Super-light inks display structural colour from bizarre physics

Line of glass slides with coloured paint from different angles
A single layer of silicon nanospheres produces bright structural colors that are independent of the viewing angle. The color can be controlled by the diameter of the spheres, where smaller particles are bluer and larger ones redder. Credit: FUJII Minoru

A team of Japanese researchers has made extremely lightweight, non-fading inks in a rainbow of colours – in the same way as animal colouration.

The invention could dramatically lower the weight of paints required for big objects.

The silicon inks use “structural colour”, a form of colouration that doesn’t depend on pigments or dyes.

Structural colours come from light waves bouncing off nanometre-sized structures inside a substance. Creatures like beetles, birds and butterflies have structural colour to thank for their iridescent hues.

Unlike pigments, which create colour by absorbing some light and reflecting the rest, structural colour doesn’t fade over time.

But structural colour usually changes depending on the angle at which it’s viewed,  and it’s also usually very reliant on the physical structure of the substance it’s in – such as carefully stacked crystals. This makes structural colours hard to print industrially.

These inks, described in ACS Applied Nano Materials, can be printed, and they don’t change their hue depending on angle.

They’re made by producing “nanospheres” of silicon, each about a tenth the size of a small bacterium.

Black and white image of spheres
A scanning electron micrograph of the nanosphere monolayer shows almost perfectly round particles. Credit: FUJII Minoru

“A single layer of sparsely distributed silicon nanoparticles with a thickness of only 100-200 nanometres shows bright colours but weighs less than half a gram per square metre,” says co-author Dr Hiroshi Sugimoto, a materials engineer at Kobe University.

“This makes our silicon nanospheres one of the lightest colour coats in the world.”

The nanoparticles use a phenomenon called “Mie resonance”, where spheres that are similar in size to a wavelength of light reflect that light very strongly, to deliver their colour.

Line of vials with different coloured liquids
The nanospheres in methanol have different colors than when applied to a surface. “This is due to the multiple scattering, i.e., blue light subsides during consecutive scattering by absorption, while red light survives,” explain the researchers. Credit: FUJII Minoru

The researchers are developing their inks further with the hope of commercialising them.

“We can apply it to the coating of, for example, airplanes,” says Sugimoto.

“The pigments and coatings on an airplane have a weight of several hundreds of kilograms. If we use our nanosphere-based ink, we might be able to reduce the weight to less than 10% of that.”

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