Nano Lett. 13, 600–606 (2013)

Ensembles of metallic nanoparticles that exhibit plasmonic properties are often called metamolecules because their building blocks interact with each other in a way that is similar to how atoms interact with each other in a molecule. This chemical analogy has also been extended to the concept of chirality and has led to the synthesis of chiral metamolecules that have a single chiral centre. Mario Hentschel and colleagues at the University of Stuttgart and the Max Planck Institute, Stuttgart have now created plasmonic diastereoisomers — structures that have multiple chiral centres.

The researchers start by placing a set of three nanodisks on a surface to form an L-shape and then place a fourth on top. It has previously been shown that this type of ensemble can either be chiral or achiral depending on the position of the fourth nanodisk. However, by adding a second set of four nanodisks to create an S-shaped ensemble (pictured) the system can be endowed with diasteroisomerism. The circular dichroism spectrum of the ensembles strongly depends on the position of the two nanodisks in the top layer and its intensity is equal to the sum of the intensities of the single L-shaped chiral centres, as long as the electric fields of the nanodisks do not overlap too much.

Notably, all metamolecules created with this method are chiral and show a marked circular dichroism. The absence of mesostructures — which have two or more chiral centres but are achiral because of a symmetry plane in the molecule — is due to the fact that the bottom layer of nanodisks sit on a surface and therefore the structures lack the necessary symmetry plane.