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SeleniumRedox Reactivity on Colloidal CdSe Quantum Dot Surfaces

View Article: PubMed Central - PubMed

ABSTRACT

Understanding thestructural and compositional origins of midgapstates in semiconductor nanocrystals is a longstanding challenge innanoscience. Here, we report a broad variety of reagents useful forphotochemical reduction of colloidal CdSe quantum dots, and we establishthat these reactions proceed via a dark surface prereduction stepprior to photoexcitation. Mechanistic studies relying on the specificproperties of various reductants lead to the proposal that this surfaceprereduction occurs at oxidized surface selenium sites. These resultsdemonstrate the use of small-molecule inorganic chemistries to controlthe physical properties of colloidal QDs and provide microscopic insightsinto the identities and reactivities of their localized surface species.

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(A) Time-resolved PLfrom a stirred anaerobic solution of unphotodopedCdSe QDs (0.5 μM in toluene, d ≈ 4.1nm) with added Li[Et3BH] (0, 30, 60, 90 equiv/QD, arrow).Inset: Normalized integrated PL intensities (I/I0) and PL single-exponential decay time constants(τ/τ0) vs Li[Et3BH], neglectingthe first few nanoseconds of decay. (B) 11B NMR spectraof Li[Et3BH] (top)16 and ofCdSe QDs treated with Li[Et3BH] (100 equiv) in C6D6 (bottom).
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fig2: (A) Time-resolved PLfrom a stirred anaerobic solution of unphotodopedCdSe QDs (0.5 μM in toluene, d ≈ 4.1nm) with added Li[Et3BH] (0, 30, 60, 90 equiv/QD, arrow).Inset: Normalized integrated PL intensities (I/I0) and PL single-exponential decay time constants(τ/τ0) vs Li[Et3BH], neglectingthe first few nanoseconds of decay. (B) 11B NMR spectraof Li[Et3BH] (top)16 and ofCdSe QDs treated with Li[Et3BH] (100 equiv) in C6D6 (bottom).

Mentions: Li[Et3BH] is a “complex” reductant anddoes not display reversible one-electron chemistry.14 The photodoping process described in Figure 1 must therefore be more complicated thana simple electron-transfer reaction. The mechanism of hole quenchingwas explored by monitoring the impact of triethylborohydride on theCdSe QD PL. Figure 2A summarizes the PL intensity and decay dynamics at various concentrationsof Li[Et3BH] in the dark before photodoping. Although theintegrated PL intensity decreases with increasing [Et3BH–], consistent with hole quenching, the PL decay dynamicsare found to remain approximately independent of [Et3BH–]. This result indicates that the visible PL quenchingby Li[Et3BH] does not result from a diffusion-limited bimolecularreaction but instead involves a static, or preassociation, mechanism.NMR spectroscopy rules out the possibility that the Et3BH– is simply adsorbed to the QD surfaces. Figure 2B compares the 11B NMR spectrum of a solution of Li[Et3BH] in C6D6 (δ = −12 ppm) with the spectrum of a C6D6 solutionof CdSe QDs treated with 100 equiv/QD of Li[Et3BH] in thedark. In the QD sample, no Et3BH– isdetected; instead, only free triethylborane is observed (δ =84 ppm).15 These experiments implicatea dark reaction between the CdSe QDs and Li[Et3BH], allowingthe conclusion that the product of this dark reaction is responsiblefor hole quenching during photodoping, rather than electron transferfrom Et3BH– itself.


SeleniumRedox Reactivity on Colloidal CdSe Quantum Dot Surfaces
(A) Time-resolved PLfrom a stirred anaerobic solution of unphotodopedCdSe QDs (0.5 μM in toluene, d ≈ 4.1nm) with added Li[Et3BH] (0, 30, 60, 90 equiv/QD, arrow).Inset: Normalized integrated PL intensities (I/I0) and PL single-exponential decay time constants(τ/τ0) vs Li[Et3BH], neglectingthe first few nanoseconds of decay. (B) 11B NMR spectraof Li[Et3BH] (top)16 and ofCdSe QDs treated with Li[Et3BH] (100 equiv) in C6D6 (bottom).
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC5016805&req=5

fig2: (A) Time-resolved PLfrom a stirred anaerobic solution of unphotodopedCdSe QDs (0.5 μM in toluene, d ≈ 4.1nm) with added Li[Et3BH] (0, 30, 60, 90 equiv/QD, arrow).Inset: Normalized integrated PL intensities (I/I0) and PL single-exponential decay time constants(τ/τ0) vs Li[Et3BH], neglectingthe first few nanoseconds of decay. (B) 11B NMR spectraof Li[Et3BH] (top)16 and ofCdSe QDs treated with Li[Et3BH] (100 equiv) in C6D6 (bottom).
Mentions: Li[Et3BH] is a “complex” reductant anddoes not display reversible one-electron chemistry.14 The photodoping process described in Figure 1 must therefore be more complicated thana simple electron-transfer reaction. The mechanism of hole quenchingwas explored by monitoring the impact of triethylborohydride on theCdSe QD PL. Figure 2A summarizes the PL intensity and decay dynamics at various concentrationsof Li[Et3BH] in the dark before photodoping. Although theintegrated PL intensity decreases with increasing [Et3BH–], consistent with hole quenching, the PL decay dynamicsare found to remain approximately independent of [Et3BH–]. This result indicates that the visible PL quenchingby Li[Et3BH] does not result from a diffusion-limited bimolecularreaction but instead involves a static, or preassociation, mechanism.NMR spectroscopy rules out the possibility that the Et3BH– is simply adsorbed to the QD surfaces. Figure 2B compares the 11B NMR spectrum of a solution of Li[Et3BH] in C6D6 (δ = −12 ppm) with the spectrum of a C6D6 solutionof CdSe QDs treated with 100 equiv/QD of Li[Et3BH] in thedark. In the QD sample, no Et3BH– isdetected; instead, only free triethylborane is observed (δ =84 ppm).15 These experiments implicatea dark reaction between the CdSe QDs and Li[Et3BH], allowingthe conclusion that the product of this dark reaction is responsiblefor hole quenching during photodoping, rather than electron transferfrom Et3BH– itself.

View Article: PubMed Central - PubMed

ABSTRACT

Understanding thestructural and compositional origins of midgapstates in semiconductor nanocrystals is a longstanding challenge innanoscience. Here, we report a broad variety of reagents useful forphotochemical reduction of colloidal CdSe quantum dots, and we establishthat these reactions proceed via a dark surface prereduction stepprior to photoexcitation. Mechanistic studies relying on the specificproperties of various reductants lead to the proposal that this surfaceprereduction occurs at oxidized surface selenium sites. These resultsdemonstrate the use of small-molecule inorganic chemistries to controlthe physical properties of colloidal QDs and provide microscopic insightsinto the identities and reactivities of their localized surface species.

No MeSH data available.


Related in: MedlinePlus