The Effect of a Common Purification Procedure on the Chemical Composition of the Surfaces of CdSe Quantum Dots Synthesized with Trioctylphosphine Oxide

This paper describes a quantitative analysis of the chemical composition of organic/inorganic interfaces of colloidal 3.1-nm CdSe quantum dots (QDs) synthesized with trioctylphosphine oxide (TOPO) as the coordinating solvent and purified by successive precipitations from a chloroform/methanol solvent/nonsolvent system. A combination of X-ray photoelectron spectroscopy, inductively coupled plasma-atomic emission spectroscopy, and NMR (both 1 H and 31 P) reveals that the only ligands that form a stable population on the surface of the QDs are X-type alkylphosphonate and carboxylate ligands. n -Octylphosphonate (OPA), a known impurity in technical-grade (90%) TOPO, and P ′- P ′-(di- n -octyl) pyrophosphonate (PPA), the self-condensation product of OPA, cover ∼84% of the atoms on the surface of the QDs, whereas few of the L-type (datively bound) ligands hexadecylamine (HDA), TOPO, and trioctylphosphine selenide (TOPSe) are present as bound ligands once the excess free surfactant is removed from the reaction mixture. Purified QDs synthesized in 99% TOPO (with no alkylphosphonates present) have no phosphorus-containing ligands on the surface. Despite the approximately constant surface coverage of phosphorus-containing ligands, the photoluminescence quantum yield of the solution of QDs steadily decreases during purification from ∼15% to less than 1%. Proton NMR analysis of the QD samples and photoluminescence spectra of QDs exposed to various concentrations of methanol suggest that this decrease is due to a combination of progressive loss of small amounts of HDA and adsorption of methanol to the surface of the QDs during purification.