Hyperspectral dark-field optical microscopy correlated to atomic force microscopy for the analysis of single plasmonic nanoparticles: a tutorial

Plasmonic nanostructures are actively investigated for their optical properties and for a wide range of applications in nanophotonics, biosensing, photocatalysis, hot carrier physics, advanced cancer therapies. The localized surface plasmon resonance (LSPR) can be excited in gold or silver nanoparticles or in more complex nanostructures and give rise to a wide range of unique optical properties. It is often critical to be able to localize individual plasmonic nanoparticles and simultaneously measure their spectrum. This is known as hyperspectral microscopy. In this tutorial, we describe and carefully explain how to achieve this goal with an optical microscope equipped with a dark field objective and an optical spectrometer. The images and the scattering spectra of spherical gold nanoparticles of diameters of 90, 70, 50 and 25 nm are recorded. We compare them with the scattering spectra predicted with the Mie formula (LSPR peaks measured at 553, 541, 535 and 534 nm respectively). The optical images are limited by the diffraction and this is discussed in the framework of the Abbe equation. We also describe a strategy to easily correlate the optical images with AFM images of the samples. This allows to precisely relate the morphology of the nanoparticles with their optical images, their color and their optical spectrum. The case of non-spherical nanostructures, namely dimers of nanoparticles is also discussed. This approach allows a relatively low-cost setup and efficient characterization method that will be helpful for teachers who want to introduce their students to the wide topics of plasmonics. This will also be useful for labs seeking an affordable method to investigate the plasmonic properties of single nanostructures.