C O M P L E X S Y S T E M S A N D B I O M E D I C A L S C I E N C E S

S C I E N T I F I C H I G H L I G H T S

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Monitoring the formation and growth of nanocrystals

The nucleation and growth of colloidal quantum dots were probed in situ using time-resolved X-ray scattering experiments. It was found that the nucleation phase extends much longer than previously assumed and that diffusion in the bulk does not limit growth. The findings starkly contrast with widely accepted models of nucleation and growth and are an important step toward an evidence-based understanding of nanocrystal formation mechanisms.

The formation of particles from a supersaturated solution is a ubiquitous process that occurs in a wide variety of fields and industrial domains. It can be decomposed into two elementary steps: the nucleation in which new particles are created, and their growth to reach their final size. Obtaining a final size distribution with the lowest polydispersity is particularly important in nanoparticle synthesis since their properties are highly dependent on their size. For example, quantum dots (colloidal semiconductor nanocrystals) have outstanding optical properties such as intense and narrow fluorescence, which can be used in luminescent displays, solid-state lighting, biological imaging and photodetectors. Their emission wavelength depends on the nanocrystal size, and the emission spectral linewidth is broadened by the polydispersity. Hence, it is highly desirable to obtain nanocrystals with the lowest possible polydispersity. It is commonly accepted that narrow polydispersities are caused by two phenomena: a very short burst of nucleation, such that nanoparticles all appear at the same time, and, prior to their growth, size distribution focusing caused by diffusion of monomers within the bulk

of the solution to the nanocrystal surface [1]. However, this mechanism had never received rigorous experimental verification.

The time-resolved ultrasmall-angle X-ray scattering (TRUSAXS) beamline ID02 was used to probe the formation mechanism of PbS nanocrystals in situ using small- and wide-angle X-ray scattering (SAXS/WAXS). The synthesis involved a novel class of sulfur precursors (substituted thioureas [2]) whose reactivity with lead oleate can be adjusted to afford very monodisperse nanocrystals of 2-7 nm in size over a range of temperatures (90-150°C) (Figure 43). A dedicated setup involving a peristaltic pump enabled the acquisition of SAXS and WAXS patterns during the reaction with a high time resolution (> 1 per second). Fitting the SAXS patterns with a model of polydisperse spheres provided the mean radius of the size distribution, its polydispersity and the nanocrystal concentration at each time point. WAXS data indicated that the PbS was crystalline rather than amorphous throughout the reaction (Figure 44).

Interestingly, the concentration of nanocrystals increased over 200 seconds, concurrent with more than 40% of the total growth time, and then plateaued. This slow nucleation contrasts with the LaMer model described above in the sense that the nucleation phase is slow and concurrent with the growth and yet the final polydispersity is very low. To understand this, the solute production rate was measured ex situ using 13C nuclear magnetic resonance (NMR) spectroscopy for the three different thioureas. These data and the evolution of the nanocrystal concentration were fit using a population balance model that predicts the evolution of size and polydispersity with only two unknown parameters. This approach allows the growth law to be evaluated. Size-dependent growth kinetics (1/rx)

Fig. 43: Thiourea derivatives control the reaction kinetics of PbS formation and the final nanocrystal size.