Spectral diffusion of single Ag–In–Zn–S quantum dots elucidates the photoluminescence mechanism

Alloyed Ag–In–Zn–S colloidal quantum dots (QDs) have recently emerged as bright fluorophores with properties compatible with various applications. Although the synthetic procedures are well developed and allow achieving near-unity photoluminescence quantum yields, further development of these nanostructures is hindered by poor understanding of the light emission mechanism. In this work, we employ a tool of single particle spectroscopy—studies of spectral diffusion—to elucidate the nature of the luminescent excited state. By analyzing temporal fluctuations and correlations of the photoluminescence intensity, peak position, and linewidth, we show that this state comprises an electron delocalized over the QD volume and a hole localized at a midgap trap state. We thus challenge the view prevailing in the literature that the photoluminescence in alloyed Ag–In–Zn–S QDs occurs via a donor–acceptor pair recombination mechanism. Furthermore, our single dot measurements reveal various contributions to the photoluminescence line broadening.