Illinois researchers in the Department of
Materials Science and Engineering--post-doctoral fellow LinLin
Wang and professor
Duane Johnson--and their collaborators, have made an important
breakthrough in understanding how "Bucky balls" modify metal
surfaces to create there own attachment points and self-assemble
into perfect single layers, opening the way to their use in
nanoscale electronic devices.
Ever
since the 1985 discovery of C60 Buckminsterfullerene--with its
perfect shape and high symmetry (60 atoms of carbon arranged in the
pattern of a soccer ball, having 20 hexagons and 12
pentagons)--these "Bucky balls" have fascinated scientists,
physicists, and chemists alike. Due to its high symmetry and
conjugated bond structure, the electronic properties of C60 are
very unusual, and there is a massive research effort toward
integrating it into molecular-scale electronic devices.
Only in recent months have scientists put together the theoretical
models with the experimental images of the surface to understand
how the molecule forms bonds with a metal substrate, such as
silver, which is commonly used as an electrode material. The
arrangement of silver and carbon atoms at such an interface affect
the strength and stability of the metal-molecule bond as well as
its electronic properties. The silver electrons are usually too low
in energy to have significant intermingling with electrons in
organic molecules, and prevents organic molecules from forming
strong bonds with silver surfaces. Hence, silver is commonly
considered a relatively inert (noble) metal that only forms strong
bonds with very corrosive atoms such as oxygen, sulfur, or
chlorine.
However, C60 does form bonds with silver surfaces and this mystery
has now been explained. It turns out that C60 digs a hole of
exactly one atom in the silver surface and settles into the hole by
binding to the six remaining atoms around the vacancy. This
previously unimagined process has been discovery by performing
quantum mechanical calculations for the C60 molecules on a silver
surface and comparing to experimental images.
The theoretical calculation showed that the binding strength
increases dramatically at such a hole, so much so that the C60 atom
actually causes the hole to be created. Because the hole is beneath
the large Bucky ball, it has been previously difficult to identify.
Now, calculations show that this data does match this arrangement
and even predicts that such self-adhering processes may be present
between C60 and other noble metal surfaces, leading the way to
their use in molecular-scale electronic devices.
__________________________
The work was described in a "Viewpoint in Physics" article in
Nature Physics 2, 64 (2009), by Georg Held. The research was
performed by H. I. Li, K. Pussi, K. J. Hanna, L.-L.Wang, D. D.
Johnson, H.-P. Cheng, H. Shin, S. Curtarolo,W. Moritz, J. A.
Smerdon, R. McGrath and R. D. Diehl, "Surface Geometry of C60 on
Ag(111)," Phys. Rev. Lett. 103, 056101 (2009) - Published July 27,
2009.
New recipe for self-healing plastic includes dash of
food additive
The research team (l to r): left, Jeffrey Moore, Nancy Sottos,
Scott White, with graduate research assistants Mary Caruso and
Benjamin Blaiszik.
Adding a food additive to damaged polymers can help restore them
to full strength, say scientists at the University of Illinois who
cooked up the novel, self-healing system.
The repair process, in which solvent-filled microcapsules embedded
in an epoxy matrix rupture when a crack forms, is a major
improvement over the original self-healing process first described
in February 2001.
"While our previous solvent worked well for healing, it was also
toxic," said Scott
White, a professor of aerospace engineering and a
researcher at the university's Beckman
Institute. "Our new solvent is both non-toxic and less
expensive."
During normal use, epoxy-based materials experience stresses that
can cause cracking, which can lead to mechanical failure. Autonomic
self-healing--a process in which the damage itself triggers the
repair mechanism--can retain structural integrity and extend the
lifetime of the material.
Designed to mimic the human body's ability to repair wounds,
self-healing materials release a healing agent into the crack plane
when damaged, and through chemical and physical processes, restore
the material's initial fracture properties.
In November 2007, White and collaborators reported the use of
chlorobenzene, a common--but toxic--organic solvent, which in epoxy
resins achieved a healing efficiency of up to 82%. In their latest
work, which combined a non-toxic and Kosher-certified food additive
(ethyl phenylactate) and an unreacted epoxy monomer into
microcapsules as small as 150 microns in diameter, the researchers
achieved a 100% healing efficiency.
"Previously, the microcapsules contained only solvent, which flowed
into the crack and allowed some of the unreacted matrix material to
become mobile, react and repair the damage," said graduate research
assistant Mary Caruso. "By including a tiny amount of unreacted
epoxy monomer with the solvent in the microcapsules, we can provide
additional chemical reactivity to repair the material."
