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Honey, I Shrank the Eiffel Tower

25 Jan 2020

The Straits Times, 25 Jan 2020, Honey, I Shrank the Eiffel Tower

SUTD Team's Work in Nanostructures Could Help in Preventing Counterfeiting

Underneath a microscope in the corner of a laboratory in Upper Changi sits a model of the Eiffel Tower no wider than a strand of human hair, and about 10 million times shorter than the actual monument in Paris.

The vibrantly coloured mini-monument is the result of two years of research led by Associate Professor Joel Yang from the Singapore University of Technology and Design (SUTD). What is significant about the model is not just its size - it is the smallest 3D model of the Eiffel Tower in the world - but also the way in which its brilliant colours are produced.

Prof Yang told The Straits Times: "Most of the colours we see around us come from pigments and dyes. These colours come from the absorption of certain frequencies of light. So we see the frequencies of light that are reflected, not the ones that are absorbed."

Unfortunately, the absorption of high frequencies of light such as ultraviolet light from the sun will break chemical bonds within the pigments or dyes, causing the colours to fade with time.

"Once the bonds break, the dye molecule is no longer the same molecule - your colours are gone," said Prof Yang.

Not so with his Eiffel Tower model.

Created through a special heat-shrinking process, its brilliant colours are known as structural colours, the result of light scattering off tiny nanostructures, each about 100 nanometres in size - about 1,000 times smaller than the width of a strand of human hair. Prof Yang said: "The colours arising from these structures are more robust. The structures don't get damaged as they're much larger than molecules.

"They also don't absorb the light, so light doesn't impart energy into the material... The light will either pass through or get reflected."

Nanostructures also do not produce the smearing effect that pigments do. Pigments tend to spread when they are applied to a surface, limiting how closely spaced two different colours can be put next to each other.

This in turn limits the resolution of images, said Prof Yang.

He and his team of 11 researchers drew inspiration from instances in nature where nanostructures are used to create brilliant colours, such as on the wings of the morpho butterfly. He said: "That got us thinking... Can we actually make colours on the smallest possible scale, like building blocks, and rearrange them in three dimensions?"

The team first attempted to 3D-print the structures using a specialised machine, but this resulted in products that were relatively large and colourless. Structures that are too large cause the scattering of all wavelengths of light, resulting in their colourless appearance, said Prof Yang.

So the team had to create structures smaller than the wavelength of light, in order to produce colour. But simply printing structures of that minuscule size would result in them crumbling and collapsing, as they would not be rigid enough to survive the process. The researchers then decided to heat-shrink the 3D-printed structures down to size. The experiment worked and in the process, the team also found that the colour of the structures would change as they got smaller.

"This allows us to make structures and features much smaller than the wavelength of light... We can make the building blocks of colour at will, and put them exactly where we want them," said Prof Yang, adding that this is the first time in the world something like this has been done.

The unique nature of the structures means the team's process could potentially be applied in anti-counterfeiting efforts, where minuscule sculptures are sometimes placed in luxury watches to prove their authenticity. There are also potential applications in specialised devices like spectrometers, which require light to be split into different colours.

Prof Yang said he will continue researching to see if the colours can be made more vibrant or if other optical effects can be produced. "Research is never done... There's a lot more to be explored," he said.