UK researchers have measured electrical resistance of films of single-walled carbon nanotubes whose sidewalls have been functionalised with different chemical groups.
UK researchers are the first to measure electrical resistance of films of single-walled carbon nanotubes whose sidewalls have been functionalised with different chemical groups.
A huge range of resistance was observed - across several orders of magnitude - depending on the functional groups used. The work provides a key insight into how the electronic and electrical properties of nanotubes can be controlled; a prerequisite for the fabrication of new devices from this class of nanostructure.
Michael Hunt and colleagues at the University of Durham, together with collaborators at the University of Newcastle and the Daresbury Laboratory, examined the resistivity of micrometre-thick films of nanotubes whose external sidewalls had been functionalised with either fluorine, ethyl groups or hexyl groups.
’Unmodified, pristine nanotubes have extremely low resistivity, with film resistances of the order of less than an ohm,’ Hunt told Chemistry World. ’When the side walls are fluorinated we see the resistivity rise a massive seven orders of magnitude, to around 25 megaohms. By contrast, films of hexylated nanotubes have a value of about 2 megaohms, and ethylated nanotubes around 2.5 kiloohms. This is a remarkable span of values across the range of resistance by simple modification of the side walls.’
The researchers used photoemission spectroscopy to probe the molecular and electronic structure of the nanotube surface. ’There appear to be two sources for the large variation in resistance,’ said Hunt. ’First, the covalent addition of species to the sidewalls leads to a change from sp2 to sp3 hybridisation for carbon atoms at bonding sites. This rehybridisation redistributes the electron density from the delocalised pi bonding states to deep-lying sigma bonding states where the electrons can no longer contribute to conduction. In other words,’ he said, ’the bonds formed by the carbon atoms go from graphite-like to diamond-like.’
The degree to which conduction is affected is controlled by the extent of coverage of the side walls by the covalently-bound species. Fluorination results in high coverage, and hence high resistance. So you might expect that smaller species would cover a greater proportion of the sidewalls, and ethyl chains would therefore produce higher resistance than hexyl chains. But this is not the case, says Hunt.
’The second contribution to the change in resistance appears to be associated with how well the nanotubes are in contact with each other within the film,’ he said. ’We observed that hexyl groups gave rise to higher film resistance than ethyl groups. It seems that the longer side chains push the nanotubes apart, making it more difficult for electrons to move freely between individual tubes, and hence across the film as a whole.’
The researchers also showed that the covalently bound side-groups can be desorbed relatively easily by annealing the functionalised films under vacuum. ’It is possible to obtain complete desorption,’ said Hunt. ’This suggests that we should be able to have close control over the degree of functionalisation.’
The unpublished data make an important contribution towards the ultimate goal of creating devices from single nanotubes, says Hunt. ’Our aim is to control the electronic structure of single nanotubes locally, which will enable us to create nano-electronic devices such as diodes. In our next series of experiments we will be using techniques such as atomic force microscopy to examine individual nanotubes.’ Simon Hadlington
Image: 5 micrometre x 5 micrometre atomic force microscope image of a typical nanotube film (in this case the unmodified nanotubes)