Ligand-Controlled Growth
of ZnSe Quantum Dots in Water
during Ostwald Ripening
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Abstract
A strong ligand effect was observed for the aqueous-phase
growth
of ZnSe quantum dots (QDs) in the Ostwald ripening (OR) stage. The
QDs were made by injecting Se monomer at room temperature followed
by a ramp to 100 °C. The ramp produced a second, more gradual
increase in the concentrations of both Zn and Se monomers fed by the
dissolution of QDs below the critical size. The dissolution process
was followed using measurements of the mass of Zn in QDs and in the
supernatant by inductively coupled plasma optical emission spectroscopy
(ICP-OES). Despite the flux of monomers, there was little growth in
the QDs of average size based on UV–vis absorption spectra,
until the temperature reached 100 °C, when there was a period
of rapid growth followed by a period of linear growth. The linear
growth stage is the result of OR as the total mass of Zn in QDs and
in the solvent remained constant. The growth data were fit to a continuum
model for the limiting case of surface reaction control. The rate
is proportional to the equilibrium coefficient for ligand detachment
from the QD surface. The ligand 3-mercaptopropionic acid (MPA) was
the most tightly bound to the surface and produced the lowest growth
rate of (1.5–2) × 10<sup>–3</sup> nm/min in the
OR stage, whereas thiolactic acid (TLA) was the most labile and produced
the highest growth rate of 3 × 10<sup>–3</sup> nm/min.
Methyl
thioglycolate (MTG) and thioglycolic acid (TGA) produced rates in
between these values. Ligands containing electron-withdrawing groups
closer to the S atom and branching promote growth, whereas longer,
possibly bidendate, ligands retard it. Mixed ligand experiments confirmed
that growth is determined by ligand bonding strength to the QD. Photoluminescence
spectroscopy showed that the more labile the ligand, the more facile
the repair of surface defects during the exposure of the QDs to room
light