
nanoelectronics involves aligning nanowires on a stretchable rubber substrate. Similarly,
rubber is also the material used as “stamps” in transfer printing—a critical
fabrication method used in manufacturing nanodevices on diverse substrates
ranging from silicon to glass to plastic.
Zhu notes that the initial step of the method, when the nanowires are first
deposited on stretched rubber, sometimes yields an inconsistent degree of
nanowire alignment. The team is currently working to understand the fundamental
interface mechanics -including adhesion and static friction -between nanowires
and rubber substrates, which is expected to lead to a better control of the
assembly process and hence a higher yield of the nanowire assembly.
The paper, “Strain-Release Assembly of Nanowires on Stretchable Substrates,”
was published in ACS Nano. The paper was co-authored by Zhu, Xu, NC State Ph.D.
student John Durham, and Dr. Benjamin Wiley, an assistant professor at Duke Univ.
The research was funded by the National Science Foundation.
NC State’s Department of Mechanical and Aerospace Engineering is part of the