Abstract
Synthetic studies of colloidal nanoparticles that
crystallize in metastable structures represent an emerging area of interest in
the development of novel functional materials, as metastable nanomaterials may
exhibit unique properties when compared to their counterparts that crystallize
in thermodynamically preferred structures. Herein, we demonstrate how phase
control of colloidal AgInSe2 nanocrystals can be achieved by
performing reactions in the presence, or absence, of 1-dodecanethiol. The thiol
plays a crucial role in formation of metastable AgInSe2
nanocrystals, as it mediates an in-situ topotactic cation exchange from an
orthorhombic Ag2Se intermediate to a metastable orthorhombic phase
of AgInSe2. We provide a detailed mechanistic description of this cation
exchange process to structurally elucidate how the orthorhombic phase of AgInSe2
forms. Density functional theory calculations suggest that the metastable
orthorhombic phase of AgInSe2 is metastable by a small margin, at 10
meV/atom above the thermodynamic ground state. In the absence of 1-dodecanethiol,
a mixture of Ag2Se nanocrystal intermediates form that convert
through kinetically slow, non-topotactic exchange processes to yield the
thermodynamically preferred chalcopyrite structure of AgInSe2. Finally,
we offer new insight into the prediction of novel metastable multinary
nanocrystal phases that do not exist on bulk phase diagrams.