Professor David McKenzie - What is my research about?

SURFACES MADE TO MEASURE

My research is a quest to design and make custom surfaces. Why is this important? The simple answer is that it opens up a whole new dimension of applications for all the materials we currently use, as well as helping to improve their performance in their traditional applications. For example, making the surface of an object chemically inert and biocompatible allows it to be safely placed in the human body. This is not an easy task because the environment inside the body is highly corrosive and most materials trigger immune system reactions. The need for functional artificial organs is great as the demand for transplantable human organs far outstrips supply.

Other more traditional applications for modified surface layers include low-friction, high-hardness coatings for cutting and grinding tools, which improve performance and reduce wear, as well as anti-corrosion coatings for hard disks and read-write heads. There are also a host of device applications in the electronics and optical industries, where a series of coatings and interfaces are used to produce devices such as thin film transistors and optical band pass filters. Thin layers of high temperature superconductors on long reels of tape may drastically reduce losses associated with the transmission of electricity in the years to come.

Surface-modifying layers are almost always created by allowing atoms or ions from a vapour to settle on the surface and bond to it and to each other. In this way the coating is gradually built up or “deposited”. The properties of the modified surface depend on the details of how the atoms bond and this is known as microstructure. The microstructure is determined by the rate at which atoms arrive, the energy with which they impact on the surface and their ability to move around once on the surface. When the material is deposited using a plasma (ionised gas), all of these factors can be controlled. The arrival rate depends on the plasma gas density; the energy depends on the electric fields applied between the plasma and the surface; and the mobility is affected by the temperature of the surface. By varying these process parameters, we can explore the range of achievable microstructures. For example, recent work with very high impact energies (a few tens of kilovolts) has shown that the adhesion of the coating to the surface is vastly improved and its internal stress simultaneously reduced, making it much less susceptible to the formation of cracks. Both of these properties are vital to biomedical applications.

A new and exciting development in the field is the emergence of computer codes able to calculate the quantum mechanics of interactions between atoms arriving on the surface and so to predict the microstructures and properties of the coating. As computers become faster and the codes further developed, we will be able to predict microstructures arising from a much wider range of deposition conditions, making the concept of truly ‘designer’ materials a reality.

Electron density plot showing distorted covalent bonds in hydrogenated silicon-carbide, calculated using ab intio molecular modelling techniques. This material is a variable band gap semiconductor with applications in solar cells and other electronic devices. (Positions of the carbon nuclei are marked C and positions of the silicon nuclei are marked S).