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Reversed oxygen sensing using colloidal quantum wells towards highly emissive photoresponsive varnishes.

Lorenzon M, Christodoulou S, Vaccaro G, Pedrini J, Meinardi F, Moreels I, Brovelli S - Nat Commun (2015)

Bottom Line: Spectroelectrochemical experiments allow us to directly relate the sensing response to the occupancy of surface states.Magneto-optical measurements demonstrate that, under vacuum, heterostructured CdSe/CdS colloidal quantum wells stabilize in their negative trion state.The high starting emission efficiency provides a possible means to enhance the oxygen sensitivity by partially de-passivating the particle surfaces, thereby enhancing the density of unsaturated sites with a minimal cost in term of luminescence losses.

View Article: PubMed Central - PubMed

Affiliation: Dipartimento di Scienza dei Materiali, Università degli Studi di Milano-Bicocca, via Cozzi 55, I-20125 Milano, Italy.

ABSTRACT
Colloidal quantum wells combine the advantages of size-tunable electronic properties with vast reactive surfaces that could allow one to realize highly emissive luminescent-sensing varnishes capable of detecting chemical agents through their reversible emission response, with great potential impact on life sciences, environmental monitoring, defence and aerospace engineering. Here we combine spectroelectrochemical measurements and spectroscopic studies in a controlled atmosphere to demonstrate the 'reversed oxygen-sensing' capability of CdSe colloidal quantum wells, that is, the exposure to oxygen reversibly increases their luminescence efficiency. Spectroelectrochemical experiments allow us to directly relate the sensing response to the occupancy of surface states. Magneto-optical measurements demonstrate that, under vacuum, heterostructured CdSe/CdS colloidal quantum wells stabilize in their negative trion state. The high starting emission efficiency provides a possible means to enhance the oxygen sensitivity by partially de-passivating the particle surfaces, thereby enhancing the density of unsaturated sites with a minimal cost in term of luminescence losses.

No MeSH data available.


Related in: MedlinePlus

Reversed O2 sensing using colloidal quantum wells.(a) Integrated Photoluminescence (PL) intensity of core-only (green circles) and core-shell (red circles) CdSe/CdS CQWs as a function of the chamber pressure (logarithmic scale). (b) Integrated PL intensity of CdSe (green) and CdSe/CdS (red) CQWs during a stepwise pressure scan. The pressure (black line, logarithmic scale) is reduced rapidly and then maintained constant for ~200 s while the PL transient is simultaneously monitored. (c) Integrated PL intensity of CdSe (green) and CdSe/CdS (red) CQWs during ‘ON/OFF’ O2/vacuum cycles starting from atmospheric pressure (1 bar) down to 10−4 bar. The pressure during the scan is shown as a black line. PL decay curves of (d) core-only and (e) core/shell CQWs measured at the respective steps of the O2/vacuum cycles in ‘c’ (highlighted with sequential numbers). (f) Decay rate of the slow component of the biexponential dynamics (triangles) and initial PL intensity, IPL(t=0 ps; triangles) for both core-only and core/shell CQWs. The same trends are observed for the fast decay contribution (Supplementary Fig. 11). All measurements are performed at room temperature using 3.1 eV excitation.
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f2: Reversed O2 sensing using colloidal quantum wells.(a) Integrated Photoluminescence (PL) intensity of core-only (green circles) and core-shell (red circles) CdSe/CdS CQWs as a function of the chamber pressure (logarithmic scale). (b) Integrated PL intensity of CdSe (green) and CdSe/CdS (red) CQWs during a stepwise pressure scan. The pressure (black line, logarithmic scale) is reduced rapidly and then maintained constant for ~200 s while the PL transient is simultaneously monitored. (c) Integrated PL intensity of CdSe (green) and CdSe/CdS (red) CQWs during ‘ON/OFF’ O2/vacuum cycles starting from atmospheric pressure (1 bar) down to 10−4 bar. The pressure during the scan is shown as a black line. PL decay curves of (d) core-only and (e) core/shell CQWs measured at the respective steps of the O2/vacuum cycles in ‘c’ (highlighted with sequential numbers). (f) Decay rate of the slow component of the biexponential dynamics (triangles) and initial PL intensity, IPL(t=0 ps; triangles) for both core-only and core/shell CQWs. The same trends are observed for the fast decay contribution (Supplementary Fig. 11). All measurements are performed at room temperature using 3.1 eV excitation.

