Plamsonic resonance is the collective oscillation of free electron in metal nanoparticles, which gives rise to unique optical properties such as light absorption and local field enhancement. Thanks to the advances in experimental techniques, plasmonic properties can now be precisely controlled by tuning the size, shape, and surrounding environment of the nanoparticles, and thus are rationally adapted for many applications. Our group is interested in how the microscopic structure of a nanoparticle impacts its plasmonic properties from a perspective at the intersection of atomistic electrodynamics and quantum mechanics.
Size effect: plasmonic absorption properties are strongly dependent on the size of the nanoparticles. Small clusters with diameters less than 2 nm exhibit molecule-like discrete absorption peaks. Large nanoparticles ( > 10 nm) have broad absorption bands due to their continuous excited states and high density of states. In the transition region between 2-10 nm, or quantum size regime, the plasmon frequency blue shifts. Accurate description of these unique features in the quantum size regime sets stage for further studies plasmon-enhanced spectroscopies and other plasmon-assisted chemical processes. (Figure 1)
Shape effect: the microscopic structure of the nanoparticles in the quantum size regime determines how the induced near field distributes on the surface and impacts the absorption spectrum, whereas the optical properties of very large nanoparticles are usually determined by plamson lengths. Atomistic representation of the nanoparticles is necessary to fully incorporate the shape effects on the plasmonic properties.
Environment: ligands are commonly used in experiments to stabilize as-synthesized nanoparticles in colloids and can also be used to control the spacing between nanoparticles in assemblies. The ligand molecule modifies the local dielectric properties on the metal surface and consequently the optical properties. One example is the near field being significantly perturbed by the ligand molecules compared with bare nanoparticles (Figure 2).
Selected publications: