Dr Chris Ewels current research

Recent Projects

1. Chemical doping of carbon nanotubes (nitrogen impurities)
2. Intrinsic defect behaviour in graphite and nanotubes
3. Behaviour of fluorine on carbon nanotube surfaces
4. Intrinsic irradiation induced defects in BN nanotubes
5. Modified fullerene structures (azafullerenes, paired pentagon structures)
6. Fluorination of fullerenes - isomer families, addition routes
7. Catalysed structural changes in fullerenes and nanotubes
8. Theoretical and experimental EELS studies of nanomaterials
9. Metal absorption and diffusion on carbon nanotubes
10. Novel oxide nanostructures (TiOxHy nanotubes, scrolls and rods, FeOx nanorods, etc)
11. Hydrogen interaction with dislocations in silicon and diamond
12. Science communication and science policy
13. Public participation in scientific research

Nanotube based composites

Carbon nanotubes are the flagship in the new world of nanotechnology research - molecular tubes of carbon their strength is theoretically 100 times that of steel with only one sixth of the weight. One of their first promising applications will be in composite materials - what if we could add them to plastics, ceramics and metals we currently use, and find these materials suddenly stronger, harder, tougher? With other added benefits (electrical conductivity, improved thermal conductance) then suddenly we could have lighter more efficient planes, cheaper more efficient cars, bridges that could span vast distances, earthquake 'proof' buildings, etc.

In reality we still have a few hurdles to overcome - nanotubes often prefer to stick to each other rather than the material we add them to, they are still expensive (but getting cheaper), and we need to improve our techniques for lining them up and effectively binding them to their host material.

In my composites work we are experimenting with all of this, using surfactants to improve the mixing of the nanotubes, testing various mechanical routes to align the tubes in the composite, various mechanical testing techniques to assess the improvement in material properties after adding the nanotubes, and state-of-the-art microscopy to see and understand what's happening in these new materials at the molecular scale.

Nanoengineering : Shape change in carbon materials

As we develop an increasing range of carbon structures with wide ranging properties, we need to address the dynamics - how do such systems form and evolve? How do they restructure under external factors such as applied stress, irradiation, or heating?

In this work we explore the behaviour of point defects such as ad-atoms, vacancies and impurity atoms, as well as topological defects such as disclinations, dislocations and dislocation dipoles, in carbon systems [2]. Using supercomputer modelling techniques (density functional theory) we examine their structure, the nature of their interaction, and try to determine low energy transformation paths between various configurations.

Recent work has led to the discovery of a catalysed exchange process in carbon networks, whereby a single carbon ad-atom can aid Stone Wales type bond rotations, lowering the migration barrier by a factor of about four. This leads to qualitatively different behaviour [1].

The discovery has important implications for many carbon systems including the formation of C₆₀, the growth and distortion of nanotubes and buckyonions, and the behaviour of graphite under irradiation. (poster).

1. Nanoengineering : Chemical Physics Letters (2001).
2. `LDF Calculations of point defects in graphites and fullerenes'
   M. Heggie, B. R. Eggen, C. P. EWELS, P. Leary, S. Ali, G. Jungnickel, R. Jones, P. R. Briddon, Electrochem. Soc. Proc. Vol. 98-8, 60-67 (1998).

Mechano-chemistry : Hydrogen in silicon and diamond

Conventional theory says that pre-existing dislocations can move through a semiconductor, sweeping up impurities as they pass. Meanwhile other impurities can diffuse through the lattice to the dislocations. In this way impurities modify the behaviour of pre-existing dislocation structures and hence affect material plasticity. Our latest work in the field of hydrogen impurities in silicon and diamond is pointing to a radical new field of mechano-chemistry. In this case impurities are able to nucleate dislocations, and propagate their growth; i.e. the impurities both initiate and control plastic behaviour in a material.

Hydrogen is a very important impurity in both diamond and silicon, yet little is known about it since it is so hard to detect. Hydrogen is deliberately implanted into silicon during the smart cut process, where implantation occurs at a fixed depth, the silicon sample is annealed, and then fractures along the implantation plane, giving an atomically smooth hydrogenated surface. The mechanism for this process is not properly understood. In silicon the behaviour of dislocations has been studied before and after deliberate hydrogen introduction; both the pre-factor and the activation energy for motion change radically. Hence hydrogen can change the plasticity of silicon.

Our early investigations of H interaction with dislocations showed that H binds strongly to dislocations, and modifies the atomic rearrangements necessary for their motion. These results were in quantitative agreement with the shifts observed in experiment [1].

In later work we extended this to dislocation behaviour in diamond [2], which again shows interesting changes in behaviour. This may have implications for processing of artificial diamonds.

Our current research is following two directions. Firstly we are developing LINEWISE, a kinetic monte carlo simulation of dislocations in semiconductors. We are feeding into this the results of our high level density functional calculations, and using LINEWISE to study the dynamics of dislocation motion in different regimes. The other research centres on further studies of dislocation nucleation and growth in the presence of hydrogen.

Mechano-chemistry may well explain unusual behaviour in other materials, such as sudden quartz fracture deep in the earth's core, occurring during earthquakes (quartz is often the first material to break, despite its extreme hardness). We are also interested in sudden failure in ice, for example fracture leading to avalanche. It is clear that this is an interesting new line of research that could shed light on many previously unexplained materials phenomena.