Mentions: To investigate the effect of oxidative environments on the luminescence properties of CQWs and to demonstrate their reversed oxygen-sensing capability, we monitored the evolution of the PL upon lowering the O2 pressure in the sample chamber from atmospheric pressure to 10−4 bar (Fig. 2a). Interestingly, both systems undergo significant PL quenching with decreasing pressure, thus confirming the surface passivating role of oxygen, whose removal progressively activates surface-quenching sites. The nature of the defects responsible for quenching is provided by the comparison between core-only and core/shell materials, the first showing a much stronger dimming (~90%) than the latter (~55% with respect to the initial value in O2). In both core/shell and core-only CQWs, the electron wave function is expected to explore the surfaces essentially equally, given the ca. 1 nm thickness of the CdS shell (3 monolayers of CdS; Fig. 1b). Therefore, electron trapping alone cannot be held accountable for the observed different sensitivity to surface chemistry. The strong difference between the two samples lies instead in the accessibility of surface traps for core-localized holes, which is drastically reduced in CdSe/CdS CQWs. The observed stronger quenching for core-only materials, therefore, points to a key role of hole trapping in the quenching mechanism and outlines the ability of oxygen to passivate excess electrons on the CQW’s surfaces. This picture is further confirmed by oxygen-sensing measurements on core/shell CQWs with thinner shell (h=0.32 nm, corresponding to ~1 monolayer of CdS) that shows intermediate behaviour (~65% PL quenching) between core-only and core/shell CQWs with h=0.95 nm (Supplementary Fig. 1). Importantly, no shift of the PL spectrum of both core-only and core/shell CQWs is observed during the pressure ramp (Supplementary Fig. 2), which indicates that the observed trends are due to activation/passivation of surface traps and not to oxidation/reduction of the CQW surfaces, as instead observed for spherical nanocrystals exposed to humid air35.


Reversed oxygen sensing using colloidal quantum wells towards highly emissive photoresponsive varnishes.

Lorenzon M, Christodoulou S, Vaccaro G, Pedrini J, Meinardi F, Moreels I, Brovelli S - Nat Commun (2015)

Reversed O2 sensing using colloidal quantum wells.(a) Integrated Photoluminescence (PL) intensity of core-only (green circles) and core-shell (red circles) CdSe/CdS CQWs as a function of the chamber pressure (logarithmic scale). (b) Integrated PL intensity of CdSe (green) and CdSe/CdS (red) CQWs during a stepwise pressure scan. The pressure (black line, logarithmic scale) is reduced rapidly and then maintained constant for ~200 s while the PL transient is simultaneously monitored. (c) Integrated PL intensity of CdSe (green) and CdSe/CdS (red) CQWs during ‘ON/OFF’ O2/vacuum cycles starting from atmospheric pressure (1 bar) down to 10−4 bar. The pressure during the scan is shown as a black line. PL decay curves of (d) core-only and (e) core/shell CQWs measured at the respective steps of the O2/vacuum cycles in ‘c’ (highlighted with sequential numbers). (f) Decay rate of the slow component of the biexponential dynamics (triangles) and initial PL intensity, IPL(t=0 ps; triangles) for both core-only and core/shell CQWs. The same trends are observed for the fast decay contribution (Supplementary Fig. 11). All measurements are performed at room temperature using 3.1 eV excitation.
© Copyright Policy - open-access
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC4382706&req=5

