Mystifying Photoluminescence Mechanisms: Unraveling and exploiting non-ideal behavior of luminescent nanomaterials

Developing quantum dots (QDs) for applications in future devices is a challenge on several frontiers. Control over particle sizes and shapes is essential, but rational design of next-generation QDs also requires a deep understanding of photoluminescence mechanisms. Whereas the scientific community has so far mostly studied the photoluminescence mechanism of quantum-dot structures based on the workhorse material CdSe, there is now a great urge for new types of QDs of different compositions. For instance, we need to explore alternative materials with reduced toxicity, such as the CuInS2- and InP-based QDs that we studied in this thesis. We have shown that optical spectroscopy at the single-particle level is an important technique to uncover the mechanisms of photoluminescence of such QD structures. However, large particle-to-particle variations of emission properties within a batch of nominally identical QDs remain puzzling. In this thesis, we also studied lanthanide doped materials. These materials have well-defined emission energies that are barely affected by the host crystal in which the lanthanides are incorporated. The general belief is that, just like in bulk crystals, the emission properties of lanthanide ions in nanocrystals (NCs) are independent of their local environment. We challenged this viewpoint, first of all by showing that dopant-to-dopant energy transfer (ET) is hampered in NCs, which alters the photoluminescence output. We also investigated how a special type of ET—from dopant ions to molecular vibrations at the NC surface—quenches the luminescence from specific lanthanide levels. Importantly, once a lanthanide ion has acted as a donor for ET, it can no longer emit light (of the same color), and vice versa. In other words, ET and radiative decay are competing processes, and the PL output is determined by the relative efficiencies of radiative decay and ET.