Ellipsometry and polarimetry – the key to metal nanostructure characterization

Citation

Saxl, O. (2009), "Ellipsometry and polarimetry – the key to metal nanostructure characterization", Sensor Review, Vol. 29 No. 4. https://doi.org/10.1108/sr.2009.08729daa.002

Ellipsometry and polarimetry – the key to metal nanostructure characterization

Article Type: Viewpoint From: Sensor Review, Volume 29, Issue 4

Keywords Nanotechnology, Metals, Sensors

Metallic nanoparticles and thin films offer almost infinite possibilities for new sensor development. But how do you define and predict their properties?

Many new developments in sensing relate to the utilization of the novel properties metals exhibit when they are in the form of nanoparticles or thin films. Metals such as platinum, gold and silver are not only desirable for their glamour or investment potential, but also for the astonishing and useful range of properties they exhibit when produced at the nanoscale. These myriad properties have captured the imagination and interest of nanoscientists, who are faced with an array of seemingly endless and delightful possibilities to explore.

Applications of precious and other metals at the nanoscale are found in a range of ultrasensitive chemical and biological sensors and have been chronicled in many scientific journals and in the popular media. For example, in May this year, publicity was given to a gold-flecked nanosensor developed by Royal Melbourne Institute of Technology that can precisely measure the presence and concentration of mercury, one of the world’s most poisonous substances, routinely emitted from coal-burning power stations and alumina refineries.

However, the treasure trove of potential applications of nanoparticulate metals requires the ability to properly characterize the nanoparticle or thin film under investigation, and if possible, derive an insight into the size-related properties from the characterization process.

Correlating the properties of metallic nanostructures with their size, shape and composition is exceptionally challenging. Naturally, this represents a major problem for commercial applications – neatly encapsulated in the old adage – “What you can’t measure, you can’t control”. However, for metallic nanoparticle and thin film users and producers alike, there is literally light at the end of the tunnel.

The optical properties of metal nanomaterials depend very much on their size and shape. A bulk metal will reflect most of the impinging light, if the surface is smooth. Metal nanoparticles and nanostructures on the other hand behave quite differently. They absorb different wavelengths of light, depending on their composition, size and shape, reflecting back the light that is not absorbed, thus appearing in many dramatically different colours. These colour effects have been known and utilized for many centuries and can be seen in the Lycurgus Cup (fourth century AD) and in medieval “stained” glass windows.

Recently, microscopy techniques are used to image nanostructures, but it is not enough to “see” nanoparticles, we need to know how they obtain their properties. This requires an understanding of how the novel optical phenomena nanostructures exhibit relate to their nanodimensionality; as controlling the optical behaviour of nanoparticles is of key importance in exploiting their properties. For example, some biosensors use the optical properties of gold nanoparticles that derive from surface plasmon resonances, which in turn depend strongly on the anisotropy of the particle shape.

Despite its importance, it can be very difficult to accurately characterize the optical response and other properties of nanoparticles and nanostructures. However, the optical, non-destructive and non-invasive diagnostic techniques of ellipsometry and polarimetry offer a vital way forward.

In polarimetry and ellipsometry, a light beam is shone on the surface of a material. When reflected, the beam changes its polarization states, allowing the optical properties (the refractive index and absorption) of the material to be extracted in a quantitative way. Polarimetry and ellipsometry techniques are very sensitive indeed to thin layers and small optical index variations; from one single measurement on a substrate, various details can be inferred including refractive index, absorption, the thickness of thin layers and interfaces, and the composition and nano dimensionality of the material.

The use of polarimetry and ellipsometry techniques are highly promising in the characterization of metal nanoparticles and nanostructures, so essential to the generation of highly specific nanostructures for many applications, including sensors. As a measure of the importance with which polarimetry and ellipsometry are viewed with regard to the successful commercialization of new nanomaterials, the EU has funded a project called NanoCharM, to disseminate information on these techniques and promote new research into their application. More information, including grant-supported training courses and summer schools, can be found at: www.nanocharm.org

Ottilia SaxlCEO of NANO Magazine, Stirling, UK, available at: www.nanomagazine.co.uk and Founder of the Institute of Nanotechnology (IoN), www.nano.org.uk. IoN is a partner in the EU-funded NanoCharM project, led by Dr Maria Losurdo, University of Bari.