The formation of nanocrystal assemblies, or superstructures, with optical functionality requires methods for the formation of the superstructures with high precision and in high purity. This encompasses challenges within the assembly of nanocrystals, which is a key enabling step, as well as for the full elucidation of the fundamental interactions between the component nanocrystals.
Self-assembly of nanoparticle superstructures becomes more challenging for asymmetric structures. Asymmetry can be introduced into the superstructures from asymmetry in the nanoparticle shape, the incorporation of nanoparticles of different materials or asymmetry inherent in the final geometrical arrangement of the nanoparticles with respect to one another. Asymmetry in the arrangement of the nanocrystals may take the form of large geometrical variations which, whilst remaining largely symmetrical, exhibit markedly different optical properties.
Developments in DNA nanotechnology offer control of the self-assembly of nanocrystals into discrete structures. We report an approach to construct multiple, structurally different, nanoparticle assemblies from just a few complementary nanoparticle-functionalised DNA strands. The approach exploits local minima in the potential energy landscape of hybridised nanoparticle-DNA structures by employing kinetic control of the assembly. This approach leads to the potentially facile production of a number of discrete three-dimensional isomeric assemblies of nanoparticles from a given set of ss-DNA, akin to molecular structural isomers, in extremely high (structure) yield. Using a four-strand DNA template, we synthesise five different 3D gold nanoparticle (plasmonic) tetrameric isomers. The 3D organisation of the nanoparticles is controlled by the electrostatic repulsions and steric hindrance of the charged nanoparticles.
A general method for the incorporation of different materials into DNA-based assembly is also presented, allowing access to different hetero-assemblies in high purity. The method has the flexibility to assemble nanoparticles of different sizes, shapes and materials in the one assembly. This paves the way for a wide range of nanoparticle interactions and the optical properties of various assemblies (containing both metal nanoparticles and/or semiconductor quantum dots) will be outlined. The results show that the geometrical requirements to achieve a specifically designed coupled optical signature from a nanocrystal assembly are strict. Additionally, the effect of the interparticle separation, able to be modulated within these assemblies, will be discussed. These results have implications for the future design and realisation of functional nanocrystal superstructures.