Publication date: 3rd July 2020
Improved mechanistic understanding of the colloidal synthesis of PbS nanocrystals will guide synthetic progress, and advance their use in near-infrared optoelectronic applications. Here we show that the conventional nucleation and growth of PbS nanocrystals per Hines exhibits two-step kinetics involving an intermediate species [1]. The metastable intermediate is small, lead-rich, and has characteristic, reproducible visible-wavelength emission—all consistent with a PbS pre-nucleation cluster. Growth in the presence of the cluster permits a long size-focusing period and has historically yielded large nanocrystals (⌀ > 4nm, hνpeak,abs<1.05eV, λpeak,abs>1180nm) with the narrowest reported ensemble excitonic absorption peak widths. However, we find that the competing generation of the meta-stable cluster suppresses the nucleation rate of nanocrystals. This ultimately requires that the reaction be quenched when small nanocrystals are desired—short-circuiting size-focussing growth and affecting size-dispersity, yield, and scalability.
We then show that the introduction of Lewis bases to the reaction can selectively disrupt the cluster, offering a new route for synthetic control. By tuning the strength of the interaction through the pKa of amines or the multidentate interaction of glycol ethers, the cluster can be effectively suppress in favor direct nucleation from a molecular intermediate, and permitting the synthesis of PbS nanocrystals as small as (⌀~1.7 nm, hνpeak,abs=2.2eV, λpeak,abs=560 nm) with ensemble linewidths that are up to 25% narrower than previous leading reports.
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