Almost all materials have defects of one kind or another. This is especially true in nature at the atomic level, where perfect crystals are rare. Being able to identify these flaws, as well as chemical order/disorder, is key to understanding how they affect the properties and function of a material.
Now, for the first time, an international team of scientists including Birmingham physicist Dr Wolfgang Theis has successfully mapped the 3D coordinates of more than 23,000 individual atoms in a tiny iron-platinum nanoparticle – more than a trillion of which could fit within a grain of sand – to reveal the material's innate imperfections.
The team’s research, which was published in the journal Nature and recently won the College’s Paper of the Month award, shows that the positions of tens of thousands of atoms can be precisely identified. Not only that, the results can be fed into quantum mechanics calculations to correlate imperfections and defects with material properties at the single-atom level.
‘While we have a good understanding of pure materials with highly ordered structure, it is the imperfections that define the material’s detailed properties and allow us to tailor these to our needs,’ explains Wolfgang, Head of the Nanoscale Physics Research Laboratory within the School of Physics and Astronomy, and the project’s only UK collaborator. ‘This is particularly promising for nanoparticles that have found applications in industrial processes, medicine and consumer products.’
What had been lacking the past, he says, was the ability to identify the exact position and chemical species of every atom in such a nanoparticle. But the groundbreaking research behind his paper, ‘Deciphering chemical order/disorder and material properties at the single-atom level’, has changed that. It explains how Wolfgang and his American co-authors used transmission electron microscopy and reconstruction algorithms to determine the position and species of every atom in an iron-platinum nanoparticle made of 6,569 iron and 16,627 platinum atoms.
They were able to demonstrate how the atoms are arranged in nine grains, each of which contains different ratios of iron and platinum atoms. This showed that atoms closer to the interior of the grains are more regularly arranged than those near the surfaces. They also observed that the interfaces between grains, called grain boundaries, are more disordered.
‘This complete characterisation of the atomic structure of the nanoparticle allows us to link observed properties to the detailed atomic structure and thus open a new window to advance our fundamental understanding and our ability to design nanoparticles that meet our needs,’ says Wolfgang. For example, an iron-platinum alloy is a suitable material for next-generation magnetic storage media and permanent magnet applications, and might also in future produce better, more robust catalysts.
Wolfgang, whose contributions to the project span all aspects with particular focus on innovations in the microscopy methodology, says he and the team – from the University of California Los Angeles and the National Center of Microscopy of the Molecular Foundry at the Lawrence Berkeley National Laboratory, Berkeley – will continue with their work, aiming to determine the 3D atomic coordinates of more materials. They also plan to set up an online databank for the physical sciences so that other scientists can use it to study material properties on the single-atom level.