Structural study of quantum dot: polymer nanocomposite films

Abstract

Nanocomposite films consisting of quantum dots (QDs) embedded in a host polymer matrix have attracted immense research attention over the last few decades due to the tunable properties and potential of these films. Such hybrid composite films can combine the unique properties of polymers, which include flexibility, low cost, being lightweight and easily processible, with the quantum size confinement effects of QDs. QDs have rational design rules that enable tailored and highly controlled optical properties. In terms of potential applications, these films have potential in photonic devices; this includes laser diodes, light emitting diodes, solar and photovoltaic cells, photodetectors, photon-upconversion, and photon-multipliers. However, these films suffer from structural/ morphological issues; the QDs tend to aggregate when incorporated into the polymer matrix, due to the presence of cohesive forces, which hinder the optical properties of these nanocomposite films, leading to loss of the nanocomposite films' optical, electronic and mechanical properties. This thesis aims to develop and discover processing routes that could optimise the performance of optoelectronic devices by controlling the morphology and dispersion of QDs in nanocomposite films. The experiments used device relevant Lead Sulphide quantum dots (PbS QDs) dispersed into three host polymers (PS, PMMA and F8BT); the nanocomposite films were characterised via ellipsometry, Atomic Force Microscopy (AFM) and Grazing Incident Small angle X-ray scattering (GISAXS). It was found that the QDs are well dispersed in the initial nanocomposite casting solution in the presence of a solvent, with a measured quantum dot core diameter of 4.8nm, and that aggregation occurs during the casting process; this casting was done via spin coating. The thickness of the spin-coated films, the QDs distribution at the film surface and the QD aggregation size were partially controlled by the host polymer Mw, which all increased with increasing polymer Mw and decreased with it. Exchanging the native stabilising ligands (OA) with shorter length carbon ligands Hexanoic and Decanoic acid ligands (HA and DA) reduced the film thickness, it also gave QD-poor domain in the film’s surface morphology, aggregation size and interparticle separation compared to films with native OA ligands. This indicates the increased miscibility of the QDs with the polymer matrix. Lastly, the effect of QDs particle volume fractions and polymers matrix was investigated; it was found that increasing the QD volume fraction and decreasing the volume fraction of polymer results in better miscibility and also with a notable reduction in aggregation sizes than when reducing the QDs volume fraction and increasing the volume fraction of polymer. Two films in this experiment show well-dispersed QDs in the polymer matrix, PMMA (2600 Mw): PbS QDs-DA (PbS QDs volume fraction ~ 0.024) and PS (1800 Mw): PbS QDs-DA (PbS QDs volume fraction ~ 0.024), where the crystal grain size is around 5nm and nearly reaches the QDs core diameter (4.8nm). Overall, these results enable us to understand some of the fundamental factors influencing the self-assembly of the QDs: polymer nanocomposite films and provide strategies for producing well-dispersed QDs in a polymer matrix.