The surface plasmon modes of self-assembled gold nanocrystals.

The three-dimensional (3D) self-assembly of nanocrystals constitutes one of the most important challenges in materials science. A key milestone is the synthesis of simple, regular structures, such as platonic solids, composed of nanocrystal building blocks. Such objects are predicted to have unique optical and electronic properties such as polarization-independent light-scattering and intense local fields. Here we present a two-stage process for fabricating well-defined and highly symmetric, 3D gold nanocrystal structures, including tetrahedra, 3D pentamers and 3D hexamers. Polarized scattering spectra are used to elucidate the plasmon modes present in each structure, and these are compared with computational models. We conclude that self-assembly of highly symmetric, polarization-independent structures with interparticle spacings of order 0.5 nm can now be fabricated. Drastically, enhanced local fields, 1000 times higher than the incident field strength, are produced within the interstices. Fano resonances are generated if the symmetry is broken.