  Colloidal nanoplatelets - quasi-two-dimensional sheets of semiconductor
exhibiting efficient, spectrally pure fluorescence - form when liquid-phase
syntheses of spherical quantum dots are modified. Despite intense interest in
their properties, the mechanism behind their anisotropic shape and precise
atomic-scale thickness remains unclear, and even counterintuitive when their
crystal structure is isotropic. One commonly accepted explanation is that
nanoclusters nucleate within molecular templates and then fuse. Here, we test
this mechanism for zincblende nanoplatelets and show that they form instead due
to an intrinsic instability in growth kinetics. We synthesize CdSe and
CdS1-xSex nanoplatelets in template- and solvent-free isotropic melts
containing only cadmium carboxylate and chalcogen, a finding incompatible with
previous explanations. Our model, based on theoretical results showing enhanced
growth on narrow surface facets, rationalizes nanoplatelet formation and
experimental dependencies on temperature, time, and carboxylate length. Such
understanding should lead to improved syntheses, controlled growth on surfaces,
and broader libraries of nanoplatelet materials.
