Nanomaterials and MEMS: the changing face of gas sensing -John Saffell

Hardly a day goes by without the media telling us of the thrill or the dangers of nanotechnology. Likewise, Micro-Electro-Mechanical Systems (MEMS) are showing up in our daily lives with devices such as tyre pressure gauges and car airbag sensors. As these technologies develop, opportunities for improved gas sensors are being explored by companies and academics alike. So should we expect to see improved gas sensors over the next years, and if so, what can we realistically expect?

Nanomaterials: shrinking our world
Nanotechnology has been around for many years, and our analytical tools and processing technologies have improved, so whereas before we were capable of arranging crystals, we are now capable of arranging atoms. Nanotechnology is the study of things very small, with the generally accepted definition that items smaller than 100nm are nanomaterials, which can be inorganic, organic or biological. Our bodies are full of nanotechnology: DNA is an excellent example of a nanoprogramme for creating proteins (nanomate-rials) to very specific design rules. With over $1 billion/annum research effort by the USA alone, nanotechnology (more correctly titled Nanoscale Science and Engineering (NSE)) is THE buzz word in research.

The important feature of nanomaterials is that as particle size decreases, the percentage of bulk material decreases and the surface area increases, with the extreme being materials such as single walled carbon nanotubes (SWCNTs) which, being only one atom thick, are only a surface. As a material becomes mostly surface area, then quantum mechanics dominates, with electronic and thermal conduction, and optical and mechanical properties no longer simply predicted from the elements’ position in the periodic table.

Nanotechnology can be approached from the ’top down’ attitude where we just keep shrinking processes: integrated circuit track widths are now below 100nm and new, quantum mechanical problems appear at these small sizes. The alternative "bottom up" approach uses atoms deposited or synthesised into a nanomaterial; an example is carbon nanotubes (CNTs), small tubes of pure carbon, six or more atoms in diameter and thousands of atoms long. Inorganic nanodots are spheres of atoms, with diameters from ten to hundreds of atoms, normally synthesised using toxic and exotic organometallic precursors.