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 1H and 31P) 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. ...