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the separation in time of a short, burst-like nucleation and a long growth phase. Furthermore, diffusion-limited growth, which makes smaller nanocrystals grow faster than larger nanocrystals, is thought to further focus QD size distributions. Although often invoked and widely accepted, little experimental evidence supports this burst-nucleation/ size focusing description of colloidal QD synthesis.
An apparently straightforward approach to investigating synthesis mechanisms is the monitoring of the concentration, size and size dispersion of QDs during synthesis. In practice, however, quantitative reaction
monitoring relies on calibration curves to derive these characteristics from UV-Vis absorption spectra (Figure 60b). These calibration curves are unknown at the reaction temperature, but even when applied at room temperature, converting QD sizes into QD volumes magnifies calibration errors.
In this work, CdSe QD synthesis was monitored in situ using time-resolved small-angle X-ray diffraction (SAXS) at beamline ID02. A dedicated reaction flask was developed (Figure 61a) in order to fully reproduce typical lab-scale synthesis conditions, such as rapid precursor injection at
Fig. 60: a) Monodisperse batches of CdSe QDs under UV illumination with
sizes increasing from left to right. b) Absorption spectra recorded on
different aliquots of a CdSe QD synthesis. The redshift of the absorption peak
reflects the increasing QD size.
Fig. 61: In-situ time-resolved small-angle X-ray scattering. a) Schematic of the experimental setup. b-c) Representation of average nanocrystal radius (blue), concentration of QDs (orange, green) and the reaction yield for the formation of CdSe QDs (red) as function of
reaction time as extracted from successive SAXS patterns.