Efficient Addition of Desired Carboxylate Ligands to CdSe Quantum Dots Passivated with Phosphonic Acids

Abstract

Phosphonic acids bind strongly to quantum dots (QDs), improving stability. However, it is difficult to introduce desired ligands to QDs passivated with phosphonic acids via ligand exchange. Here, we have investigated the efficient addition of X-type ligands to wurtzite CdSe QDs passivated with phosphonic acids using Z-type ligands. Oleic acid (OA) and Cd­(OA)2 were used as examples of the desired X-type and Z-type ligands, respectively, and octadecylphosphonic acid (ODPA) was used as a passivating ligand for CdSe QDs. OA ligands were not introduced at all into the QDs when OA itself was used as the X-type ligand. However, after Z-type ligand addition, the added OA ligand accounted for 35% of the total ligands and only 20% of ODPA was dissociated from QDs. In addition, photoluminescence (PL) and its lifetime increased 6.1 and 2.8 times, respectively, indicating that Z-type ligands bound to trap sites to increase PL. Thus, we suggest that a sufficient amount of the desired ligand can be introduced, enhancing the quantum yields of CdSe QDs substantially. Our approach can be applied to various applications requiring energy input/output control and exciton lifetime extension in QDs.

Binding Affinity of Monoalkyl Phosphinic Acid Ligands toward Nanocrystal Surfaces

“…It is interesting to look at the quantification of all free and bound species during the titration (Figure C,F). The amounts of free (released) phosphinic acid and the bound phosphonate are equal (within error) over the course of the titration, indicating again a one-for-one exchange, which is consistent with the previous literature on phosphonate ligand binding. ,,− There is an exception for smaller clusters since a two-for-one exchange was observed on InP clusters . Interestingly, our exchange reaction does not go to completion, and the bound phosphinate saturates at 40 and 30% for HfO 2 and CdSe, respectively.…”

“…Not surprisingly, an equilibrium is observed for different ligands with the same binding group, for example, carboxylate ligands are in equilibrium with most other carboxylic acids. − Interestingly, the driving force for X-type ligand exchange in nonpolar solvents is often the proton transfer from the incoming proligand to the surface-bound ligand . In non-polar solvents, carboxylate ligands (p K a ≈ 4.75) are quantitatively exchanged for a stronger acid such as phosphonic acid (p K a ≈ 1.1–2.3), − ,− trifluoroacetic acid (p K a ≈ 0.5), − or hydrochloric acid (p K a ≈ −8) . The reverse reaction (e.g., chloride replaced by carboxylate) is observed when a base (e.g., an alkylamine) is added. ,, Under such basic conditions, the proton transfer is no longer part of the ligand exchange.…”

The Surface Chemistry of Colloidal Nanocrystals Capped by Organic Ligands

“…Cadmium phosphonate was exchanged for cadmium oleate, to some extent, on the surface of CdSe nanocrystals. 82 Addition of oleic acid to cadmium phosphonate capped nanocrystals did not result in any exchange, as expected.…”

Waveguide-Based Spectroelectrochemical Characterization of Band Edge Energies in Submonolayers of CdSe Quantum Dots Tethered to Indium–Tin Oxide Electrodes

“…Previous studies have suggested that CdSe ligand surface coverages up to ca. 2–4 ligands per nm 2 depending upon the ligand footprint. ,,− For a 5 nm CdSe QD in contact with a planar oxide with a surface linker coverage of ca. 10 –10 moles/cm 2 , as few as 3–6 linkages per QD are expected, but as many as ca.…”