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Top view of the ruthenium tris-bipyridine
light-emitting device created by Cornell researchers. The
ruthenium metal complex is represented by red spheres, and counter
ions are represented by green spheres. The material is sandwiched
between two gold electrodes. Also visible is the probe of the
electron force microscope used to measure the electric field of
the device.
Image � by
Cornell Chronicle
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The researchers set out to understand the
fundamental physics of the material - that is, what happens when it
encounters an electric field, both at the interfaces and inside the
film. By fabricating a device out of the ruthenium metal complex that
was spin-coated onto an insulating substrate with pre-patterned gold
electrodes, the scientists were able to use electron force microscopy
to measure directly the electric field of the device.
A long-standing question, according to George G. Malliaras, associate
professor of materials science and engineering, director of the
Cornell NanoScale Science and Technology Facility and one of the
co-principal investigators, was whether an electric field, when
applied to the material, is concentrated at the interfaces or in the
bulk of the film.
The researchers discovered that it was at the interfaces - two gold
metal electrodes sandwiching the ruthenium complex film - which was a
huge step forward in knowing how to build and engineer future devices.
"So when you apply the electric field, ions in the material move about,
and that creates the electric fields at the interfaces," Malliaras explained.
Essential to the effort was the ability to pattern the ruthenium
complex using photolithography, a technique not normally used with
such materials and one that took the researchers more than three years
to perfect, using the knowledge of experts in nanofabrication,
materials and chemistry.
The patterning worked by laying down a gold electrode and a polymer
called parylene. By depositing the ruthenium complex on top of the
parylene layer and filling in an etched gap between the gold
electrodes, the researchers were then able to peel the parylene
material off mechanically, leaving a perfect device.
Ruthenium tris-bipyridine has energy levels well suited for efficient
light emission of about 600 nanometers, said H�ctor D. Abru�a, the E.M.
Chamot Professor of Chemistry, and a principal co-investigator. The
material, which has interested scientists for many years, is ideal for
its stability in multiple states of oxidation, which, in turn, allows
it to serve as a good electron and hole transporter. This means that a
single-layer device can be made, simplifying the manufacturing process.
"It's not fabulous, but it has a reasonable emission efficiency,"
Abru�a said. "One of the drawbacks is it has certain instabilities,
but we have managed to mitigate most of them."
Among the other authors were co-principal investigators Harold G.
Craighead, the C.W. Lake Jr. Professor of Engineering, and John A.
Marohn, associate professor of chemistry and chemical biology.
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