4-point conductivity measurement of the new transparent conducting film developed by prof. Cor Koning (left) and prof. Paul van der Schoot (right). The black pot contains a dispersion of carbon nanotubes in water, and the white pot contains the conducting latex. Photo: Bart van Overbeeke. |
Researchers
at Eindhoven University of Technology (TU/e, Netherlands) have
developed a replacement for indium tin oxide (ITO), an important
material used in displays for all kinds of everyday products such as
TVs, telephones and laptops, as well as in solar cells. Unfortunately
indium is a rare metal, and the available supplies are expected to be
virtually exhausted within as little as ten years. The replacement
material is a transparent, conducting film produced in water, and based
on electrically conducting carbon nanotubes and plastic nanoparticles.
It is made of commonly available materials, and on top of that is also
environment-friendly. The results, which also provide new insights into
conduction in complex composite materials, were published online
yesterday 10 April by the scientific journal Nature Nanotechnology.
The
research team has been able to achieve higher conductivity by combining
low concentrations of carbon nanotubes and conducting latex in a
low-cost polystyrene film. The nanotubes and the latex together account
for less than 1 percent of the weight of the conducting film. That is
important, because a high concentration of carbon nanotubes makes the
film black and opaque, so the concentration needs to be kept as low as
possible. The research team was led by theoretical physicist Paul van
der Schoot and polymer chemist Cor Koning. Post-doc Andriy Kyrylyuk is
the first author of the paper in Nature Nanotechnology.
The
researchers use standard, widely available nanotubes which they
dissolve in water. Then they add conducting latex (a solution of polymer
beads in water), together with a binder in the form of polystyrene
beads. When the mixture is heated, the polystyrene beads fuse together
to form the film, which contains a conducting network of nanotubes and
beads from the conducting latex. The water, which only serves as a
dispersing agent in production, is removed by freeze-drying. The
‘formula’ is not a question of good luck, as the researchers first
calculated the expected effects and also understand how the increased
conductivity works.
The
conductivity of the transparent e film is still a factor 100 lower than
that of indium tin oxide. But Van der Schoot and Koning expect that the
gap can quickly be closed. “We used standard carbon nanotubes, a
mixture of metallic conducting and semiconducting tubes”, says Cor
Koning. “But as soon as you start to use 100 percent metallic tubes, the
conductivity increases greatly. The production technology for 100
percent metallic tubes has just been developed, and we expect the price
to fall rapidly.” However the conductivity of the film is already good
enough to be used immediately as an antistatic layer for displays, or
for EMI shielding to protect devices and their surroundings against
electromagnetic radiation.
The
film has an important advantage over ITO: it is environment-friendly.
All the materials are water based, and no heavy metals such as tin are
used. The new film is also a good material for flexible displays.
The
researchers themselves are very positive about the diversity of their
team, which they believe made an important contribution to the results.
“We had a unique combination of theoreticians, modeling specialists and
people to do practical experiments”, says Paul van der Schoot. “Without
that combination we wouldn’t have succeeded.”
The
paper ”Controlling Electrical Percolation in Multi-Component Carbon
Nanotube Dispersions’ was published yesterday, Sunday 10 april, on the
website of the journal Nature Nanotechnology (DOI:
10.1038/NNANO.2011.40). The research forms part of the Functional
Polymer Systems research program at the Dutch Polymer Institute (DPI),
which provided financial support for this project. Prof. Cor Koning is
with the Polymer Chemistry group (Department of Chemical Engineering and
Chemistry) and prof. Paul van der Schoot is with the Theory of Polymers
and Soft Matter group (Department of Applied Physics) of Eindhoven
University of Technology. The other authors of the article are Andriy
Kyrylyuk (first author), Marie Claire Hermant, Tanja Schilling and Bert
Klumperman.