Making high quality lighting with nanotechnology and…old fish scales?

You’ve heard of carbon nanotubes, now get ready for carbon nano-onions. The nanometre-sized particles (made from layers of carbon atoms connected in sheets, hence “onions”) are excellent electrical conductors and have a bevy of other weird chemical properties, but are largely harmless to people and the environment. This makes them encouraging candidates for new electronics, energy storage, and biomedical technologies.

What’s the catch? While carbon nano-onions were first made in 1980, they remained difficult to synthesise. Typically, they need high temperatures, a vacuum, and a lot of time to make – or expensive feedstock and very corrosive acids and bases.

But a team of Japanese researchers has figured out how to make carbon nano-onions in a few seconds – by microwaving fish waste.

Ultimately, the researchers hope their cheap and environmentally friendly method of making carbon nano-onions could be useful in next generation LEDs and QLED displays.

In a paper published in Green Chemistry, the researchers describe a method for synthesising carbon nano-onions (CNOs) from fish scales, which have been extracted from fish waste and cleaned. It’s done via a technique called microwave pyrolysis: submitting the scales to microwave radiation.

The researchers aren’t exactly sure why this method works. They suspect it’s got to do with the collagen in the fish scales, which is an excellent absorber of microwave radiation. The collagen heats up so quickly it triggers pyrolysis: the breaking down of the collagen into gases. These gases then promote the formation of CNOs.

Diagram showing carbon nano-onion synthesis, electron microscope picture, photo of bottle of ethanol glowing bright blue, thin transparent film glowing bright blue, led glowing blue
Left: The whole pyrolysis scheme mapped out. The top inset shows the rise in temperature of the fish scales due to microwave absorption over a period of 10 seconds, as well as a suggested mechanism forhow the nano-onions form. Right: transmission electron microscopy images showing the morphology of the carbon nano-onions, alongside photographs of CNO dispersion in ethanol, a flexible film, and an LED containing CNOs – all displaying a characteristic bright blue light. Credit: Takashi Shirai from NITech, Japan

The CNOs made have a couple of impressive chemical properties. They have a high crystallinity (meaning they form ordered and structured patterns when clumped together), and high “functionalisation” (meaning they’re bonded to other small molecules on their surface).

The combination of these properties means that the CNOs can do some interesting optical things – like glow bright blue.

“The CNOs exhibit ultra-bright visible-light emission with an efficiency (or quantum yield) of 40%,” says co-author Dr Takashi Shirai, an associate professor at the Nagoya Institute of Technology, Japan.

“This value, which has never been achieved before, is about 10 times higher than that of previously reported CNOs synthesised via conventional methods.”

The researchers used this property to make LEDs and blue-light-emitting thin films with their CNOs.

“The stable optical properties could enable us to fabricate large-area emissive flexible films and LED devices,” says Shirai.

“These findings will open up new avenues for the development of next-generation displays and solid-state lighting.”

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