Figure 1: Optical properties and material characterization of Bi-doped PbS QDs.

(a) Measured from ICP-OES Bi/Pb atomic ratio of QDs as a function of nominal precursor ratio. Different symbols indicate different sets of samples. Line represents a hypothetical 1:1 relationship between measured and precursor ratios. (b) TEM image of Bi:PbS (Bi/Pb=3.2%) QDs (10 nm scale bar). (c) Typical optical absorption and photoluminescence spectra of reference PbS and Bi:PbS QDs at 5 g l⁻¹ in toluene (all have a size of 3.5±0.3 nm as measured from TEM images). (d) Measured shifts of excitonic absorption and photoluminescence peaks and quenching of the later of Bi:PbS with respect to reference PbS QDs, as a function measured from ICP-OES Bi/Pb ratios (average ratios where applicable). Lines in the graph are guides to the eye. (e) XPS spectral regions of Bi4f and S2p for PbS and Bi:PbS QDs (precursor Bi/Pb=3.2%) and (f) the XPS spectral region of S2s of the same samples as in Fig. 1e as well as Bi₂S₃ nanocrystals²⁸, demonstrating the absence of a distinct bismuth sulphide feature for the case of Bi:PbS QDs.