We use 1H and 31P solution nuclear magnetic resonance spectroscopy to analyze the binding of phosphonic acid ligands to wurtzite CdSe quantum dots (CdSe QDs). CdSe QDs synthesized with phosphonic acids as a surfactant have a ligand shell composed of phosphonic acid and phosphonic acid anhydride moieties. Titrations of as-synthesized QDs with excess oleic acid do not induce desorption of phosphonic species, whereas titration of oleic-acid-exchanged QDs with excess phosphonic acid shows that the latter quantitatively replaces the oleic acid with a 1:1 stoichiometry. Both the stoichiometry of the oleic acid/phosponic acid exchange interaction and the ratio between the Cd surface excess and the ligand density indicate that phosponic acids bind as hydrogen phosphonates to the CdSe surface.

Inductively coupled plasma mass spectrometry (ICP-MS) was combined with UV-vis absorption spectroscopy and transmission electron microscopy to determine the size, composition, and intrinsic absorption coefficient μ of 4 to 11 nm sized colloidal CsPbBr nanocrystals (NCs). The ICP-MS measurements demonstrate the nonstoichiometric nature of the NCs, with a systematic excess of lead for all samples studied. Rutherford backscattering measurements indicate that this enrichment in lead concurs with a relative increase in the bromide content. At high photon energies, μ is independent of the nanocrystal size. This allows the nanocrystal concentration in CsPbBr nanocolloids to be readily obtained by a combination of absorption spectroscopy and the CsPbBr sizing curve.

Colloidal quantum dots (QDs) made from In-based III–V semiconductors are emerging as a printable infrared material. However, the formulation of infrared inks and the formation of electrically conductive QD coatings is hampered by a limited understanding of the surface chemistry of In-based QDs. In this work, we present a case study on the surface termination of IR active III–V QDs absorbing at 1220 nm that were synthesized by reducing a mixture of indium halides and an aminoarsine by an aminophosphine in oleylamine. We find that this recently established synthesis method yields In­(As,P) QDs with minor phosphorus admixing and a surface terminated by a mixture of oleylamine and chloride. Exposing these QDs to protic surface-active compounds RXH, such as fatty acids or alkanethiols, initiates a ligand exchange reaction involving the binding of the conjugate base RX– and the desorption of 1 equiv of alkylammonium chloride. Using density functional theory simulations, we confirm that the formation of the alkylammonium chloride salt can provide the energy needed to drive such acid/base mediated ligand exchange reactions, even for weak organic acids such as alkanethiols. We conclude that the unique surface termination of In­(As,P) QDs, consisting of a mixture of L-type and X-type ligands and acid/base mediated ligand exchange, can form a general model for In-based III–V QDs synthesized using indium halides and aminopnictogens.

We analyze the surface chemistry of CuInS 2 nanocrystals synthesized in the presence of amines. Using solution NMR spectroscopy and elemental analysis, we come to the conclusion that as-synthesized CuInS 2 nanocrystals have charge neutral inorganic cores and are stabilized by a layer of tightly bound L-type amines. In situ NMR heating-up experiments show that desorption of amines can be induced by increasing the temperature, which makes the partial exchange of amines for thiols possible. On the other hand, we find that carboxylic acids are unable to bind as L-type ligands to the CuInS 2 surface. In addition, we demonstrate that the use of technical oleylamine in the synthesis of CuInS 2 nanocrystals leads to nonstoichiometric nanocrystals which have, next to oleylamine ligands, also X-type impurities on the surface that can be exchanged for carboxylic acids.

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.

We analyze the optical properties of CdTe quantum dots, including the sizing curve, the absorption coefficient, and the oscillator strength of the band gap transition, by combining absorption spectroscopy, elemental analysis, and electron microscopy imaging. At short wavelengths, the absorption coefficient spectrum is still affected by quantum confinement, yet a largely constant value, close to that of bulk CdTe, is found at around 410 nm. At shorter wavelengths, remaining quantum confinement effects on the CdTe E 1 transition are present even for the largest quantum dots studied (11 nm). For the band gap transition, we find an integrated absorption coefficient μ gap that scales almost proportionally to the inverse of the quantum dot volume. Especially for the smaller diameters, deviations up to a factor of 3 are found as compared to widely used literature values. The corresponding oscillator strength f gap is almost size-independent in the diameter range 3−7 nm. The correspondence between radiative lifetimes predicted based on f gap and literature values is discussed.

Although wurtzite cadmium selenide quantum dots (wz-CdSe QDs) are one of the best explored colloidal nanomaterials, no detailed investigation of the optical properties of zincblende cadmium selenide quantum dots (zb-CdSe QDs) has been performed until now. Typically, it is assumed that this material shows the same behavior as the wurtzite modification. To investigate this, we present a study on the optical properties of zb-CdSe QDs, yielding the electronic band gap to size relation (sizing curve), the extinction coefficient at short wavelengths, and the oscillator strength of the band gap transition. Comparing these results with literature data on wz-CdSe QDs we observe, despite a deviation of the sizing curve for diameters above 4 nm, a similar extinction coefficient at short wavelengths and a similar oscillator strength.

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