Nanowires fabricated using the new techniques developed by Grade?ak and her team can have varying widths, profiles, and composition along their lengths, as illustrated here, where different colors are used to indicate compositional variations. Image: Grade?ak laboratory |
Nanowires
are a hot research topic today, with a variety of potential applications
including light-emitting diodes (LEDs) and sensors. Now, a team of Massachusetts
Institute of Technology (MIT) researchers has found a way of precisely controlling
the width and composition of these tiny strands as they grow, making it
possible to grow complex structures that are optimally designed for particular
applications.
The
results are described in a new paper authored by MIT assistant professor of
materials science and engineering Silvija Grade?ak and her team, published in Nano
Letters.
Nanowires
have been of great interest because structures with such tiny dimensions—typically
just a few tens of nanometers in diameter—can have very different properties
than the same materials have in their larger form. That’s in part because at
such minuscule scales, quantum confinement effects—based on the behavior of
electrons and phonons within the material—begin to play a significant role in
the material’s behavior, which can affect how it conducts electricity and heat
or interacts with light.
In
addition, because nanowires have an especially large amount of surface area in
relation to their volume, they are particularly well-suited for use as sensors,
Grade?ak says.
Her
team was able to control and vary both the size and composition of individual
wires as they grew. Nanowires are grown by using “seed” particles, metal
nanoparticles that determine the size and composition of the nanowire. By
adjusting the amount of gases used in growing the nanowires, Grade?ak and her
team were able to control the size and composition of the seed particles and, therefore,
the nanowires as they grew. “We’re able to control both of these properties
simultaneously,” she says. While the researchers carried out their
nanowire-growth experiments with indium nitride and indium gallium nitride,
they say the same technique could be applied to a variety of different
materials.
These
nanowires are far too small to see with the naked eye, but the team was able to
observe them using electron microscopy, making adjustments to the growth
process based on what they learned about the growth patterns. Using a process
called electron tomography, they were able to reconstruct the three-dimensional
shape of individual nanoscale wires. In a related study recently published in Nanoscale,
the team also used a unique electron microscopy technique called
cathodoluminescence to observe what wavelengths of light are emitted from
different regions of individual nanowires.
Precisely
structured nanowires could facilitate a new generation of semiconductor
devices, Grade?ak says. Such control of nanowire geometry and composition could
enable devices with better functionality than conventional thin-film devices
made of the same materials, she says.
One
likely application of the materials developed by Grade?ak and her team is in
LED light bulbs, which have far greater durability and are more energy efficient
than other lighting alternatives. The most important colors of light to produce
from LEDs are in the blue and ultraviolet range; zinc oxide and gallium nitride
nanowires produced by the MIT group can potentially produce these colors very
efficiently and at low cost, she says.
While
LED light bulbs are available today, they are relatively expensive. “For
everyday applications, the high cost is a barrier,” Grade?ak says. One big
advantage of this new approach is that it could enable the use of much less
expensive substrate materials—a major part of the cost of such devices, which
today typically use sapphire or silicon carbide substrates. The nanowire
devices have the potential to be more efficient as well, she says.
Such
nanowires could also find applications in solar-energy collectors for
lower-cost solar panels. Being able to control the shape and composition of the
wires as they grow could make it possible to produce very efficient collectors:
The individual wires form defect-free single crystals, reducing the energy lost
due to flaws in the structure of conventional solar cells. And by controlling
the exact dimensions of the nanowires, it’s possible to control which
wavelengths of light they are “tuned” to, either for producing light in an LED
or for collecting light in a solar panel.
Complex
structures made of nanowires with varying diameters could also be useful in new
thermoelectric devices to capture waste heat and turn it into useful electric
power. By varying the composition and diameter of the wires along their length,
it’s possible to produce wires that conduct electricity well but heat poorly—a
combination that is hard to achieve in most materials, but is key to efficient
thermoelectric generating systems.
The
nanowires can be produced using tools already in use by the semiconductor
industry, so the devices should be relatively easy to gear up for mass
production, the team says.
