title: Steric-hindrance-driven shape transition in PbS quantum dots: understanding size-dependent stability.
abstract: Ambient stability of colloidal nanocrystal quantum dots (QDs) is imperative for low-cost, high-efficiency 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. X-ray 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 size-dependent 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.
authors.affiliation: Nanomechanical Systems Research Division, Korea Institute of Machinery and Materials, Daejeon 305-343, Republic of Korea.
authors.forename: Hyekyoung
authors.initials: H
authors.lastname: Choi
authors.forename: Jae-Hyeon
authors.initials: JH
authors.lastname: Ko
authors.forename: Yong-Hyun
authors.initials: YH
authors.lastname: Kim
authors.forename: Sohee
authors.initials: S
authors.lastname: Jeong
mesh_terms: D007854:Lead / Q000737:chemistry*; D010316:Particle Size; D045663:Quantum Dots; D013440:Sulfides / Q000138:chemical synthesis / Q000737:chemistry*; D013499:Surface Properties
chemical_list: D013440:Sulfides; C018391:lead sulfide; D007854:Lead