Photoluminescence quantum yield optimization techniques, influential effects involving upconversion materials, and investigation for nanothermometry

Abstract

Photoluminescent materials, which possess the ability to emit optical radiation after a photon absorption event, have garnered high levels of attention for applications such as sensing. Initially, this thesis examines the latest generation of photoluminescent probes, including various organic dyes, quantum dots (QDs), and upconversion (UC) materials, to establish the compositions and synthesis routes that yield optimal performance to increase their application potential. Characterising the photoluminescent quantum yield (PLQY) is particularly important as it determines the efficiency of their photoluminescence mechanism. Due to the low efficiency of UC materials, an aim of this thesis is to explore new PLQY optimisation techniques. This was carried out by investigating effects such as excitation beam scattering, which proved to be advantageous to the UC mechanism due to its non-linear dependence on the excitation power. However, limitations also arose as the scattering limited the excitation beam penetration depth. Other factors such as UC emission self-absorption, inner-filter effects, and thermal effects, were then explored and found to limit the maximum PLQY of these materials as well as reduce the measurements’ reliability. Overall, an in-depth summary of all the effects that influence these characterisations was produced as a steppingstone towards acquiring PLQY standards and improving comparability in the UC field. Finally, due to the growing interest of using photoluminescent materials for temperature sensing, this thesis also aimed to advance characterisation methods in this sub-field. This was achieved through modifying the experimental setup to acquire novel PLQY measurements in relation to photoluminescent probe’s temperature.