<Abstrate>

Size and shape tunability and low-cost solution processability make colloidal

lead chalcogenide quantum dots (QDs) an emerging class of building blocks

for innovative photovoltaic, thermoelectric and optoelectronic devices. Lead

chalcogenide QDs are known to crystallize in the rock-salt structure, although

with very different atomic order and stoichiometry in the core and surface

regions; however, there exists no convincing prior identification of how extreme

downsizing and surface-induced ligand effects influence structural distortion.

Using forefront X-ray scattering techniques and density functional theory

calculations, here we have identified that, at sizes below 8 nm, PbS and PbSe

QDs undergo a lattice distortion with displacement of the Pb sublattice, driven

by ligand-induced tensile strain. The resulting permanent electric dipoles may

have implications on the oriented attachment of these QDs. Evidence is found

for a Pb-deficient core and, in the as-synthesized QDs, for a rhombic dodecahedral

shape with nonpolar {110} facets. On varying the nature of the surface ligands,

differences in lattice strains are found.

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