  Due to the spatial confinement, two-dimensional metal chalcogenides display
an extraordinary optical re-sponse and carrier transport ability.
Solution-based synthesis techniques such as colloidal hot injection and ion
exchange provide a cost-effective way to fabricate such low-dimensional
semiconducting nanocrystals. Over the years, developments in colloidal
chemistry made it possible to synthesize various kinds of ultrathin colloidal
nanoplatelets, including wurtzite- and zinc blende-type CdSe, rocksalt PbS,
black phosphorus-like SnX (X=S or Se), hexagonal copper sulfides, selenides and
even transition metal dichalcogenides (TMD) like MoS2. By altering experimental
conditions and applying capping ligands with specific functional groups, it is
possible to accurately tune the dimensionality, geometry and consequently the
optical prop-erties of these colloidal metal chalcogenide crystals. Here, we
review recent progresses in the syntheses of two-dimensional colloidal metal
chalcogenides (CMCs) and property characterizations based on optical
spectroscopy or device-related meas-urements. The discoveries shine a light on
their huge prospect for applications in areas such as photovoltaics,
optoelectronics and spintronics. In specific, the formation mechanisms of
two-dimensional CMCs are discussed. The growth of colloidal nano-crystals into
a two-dimensional shape is found to require either an intrinsic structural
asymmetry or the assist of coexisted ligand molecules, which act as lamellar
double-layer templates or "facet" the crystals via selective adsorption. By
performing optical characterizations and especially ultrafast spectroscopic
measurements on these two-dimensional CMCs, their unique electronic and
excitonic features are revealed.
