The big question raised by a nanoscale pattern
In recent times nanoscientists have been able to manipulate flat nanocrystals to create tiled patterns. There’s an inherent beauty in the result – dense, regular patterns, almost impossibly small. But another marvel caught the interest of researchers at the University of Pennsylvania – the fact that particular crystals produce a precise, tight herringbone pattern.
A team of computer simulation experts from the Massachusetts Institute of Technology were asked to help explain this conundrum, and their computer modelling proved invaluable in finding the answer – but it took further work by another team from the University of Michigan to cement the findings.
Christopher Murray, the Richard Perry University Professor and professor of chemistry at the University of Pennsylvania, leads a team of researchers who specialise in making nanocrystals and combining them to form a larger crystal superstructure. The team have created patterns with flat nanocrystals comprised of lanthanides and fluorine atoms. Using lanthanides with different atomic radii allows the researchers to control the top and bottom faces of the hexagonal crystals. Manipulating the crystals in this way gives them the scope to either make these faces longer than the other four sides, or to make them non-existent – resulting in a diamond shape.
By spreading a layer of nanocrystals and solvent upon a thick fluid, the crystals pack together as the solvent evaporates, forming a tiled pattern. Diamond-shaped nanocrystals form an argyle pattern, and hexagonal nanocrystals with lengthened faces form a pattern akin to honeycomb. Hexagonal nanocrystals with sides that are almost all the same length don’t, however, also form a honeycomb-like pattern as one might assume – they create a complex, alternating herringbone pattern.
A team led by the University of Michigan's Sharon Glotzer, the Stuart W. Churchill Collegiate Professor of Chemical Engineering, were asked to help explain this phenomenon. Her team use computer simulations to design and comprehend nanoparticle ‘patchiness’ – a concept Glotzer herself introduced in 2004.
Initially, simulations showed nanocrystals of near-regular hexagons should tend towards a foreshortened honeycomb pattern. This inspired Glotzer and her team to investigate the interactions between the edges of the particles. They discovered that if the edges that formed the points were stickier than the other two sides, the hexagons would naturally arrange in the herringbone pattern.
Both teams felt that the stickiness must be caused by carbon and hydrogen chains attaching more readily to the pointed ends. Measuring this hypothesis is not yet possible due to the scale involved, so Murray asked Ju Li, now the Battelle Energy Alliance Professor of Nuclear Science and Engineering at the Massachusetts Institute of Technology, to calculate how the chains would attach to the edges at a quantum mechanical level. Li and his team confirmed that because of the way that the different facets cut across the lattice of the metal and fluorine atoms, more hydrocarbon chains could stick to the four edges that led to points than the other two sides.
‘Our study shows a way forward making very subtle changes in building block architecture and getting a very profound change in the larger self-assembled pattern,’ Glotzer said. ‘The goal is to have knobs that you can change just a little and get a big change in structure, and this is one of the first papers that shows a way forward for how to do that.’
Image courtesy of the University of Pennsylvania. Credit:Xingchen Ye