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 (K<inf>4</inf>SnS<inf>4</inf>, K<inf>4</inf>Sn<inf>2</inf>S<inf>6</inf>, K<inf>6</inf>Sn<inf>2</inf>S<inf>7</inf>) 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)<inf>core</inf>[Cd<inf>m</inf>(Sn<inf>2</inf>S<inf>7</inf>)<inf>y</inf>K<inf>(6y-2m)</inf>]<inf>shell</inf> (taking Sn<inf>2</inf>S<inf>7</inf>6- ligand as an example). Thiostannates SnS<inf>4</inf>4- and Sn<inf>2</inf>S<inf>7</inf>6- retain (distorted) tetrahedral SnS<inf>4</inf> geometry upon binding to NC surface. At the same time, experiments and simulations point to lower stability of Sn<inf>2</inf>S<inf>6</inf>4- (and SnS<inf>3</inf>2-) in most solvents and its lower adaptability to the NC surface caused by rigid Sn<inf>2</inf>S<inf>2</inf> rings. © 2015 American Chemical Society.

Published in:
Journal of the American Chemical Society, 137, 5, 1862-1874

 Record created 2015-05-27, last modified 2018-09-13

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