Before there was light: Excited state dynamics in luminescent (nano)materials

Abstract

In this thesis we examine two types of luminescent materials: colloidal semiconductor nanocrystals (also known as quantum dots), and crystals doped with lanthanide ions. These materials convert one color of light to another. By investigating the dynamics of the excited state, we gain new insights into the physical processes that precede or compete with the emission of light. Such insights are necessary to enhance the efficiency of luminescent materials. We study blinking in individual semiconductor nanocrystals, i.e. the seemingly random switching between an emissive and a dark (non-emissive) state under continued excitation. The switching events are due to processes of charge carrier trapping and detrapping in or nearby the quantum dot, which we show occur much more frequently than usually assumed. After charge carrier trapping the nanocrystal is dark because of quenching by Auger processes. We investigate how losses due to Auger processes can be reduced: they are less efficient in CdSe/CdS core–shell nanocrystals or CdSe/CdS dot-in-rods than in bare CdSe nanocrystals. Crystals doped with lanthanide ions find numerous applications as luminescent materials in lighting, bioimaging and lasers. The properties of the material often depend on energy transfer processes between the lanthanide ions. These can be necessary to achieve the desired color output, but they can also reduce the photoluminescence efficiency. The quantitative understanding of energy transfer processes is however limited. We present and apply models to understand energy transfer processes in detail from the analysis of photoluminescence decay curves. In particular, we analyze the energy transfer processes that can result in downconversion (i.e. photoluminescence with a photon-to-photon conversion efficiency higher than 100%), between Tm3+ ions in Gd2O2S or from Tb3+ to Yb3+ in YPO4. Furthermore, we demonstrate that the efficiency of energy transfer processes doped nanocrystals is influenced by the refractive index of the environment of the crystal: the lower the refractive index of the environment, the higher the efficiency of energy transfer.