NANOTECHNOLOGY - HERALDING A NEW ERA OF LIGHT AND COLOUR
By Dr Maria Losurdo, CNR-IMIP, Institute of Inorganic Methodologies and Plasmas, University of Bari, Italy; Norbert Esser, ISAS, Institute for Analytical Sciences, Berlin, Germany; Ottilia Saxl, Institute of Nanotechnology, Stirling, UK.

Metal nanostructures and the colour of size
A bulk metal typically reflects more or less all of the light that impinges on it, as long as its surface is smooth. We all see this effect when we look in the mirror. So the use of metallic structures to transmit light might seem a totally impractical notion. Well, not if the metal is in nanoparticle form. Metal nanoparticles absorb only certain wavelengths of light (depending on their composition, size and shape), so they reflect back the light that is not absorbed. This means they can appear in many dramatically different colours.
The origins of the colours exhibited by metal nanoparticles relate to their composition, particle size and shape. When visible light strikes some metal nanoparticles, such as gold, silver, copper, indium, gallium, palladium etc, the synchronized oscillations of the conduction electrons are displaced. This gives rise to extra absorption at certain wavelengths, and the colours that are characteristic of each material. This effect is called Surface Plasmon Resonance (SPR), and the field of research is known as “Plasmonics”. The study of plasmonics is leading to a plethora of intriguing physical phenomena and their application in industry. For example, metal nanoparticles are being used for special effects in paints and cosmetics, as tracers in pharmaceuticals and in spectacular self-cleaning surface coatings.

Bringing ancient technologies into the 21st century
The colours obtained from some metal nanoparticles were known to medieval artisans, who – totally unconscious of the fact that they were early nanotechnologists – mixed gold and copper nanoparticles into molten glass, creating materials that absorbed and reflected light in a way that produced a rich ruby colour. Those effects have been used since the Middle Ages for producing glowing colours, particularly for ‘stained’ glass in church windows.
Another famous example of the use of metal nanoparticles is the early Lycurgus Cup (British Museum; fourth century AD) containing gold nanoparticles of typically 5nm – 60nm in size, whose colour changes from greenish to red when it is illuminated from the inside because of the plasmonic excitation of the gold nanoparticles within the glass.
Nowadays, metal nanostructures are created based on a rational design of size and shape, and various metals can be used, not just gold and silver, but also gallium, indium, copper and palladium. It is now possible to control the production of nanostructures of many kinds of metals in the quality and quantity needed for the investigation of their intriguing properties.
Shining a light on the mysteries of the nanoverse
Controlling the optical behaviour of nanoparticles is of key importance if their properties are to be exploited in several emerging applications. For example, selective optical filters, novel solar cells, biosensors and nanoparticle tracers for cancer detection are among the many applications that use the optical properties of gold nanoparticles that derive from its surface plasmon resonances.
Despite the great importance of understanding the optical response of nanoparticles, it is inherently difficult to accurately characterise their properties. However, optical techniques such as ellipsometry and polarimetry, which are non-destructive and non-invasive, can be used to define the optical and physical properties of nanoparticles and nanomaterials.
How do these techniques work? In ellipsometry and polarimetry, a light beam of known polarization impinges on the surface of a material. When reflected, it changes its polarization state, allowing the optical properties to be ‘measured’ in a quantitative way. These techniques are very sensitive to thin layers and small optical index variations. From one single measurement on a substrate, details can be inferred, including the refractive index, absorption, thickness of thin layers and interfaces, and even the composition and nano-dimensionality of the material.
Shown to the right is an example of 50nm gold nanoparticles assembled to mimic a raspberry on a silicon surface. Its ellipsometric spectra shows the SPR absorption peak at a wavelength of 620nm, corresponding to a ruby colour. It is worth noting that the ellipsometry analysis simultaneously gives the optical response, the SPR characteristics and also the thickness or height of the nanoparticle assembly.
How “nano” is a nanostructure? Ellipsometry and polarimetry can give a direct answer to this question. The size of the nanoparticles can be inferred by the position of the SPR peak, while the amplitude of the peak gives information on the density of the nanoparticles, and is therefore an extremely versatile tool for determining the size of nanoparticles in real time.

To sum up…
Increasingly, new nanostructures are being engineered with tailored, functional optical properties and colours for optics, photonics, and biomedical applications ranging from therapeutics to diagnostics. The versatile nature of ellipsometry as a functional, nanoscale sensitive and non-destructive technique, is paving the way for the application of these new nanostructures in a widening field of applications from breakthroughs in knowledge of thin film multilayer surfaces, composite and smart materials and materials engineering at the nanoscale.
To address the need for a wider understanding of nanoparticle characterisation techniques for the rapidly expanding field of nano materials and their applications, a world class consortium of experts has joined together in a European Coordination Action called NanoCharM (Multifunctional Nanomaterials Characterization Exploiting EllipsoMetry and Polarimetry. A Concerted Action of the EU’s Framework Programme 7).

www.nanocharm.org
E: Maria Losurdo, maria.losurdo@nanocharm.org


Industrial Focus May/June 2008