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New Study Suggests Ways to Brighten Nanotube Light Sources
Nanotubes are the poster children of
the nanotechnology revolution. These tiny carbon tubes
– less than 1/50,000 the diameter of a human hair
– possess novel properties that have researchers
excitedly exploring dozens of potential applications
ranging from transistors to space elevators.
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).
“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.
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| Tobias Hertel in the lab (/Photo
by Neil Brake/) |
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 this energy drain,
so it should be possible to plug these energy sinks
and improve their overall quantum 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.
Funding was provided by Vanderbilt University,
the Max Planck Society and DFG, the German National
Science Foundation.
(By David F. Salisbury)
Visit http://people.vanderbilt.edu/~tobias.hertel.
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