Steric-Hindrance-Driven Shape Transition in PbS Quantum Dots: Understanding Size-Dependent Stability

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.