Gold nanoparticles turn mirrors into windows at the touch of a switch


By using electrical current to change the spacing of a layer of floating nanoparticles, researchers can change its reflectivity at will, writes Andrew Masterson.


The layer of nanoparticles in mirror mode.
The layer of nanoparticles in mirror mode, reflecting an image of a coin.
Montelongo et al., Nature Materials (2017)

Sheets of glass that can flip between being windows and mirrors at the tap of an app are a step nearer following research by scientists at Imperial College London and published in the journal Nature Materials.

Joshua Edel and colleagues report successfully creating a single layer of gold nanoparticles in which each particle is exactly equidistant from its neighbours – a feat made challenging because of the minute scales involved. Each particle is only tens of nanometres in size.

Edel’s team successfully created the layer by forcing the particles to gather at the intersection of two liquids that do not intermix. By passing a small electrical current through the layer, the particles could be induced to either form a dense concentration or disperse – resulting in either transparency or opacity.

“It’s a really fine balance,” explains Edel. “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.”

The transition between window and mirror states.
The transition between window and mirror states.
Montelongo et al., Nature Materials (2017)

The density of the nanoparticles – measured by the distance between them – affects light wavelengths the layer reflects. By increasing density to maximum, all wavelengths are reflected, creating a mirror. At the other end of the scale, reflectance is minimised, creating a window.

Because the catalyst for adjusting the density is electrical rather than chemical, the process 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,” says co-author Alexei Kornyshev. “It was remarkable how closely the theory matched experimental results.”

Tuneable nanoparticle layers could one day be deployed in a range of applications, such as optical filters and chemical sensors.

And while commercial applications are still a long way off, the proof-of-concept demonstrated by the represents a significant move forward.

“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,” says another co-author, Anthony Kucernak.

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Andrew Masterson is an author and journalist based in Melbourne, Australia.
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