posted on 2021-01-20, 15:16authored byAniket
S. Mule, Sergio Mazzotti, Aurelio A. Rossinelli, Marianne Aellen, P. Tim Prins, Johanna C. van der Bok, Simon F. Solari, Yannik M. Glauser, Priyank V. Kumar, Andreas Riedinger, David J. Norris
Magic-sized
clusters (MSCs) of semiconductor are typically defined
as specific molecular-scale arrangements of atoms that exhibit enhanced
stability. They often grow in discrete jumps, creating a series of
crystallites, without the appearance of intermediate sizes. However,
despite their long history, the mechanism behind their special stability
and growth remains poorly understood. It is particularly difficult
to explain experiments that have shown discrete evolution of MSCs
to larger sizes well beyond the “cluster” regime and
into the size range of colloidal quantum dots. Here, we study the
growth of MSCs, including these larger magic-sized CdSe nanocrystals,
to unravel the underlying growth mechanism. We first introduce a synthetic
protocol that yields a series of nine magic-sized nanocrystals of
increasing size. By investigating these crystallites, we obtain important
clues about the mechanism. We then develop a microscopic model that
uses classical nucleation theory to determine kinetic barriers and
simulate the growth. We show that magic-sized nanocrystals are consistent
with a series of zinc-blende crystallites that grow layer by layer
under surface-reaction-limited conditions. They have a tetrahedral
shape, which is preserved when a monolayer is added to any of its
four identical facets, leading to a series of discrete nanocrystals
with special stability. Our analysis also identifies strong similarities
with the growth of semiconductor nanoplatelets, which we then exploit
to further increase the size range of our magic-sized nanocrystals.
Although we focus here on CdSe, these results reveal a fundamental
growth mechanism that can provide a different approach to nearly monodisperse
nanocrystals.