Photoexcited Charge Trapping Induced Quenching of Radiative Recombination Pathways in CuInS₂/ZnS-Dye Nanoassemblies

Photoluminescence (PL) quenching of nanoassemblies of CuInS₂/ZnS quantum dots (CIS QDs) and rhodamine 560 molecules (Rh560) is spectroscopically investigated by steady-state and femtosecond-to-nanosecond time-resolved techniques. Fluorescence lifetime measurements of CIS QDs show a bi-exponential decay (time constants ca. 650 ns and 210 ns) that are assigned to the radiative recombination of delocalized CB electrons with localized holes, presumably associated with Cu-related defect sites. A trapped electron recombines nonradiatively with the localized hole. That means, electron trapping is the first step in the nonradiative recombination pathway in CIS QDs. The traps are of surface origin and are likely associated with unpassivated dangling bonds. In this work, we controlled the trap density by varying the amount of Rh560 on the QD surface and monitoring the electron trapping in different time scales. Transient absorption measurements of the CIS-Rh560 assemblies resolved the fast component of electron trapping that occurs in tens to hundreds of picoseconds, while fluorescence lifetime measurements resolved the slow components of trapping that occur in hundreds of nanoseconds. Unlike the case of more traditional CdSe/ZnS QDs, the PL lifetime of CIS QDs approaches the typical time scale of fluorescence intermittency. As a result, the excited state of CIS QDs is vulnerable to the blinking process. In the CIS-Rh560 assembly, trapping of CB electrons increases with dye loading which eventually prolongs the dark (or dim) period and therefore reduces the fluorescence quantum yield of CIS QDs. The appearance of short lifetime components (ca. 0.5–6.9 ns) in the QD-dye assembly hints that Auger quenching process and/or electron-phonon coupling seems to play a major role in the PL quenching process.