Atomistic Description of Thiostannate-Capped CdSe Nanocrystals: Retention of Four-Coordinate SnS4 Motif and Preservation of Cd-Rich Stoichiometry

Colloidal semiconductor nanocrystals
(NCs) are widely studied as
building blocks for novel solid-state materials. Inorganic surface
functionalization, used to displace native organic capping ligands
from NC surfaces, has been a major enabler of electronic solid-state
devices based on colloidal NCs. At the same time, very little is known
about the atomistic details of the organic-to-inorganic ligand exchange
and binding motifs at the NC surface, severely limiting further progress
in designing all-inorganic NCs and NC solids. Taking thiostannates
(K4SnS4, K4Sn2S6, K6Sn2S7) as typical examples of
chalcogenidometallate ligands and oleate-capped CdSe NCs as a model
NC system, in this study we address these questions through the combined
application of solution 1H NMR spectroscopy, solution and
solid-state 119Sn NMR spectroscopy, far-infrared and X-ray
absorption spectroscopies, elemental analysis, and by DFT modeling.
We show that through the X-type oleate-to-thiostannate ligand exchange,
CdSe NCs retain their Cd-rich stoichiometry, with a stoichiometric
CdSe core and surface Cd adatoms serving as binding sites for terminal
S atoms of the thiostannates ligands, leading to all-inorganic (CdSe)core[Cdm(Sn2S7)yK(6y‑2m)]shell (taking Sn2S76– ligand as an example). Thiostannates SnS44– and Sn2S76– retain (distorted) tetrahedral SnS4 geometry upon binding
to NC surface. At the same time, experiments and simulations point
to lower stability of Sn2S64– (and SnS32–) in most solvents and its
lower adaptability to the NC surface caused by rigid Sn2S2 rings.