The effect of a common purification procedure on the chemical composition of the surfaces of cdse quantum dots synthesized with trioctylphosphine oxide

Adam J. Morris-Cohen, Martin D. Donakowski, Kathryn E. Knowles, Emily A Weiss

Research output: Contribution to journalArticle

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Abstract

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. 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.

Original languageEnglish
Pages (from-to)897-906
Number of pages10
JournalJournal of Physical Chemistry C
Volume114
Issue number2
DOIs
Publication statusPublished - Jan 21 2010

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purification
Semiconductor quantum dots
Purification
chemical composition
quantum dots
Oxides
oxides
Ligands
Chemical analysis
ligands
Methanol
methyl alcohol
Phosphorus
phosphorus
Photoluminescence
Nuclear magnetic resonance
Atomic emission spectroscopy
photoluminescence
nuclear magnetic resonance
trioctyl phosphine oxide

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Electronic, Optical and Magnetic Materials
  • Surfaces, Coatings and Films
  • Energy(all)

Cite this

The effect of a common purification procedure on the chemical composition of the surfaces of cdse quantum dots synthesized with trioctylphosphine oxide. / Morris-Cohen, Adam J.; Donakowski, Martin D.; Knowles, Kathryn E.; Weiss, Emily A.

In: Journal of Physical Chemistry C, Vol. 114, No. 2, 21.01.2010, p. 897-906.

Research output: Contribution to journalArticle

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abstract = "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. 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.",
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