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Alan Kaiser: Nanodetective

Alan surrounded by multiwalled nanotubes

Alan Kaiser works at the boundary of experiment and theory. He sees experimental data as a riddle, a mystery to be solved. Nanotubes and conducting polymers are his main area of research. These little characters have huge potential in technology with applications ranging from nanoscale electronics, which could break the size barrier in computing to large flexible display screens using nanotubes as LED's. Alan's job is to find out what's going on in these fascinating new materials and expose the fundamental physics at work in them.

Conduction Mechanisms in Carbon Nanotube Networks:

This programme has shed light on the conduction of electricity through bulk samples of single wall carbon nanotubes (SWCN's). The tubes themselves can be less than one nm in diameter but as they grow they self-organize themselves into bundles or “ropes” around 40nm in diameter. These ropes form bulk samples or “mats” of SWCN's consisting of networks of ropes of varying orientations. Alan works with chemists John Spencer and PHD student Kirsten Edgar who synthesise the nanotube networks. The pictures above are transition electron microscope (TEM) images of a single multiwalled nanotube and a multi-walled nano-tube rope.
Alan has found that the process of conduction in nanotube networks shows striking similarity to that in organic conducting polymers. Both nanotubes and conducting polymers display ballistic (non-resistive) conduction as long as their structure is regular. In reality however structures are littered with defects. Unlike in three or two-dimensional samples where conduction electrons can simply go around defects, electrons in one dimensional conductors have no choice but to go straight through the defects. This causes disturbances in the electronic wavefunction and results in a reduced conductivity that is the sum of the weighted conductivity contributions of the barriers and the metallic parts in between.
Unlike organic conducting polymers the barriers to conduction in a nanotube are only small and electrons tunnel readily through them when given a little nudge by thermal energy. The temperature dependence of conduction in nanotube ropes reflects this. At high temperatures when thermal energy far exceeds the energy barrier conduction is metallic in character whereas at low temperatures the barriers have more impact and the conduction is non-metallic. Such barriers could be caused by nanotube defects such as the twisting and flattening of tubes shown in the TEM image of an individual carbon nanotube by Kirsten Edgar. Alan's models have been used by SONY Corporation Materials Science Laboratories, Yokohama, and in several other laboratories to help understand electronic transport in their nanotube and conducting polymer samples.


The twisting and flattening of a single walled nanotube at a defect

Conducting Polyacetylene Nanofibres

Chains of carbon atoms in the conducting polymer, polyacetylene are about the thinnest conductors conceivable, less than 1 nm wide. Alan has been working in collaboration with researchers from NanoTransport Laboratory at Seoul National University to study conduction in nano-fibres made of polyacetylene. They have shown that below a certain temperature (about 30K) these fibres display temperature independent conduction. Alan and his collaborator at Seoul University have proposed that the origin of this unusual behaviour is quantum mechanical tunnelling of the bond pattern along individual polyacetylene chains. The bond pattern down each chain has alternating single and double bonds. The Energy of the state would be identical if the double and single bond positions were swapped but there is an energy barrier that prevents this from spontaneously occurring. It would usually be assumed that thermal energy overcomes this barrier and bumps the bond configuration from one position to the next but in this case conduction is temperature independent. Alan has postulated that conduction is caused by a portion of the electronic bond pattern tunnelling through these barriers creating a negatively charged ‘soliton' at one end of the tunnelling portion and a positively charged anti-soliton at the other end. Repetition of this process sees charge moving along the polyacetylene chain.

The soliton tunneling mechanism

The difference between this model and other models* for conduction in polyacetylene is that other models require tunneling segments made of large bundles of chains. For polyacelylene tunnelling, conduction could occur in much smaller samples. This is consistent with the experimental results described above. It appears the mystery has a solution.

Alligning Carbon Nanotubes Using Electric Fields

One of the essential requirements if carbon nanotubes are to be used in electronic devices is the ability to align tubes and position them at predetermined locations.
Xianming Liu, (a PhD student supervised by Profs Alan Kaiser and John Spencer) has been working with Dr Mike Arnold from IRL using a technique called dielectrophoresis. This technique uses electric fields to align carbon nanotubes.

A dielectric (insulating) solution containing nanotubes is placed over an array of electrodes and the nanotubes are allowed to settle onto the array while voltage is applied. The nanotubes are electrically polarised by the applied electric field, which causes them to align in bundles along the field lines. It was found that when the field was turned off the nanotubes reverted to a more disordered state but this field-induced alignment was superimposed on an underlying irreversible increase of alignment that continued even when the field was turned off. It appeared that the parallel capacitance and conductance between opposing electrodes increased as the degree of alignment increases so capacitance and conductance proved to be good quantitative measures of alignment.


The electrode structure and an optical micrograph of aligned carbon nanotubes

It has been possible to immobilise the oriented carbon nanotubes by setting them in a solidifying polymer and this has created a carbon nanotube-containing composite. With continued research these will be able to be used in electronic devices.

* Eg. Charge density waves (CDW): Bardeen and Maki