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Extended Nucleation and Superfocusing in Colloidal Semiconductor Nanocrystal Synthesis
preprintsubmitted on 16.12.2020, 16:49 and posted on 18.12.2020, 09:19 by P. Tim Prins, Federico Montanarella, Kim Dümbgen, Yolanda Justo, Johanna C. van der Bok, Stijn Hinterding, Jaco Geuchies, Jorick Maes, Kim De Nolf, Sander Deelen, Hans Meijer, Thomas Zinn, Andrei Petukhov, Freddy Rabouw, Celso de Mello Donega, Daniel Vanmaekelbergh, Zeger Hens
The hot injection synthesis of colloidal semiconductor nanocrystals is renowned for producing nanocolloids with superb size dispersions. Burst nucleation and diffusion-controlled size focusing during growth have been invoked to rationalize this characteristic, yet experimental evidence supporting the pertinence of these concepts is scant. Using a well-established CdSe synthesis followed with in situ X-ray scattering, we show that nucleation is an extended event that overlaps with growth and can last for 15-20% of the reaction time. Moreover, we find that size focusing outpaces predictions of diffusion-limited growth. This observation supports the conclusion that nanocrystal growth is dictated by the surface reactivity, which drops sharply for larger nanocrystals. Kinetic reaction simulations demonstrate this so-called superfocusing can lengthen the nucleation period and promote size-focusing. The finding that narrow size dispersions can emerge from the counteracting effects of extended nucleation and reaction-limited size focusing ushers in an evidence-based perspective that turns hot injection into a rational scheme to produce monodisperse nanocolloids.