Relations between Dewetting of Polymer Thin Films and Phase-Separation of Encompassed Quantum Dots

Abstract

Incorporation of semiconductor quantum dots (QDs) at high densities in polymer thin films is promising for the development of nanoscale sensors, light-emitting diodes, and photovoltaic devices. However, inhomogeneous distribution and aggregation of QDs in polymers limit the applications of QD−polymer composites. We investigated the origin of the aggregation of CdSe-ZnS QDs in polybutadiene (PB) thin films and its relations with the dewetting of polymer thin films on inorganic surfaces. Microscale dewetting of PB thin films on glass surfaces resulted in the formation of honeycomb structures of PB, and nanoscale dewetting on the QD surface resulted in phase-separation and aggregation of QDs. The formation of the honeycomb structures of PB is attributed to phase-separation between PB and glass during the dewetting on the glass surface, and the aggregation of QDs is attributed to the phase-separation between PB and QDs during the dewetting on the QD surface. The honeycomb structures of PB are characterized using optical microscopy and atomic force microscopy (AFM) imaging, and the phase-separation and aggregation of QDs are characterized using transmission electron microscopy (TEM) imaging and inter-QD Förster resonance energy-transfer (FRET). FRET is confirmed from photoluminescence (PL) spectral shifts and decreases of PL intensity and lifetime of QDs. Without the phase-separation and aggregation of QDs, inter-QD FRET is unexpected under the selected densities of QDs; the calculated inter-QD distance (>60 nm) for a homogeneous distribution of QDs is beyond the Förster distance (<10 nm). The phase-separation and aggregation of QDs in PB thin films are further characterized based on a decrease of FRET from QD to rhodamine 590 (R590) with an increase in the density of QDs. The aggregation of QDs occurs because of the rejection of QDs by PB molecules during the release of recoiling energy, which is distributed among entangled polymer chains and between polymer chains and QDs in the thin films. In the current work, we characterized the nanoscale dewetting of PB on the surface of QDs, the phase-separation and aggregation of QDs in PB thin films, and the limit of close-packing of QDs in polymer thin films without aggregation that are valuable during the construction of QD and polymer-based thin-film devices.