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elevated temperature. With this cell, quantitative SAXS pattern series were recorded with a time resolution that gradually shifted from 100 ms at the beginning to 100 s at the end of the experiment. Assuming a Gaussian distribution of QD radii, the SAXS patterns were analysed to obtain the time evolution of the QD size, size dispersion and concentration in the reaction mixture (Figure 61b-c).
A striking outcome of this study was the surprisingly long nucleation time, which lasts for ~15% of the reaction time (Figure 61c). In addition, the reduction of the standard deviation of the particle distribution (Figure 61b) during growth gave the first direct evidence that size focusing effectively occurs during QD synthesis. However, despite such clear observations, directly linking evolutions of size, size dispersion and concentration to underlying mechanisms is not straightforward. Therefore, the in-situ SAXS data were used to calibrate quantitative, ex-situ reaction monitoring. Through this approach, more complex synthetic schemes were studied, in which bimodal size
distributions were formed and analysed during growth. What this analysis showed is that the absolute size focusing seen in in-situ SAXS is stronger than expected for diffusion- limited growth. Such an outcome indicates that QD growth concurs with a steep drop in surface reactivity for larger sizes. The resulting superfocusing is what counteracts the size broadening induced by the extended nucleation period to obtain the narrow size dispersions well known for QD synthesis.
The development of the QD size, size dispersion and concentration was probed in situ using SAXS while replicating actual synthesis conditions. In combination with ex-situ experiments calibrated using in-situ SAXS monitoring it was found that monodisperse QD nanocolloids form by superfocusing counteracting long nucleation times. Underscoring the role of size-dependent surface reactivity creates a new perspective on QD synthesis that can be further studied using the in-situ monitoring method established.
PRINCIPAL PUBLICATION AND AUTHORS
Extended Nucleation and Superfocusing in Colloidal Semiconductor Nanocrystal Synthesis, P.T. Prins (a), F. Montanarella (a), K. Dümbgen (b), Y. Justo (b), J.C. van der Bok (a), S.O.M. Hinterding (a), J.J. Geuchies (a), J. Maes (b), K. De Nolf (b), S. Deelen (c), H. Meijer (c), T. Zinn (d), A.V. Petukhov (a,e), F.T. Rabouw (a), C. De Mello Donega (a), D. Vanmaekelbergh (a), Z. Hens (b), Nano Lett. 21(6), 2487-2496 (2021); https:/doi.org/10.1021/acs.nanolett.0c04813 (a) Debye Institute for Nanomaterials Science, Utrecht University, Utrecht (Netherlands) (b) Physics and Chemistry of Nanostructures, Ghent University, Ghent (Belgium) (c) Scientific Instrumentation, Faculty of Science, Utrecht University, Utrecht (The Netherlands) (d) ESRF (e) Laboratory of Physical Chemistry, Eindhoven University of Technology, Eindhoven (The Netherlands
