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Abstract
A characteristic property of colloidal semiconductor nanocrystal quantum dots (QDs) is their emission intermittency. Although a unifying theory of QD photoprocesses remains elusive, the importance of charged states is clear. We now report a new approach to directly study the role of surface charge on QD emission by adding metal ions to individual, core-only QDs immobilized in aqueous solution in an agarose gel. The CdTe QDs show very stable emission in the absence of metal ions but a dramatic and reversible increase in blinking due to the presence of trivalent metal ions. Our results support a charge-separation model, in which the major blinking pathway is the surface trapping of electrons; transiently bound metal ions close to the QD surface enhance this process.
Highlights
Photoluminescence (PL) intermittency, termed blinking, is a universal feature of emitters ranging from small molecular fluorophores,[1,2] fluorescent proteins,[3] and conjugated polymers[4] to nanoscale emitters such as semiconductor quantum dots (QDs)[5] and diamond nanocrystals.[6]
Since the QD binds to the anionic agarose, it might be expected that binding of a sufficient number of positive metal ions to the agarose would disrupt this interaction and lead to QD loss, which is exactly what we observed on addition of 10 μM Y3+
Embedding the core-only CdTe QDs in agarose leads to Article very stable emission because the negatively charged agarose suppresses blinking by slowing electron trapping to surface sites
Summary
Photoluminescence (PL) intermittency, termed blinking, is a universal feature of emitters ranging from small molecular fluorophores,[1,2] fluorescent proteins,[3] and conjugated polymers[4] to nanoscale emitters such as semiconductor quantum dots (QDs)[5] and diamond nanocrystals.[6]. The first model that gained acceptance involved the long-time charging of a single QD core, with subsequent excited-state energy lost nonradiatively via Auger recombination.[21,22] various studies have questioned this model.[23−25] The recently developed multiple recombination centers (MRC)[26−28] model invokes multiple surface hole traps with fluctuating trapping rates This has been recently extended to include additional electron trapping pathways,[16] in part to accommodate experimental evidence that QD blinking can be altered through surface or solution modifications.[29−31] More recent experiments[32,33] have confirmed the existence of delayed emission,[34,35] which can only be explained by longlived charge-separated states; a modified version of the Auger quenching model was proposed, with a suggested assignment of the electron as the trapped carrier.[32,33]. Our data are consistent with a model in which blinking results from the surface trapping of electrons.[32,33]
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