Welcome to the nanoscale
When certain materials are shrunk from the macroscale to the nanoscale (1000 times smaller than the width of a human hair), the properties of those materials can change dramatically. A simple example is silver: while we are all familiar with the shiny gray metal at the macroscale, nanoscale silver can achieve an array of colors based on the shape and size of the nanocrystals (see photo above). This behavior is due to the formation of a plasmon resonance, which allows these materials to interact with light quite differently than their macroscale counterparts. Plasmonic nanocrystals are currently used in diverse fields from clean energy to nanomedicine—and are even the basis of the visual response in many COVID-19 rapid tests!
What is a plasmon?
A plasmon is a collective oscillation of surface conduction electrons in materials with a small positive and negative real dielectric constant. Easy, right?
Let's consider a metal sphere with dimensions smaller than the wavelength of light, as drawn above. The surface conduction electrons behave like an electron gas (they aren't bound to their nuclei and are free to move), so when an electric field (shown in red) impinges on the metal, the electrons will respond (the green cloud in the diagram above). Because the metal nanoparticle is small compared to the wavelength of light, the electrons will all move together from their equilibrium position, setting up a dipole at the surface of the particle. There is an attractive force between the electrons on one side of the particle and the nuclei on the other, but when the electrons try to return to their equilibrium position, they overshoot, setting up a collective oscillatory motion. This collective electron oscillation behaves like a quasi-particle, even though it involves the motion of many electrons, and was termed a plasmon by David Pines in 1956 (here is a cool historical perspective). In the framework described here, we are considering a localized surface plasmon because the oscillating electron density wave is confined to the surface of the particle (in other words, it behaves like a standing wave). Moreover, plasmons can involve higher order multipoles (quadrapoles) in addition to the dipole mode shown above.
The wavelength of light that drives the plasmon is known as the localized surface plasmon resonance and will depend on the size, shape, composition, and environment surrounding the nanoparticle. By tuning these properties, we can shift the resonance (and thus the color) of the nanoparticles throughout the visible (and beyond).
For more details and the physics behind this process, check out this review (especially the SI if you like math):
The heterogeneity challenge
The challenge with nanoparticles is that heterogeneity is inherent to their synthesis. Unlike molecules, which have well-defined numbers and spatial arrangements of atoms, nanoparticles have polydispersity in their size, shape, and even surface chemistry. These small differences create a challenge in determining structure-function relationships, especially since identical synthetic protocols can generate different distributions of products. Our lab devises strategies to characterize this heterogeneity and understand its impact on the function and properties of plasmonic materials.