Nanodiamonds forever

Cath O’Driscoll

A new method for producing nanodiamonds (NDs) of uniform size could pave the way for applications in drug delivery, sensors and quantum computing.

Currently, NDs are widely made by detonating an explosive such as TNT in the presence of graphite in a sealed steel tube, but this process is difficult to control, University of North Texas project leader Hao Yan told the ACS Spring 2022 Meeting. The NDs formed this way have a broad size distribution in the range 5-500nm and high impurity levels.

‘You have a lot of uncontrolled defects and impurities, so in general what you end up getting is more like a pile of soot than the diamond you imagine,’ says Yan. ‘We realised that places where diamonds are formed in the Earth’s mantle contain a lot of iron and iron-carbon compounds, including carbides and carbonates.’

Using this information, the Texas team created evenly sized nanoparticles of iron carbide as the carbon source for the diamonds and distributed them in an iron oxide matrix. Under high temperature and pressure, the compounds reacted to produce uniform NDs as small as 2nm wide - an order of magnitude better than without additional post-synthetic treatment or purification steps, Yan says.

But although creating uniform, perfect nanodiamonds is a step forward, materials can be even more useful when they have defects, such as empty spots in the diamond’s structure or when neighbouring carbon atoms are replaced with nitrogen, silicon or nickel, for example. Non-carbon atoms impart colour to the NDs and are therefore called colour centres. Nanoparticles with only one colour centre are highly desirable because they can securely store information in quantum computers and telecommunication devices.

Yan estimates the new method could make enough single colour centre NDs for a couple of thousand quantum computers with one synthesis experiment, although the tiny crystals would need to be arranged properly before computations could be made.

‘We now have an ideal platform to devise a way for making a single colour centre nanodiamond, which is a breakthrough for a number of diamond-related technologies. But also, in a broader sense, it would be a fascinating demonstration of how you can control a single atom in a much larger structure,’ says Yan, commenting that the infrastructure is already in place for making synthetic diamonds.

‘We are already in contact with researchers and industrial partners in trying to demonstrate that this method can really be scaled up to produce gram to even kilogram scales of NDs with high uniformity, high crystallinity and high quality,’ he concludes.