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).
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.
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