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."

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.

Principal Investigator > Prof Zikang Tang> Email > phzktang@ust.hk