Radiative and nonradiative properties of single plasmonic nanoparticles and their assemblies

A surface plasmon is the coherent oscillation of the conduction band electrons. When a metal nanoparticle is excited to produce surface plasmons, incident light is both scattered and absorbed, giving rise to brilliant colors. One available technique for measuring these processes, ensemble extinction spectroscopy, only measures the sum of scattering and absorption. Although the spectral responses of these processes are closely related, their relative efficiencies can differ significantly as a function of nanoparticle size and shape. For some applications, researchers may need techniques that can quantitatively measure absorption or scattering alone. Through advances in single particle spectroscopy, researchers can overcome this problem, separately determining the radiative (elastic and inelastic scattering) and nonradiative (absorption) properties of surface plasmons. Furthermore, because we can use the same sample preparation for both single particle spectroscopy measurements and electron microscopy, this technique provides detailed structural information and a direct correlation between optical properties and nanostructure morphology. In this Account, we present our quantitative investigations of both radiative (scattering and one-photon luminescence) and nonradiative (absorption) properties of the same individual plasmonic nanostructures employing different single particle spectroscopy techniques. In particular, we have used a combined setup to study the same structure with dark-field scattering spectroscopy, photothermal heterodyne imaging, confocal luminescence microscopy, and scanning electron microscopy. While Mie theory thoroughly describes the overall size dependence of scattering and absorption for nanospheres, our real samples deviate significantly from the predicted trend: their particle shape is not perfectly spherical, especially when supported on a substrate. Because of the high excitation rate in laser based single particle measurements, we can efficiently detect one-photon luminescence despite a low quantum yield. For gold nanoparticles, the luminescence spectrum follows the scattering response, and therefore we assigned it to the emission of a plasmon. Due to strong near-field interactions the plasmonic response of closely spaced nanoparticles deviates significantly from that of the constituent nanoparticles. This response arises from coupled surface plasmon modes that combine those of the individual nanoparticles. Our correlated structural and optical imaging strategy is especially powerful for understanding these collective modes and their dependence on the assembly geometry.