Ligand-induced symmetry breaking, size and morphology in colloidal lead sulfide QDs: from classic to thiourea precursors

Colloidal lead chalcogenide quantum dots (CQDs) exhibit promising optoelectronic properties for applications in solar cell devices and as thermoelectrics. Herein, we report and discuss a ferroelectric structural distortion, at the picometer scale resolution, in PbS CQDs prepared using both classic and new synthetic pathways. The investigation was performed using synchrotron X-ray total scattering data and advanced methods of analysis that rely on a homo-core-shell model and evaluate the atomic arrangement, stoichiometry, size and morphology of nanocrystals. The CQDs show comparable size-dependent relative elongation, up to 0.7 % of one body diagonal of the cubic rock-salt structure, which corresponds to a rhombohedral lattice deformation. The findings suggest a joint role for the oleate ligands (which induce surface tensile strain) and the Pb(II) lone pair as the driving forces of the deformation. Pb displacements along the [111] direction, which provoke a ferrolectric distortion related to the lattice change, fall in the 0.0 – 0.1 Å range. Overall, the findings suggest the local nature of the metal off-centering, leading to different average displacements which depend on the synthetic conditions.