The nature of non-FRET photoluminescence quenching in nanoassemblies from semiconductor quantum dots and dye molecules

Abstract

Nanoassemblies formed via self-assembly based on colloidal CdSe quantum dots (QDs) and porphyrin (H₂P) dye molecules show Fluorescence Resonant Energy Transfer (FRET) and non-FRET quenching of QD photoluminescence (PL). We present a procedure to unravel and quantify these two relaxation pathways via dynamic and static PL quenching experiments. Accordingly, FRET amounts at maximum to 10% of the total quenching efficiency. Since the degree of ligand coverage is inhomogeneously distributed across the QD ensemble PL quantum yields vary broadly. The attachment of H₂P molecules occurs preferentially to those QDs with low ligand coverage. Along with that, nanoassembly formation deviates strongly from Poisson statistics. Like FRET, non-FRET depends on the QD size. We assign non-FRET quenching to the formation of specific new Cd²⁺ trap states following depletion of several ligands by the spacious dye molecules. While FRET follows quantitatively the Förster model, non-FRET appears on time scales of 1–3 ns in new and enhanced non-radiative near-band-edge QD PL decay channels caused by a trapping of the electrons in long-lived intra-gap states which then manifests itself in a subsequent weak PL emission. We assign the related intra-band emission to a recombination of deep-trap electrons and shallow-trap holes.