Journal
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
Volume 135, Issue 49, Pages 18536-18548Publisher
AMER CHEMICAL SOC
DOI: 10.1021/ja4086758
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Funding
- Department of Energy [DE-SC0006410]
- National Science Foundation [DGE07-07425]
- Columbia University
- National Center for Research Resources, National Institutes of Health [C06 RR017528-01-CEM]
- U.S. Department of Energy (DOE) [DE-SC0006410] Funding Source: U.S. Department of Energy (DOE)
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We demonstrate that metal carboxylate complexes (L-M(O2CR)(2), R = oleyl, tetradecyl, M = Cd, Pb) are readily displaced from carboxylate-terminated ME nanocrystals (ME = CdSe, CdS, PbSe, PbS) by various Lewis bases (L = tri-n-butylamine, tetrahydrofuran, tetradecanol, N,N-dimethyl-n-butylamine, tri-n-butylphosphine, N,N,N',N'-tetramethylbutylene-1,4-diamine, pyridine, N,N,N',N'-tetramethylethylene-1,2-diamine, n-octylamine). The relative displacement potency is measured by H-1 NMR spectroscopy and depends most strongly on geometric factors such as sterics and chelation, although also on the hard/soft match with the cadmium ion. The results suggest that ligands displace L-M(O2CR)(2) by cooperatively complexing the displaced metal ion as well as the nanocrystal. Removal of up to 90% of surface-bound Cd(O2CR)(2) from CdSe and CdS nanocrystals decreases the Cd/Se ratio from 1.1 +/- 0.06 to 1.0 +/- 0.05, broadens the 1S(e)-2S(3/2h) absorption, and decreases the photoluminescence quantum yield (PLQY) from 10% to <1% (CdSe) and from 20% to <1% (CdS). These changes are partially reversed upon rebinding of M(O2CR)(2) at room temperature (similar to 60%) and fully reversed at elevated temperature. A model is proposed in which electron-accepting M(O2CR)(2) complexes (Z-type ligands) reversibly bind to nanocrystals, leading to a range of stoichiometries for a given core size. The results demonstrate that nanocrystals lack a single chemical formula, but are instead dynamic structures with concentration-dependent compositions. The importance of these findings to the synthesis and purification of nanocrystals as well as ligand exchange reactions is discussed.
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