British chemists have successfully fabricated a series of novel molecular structures ranging from hollow cylinders to solid wires and tubes containing metal cores. These will boost the spectrum of materials available for developing sub-microscopic devices for the emerging field of nanotechnology. The work is being carried out by researchers at Sussex University led by Harry Kroto and David Walton. The project is funded by the Engineering and Physical Sciences Research Council.
Nanotubes are extremely strong yet very light, making them candidates for new construction or storage materials. They also have unique electronic properties. Filling the hollow core with other substances, such as metals or even proteins, is also possible.The Sussex team is investigating new ways of making nanoscale structures consisting of carbon and of other elements. The aim is to improve the understanding of how better to control the dimensions and geometrical alignment of the structures, factors which are essential pre-requisites to any future applications.
‘We have devised a new way to produce nanotubes,’ says Dr Walton. ‘If you immerse graphite rods into a molten salt of a metal and pass a direct current, the anode is destroyed and nanotubes are formed in the melt. On cooling, the metal salt can be removed with water, leaving the nanotubes.’This method has two main attractions. Firstly it requires temperatures only of around 500C, whereas previous methods involved electrically vaporising graphite at several thousand degrees, or a process called thermolysis, which requires temperatures of above 1000C. Secondly the metal from the salt becomes enclosed inside the nanotubes, which themselves have sealed caps on each end.
Until now, to introduce metals inside the tube required tubes to be created, their end-caps to be removed, the metal to be inserted and the caps replaced – a far more complex process.’We are also looking at making nanotubes from things other than carbon,’ says Dr Walton. ‘We have discovered that if you vaporise a compound called boron nitrite under specific conditions you can make a nanotube whose structure superficially resembles that of the carbon nanotube but actually consists purely of boron and nitrogen atoms.’
The Sussex team has also created hybrid nanotubes, containing boron, nitrogen and carbon. As well as making hollow and filled tubes, the research team has fabricated a range of solid nanowires containing various metals. In an attempt to obtain ordered arrays of nanotubes the researchers coated a silica sheet with cobalt and cut tracks in the surface with a laser. When appropriate hydrocarbons were heated over the cobalt film, nanotubes grew along the edges of the laser-etched tracks, giving a high degree of alignment.
‘This sort of alignment is crucial if eventually we want to make structurally robust materials and exploit the unique properties of nanotubes,’ says Dr Walton. ‘The magnetic properties of this material suggest applications in data storage.’