Grow your own carbon nanotubes

Nanotechnology takes off in US air force.

US Air Force researchers claim to have come up with a simple and efficient way to grow single-wall carbon nanotubes (SWNTs) on a range of substrates, which could soon see the nano-wonders being grown inside microelectronic devices.

Growing SWNTs directly into devices using catalysts is a tempting prospect and many research teams have come very close to making a success of the technique. However, there are still technical issues which need to be addressed, including controlling the catalyst particle size. Many of the ways used so far to control particle size, including sputtering sub-nanomtre metal films on surfaces, are not only difficult, expensive or time-consuming but can also give rough surfaces and large amounts of carbon deposition, which are no good for microelectronic devices.

A couple of years ago, researchers came up with a flexible way to grow SWNTs using a polymer template to ensure that the catalyst sits in the desired areas. In one example, a polymer decomposed at high temperatures to leave catalyst islands which guide SWNT growth. The problem with this technique is that the high-temperature step could damage electronic devices.

Now researchers from the Air Force Research Laboratory, Ohio, US, have combined several ideas and grown SWNTs using a so-called ’spin-on’ catalyst system. To make the catalyst they spin a mixture of iron nitrate and a commercially available methoxysilane polymer solution onto silicon wafers. This creates SiO _x and iron particles which catalyse the growth of carbon nanotubes when the environment is flooded with methane and hydrogen. They discovered that they could control the number of SWNTs per unit area and the particle size of the catalyst simply by varying the concentration of the iron salt. They also found that the flexibility of the polymer enables complex 3D SWNT-coated structures to form.

The researchers say that the spin-on catalyst system can also be used on other substrates such as quartz and graphite. They envisage that the flexibility of the new technique ’could lead to replacements for traditional materials and components in electrostatic discharge, electromagnetic shielding and transparent electrode applications’. Also, because spin-on glass is already used in microelectronic fabrication processes, the researchers suggest that their spin-on catalyst system could be easily integrated into current processes.

Emma Davies