Yadong Yin (left), Le He (center), and Yongxing Hu examining a solution of iron oxide particles that changes color when an external magnetic field is applied to it. Credit: Yin lab, UC Riverside.
Chemists at the Univ.
of California, Riverside have developed nanoscale-size rods
of iron oxide particles in the lab that respond to an external magnetic field
in a way that could improve how visual information is displayed in the future.
Previously, Yadong Yin’s lab showed that when an external
magnetic field is applied to iron oxide particles in solution, the solution
changes color in response to the strength and orientation of the magnetic
field. Now his lab has succeeded in applying a coating of silica to the iron
oxide particles so that when they come together in solution, like linearly
connected spheres, they eventually form nanorods that permanently retain their
When an external magnetic field is applied to the solution
of nanorods, they align themselves parallel to one another like a set of tiny
flashlights turned in one direction, and display a brilliant color.
“We have essentially developed tunable photonic materials
whose properties can be manipulated by changing their orientation with external
fields,” said Yin, an assistant professor of chemistry. “These nanorods with
configurable internal periodicity represent the smallest possible photonic
structures that can effectively diffract visible light. This work paves the way
for fabricating magnetically responsive photonic structures with significantly
reduced dimensions so that color manipulation with higher resolution can be
Applications of the technology include high-definition
pattern formation, posters, pictures, energy efficient color displays, and
devices like traffic signals that routinely use a set of colors. Other
applications are in bio- and chemical sensing, as well as biomedical labeling
and imaging. Color displays that currently cannot be seen easily in sunlight will
be seen more clearly and brightly on devices that utilize the nanorod
technology since the rods simply diffract a color from the visible light
incident on them.
Study results appear online in Angewandte Chemie.
In the lab, Yin and his graduate students Yongxing Hu and Le
He initially coated the magnetic iron oxide molecules with a thin layer of
silica. Then they applied a magnetic field to assemble the particles into
chains. Next, they coated the chains with an additional layer of silica to
allow for a silica shell to form around and stabilize the chain structure.
According to the researchers, the timing of magnetic field
exposure is important to the success of the chain formation because it allows
for fine-tuning the interparticle spacing within photonic chains. They report
that the chaining of the magnetic particles needs to be induced by brief
exposure to external fields during the silica coating process so that the
particles temporarily stay connected, allowing additional silica deposition to
then fix the chains into rods or wires.
From left to right: Iron oxide (Fe3O4) particles are coated with silica (SiO2) to form tiny linear chains that grow into peapod-like structures with the application of more silica. Credit: Yin lab, UC Riverside.
They also report in the research paper that the
interparticle spacing within the chains in a sample can be fine-tuned by
adjusting the timing of the magnetic field exposure; the length of the
individual chains, which does not affect the color displayed, can be controlled
by changing the duration of the magnetic field exposure.
“The photonic nanorods that we developed disperse randomly
in solution in the absence of a magnetic field, but align themselves and show
diffraction color instantly when an external field is applied,” Yin said. “It
is the periodic arrangement of the iron oxide particles that effectively
diffracts visible light and displays brilliant colors.”
He explained that all the one-dimensional photonic rods
within a sample show a single color because the particles arrange themselves
with uniform periodicity—that is, the interparticle spacing within all the
chains is the same, regardless of the length of the individual chains. Further,
the photonic chains remain separated from each other in magnetic fields due to
the magnetic repulsive force that acts perpendicular to the direction of the
The researchers note that a simple and convenient way to
change the periodicity in the rods is to use iron oxide clusters of different
sizes. This, they argue, would make it possible to produce photonic rods with
diffraction wavelengths across a wide range of spectrum from near ultraviolet
to near infrared.
“One major advantage of the new technology is that it hardly
requires any energy to change the orientation of the nanorods and achieve
brightness or a color,” Yin said. “A current drawback, however, is that the
interparticle spacing within the chains gets fixed once the silica coating is
applied, allowing for no flexibility and only one color to be displayed.”
His lab is working now on achieving bistability for the
nanorods. If the lab is successful, the nanorods would be capable of diffracting
two colors, one at a time.
“This would allow the same device or pixel to display one
color for a while and a different color later,” said Yin, a Cottrell Scholar.