Light trapping pixels

C&I Issue 7, 2019

The smallest pixels yet have been created by trapping light with gold nanoparticles. They could be used to change the colour of buildings while consuming very little energy.

The gold nanoparticles are coated in a sticky conductive polymer and sit on top of a reflective, mirror-like surface, which traps light (Science Advances, doi: 10.1126/sciadv.aaw2205). ‘We deployed potential to control the redox state of the polymer, and that in turn changes the refractive index and essentially the light colour,’ says Hyeon-Ho Jeong, a co-author of the research at Cambridge University. ‘We don’t need a backlight or much energy to operate the light, just ambient light.’

The polymer, polyaniline, and nanoparticle are sprayed onto the flexible mirror-coated plastic. Initial applications include lighting up buildings, or as electrical panels in supermarkets. Gold absorbs light in the blue region, so ‘we are currently operating colours tinted from red to green, but we are looking to see what type of material could generate blue,’ Jeong says. Aluminium and silver nanoparticles are other possibilities.

The nanoparticle mirror system is based on gold particles that possess well understood ‘surface plasmon resonances’, says Martyn Pemble, materials chemist at the Tyndall National Institute at University College Cork, Ireland, commenting on the research. These are oscillations of electrons that occur at wavelengths linked directly to their size, shape and – importantly for this application – the spacing between the particles and the underlying mirror. ‘This work represents a potential paradigm shift in terms of the design and fabrication of colour displays, which could have enormous commercial potential,’ notes Pemble.

The nanoparticle on mirror, or NPoM, structure, he says, ‘opens up huge possibilities in terms of the construction and manufacture of ultra-thin colour changing devices, which show changes in colour and response times suitable for most display applications, yet would be perhaps 100 times thinner than comparable conventional displays’.

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