On heterogeneity in fluorescent few-atom silver nanoclusters.

Fluorescent clusters of several Ag atoms are fluorophores with a large set of attractive properties including sub-nanometer size, high fluorescence quantum yield and large Stokes shift. When formed on polymeric scaffolds they exhibit a broad range of heterogeneities in excitation and emission spectra. Addressing this issue we provide detailed spectroscopic studies of these clusters produced by photochemical reduction of silver salt on amphiphilic polystyrene-block-poly(methacrylic acid) block copolymer in N,N-dimethylformamide. Our results allow discriminating three types of emitters differing in positions of their excitation and emission bands. In addition, each band exhibits characteristic features of inhomogeneous broadening, such as strong dependences of excitation spectra on emission wavelength and of emission spectra on excitation wavelength in a mirror-image manner. The latter dependence does not appear on excitation at the 2nd band in the near-UV region. A qualitative model based on inhomogeneous broadening of spectra is suggested to explain these effects.