When the epoxy monomer enters the crack plane, it bonds with
material in the matrix to coat the crack and regain structural
properties. In tests, the solvent-epoxy monomer combination was
able to recover 100% of a material's virgin strength after damage
had occurred.
"This work helps move self-healing materials from the lab and into
everyday applications," said graduate research assistant Benjamin
Blaiszik. "We've only begun to scratch the surface of potential
applications using encapsulated solvent and epoxy resin.
In addition to White, Caruso and Blaiszik, the other co-authors of
the paper were Nancy
Sottos, a professor of materials science and
engineering, and chemistry professor Jeffrey Moore. The
researchers reported their findings in the scientific journal
Advanced Functional Materials.
The work was supported by the U.S. Air Force Office of
Scientific Research and the U.S. Department of Defense. Some of the
work was performed at the university's Center for Microanalysis of
Materials, which is partially supported by the U.S. Department of
Energy.
Foldable and stretchable--silicon circuits conform to
many shapes
Mechanically stretchable, "wavy" silicon integrated circuit on a
rubber substrate.
Scientists have developed a new form of stretchable silicon
integrated circuit that can wrap around complex shapes such as
spheres, body parts, and aircraft wings, and can operate during
stretching, compressing, folding, and other types of extreme
mechanical deformations, without a reduction in electrical
performance.
"The notion that silicon cannot be used in such applications
because it is intrinsically brittle and rigid has been tossed out
the window," said John
Rogers, a Founder Professor of Materials Science and
Engineering at Illinois.
"Through carefully optimized mechanical layouts and structural
configurations, we can use silicon in integrated circuits that are
fully foldable and stretchable," explained Rogers, who is a
corresponding author of a paper accepted for publication in the
journal Science, and posted on its Science Express
website.
The new designs and fabrication strategies could produce wearable
systems for personal health monitoring and therapeutics, or systems
that wrap around mechanical parts such as aircraft wings and
fuselages to monitor structural properties.
In December 2005, Rogers and his U of I research group reported the
development of a one-dimensional, stretchable form of
single-crystal silicon with micron-sized, wave-like geometries.
That configuration allows reversible stretching in one direction
without significantly altering the electrical properties, but only
at the level of individual material elements and devices.
Now, Rogers and collaborators at Illinois, Northwestern University,
and the Institute of High Performance Computing in Singapore,
report an extension of this basic wavy concept to two dimensions,
and at a much more sophisticated level to yield fully functional
integrated circuit systems.
"We've gone way beyond just isolated material elements and
individual devices to complete, fully integrated circuits in a
manner that is applicable to systems with nearly arbitrary levels
of complexity," said Rogers, who also is a researcher at the Beckman Institute and at
the university's Frederick Seitz
Materials Research Laboratory.
"The wavy concept now incorporates optimized mechanical designs and
diverse sets of materials, all integrated together in systems that
involve spatially varying thicknesses and material types," Rogers
added. "The overall buckling process yields wavy shapes that vary
from place to place on the integrated circuit, in a complex but
theoretically predictable fashion."
Circuit diagram (top frame) and optical images of a
stretchable, "wavy" silicon ring oscillator circuit on a rubber
substrate, in the "as fabricated" flat state (top micrograph) and
in moderate and high states of biaxial compression (middle and
bottom micrographs, respectively).
Achieving high degrees of mechanical flexibility, or
foldability, is important to sustaining the wavy shapes, Rogers
said. "The more robust the circuits are under bending, the more
easily they will adopt the wavy shapes which, in turn, allow
overall system stretchability. For this purpose, we use ultrathin
circuit sheets designed to locate the most fragile materials in a
neutral plane that minimizes their exposure to mechanical strains
during bending."
To create their fully stretchable integrated circuits, the
researchers begin by applying a sacrificial layer of polymer to a
rigid carrier substrate. On top of the sacrificial layer they
deposit a very thin plastic coating, which will support the
integrated circuit. The circuit components are then crafted using
conventional techniques for planar device fabrication, along with
printing methods for integrating aligned arrays of nanoribbons of
single-crystal silicon as the semiconductor. The combined thickness
of the circuit elements and the plastic coating is about 50 times
smaller than the diameter of a human hair.
Next, the sacrificial polymer layer is washed away, and the plastic
coating and integrated circuit are bonded to a piece of prestrained
silicone rubber. Lastly, the strain is relieved, and as the rubber
springs back to its initial shape, it applies compressive stresses
to the circuit sheet. Those stresses spontaneously lead to a
complex pattern of buckling, to create a geometry that then allows
the circuit to be folded, or stretched, in different directions to
conform to a variety of complex shapes or to accommodate mechanical
deformations during use.
