Utilizing Self-Exchange To Address the Binding of Carboxylic Acid Ligands to CdSe Quantum Dots

Abstract

We use solution NMR techniques to analyze the organic/inorganic interface of CdSe quantum dots (Q-CdSe) synthesized using oleic acid as a surfactant. It is shown that the resulting Q-CdSe are stabilized by tightly bound oleic acid species that only exchange upon addition of free oleic acid. The NMR analysis points toward a two-step exchange mechanism where free ligands are initially physisorbed within the ligand shell to end up as bound, chemisorbed ligands in a second step. Importantly, we find that every ligand is involved in this exchange process. By addition of oleic acid with a deuterated carboxyl headgroup, we demonstrate that the bound ligands are oleate ions and not oleic acid molecules. This explains why a dynamic adsorption/desorption equilibrium only occurs in the presence of excess free oleic acid, which donates the required proton. Comparing the number of oleate ligands to the excess cadmium per CdSe quantum dot, we find a ratio of 2:1. This completes the picture of Q-CdSe as organic/inorganic entities where the surface excess of Cd(2+) is balanced by a double amount of oleate ligands, yielding overall neutral nanoparticles.

Mechanistic Study of the Formation of Bright White Light-Emitting Ultrasmall CdSe Nanocrystals: Role of Phosphine Free Selenium Precursors

“…It is known that synthesis of QDs in the presence of anionic X-type ligands, e.g., the benzoate ion, results in surface metal cation rich nanocrystals to maintain the overall charge neutrality of ligand-coated QDs. [44][45][46][47][48][49] We believe that the existence of a similar atomic structure at the surface of our ultrasmall nanocrystals influenced the peak sharpness for 1S(e)-2S3/2(h) transition, as was recently demonstrated for CdSe QDs. 44 The ~8 nm red shift in the broad emission band of our nanocrystals also indicates that their surface was composed of different ligands, which influenced exciton delocalization as will be discussed later.…”

“…35 Our experimental data nicely corroborate literature results which suggest that the presence of excess metal ions on the surface of the QDs increases the PLQY. 46,77 In order to demonstrate the importance of surface ligand chemistry we now show that the presence of an excess Cd(O2CPh)2 layer on the CdSe nanocrystal surface can increase the PLQY.…”

Surface Chemistry of Colloidal Semiconductor Nanocrystals: Organic, Inorganic, and Hybrid

“…The figure indicates that the resonances of OA all have a diffusion coefficient of 9.4(1) × 10 −11 m 2 s −1 , almost 1 order of magnitude smaller than the value expected for free OA [61]. For spherical particles, D can be linked to a hydrodynamic diameter d H using the Stokes−Einstein relation (k B , Boltzmann constant; T, absolute temperature; , solvent viscosity):…”

“…These aliphatic carboxylic acids, such as oleic acid (OA) and stearic acid, can be added to the reaction either as the free acid or as a metal oleate or stearate. For example, Fritzinger et al demonstrated by solution 1 H NMR spectroscopy that CdSe NCs synthesized by the reaction of Cd(oleate) 2 with elemental selenium in non-coordinating octadecene were stabilized with oleate ligands [21]. When these oleate-stabilized CdSe NCs are dispersed in a nonpolar solvent in the absence of excess ligand, the equilibrium between the free and bound ligand states lies very strongly toward the bound state.…”

“…Typical 1 H NMR spectra of CdSe [61] and PbSe [62] NCs or QDs synthesized in the presence of OA are represented in Figures 7.7b and 7.7c, respectively. Apart from a number of sharp resonances in the PbSe NC spectrum-attributed to remainders of the synthesis and a deliberately added concentration standard-both spectra appear very similar.…”