AtomisticDescription of Thiostannate-Capped CdSeNanocrystals: Retention of Four-Coordinate SnS4 Motif and Preservationof Cd-Rich Stoichiometry

摘要

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 stoichiometricCdSe core and surface Cd adatoms serving as binding sites for terminalS atoms of the thiostannates ligands, leading to all-inorganic (CdSe)core[Cdm(Sn2S7)yK(6y-2m)]shell (taking Sn2S₇^6– ligand as an example). Thiostannates SnS₄^4– and Sn₂S₇^6– retain (distorted) tetrahedral SnS₄ geometry upon bindingto NC surface. At the same time, experiments and simulations pointto lower stability of Sn₂S₆^4– (and SnS₃^2–) in most solvents and itslower adaptability to the NC surface caused by rigid Sn₂S₂ rings.