Team changes colour of gold by altering surface structure

In what is claimed to be a world first, researchers at Southampton University have demonstrated that by embossing nanoscale raised or indented patterns onto the metal’s surface they can change its colour without using coatings or chemical treatments.

This technique, which could also be applied to other metals, could be used in jewellery manufacture or in the security industry to create banknotes or other items with sophisticated optical properties that would be almost impossible to imitate.

According to the group at the university’s Centre for Photonic Metamaterials, the behaviour of the light striking the metal (and therefore the colour that is created) can be modified simply by varying the shape and height of these patterns in order to produce a wide range of colours.

Developed through the £5m EPSRC-funded ‘Nanostructured Photonic Metamaterials’ initiative, the breakthrough is one of the latest additions to the field of structural colour, in which materials are coloured not through dye or pigmentation but through the way in which light interacts with tiny structures on a material’s surface. The iridescent greens and blues of beetles are an example of this phenomenon occurring naturally.

According to the research manager on the project, Dr Kevin Macdonald, the Southampton technique has a number of advantages over existing approaches. He told The Engineer that it can be applied in a single layer on a continuous surface and that unlike other techniques the colour can be varied throughout the structure.

Macdonald explained that the nano-patterning is carried out at the research level using techniques such as ion beam milling and electron beam lithography. For larger-scale commercial applications, he envisages the use of nano-imprint processes, whereby large areas are stamped out from a master template in a manner comparable to CD or DVD production.

Although he wouldn’t be drawn on a ‘killer application’ for the technology, he confirmed that a patent application covering the work has been filed and that the group is currently talking to potential end users in both the security and aesthetic markets.