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[Cd_m(Sn₂S₇)_yK_(6y-2m)]_shell (taking Sn₂S₇^6- ligand as an example). Thiostannates SnS₄^4- and Sn₂S₇^6- retain (distorted) tetrahedral SnS₄ geometry upon binding to NC surface. At the same time, experiments and simulations point to lower stability of Sn₂S₆^4- (and SnS₃^2-) in most solvents and its lower adaptability to the NC surface caused by rigid Sn₂S₂ rings.