This photo shows the apparatus used by the Imperial researchers to test the optical properties of their nanoparticle layer. Photo: Imperial College London.
This photo shows the apparatus used by the Imperial researchers to test the optical properties of their nanoparticle layer. Photo: Imperial College London.

By finely tuning the distance between nanoparticles in a single layer, researchers have made a filter that can change between a mirror and a window. This development could help scientists create special materials whose optical properties can be changed in real time and so could be used for applications ranging from tuneable optical filters to miniature chemical sensors.

Creating a 'tuneable' optical material – one that can be accurately controlled – has proved a challenge because of the tiny scales involved. In order to tune the optical properties of a single layer of nanoparticles – which are only tens of nanometers in size each – the space between them needs to be set precisely and uniformly.

To form the layer, the team of researchers from Imperial College London in the UK created conditions that allowed gold nanoparticles to localize at the interface between two liquids that do not mix. By applying a small voltage across this interface, the team has been able to demonstrate a tuneable nanoparticle layer that can be dense or sparse, allowing the layer to switch between a reflective mirror and a transparent surface. The research is reported in a paper in Nature Materials.

"It's a really fine balance – for a long time we could only get the nanoparticles to clump together when they assembled, rather than being accurately spaced out. But many models and experiments have brought us to the point where we can create a truly tuneable layer," said study co-author Joshua Edel, a professor in the Department of Chemistry at Imperial.

The distance between the nanoparticles determines whether the layer is transparent to or reflects different wavelengths of light. At one extreme, all the wavelengths are reflected, and the layer acts as a mirror. At the other extreme, where the nanoparticles are dispersed, all wavelengths are permitted through the interface, allowing it to act as a window.

In contrast to previous nanoscopic systems that used chemical means to change the optical properties, the team's electrical system is reversible.

"Finding the correct conditions to achieve reversibility required fine theory; otherwise it would have been like searching for a needle in a haystack. It was remarkable how closely the theory matched experimental results," said study co-author Alexei Kornyshev, also a professor in the Department of Chemistry.

"Putting theory into practice can be difficult, as one always has to be aware of material stability limits, so finding the correct electrochemical conditions under which the effect could occur was challenging," commented co-author Anthony Kucernak, another professor in the Department of Chemistry.

"The whole project was only made possible by the unique knowhow and abilities and enthusiasm of the young team members, including Dr Yunuen Montelongo and Dr Debarata Sikdar, amongst others who all have diverse expertise and backgrounds," added Kornyshev.

This story is adapted from material from Imperial College London, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.