‘Chameleon’ camouflages itself with plasmonic skin


© American Chemical Society

It may not win any prizes for subtlety yet, but a plasmonic ‘chameleon’ designed by scientists in China and the US could one day prove to be as good at adaptive camouflage as the real thing. Its skin, which is based on colour-shifting nanostructures, could also become a new type of reflective display.

In recent years there has been a surge of interest in invisibility cloaks, made from light-bending structures known as metamaterials. Since invisibility cloaks are currently limited to microwaves, the next best option could be a form of adaptive camouflage based on a reflective display, like the sort used in e-book readers. Some of these displays can generate a variety of colours, but to do so they currently need heavily layered structures, which reduce overall reflectivity and give a dull appearance.

Plasmonic metal nanostructures could be the answer. If light falling on these materials has the right wavelength – that is, the right colour – it meets a collective oscillation of charge known as a plasmon that forces it to reflect. Tuning the size and composition of the nanostructures controls the colour of light that is reflected, but normal methods of fabrication leave the structures fixed.

Now, Sheng Chu and Guoping Wang at Sun Yat-Sen University in Guangzhou, together with colleagues from there and other institutions in China and the US, have developed a way to produce colour-tuneable plasmonic metal nanostructures. They consist of a layer of nano-sized gold domes coated with a gel containing silver ions, all of which are sandwiched between two electrodes. Applying a positive voltage between the electrodes binds the silver ions to the gold domes, changing the characteristic plasmon reflectivity from red to green and finally blue; reversing the voltage takes the reflectivity back to the red end of the spectrum.

The colour changes result from silver-coated gold nanoparticles whose reflectivity changes according to applied voltage © American Chemical Society

To demonstrate the colour-shifting effect, Chu and colleagues attached lots of their cells to a mechanical chameleon that had colour sensors for eyes. As the chameleon rolled in front of a striped background, the sensors detected the stripe colour and tuned the cells’ colours automatically.

Yusuke Komazaki, a researcher at the University of Tokyo in Japan who specialises in display technology, is impressed by the display’s range of colours, but wonders whether it is as effective at different viewing angles, and whether it could display blacks and whites. ‘This display may have a great potential for various applications,’ he says.

The team now intend to streamline the fabrication process and improve the level of camouflage. ‘Right now I do not have a very clear vision,’ says Wang. ‘[But] we definitely need to keep working on this, and see what we really need to be focused on.’

References

G Wang et al, ACS Nano, 2016, DOI: 10.1021/acsnano.5b07472


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