The researchers constructed integrated circuits consisting of
transistors, oscillators, logic gates and amplifiers. The circuits
exhibited extreme levels of bendability and stretchability, with
electronic properties comparable to those of similar circuits built
on conventional silicon wafers.
The new design and construction strategies represent general and
scalable routes to high-performance, foldable and stretchable
electronic devices that can incorporate established, inorganic
electronic materials whose fragile, brittle mechanical properties
would otherwise preclude their use, the researchers report.
"We're opening an engineering design space for electronics and
optoelectronics that goes well beyond what planar configurations on
semiconductor wafers can offer," Rogers said.
The work was funded by the National Science Foundation and the U.S.
Department of Energy.
Photonic crystals key to protective eyewear
ECE researchers are using photonic crystals in special eyewear that
may one day protect soldiers.
A U of I research team is currently developing technology aimed
to improve devices used to protect U.S. soldiers' eyes during
combat. Even though the researchers have had much progress in the
past two and a half years, they are continuing to make
advances.
"At this point, we've demonstrated the concepts in the lab that we
theoretically predicted," said Brian
Cunningham, an associate professor of electrical and computer engineering
(ECE) at Illinois. "The biggest obstacle to overcome now has to
do with making the response speed of the devices fast
enough."
The research project started shortly after Cunningham visited the
U.S. Army Soldier System Center in Natick, Mass., a research
laboratory headed by the U.S. Department of Defense that
investigates and develops food, clothing, shelters, airdrop
systems, and other service member support items for the U.S.
military. While visiting the center in 2004, Cunningham gave a talk
on photonic crystals, which are multilayer films used to control
optical transmission, reflection, and refraction characteristics in
biosensors.
The presentation intrigued Brian Kimble, a program manager in
charge of technological developments for the Army's protective
eyewear. Kimble was looking for a way to solve the Army's eye
protection problem and believed that photonic crystal technology
could be the solution, Cunningham said.
Protective eyewear, which was first launched in the 1990s, is
designed to protect U.S. soldiers from enemies attempting to
disable them and United States lasers that are used when targeting.
When they hit the eye, laser pulses can cause flash blindness, a
visual impairment during and following exposure to a light flash of
extremely high intensity, which can result in permanent
blindness.
"It is a concern for the soldier not only when they are there in
the battlefield, but also for the rest of their lives," Cunningham
said. "Because these weapons could be used against us, the U.S.
Army wanted us to develop a technology that could counter
them."
After Kimble's recommendation to his superiors, the U.S. Army
granted Cunningham and his research
group, which consists of ECE graduate students Fuchyi Yang and
Gary Yen, a contract to theoretically study potential concepts for
a year. During that time, they designed devices and modeled them by
computer.
After the initial study, which took place from 2005 to 2006, the
Army extended the grant for another two years, enabling the
researchers to start building and testing hardware. They are
currently one and a half years into the second stage.
Brian Cunningham
"We have electromagnetics simulation software that allows us to
design the photonic crystal structure and incorporate all the
shapes and sizes of the materials," Cunningham said. "In the
computer, we can see how efficiently the device blocks light at
different wavelengths and angles."
While the eye protection devices currently available can rapidly
react to laser radiation, they are heavy and expensive to make,
Cunningham said. In addition, when certain wavelengths hit those
devices, which are similar to sunglasses or goggles that cover eyes
from all angles, they turn from clear to opaque, making it hard for
the soldier to see out.
"It's why (the Army) is interested in the photonic crystal
technology," he said. "We can make devices that can reflect a
specific band of wavelengths, but allow all the other wavelengths
to come through so soldiers can still see."
Cunningham said that parts of the research are highly
classified, which poses some problems. While he has a security
clearance for some information, he is unable to tell his graduate
students who aid in the research any of the classified information.
Therefore, Cunningham said that he, Yang, and Yen will most likely
make incomplete devices and send them to the Army, so other
researchers can add the additional materials.
"We're stuck working with the commercially available materials,
where the fastest response speed we can show is about a
millisecond," Cunningham said. "But that really needs to be about a
nanosecond…We're looking forward to being able to incorporate some
of the faster, more advanced materials when they can give them to
us. Some of the materials that they have other contractors working
on are subject to other agreements and they can't allow us to use
them until a certain point."
Cunningham said he is unsure how long the group's funding for
this project will last.
"It depends how well [Kimble] can persuade his bosses to grant
more money that will push the effort forward," he said. "Our group
is one piece of a big program…We focus on advanced concepts, which
are more technically risky, but would have the best pay off."
