Controlling Cluster Intermediates Enables the Synthesis of Small PbS Nanocrystals with Narrow Ensemble Line Widths
Philippe B. Green, Pournima Narayanan, Ziqi Li, Philip Sohn, Christian J. Imperiale, Mark W. B. Wilson
Abstract
PbS nanocrystals are critical materials for infrared optoelectronics, but the persistent challenge in synthesizing small nanocrystals with narrow line widths demands improved mechanistic understanding. Here, we show that the conventional hot-injection synthesis of PbS nanocrystals per Hines exhibits two-step kinetics involving an intermediate species. The intermediate is small, lead-rich, and has characteristic, reproducible, visible-wavelength emission—all consistent. with a PbS prenucleation cluster (PNC). We then demonstrate that high-pKa amines disrupt the PNC, accelerating nanocrystal nucleation and enabling the synthesis of PbS nanocrystals with diameters as small as ⌀ ∼ 1.7 nm and distinct ensemble absorption peaks (hν = 2.2 eV, λ = 560 nm) in reactions allowed to run to completion. We show that the basicity of the amine additive controls the average size of nanocrystals at reaction completion, which we understand by incorporating metastable PNCs into reaction models that partition monomers between nanocrystal nucleation and nanocrystal growth. This conceptual advance permits the routine synthesis of ultrasmall PbS NCs with excitonic absorption line widths that are up to 25% narrower than previously reported for comparable sizes (⌀: 1.7–3 nm, λpeak,abs: 560–885 nm, hνpeak,abs: 2.2–1.4 eV). This reduced electronic dispersity will enhance device performance, and the underlying insight is further evidence of the exquisite ability of metal-complexing additives to direct the bottom-up syntheses of nanostructured materials.