To demonstrate the ease of controlling and positioning colorful nanowires, the researchers created a nanoscale-sized tribute to Harvard, designing a pattern resembling the engineering school’s Veritas seal and and spelling out the acronym SEAS. Credit: Harvard Univ.
Engineers may soon be singing, “I’m going
to wash that gray right out of my nanowires,” thanks to a colorful discovery by
a team of researchers from Harvard Univ. and Zena Technologies. In contrast to
the somber gray hue of silicon wafers, Kenneth B. Crozier and colleagues
demonstrated that individual, vertical silicon nanowires can shine in all
colors of the spectrum.
The vibrant display, dependent on the
diameter of the individual wires, is even visible to the naked eye. In addition
to adding a splash of color to the lab, the finding has potential for use in
nanoscale image sensor devices, offering increased efficiency and the ability
to detect color without the use of filters.
“It is surprising,” says Crozier, John L.
Loeb Associate Professor of the Natural Sciences at the Harvard School of
Engineering and Applied Science (SEAS). “A lot of people are making nanowires, and
you really don’t think of the color so much. In this vertical configuration you
can get very strong color effects, and you can tune them over a range of
wavelengths of the visible region. The strong effects can be seen right down to
the level of the individual wire.”
The finding, published in an online
edition of Nano Letters, may
be the first experimental report that silicon nanowires can take on a variety
of colors depending on their diameter and under bright-field illumination.
Previous work has shown that nanowires can take on different colors but only by
looking at scattered, rather than directly reflected, light.
To create the multicolored array of
vertical silicon nanowires, the engineers at Harvard and Zena Technologies used
a combination of electron beam lithography and inductively coupled plasma
reactive ion etching.
A smooth wafer of silicon was plasma
etched until all that remained were the vertically protruding nanowires,
resembling bristles on a toothbrush. While the nanowires were created in arrays
of thousands for convenience, the colors they exhibited were due to the
properties of the individual wires,
not by the way light was scattered or diffracted in the group.
“Each nanowire acts as a waveguide, like a
nano-sized optical fiber—but an optically absorbing one,” explains Crozier. “At
short wavelengths there is not much optical coupling to the nanowire. At long
wavelengths, the coupling is better, but the properties of the waveguide are
such that there is not much absorption. In between, there is a range of
wavelengths where the light is coupled to the nanowire and absorbed. This range
is determined by the nanowire diameter. We made nanowires with diameters of 90,
100, and 130 nm that appeared red, blue and green, respectively.”
To demonstrate the phenomenon and the
relative ease of controlling and positioning the colorful nanowires, the
researchers created a nanoscale-sized tribute to Harvard, designing a pattern
resembling the engineering school’s Veritas seal
and spelling out the acronym
SEAS in a rainbow of
While the Harvard image closely matched
the school’s seal, the desired color eluded the engineers.
“We actually wanted to make the seal red
rather than blue, but it turned out that the diameter was a little bit wrong,”
As even small changes in the radius of a
wire can alter the color, the seal turned out to be blue, more suitable for the
famous seal of a certain other Ivy League institution.
Fortunately, the technology has other
promising applications. The researchers’ eventual aim is to use the wires in
image sensors. Traditional photodetectors in image sensor devices can gauge the
intensity of light but not determine its color without the use of an additional
filter, which throws away much of the light, limiting the device’s sensitivity.
The researchers hope to address this by
fabricating vertical nanowires containing photodetectors above standard
photodetectors formed on a silicon wafer. The nanowire and standard
photodetectors could each detect a different part of the spectrum of the
incident light. By comparing the signals from each, the color could be
determined without losing so much of the light.
“With image sensors, every little bit of
efficiency counts. Moreover, we even imagine using the colored wires to encode
data in a read-only type of information storage,” adds Crozier.
The researchers have filed a provisional
patent for their work.