Optical Spectroscopy of Single Ag Nanoclusters
Nanometer-sized, noble metal clusters are promising candidates for bright, stable, and nontoxic fluorescent labels in the field of bioimaging. Single-molecule techniques pave the way to study single nanoclusters and their photophysical properties, which often diverge strongly from the ensemble average. This chapter provides an overview related to studies of silver (Ag) nanoclusters on the single-particle level. Single-molecule spectroscopy allows for a direct measurement of transition energies, fluorescence decays, intensities, and fluorescence intermittency phenomena without any ensemble averaging and is therefore the ideal tool to study correlations between size, type of ligands, and the optical properties of Ag nanoclusters. Spectroscopic investigations of single emitters revealed spectral fluctuations and a collective multielectronic nature of excitation, which is known from plasmon resonance effects in novel metal particles. The observation of photon antibunching guaranteed detection of single emitters, while the corresponding emission spectra revealed spectral diffusion processes without any obvious trends.