Researcher at The
Hong Kong University of Science and Technology have succeeded in fabricating
the smallest single-walled carbon nanotube in the world.
The discovery
opens up the possibility of new applications in various high-tech areas
such as electronics, information technology, and biology. Principal Investigator
Prof Zikang Tang said: "Our nanotube is an ideal one-dimensional
conductor but all its novel properties are to be explored." Carbon
nanotubes have rich and varied electronic properties. Depending on size
and chirality, they can be a metal, a narrow-gap
or a moderate-gap semiconductor. The diameter of Prof Zikang's nanotube
is just 0.4 nanometer which, he says, gives it "superconducting behaviours."
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Tunnelling
electron microscope (TEM) image showing zeolite channels and the nanotubes
inside, scanning electron microscope (SEM) image of zeolite single
crystals, a diagram of the zeolite crystals and nanotubes, and a model
of zeolite framework and nanotubes inside its channels |
There
are 1 million nanometers to 1 millimetre. Prof Zikang explained the novel
properties of nanotubes: "Usually, property of a material is not
dependant on its size. For example, if a material is metallic, it is still
metallic even if you cut it into many pieces.
"Carbon
nanotubes are different. A nanotube can be metallic or semiconducting,
depending on its size and wrapping direction. By connecting the metal
and semiconductor nanotubes, you can have a nano-scaled semiconductor
diode, or a transistor which might be very useful in future nano-technologies."
The HKUST
researchers moulded the nanotubes using the channels in zeolite AFI crystals.
Zeolite comes in many forms, both natural and synthetic. A common property
is that they all have nano-sized pores in their framework. Because they
are able to absorb molecules in these pores, they are widely used as molecule
sieves.
One problem
with the AFI single crystal is that it is sensitive to growth conditions.
The researchers devoted much work to finding optimum conditions, and zeolite
channels free of defects. Fabrication of the nanotubes consists of introducing
hydrocarbon molecules into the channels and inducing pyrolysis.
Nearly 20
different kinds of hydrocarbon molecules can be used, each forming nanotubes
of different diameters and or chirality. A difficulty with ultra-small
nantubes is that it is difficult to see their structure and atomic arrangement.
To see the structure with an electron microscope, the nanotubes need to
be moved from their zeolite channels. However they can become unstable
under intense electron beams.
Through trial
and error, the researchers arrived at a method which enabled them to directly
observe their own record-breaking single-walled nanotube, the smallest
in the world. Seeing the atomic arrangement of the nanotubes is important
as it will reveal their symmetry.
While
transmission electron microscopy (TEM) allowed researchers to verify the
size of 0.4 nm diameter, they were unable to see the structure. As a possible
solution, they are turning to scanning-tunnelling microscopy (STM) to
see how the carbon atoms are arranged on the surface. From this, they
will be able to discern the symmetry.
Said Prof
Zikang: "It's exciting and challenging work. There are many who foresee
difficulties in observing the atomic structure because of the size. But
itŐs worth trying."
Because of
their size and good conducting properties, carbon nanotubes have been
used in the fabrication process of flat panel displays a few millimetres
thick for TV and computer monitor applications.
With a high
capacity for absorbing hydrogen inside the tubes, they can be used to
fabricate fuel cells with zero emissions. A carbon nanotube is the strongest
known material in the world, 20 times stronger than steel, and stronger
even than diamond.
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