Quasicrystals have long-range order but no periodicity, and were first observed by Dan Shechtman in 1982, a discovery that he was awarded the Nobel Prize in Chemistry for in 2011. The structures are typically found in metal alloys such as AlCuMn, but they have also been reported in more unusual systems such as liquid crystals, nanoparticle assemblies and perovskite thin films. Two independent research teams have now discovered different ways to create quasicrystals from a layer of molecules on a surface.
Alex Kandel and colleagues at the University of Notre Dame formed self-assembled two-dimensional quasicrystals using ferrocenecarboxylic acid molecules adsorbed on a gold surface. The carboxyl groups of the molecules can form hydrogen bonds and these bonds drive the molecules into cyclic pentamers. This leads to the formation of molecular monolayers — which the researchers imaged with a scanning tunnelling microscope (STM) — that have local five-fold symmetry, but no periodicity.
Alternatively, Joe Smerdon and colleagues at the University of Central Lancashire, the University of Liverpool and Tohoku University used the surface of a ternary metal alloy quasicrystal as a template to grow a quasicrystalline layer of C60 or pentacene molecules. The molecules preferentially adsorb on specific sites on the surface, creating molecular layers that reflect the structure of the substrate below. The resulting quasicrystalline molecular layers were also imaged with an STM.
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Vaughan, O. Now in molecular layers. Nature Nanotech (2014). https://doi.org/10.1038/nnano.2014.75