Photoluminescence Dynamics of CdSe QD/Polymer Langmuir-Blodgett Thin Films: Morphology Effects

Thin films of colloidal semiconductor CdSe quantum-dots (QDs) and a styrene/maleic anhydride copolymer were prepared by the Langmuir-Blodgett technique and their photoluminescence was characterized by confocal fluorescence lifetime microscopy. In the films, the photoluminescence dynamics are strongly affected by excitation energy migration between close-packed quantum-dots and energy trapping by surface-defective QDs or small clusters of aggregated QDs. Polymer/QD films with more aggregated QD regions exhibit lower PL intensities and faster decays, which we attribute primarily to the increased ability of excitations to find trap sites among more close-packed QD regions. This was confirmed by comparing films from bilayer or cospreading deposition, and varying the QD-to-polymer composition or the surface pressure at deposition. The more regular films, such as those obtained by bilayer deposition at high-pressure, display more surface emission intensity and decays with less accentuated curvature. Decay analysis was performed with a model that accounts for excitation energy migration and trapping in the films. In the framework of the model, the photoluminescence dynamics are related to film morphology through the density of energy traps. A lower amount of QD clustering in the films reflects on a lower density of energy traps and, thus, on more emissive QD films, as inferred here for bilayer films relative to cospreading films.