Fig. 1. (Color online) (a) The XRD pattern of the NCs with different feeding ratios of Cl and S. (b) The (111) and (200) peak shift vs. feeding ratio of Cl and S. (c) and (d) The crystal structure of PbS and Pb₃S₂Cl₂.

Fig. 2. (Color online) (a)−(c) High-resolution XPS spectrum in the (a) Cl 2p, (b) Pb 4f, (c) S 2p regions of the NCs with different feeding ratios of Cl and S. (d) The element ratios of the PbS and Pb₃S₂Cl₂ NCs obtained by XPS and SEM-EDS. (e) Cl contents measured by SEM-EDS and XPS the NCs with different feeding ratios of Cl and S. (f) Plot of the ratio between Cl concentrations determined by SEM-EDS and XPS measurements in (e).

Fig. 3. (Color online) (a) and (b) The absorbance (a) and PL (b) of the NCs with different feeding ratios of Cl and S. (c) The peak position of absorbance and PL, as well as Stokes shift. (d) The images of the NC solutions. (e)−(g) The PLQY (e) and TRPL (f) and (g) of the NCs with different feeding ratios of Cl and S.

Fig. 4. (Color online) Time-resolved emission spectroscopy (TRES) of PbS NCs. (a) and (b) TRES map of control and shelled. (c) and (d) Time-slices of the emission spectra of control and shelled. (e) Summary of dynamic red-shifts of the NC photoluminescence peak after excitation for control and shelled PbS NCs. All experiments are conducted in dilute (1 mg/mL) hexane solution.

Fig. 5. (Color online) (a) The absorbance of Pb₃S₂Cl₂, Pb₃S₂Cl₂, and Pb₃S₂I₂ (The photo of Pb₃S₂X₂ solution is shown in the inset). (b) The XRD pattern of Pb₃S₂Cl₂, Pb₃S₂Cl₂, and Pb₃S₂I₂ NCs. The peak marked with an asterisk represents unreacted PbS phase. (c)−(e) The TEM images of Pb₃S₂Cl₂, Pb₃S₂Cl₂, and Pb₃S₂I₂ NCs.