The Chemical Environments of Oleate Species within Samples of Oleate-Coated PbS Quantum Dots

Abstract

A combination of FT-IR, (1)H NMR, nuclear Overhauser effect (NOESY), and diffusion-ordered (DOSY) NMR spectroscopies shows that samples of oleate-coated PbS quantum dots (QDs) with core radii ranging from 1.6 to 2.4 nm, and purified by washing with acetone, contain two species of oleate characterized by the stretching frequencies of their carboxylate groups, the chemical shifts of their protons, and their diffusion coefficients. One of these oleate species exists primarily on the surfaces of the QDs and either chelates a Pb(2+) ion or bridges two Pb(2+) ions. The ratio of bridging oleates to chelating oleates on the surfaces of the QDs is approximately 1:1 for all sizes of the QDs we studied. The second oleate species in these samples bridges two Pb(2+) ions within clusters or oligomers of lead oleate (with a hydrodynamic radius of ~1.4 nm), which are byproducts of the QD synthesis. The concentration of these clusters increases with increasing size of the QDs because larger QDs are produced by increasing the concentration of the oleic acid ligand in the reaction mixture. The oleate molecules on the surfaces of the QDs and within the lead oleate clusters are in rapid exchange with each other. Additional washes with methanol progressively eliminate the contaminating clusters from the PbS QD samples. This work quantitatively characterizes the distribution of binding geometries at the inorganic/organic interface of the nanocrystals and demonstrates the utility of using organic ligands as probes for the composition of a colloidal QD sample as a function of the preparation procedure.

Role of PbSe Structural Stabilization in Photovoltaic Cells

“…The results suggest that the passivation process acts as a preliminary exchange process at the nanocrystal surface, and this passivation displaces half of the oleate ligands prior to treatment with BDT. [ 76,77 ] Hence, we conclude that a large fraction of the native ligands are removed by BDT treatment, likely binding through a thiolate moiety, [66][67][68]76,77 ] in agreement with the earlier report on PbS. [ 7 ] In semiconductor NCs, surface defects can lead to nonradiative decay pathways that reduce the quantum effi ciency of the band edge PL.…”

“…[66][67][68] A large fraction of the atoms in a NC is at its surface, which is dynamic and crystallographically disordered. [ 34,[69][70][71] This structural disorder leads to disorder in the electronic structure of NCs immediately after synthesis, and Figure 1.…”

Quantum dot surface engineering: Toward inert fluorophores with compact size and bright, stable emission

“…NMR spectroscopy is a key information-rich technique for surface ligand characterization because it allows direct chemical identification of ligands, quantification of ligands when using internal standards, and monitoring of dynamic interactions between ligands and QD surfaces in solution since it can distinguish between ligands that are surface-bound or free in solution. 1 H, 13 C, and 31 P NMR have been used to characterize binding mechanisms of ligands, including carboxylic acids [128-132], thiols [46, 94, 133, 134], phosphonic acids [46, 60, 128, 135, 136], amines [137-140], and phosphines/phosphine oxides [128, 135, 136]. NMR-based ligand studies combined with elemental analysis of the total QD has been instrumental in revealing the stoichiometry of ligand binding whereby metal-rich surfaces are passivated by strongly-bound anionic X-type ligands such as carboxylates [129, 141].…”

“…FT-IR has been employed to infer the structure of ligands bound to QD surfaces by probing their vibrational signatures. This has allowed the study of binding modes [131, 132, 144] and ligand tail conformation [145], as well as identification of chemical functionality [146], and monitoring of ligand exchange [147, 148]. While NMR and IR are particularly well suited for organic ligands, XPS has allowed the analysis of how inorganic ligands such as halide and hydroxide ions bind to QD surfaces [46, 61].…”

Organic-to-Aqueous Phase Transfer of Cadmium Chalcogenide Quantum Dots Using a Sulfur-Free Ligand for Enhanced Photoluminescence and Oxidative Stability

“…This spectrum contains a broad signal from bound oleate at 5.67 ppm and a sharp multiplet corresponding to free oleic acid/cadmium oleate at 5.49 ppm. 45 The width of the bound oleate signal is due to the slow diffusion and the restricted rotational motion of molecules bound to nanoparticles. 34, 40 Integration of the peak at 5.67 ppm, relative to a peak corresponding to the 18 protons of hexamethylcyclotrisiloxane at 0.13 ppm, shows that the average number of oleate ligands initially bound is 248 ± 11 per QD.…”