In the growing field of tunable metamaterials, scientists from Imperial College London have created self-assembling and disassembling nanoparticle arrays that are able to vary between a mirror and a window, a breakthrough that could lead to the development of materials with optical properties changeable in real time and applications for optical filters and switchable mirrors.

The study, which evolved from a collaboration between experts in electrochemical systems and plasmonics, nanoparticle spectroscopy and the electrochemistry of nanoparticles and liquid–liquid interfaces, was reported in the journal Nature Materials [Montelongo et al. Nat. Mater. (2017) DOI: 10.1038/nmat4969]. It showed that when the distance between nanoparticles in a single layer is finely tuned, they were able to produce a filter that moves between a mirror and a window.

To tune the optical properties of a single layer of nanoparticles, the space between the layers has to be known in a precise and uniform way, with the optical properties being controlled by the number of particles at the interface. To provide the layer, the team produced the ideal conditions for gold nanoparticles to localize at the interface between two liquids that do not mix, here oil and water.

“We have shown that it is possible to reversibly assemble nanoparticles at an interface guided by an electric field”Anthony Kucernak

When a small voltage was applied across the interface, a tuneable nanoparticle layer that could be either dense or sparse was demonstrated, thereby allowing for switching between a reflective mirror and a transparent surface. It is key that the electrical system is reversible, something not achievable by other nanoscopic systems that use a chemical approach to alter the optical properties.

The space between the nanoparticles determines if the layer allows or reflects different wavelengths of light. On one extreme, all wavelengths are reflected so that the layer behaves like a mirror, while at the other, as the nanoparticles are dispersed, all the wavelengths are permitted through the interface so that it behaves like a window.

As co-author Anthony Kucernak told Materials Today, “We have shown that it is possible to reversibly assemble nanoparticles at an interface guided by an electric field”. This let them control the optical response of the nanoparticles just by “flicking a switch” so that they either move to the oil–liquid interface or away from it.

As the findings were exactly reflected by theory of the forces and resultant optical response, the approach could suggest ways of producing other controllable systems with different optical properties. The researchers expect the field to develop to increase the speed at which the mirror can be assembled and disassembled, and also to strengthen the effect involved.