Fluorescence origin and spectral broadening mechanism in atomically precise Au8 nanoclusters

Super-small Au8 nanoclusters have shown great potential to be used in bioimaging, biosensors and catalysis. Understanding the fluorescence origin and the spectral broadening mechanism is of critical importance for the applications. Here we investigate the fluorescence origin and the spectral broadening mechanism using steady state and ultrafast time-resolved spectroscopy. For the first time we clearly elucidate the broad fluorescence of Au8 nanoclusters consisting of an intrinsic band from the Au8 core and an extrinsic band from the surface fluorophores. The emission energy of the intrinsic band is in accord with the rule of E/N(1/3) and the spectral broadening originates from ultrafast dephasing due to effective electron-electron scattering. The extrinsic band has a much larger bandwidth due to massive surface fluorophores; it is the dominant mechanism for spectral broadening in Au8. In contrast to the jellium model predictions, the overall fluorescence exhibits excitation wavelength dependence.