A Rice University team has hit
upon a method to produce nearly transparent films of electrically conductive
carbon nanotubes, a goal sought by researchers around the world.
The lab of Rice researcher
Matteo Pasquali found that slides dipped into a solution of pure nanotubes in
chlorosulfonic acid (CSA) left them with an even coat of nanotubes that, after
further processing, had none of the disadvantages seen with other methods.
The films may be suitable for
flexible electronic displays and touchscreens, according to the paper published
this month in the American Chemical Society journal ACS
"I think this could be the
way that high-performance transparent electrodes are made in the future," said
Pasquali, a professor of chemical and biomolecular engineering and of
chemistry. "The solution is straightforward. It's a very simple
The method is scalable to
high-throughput processes like slot, slide and roll coating used by industry,
A frustrating characteristic of
nanotubes, particularly long ones, is that they attract each other in common
solvents, making it a challenge to disperse them. Long nanotubes are believed
to be the key to high-performance films.
Researchers have tried other
ways to keep them from aggregating, Pasquali said. Functionalizing nanotubes –
dressing them with chemicals – can make them less attractive to each other, but
it degrades their desirable electrical properties. Combinations of surfactants and
sonication have also been tried, but the nanotubes break during sonication, and
the surfactant leaves a residue that cannot be washed away, he said.
These methods, combined with
various means of mechanical coating, have been used to create nanotube films,
but none with the level of quality achieved by the Pasquali lab. The Rice
films, which are made of nanotubes thousands of times longer than they are
wide, remain electrically stable after more than three months, said graduate
student and lead author Francesca Mirri.
The nanotubes, literally, had
to pass an acid test. "(CSA) is the acid we typically use in our lab, so
the first thing we say when we get a new type of carbon nanotubes is, 'OK,
let's put it in acid and see what happens,'" Mirri said. In previous
research, Pasquali’s lab had determined that CSA can dissolve high-quality
nanotubes because the acid induces repulsive forces between the tubes that
counterbalance the van der Waals force that draws them together.
Mirri and her colleagues
produced films by combining single- or double-walled carbon nanotubes with CSA
in various concentrations. They dipped glass slides into the nanotube solutions
with a motorized arm to ensure even coating as the slides were steadily
They used chloroform to
coagulate the acid and dry the slides, followed by a wash of diethyl ether. The
researchers were surprised to find the chloroform did not disrupt the thin
liquid layer. The result was a film several nanometers thick that provided the
best tradeoff between transparency and sheet resistance, a measure of
Mirri sees nanotube films as a
viable alternative to indium tin oxide (ITO), the current standard conductive
layer in transparent displays. "Everybody uses ITO for commercial
applications, but the problem is it's a ceramic and really fragile," she
said. "It's not good for flexible electronics, and also requires high
temperature or vacuum processes to produce; that uses more energy and makes it
"Our thin film for
something like a cell phone would need very little material -- a few micrograms
of nanotubes -- so it wouldn't be that expensive, but it would have similar
properties in transparency and conductivity to ITO," she said.
Co-authors are former
postdoctoral researcher Anson Ma, now an assistant professor at the University
of Connecticut; postdoctoral researchers Shannon Eichmann and Tienyi Theresa
Hsu; former graduate student Natnael Behabtu, now a researcher at DuPont;
graduate student Colin Young; and senior undergraduate Dmitri Tsentalovich, all
The research was supported by
the Air Force Office of Scientific Research, the Air Force Research
Laboratories and the Robert A. Welch Foundation.