Electronic Structure Origins of Surface-Dependent Growth in III–V Quantum Dots

Indium
phosphide quantum dots (QDs) have emerged as a promising candidate
to replace more toxic II–VI CdSe QDs, but production of high-quality
III–V InP QDs with targeted properties requires a better understanding
of their growth. We develop a first-principles-derived model that
unifies InP QD formation from isolated precursor and early stage cluster
reactions to 1.3 nm magic sized clusters and rationalize experimentally
observed properties of full sized >3 nm QDs. Our first-principles
study on realistic QD models reveals large surface-dependent reactivity
for all elementary growth process steps, including In-ligand bond
cleavage and P precursor addition. These thermodynamic trends correlate
well to kinetic properties at all stages of growth, indicating the
presence of labile and stable spots on cluster and QD surfaces. Correlation
of electronic or geometric properties to energetics identifies surprising
sources for these variations: short In···In separation
on the surface produces the most reactive sites, at odds with conventional
understanding of strain (i.e., separation) in bulk metallic surfaces
increasing reactivity and models for ionic II–VI QD growth.
These differences are rationalized by the covalent, directional nature
of bonding in III–V QDs and explained by bond order metrics
derived directly from the In–O bond density. The unique constraints
of carboxylate and P precursor bonding to In atoms rationalize why
all sizes of InP clusters and QDs are In-rich but become less so as
QDs mature. These observations support the development of alternate
growth recipes that take into account strong surface-dependence of
kinetics as well as the shapes of both In and P precursors to control
both kinetics and surface morphology in III–V QDs.