Self-Assembly Enabled Mie-Scattering in Hematite Nanoplatelet Stacks

In this work spectral changes following the self-assembly of hematite nanoplatelets into particle stacks are characterized and interpreted. The individual hexagonal nanoplatelets are dispersed in aqueous medium and have a dimension of 120 × 20 nm. In the framework of Mie-scattering, hematite is considered as a medium refractive index material in the visible wavelength range and the particles' ensemble optical spectrum is dominated by absorption with a small contribution from the electric dipole scattering. However, by introducing an increasing amount of inert electrolyte into the system, these platelets can be assembled into vertical stacks, dictated by the interaction-area proportional attractive dispersion force. The process is accompanied by a profound change of the optical spectrum and the evolution of a new scattering peak is observed. Based on the simulation-derived spectra and optical near-field properties, this can be attributed to the stacking-associated enhancement of the collective electric and magnetic dipole modes. This is confirmed by performing correlative electron microscopy and scattering spectroscopy on individual nanoparticles and particle stacks. The presented approach provides a yet unexplored strategy for modulating especially the magnetic mode of otherwise weak Mie-scatterer nanoparticles.