Self-assembling nanoparticles can switch between a mirror and a window

Researchers from the Department of chemistry at Imperial College, London, have made a filter that can change between a mirror and a window by finely tuning the distance between nanoparticles in a single layer.

The development could help scientists create special materials whose optical properties can be changed in real time. These materials could then be used for applications from tuneable optical filters to miniature chemical sensors.

Creating a ‘tuneable’ material that can be accurately controlled has been a challenge because of the tiny scales involved. In order to tune the optical properties of a single layer of nanoparticles the space between them needs to be set precisely and uniformly.

To form their layer, the Imperial researchers created conditions for gold nanoparticles to localise at the interface between two liquids that do not mix.

By applying a small voltage across the interface, the team have been able to demonstrate a tuneable nanoparticle layer that can be dense or sparse, allowing for switching between a reflective mirror and a transparent surface.

The distance between the nanoparticles determines whether the layer permits 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 and it acts 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.

Study co-author, Professor Joshua Edel said: “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.”

Author
Tom Austin-Morgan

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