The key to the new method of fabricating nanowire electronics is coating the surface of the silicon wafer with a thin layer of nickel before fabricating the electronic circuitry. |
Stanford
researchers have developed a new method of attaching nanowire
electronics to the surface of virtually any object, regardless of its
shape or what material it is made of. The method could be used in making
everything from wearable electronics and flexible computer displays to
high-efficiency solar cells and ultrasensitive biosensors.
Nanowire
electronics are promising building blocks for virtually every digital
electronic device used today, including computers, cameras and cell
phones. The electronic circuitry is typically fabricated on a silicon
chip. The circuitry adheres to the surface of the chip during
fabrication and is extremely difficult to detach, so when the circuitry
is incorporated into an electronic device, it remains attached to the
chip. But silicon chips are rigid and brittle, limiting the possible
uses of wearable and flexible nanowire electronics.
The
key to the new method is coating the surface of the silicon wafer with a
thin layer of nickel before fabricating the electronic circuitry.
Nickel and silicon are both hydrophilic, or “water-loving,” meaning when
they are exposed to water after fabrication of nanowire devices is
finished, the water easily penetrates between the two materials,
detaching the nickel and the overlying electronics from the silicon
wafer.
“The detachment process can be done at room temperature in water and only takes a few seconds,” said Xiaolin Zheng,
an assistant professor of mechanical engineering, who led the research
group that developed the process. “The transfer process is almost 100
percent successful, meaning the devices can be transferred without
sustaining any damage.”
After detachment, the silicon wafers are clean and ready to reuse, which should reduce manufacturing costs significantly.
Zheng is one of the authors of a paper describing the method that will be published in an upcoming issue of Nano Letters. The paper is available online now. Chi Hwan Lee and Dong Rip Kim, both graduate students in Zheng’s lab, are coauthors.
After
applying the nickel layer to the silicon chip, the researchers also
laid down an ultrathin layer of a polymer to act as an insulator and
provide mechanical support for the electronics.
The
ultrathin polymer layer is also extremely flexible, which is what
allows Zheng and her team to attach their nanowire electronics to a wide
range of shapes and materials including paper, textiles, plastics,
glass, aluminum foil, latex gloves – even a crumpled Coke can and a
mashed plastic water bottle.
“The
polymer layers we’re using are about 15 times thinner than the plastic
wrap you use to cover a plate of food,” Zheng said. “Since the polymer
has such a great degree of flexibility, you can wrap the polymer with
nanowire devices on top over anything while conformally following the
shape of any object.”
Currently
her team has been working with polymer layers about 800 nanometers
thick. A nanometer is one millionth of a millimeter.
But
what really makes the devices so flexible, what allows the devices to
bend with the flexible substrate, is the short length of the nanowires
used to fabricate the circuitry.
“The
length of these nanowires is only a couple thousandths of a millimeter
long,” Zheng said. “Compared to the curvature of the objects we’re
attaching them to, that is really short, so there is very little strain
on the nanowires.”
Because
the nanowires are so short, when they are placed on a convoluted
surface?even the sharp bends of a mashed up plastic water bottle?it is
as if the surface is practically flat.
The
devices can also easily be applied to a surface, removed and applied
again to another surface, repeatedly, without degrading the circuitry.
Some
of the major applications of the process that Zheng foresees will be in
the area of biological research. Nanowire devices could be attached
directly to heart or brain tissues to measure the electrical signals
from those tissues.
“Researchers
could measure heart arrhythmias or how a neuron fires,” she said.
”Those signals are electrical, but to measure them you need a very
conformable, very thin coating that allows the signals to propagate
across the substrate.”
The
transfer process could also be used in developing high-efficiency
flexible solar cells and would likely have uses in robotics, as well.
“The possibilities are really unlimited,” Zheng said.