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Tobias Hertel in the lab.
Nanotubes have different properties
depending on slight differences in the way that carbon atoms are
arranged.
An electron microscope image shows a
bundle of nanotubes.
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Nanotubes also produce light with a number of
interesting properties, which have led researchers to propose various
optical applications. One of the most promising is to use the tiny
tubes as fluorescent markers to study biological systems, a role
pioneered by fluorescent proteins. But there has been one primary
problem: Nanotubes have proven to be very inefficient phosphors,
absorbing a thousand photons for every photon that they emit (a ratio
called quantum efficiency).
Now, however, the latest research into nanotube luminescence has
found that there is substantial room for increasing the efficiency of
these infinitesimal light sources: The study, which is the first to
measure the luminescence of single nanotubes, was published in the
Nov. 4 issue of Physical Review Letters and reports that there is a
surprising amount of variation between the quantum efficiencies of the
15 individual nanotubes that were examined.
"We were expecting to see individual differences of only a few
percent, so we were very surprised to find that some nanotubes are a
1,000 percent more efficient than others," says Tobias Hertel,
associate professor of physics at Vanderbilt University, who conducted
the study with two German research groups.
Nanotubes are members of the fullerene family along with buckyballs,
carbon molecules shaped like soccer balls. Nanotubes, which are also
called buckytubes, are seamless cylinders made of carbon atoms and
capped on at least one end with a buckyball hemisphere. Nanotubes come
in two basic forms: single-walled and multi-walled, which have two or
more concentric shells. Slight differences in the geometric
arrangement of carbon atoms produces nanotubes with different
electrical properties, either metallic or semiconductor.
Semiconducting nanotubes are the variety that produces light.
Since nanotubes were discovered in 1991, scientists have determined
that they are relatively easy to make and have developed several
methods for doing so.
The original process that was used is called the arc-discharge
technique. Large amounts of current are passed through two graphite
rods in a container filled with high-pressure helium gas. As the rods
are brought together, an electrical arc is formed and the carbon in
the smaller rod is transformed into a tubular structure filled with
nanotubes. This produces a mixture of different types of nanotubes,
including single-walled and multiple walled, semiconductor and
metallic varieties in the form of black, sooty powder.
A more recent process uses a laser to vaporize carbon by scanning
repeated across a flat slab made from a mixture of graphite and metal.
This approach is noted for its ability to make a large proportion of
single-walled tubes. In addition, a chemical vapor deposition process
has been developed that is most suitable for producing nanotubes in
commercial quantities.
"Our analysis pinpoints structural defects as the source for most
of the energy drain that reduces nanotubes' quantum efficiency as a
light source. It should be possible to plug these energy sinks and
improve their overall efficiency by a factor of five or so by
improvements in the synthesis processes," Hertel says.
Although he doesn't know exactly what these improvements will be,
Hertel is confident that they will happen. Improving nanotube
synthesis is a big business. "There are hundreds of research groups
around the world who are working full time to improve nanotube
synthesis," he reports. As a result, improvements in the various
synthesis processes are reported regularly.
Even if improving the nanotube's quantum efficiency proves
unexpectedly difficult, there are likely to be work-arounds. For
example, another way to brighten nanotubes is to simply make them
longer, the physicist points out.
Other research groups are already experimenting with the use of
nanotubes as a replacement for fluorescent proteins in the study of
biological systems. In this application, they are competing with
another nanotechnology called quantum dots, which are tiny fluorescent
beads often made of cadmium selenide. According to Hertel, nanotubes
have several inherent advantages over quantum dots for this
application. Nanotubes are not known to be toxic to living cells,
unlike the cadmium found in quantum dots. They produce a narrower,
more precise beam of light, which makes them easier to detect. Finally,
they are more stable and continue producing light long after quantum
dots have faded.
Hertel's co-authors on the study are Mathias Steiner, Huihong Qian
and Achim Hartschuh from the University of Tuebingen, Alfred Meixner
from the University of Siegen, Markus Raschke and Christoph Lienau
from the Max Born Institute and Axel Hagen from the Fritz Haber
Institute of the Max Planck Society. |
