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.