Less Is More. Cation Exchange and the Chemistry of the Nanocrystal Surface

Abstract

We link the extent of Pb for Cd cation exchange reactions in PbS colloidal quantum dots (QDs) to their surface chemistry. Using PbS QDs with either a full or a partial surface coverage by excess Pb, we demonstrate the central role played by vacant cation sites on the QD surface. They facilitate the adsorption of cations from solution, and they act as a source of vacancies needed for the transport of cations through the crystal lattice. This model explains our finding that the cation exchange reaction runs to completion when using a low Cd excess in the exchange bath, while it is impeded by a high Cd excess. Whereas in the latter case, the QD surface is poisoned by surface Cd, the former conditions provide the mixture of surface Cd and vacant surface sites the exchange reaction needs to proceed. This understanding provides a missing link needed to build a unifying mechanistic picture of cation exchange reactions at nanocrystals.

Identifying and Eliminating Emissive Sub‐bandgap States in Thin Films of PbS Nanocrystals

“…Our calculations allow us to investigate the effect of ligands on the electronic structure of off‐stoichiometric (lead‐rich) PbS QDs and provide a fundamental explanation of previous approaches to control trap states with ligand treatments . As we (Table ) and others have shown, as‐synthesized PbS QDs tend to be lead rich. The excess Pb atoms are located on the off‐stoichiometric (111) surface (Figure S6, Supporting Information) and calculations on bare (ligand‐free) QDs show that overall charge balance is maintained by the under‐charged Pb atoms (Figure S7, Supporting Information).…”

Solid‐State Chemistry on the Nanoscale: Ion Transport through Interstitial Sites or Vacancies?

“…ForR eaction (1), the interstitial sites of the PbSe lattice form the lattice sites of the developing CdSe crystal. ForR eaction (2), the exchange has to be performed on the existing lattice sites.T he literature describes the cation exchange by ad iffusion via vacancies,f or the exchange of PbSe with cadmium ions [14,15] as well as for ZnSe with cadmium ions. [16,17] But short diffusion pathways and the resulting reorganization lead to al ow vacancy concentration The Raman band at 300 cm À1 results from the Si substrate.…”

Festkörperchemie auf der Nanoskala: Ionentransport über Zwischengitterplätze oder Leerstellen?

“…[12] Im Fall von Reaktion (2) erfolgt dagegen eine kontinuierliche Verschiebung der Beugungsreflexe und der LO-Frequenzen von ZnSe zu CdSe,was ein klares Indiz für eine Mischkristallbildung mit homogener Durchmischung ist. [13] Die Bildung einer Kern-Schale-Struktur ist auch in den TEM-Aufnahmen und den EDX-Elementverteilungsbildern (Abbildung 2e und f) von sphärischen (10 nm (2) ein Austausch auf den Gitterplätzen erfolgen muss.Die in der Literatur beschriebenen Mechanismen für den Kationenaustausch in Nanokristallen gehen von der Diffusion über Leerstellen aus,sowohl für den Austausch von PbSe mit Cadmiumionen [14,15] als auch von Abbildung 2. Vergleich der PbSe-und ZnSe-Partikel.…”