Evidence of photo- and electrodarkening of (CdSe)ZnS quantum dot composites

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Abstract
We present a study of the kinetics of photoluminescence (PL) and cathodoluminescence (CL) degradation of semiconductor quantum dot composites, formed by highly luminescent (CdSe)ZnS core-shell nanocrystals embedded in a ZnS matrix. The photoluminescence and cathodoluminescence spectra indicate that both emissions originate from the same near band-edge state of the nanocrystals. We observe a strong decrease in the PL and CL intensities with time. Photoluminescence experiments carried out at high laser fluences (0.5–10 mJ/cm2 per pulse) show that the PL intensity decay with time depends on the size of the nanocrystals and the nature of the surrounding matrix. For instance, close-packed films showed a much slower decay than composite films. The cathodoluminescence intensity degradation is enhanced at lower temperatures. Partial recoveries of the CL signal have been achieved after thermal annealing at temperatures around 120 °C, which indicates that activation of trapped carriers can be induced by thermal stimulation. We attribute the CL and PL decay in the composite films to photo- and electroionization of the nanocrystals, and subsequent trapping of the ejected electrons in the surrounding semiconductor matrix.