quantum dots (QDs) is imperative for low-cost, highefficiency QD photovoltaics. We synthesized air-stable, ultrasmall PbS QDs with diameter (D) down to 1.5 nm, and found an abrupt transition at D ≈ 4 nm in the air stability as the QD size was varied from 1.5 to 7.5 nm. Xray photoemission spectroscopy measurements and density functional theory calculations reveal that the stability transition is closely associated with the shape
transition of oleate-capped QDs from octahedron to
cuboctahedron, driven by steric hindrance and thus sizedependent surface energy of oleate-passivated Pb-rich QD facets. This microscopic understanding of the surface chemistry on ultrasmall QDs, up to a few nanometers, should be very useful for precisely and accurately controlling physicochemical properties of colloidal QDs such as doping polarity, carrier mobility, air stability, and hot-carrier dynamics for solar cell applications.