f2: Reversed O2 sensing using colloidal quantum wells.(a) Integrated Photoluminescence (PL) intensity of core-only (green circles) and core-shell (red circles) CdSe/CdS CQWs as a function of the chamber pressure (logarithmic scale). (b) Integrated PL intensity of CdSe (green) and CdSe/CdS (red) CQWs during a stepwise pressure scan. The pressure (black line, logarithmic scale) is reduced rapidly and then maintained constant for ~200 s while the PL transient is simultaneously monitored. (c) Integrated PL intensity of CdSe (green) and CdSe/CdS (red) CQWs during ‘ON/OFF’ O2/vacuum cycles starting from atmospheric pressure (1 bar) down to 10−4 bar. The pressure during the scan is shown as a black line. PL decay curves of (d) core-only and (e) core/shell CQWs measured at the respective steps of the O2/vacuum cycles in ‘c’ (highlighted with sequential numbers). (f) Decay rate of the slow component of the biexponential dynamics (triangles) and initial PL intensity, IPL(t=0 ps; triangles) for both core-only and core/shell CQWs. The same trends are observed for the fast decay contribution (Supplementary Fig. 11). All measurements are performed at room temperature using 3.1 eV excitation.
Mentions: To investigate the effect of oxidative environments on the luminescence properties of CQWs and to demonstrate their reversed oxygen-sensing capability, we monitored the evolution of the PL upon lowering the O2 pressure in the sample chamber from atmospheric pressure to 10−4 bar (Fig. 2a). Interestingly, both systems undergo significant PL quenching with decreasing pressure, thus confirming the surface passivating role of oxygen, whose removal progressively activates surface-quenching sites. The nature of the defects responsible for quenching is provided by the comparison between core-only and core/shell materials, the first showing a much stronger dimming (~90%) than the latter (~55% with respect to the initial value in O2). In both core/shell and core-only CQWs, the electron wave function is expected to explore the surfaces essentially equally, given the ca. 1 nm thickness of the CdS shell (3 monolayers of CdS; Fig. 1b). Therefore, electron trapping alone cannot be held accountable for the observed different sensitivity to surface chemistry. The strong difference between the two samples lies instead in the accessibility of surface traps for core-localized holes, which is drastically reduced in CdSe/CdS CQWs. The observed stronger quenching for core-only materials, therefore, points to a key role of hole trapping in the quenching mechanism and outlines the ability of oxygen to passivate excess electrons on the CQW’s surfaces. This picture is further confirmed by oxygen-sensing measurements on core/shell CQWs with thinner shell (h=0.32 nm, corresponding to ~1 monolayer of CdS) that shows intermediate behaviour (~65% PL quenching) between core-only and core/shell CQWs with h=0.95 nm (Supplementary Fig. 1). Importantly, no shift of the PL spectrum of both core-only and core/shell CQWs is observed during the pressure ramp (Supplementary Fig. 2), which indicates that the observed trends are due to activation/passivation of surface traps and not to oxidation/reduction of the CQW surfaces, as instead observed for spherical nanocrystals exposed to humid air35.

Bottom Line: Spectroelectrochemical experiments allow us to directly relate the sensing response to the occupancy of surface states.Magneto-optical measurements demonstrate that, under vacuum, heterostructured CdSe/CdS colloidal quantum wells stabilize in their negative trion state.The high starting emission efficiency provides a possible means to enhance the oxygen sensitivity by partially de-passivating the particle surfaces, thereby enhancing the density of unsaturated sites with a minimal cost in term of luminescence losses.

View Article: PubMed Central - PubMed

Affiliation: Dipartimento di Scienza dei Materiali, Università degli Studi di Milano-Bicocca, via Cozzi 55, I-20125 Milano, Italy.

ABSTRACT
Colloidal quantum wells combine the advantages of size-tunable electronic properties with vast reactive surfaces that could allow one to realize highly emissive luminescent-sensing varnishes capable of detecting chemical agents through their reversible emission response, with great potential impact on life sciences, environmental monitoring, defence and aerospace engineering. Here we combine spectroelectrochemical measurements and spectroscopic studies in a controlled atmosphere to demonstrate the 'reversed oxygen-sensing' capability of CdSe colloidal quantum wells, that is, the exposure to oxygen reversibly increases their luminescence efficiency. Spectroelectrochemical experiments allow us to directly relate the sensing response to the occupancy of surface states. Magneto-optical measurements demonstrate that, under vacuum, heterostructured CdSe/CdS colloidal quantum wells stabilize in their negative trion state. The high starting emission efficiency provides a possible means to enhance the oxygen sensitivity by partially de-passivating the particle surfaces, thereby enhancing the density of unsaturated sites with a minimal cost in term of luminescence losses.

No MeSH data available.


Related in: MedlinePlus