Shaping the nanoworld

Abstract

This thesis investigates the synthesis, material properties, and optical behaviour of several types of optically active nanomaterials. Such materials can convert energy into light and vice versa, making them relevant for applications in sensing, photonics, and quantum technologies. Reducing well-known bulk materials to the nanoscale increases their applicability for non-uniform surfaces, biological systems, and quantum mechanical studies. However, nanoscaling also changes material properties due to quantum confinement effects in semiconductors and the large increase in surface-to-volume ratio. While these changes expand the range of applications, they also introduce challenges such as surface-related quenching that must be addressed before nanomaterials can be effectively used in scientific or technological applications.

This thesis presents a broad exploration of different optically active nanomaterials by comparing their synthesis methods, material properties, limitations, and potential applications.

Chapter 1 introduces the fundamental concepts needed to understand the research presented in the thesis, including the differences between bulk and nanoscale materials and the challenges associated with nanoscale systems.

Chapter 2 investigates the synthesis of lithium yttrium fluoride (LiYF4) nanocrystals, an optically inactive host material. The study shows that the nanocrystals form through multiple growth stages during a one-pot synthesis. After precursor decomposition, amorphous LiYF4 nanospheres nucleate and grow through Ostwald ripening before crystallizing into bipyramidal nanocrystals. Prolonged exposure to synthesis conditions leads to dissolution of LiYF4 and recrystallization into LiF, indicating that LiYF4 nanocrystals exist in a metastable phase under these conditions.

Chapter 3 studies LiYF4 nanocrystals doped with ytterbium ions (Yb³⁺), which act as optically active emitters. While bulk Yb:YLF exhibits near-unity photoluminescence quantum yield (PLQY), nanocrystals show reduced efficiency due to surface-related quenching. The main quenching pathways are identified and minimized by eliminating water impurities and growing an undoped LiYF4 shell around the doped core. This shell suppresses energy transfer to the surface, increasing the PLQY to near unity even for highly doped nanocrystals.

Chapter 4 focuses on indium phosphide (InP) semiconductor quantum dots, where quantum confinement leads to size-dependent optical emission. Surface passivation is achieved by growing a ZnMgSe semiconductor shell around the InP core. Although the photoluminescence efficiency remains similar, increasing magnesium content improves colour purity by reducing trap-state emission and exciton delocalization.

Chapter 5 investigates electrochemical p-doping of cesium lead bromide (CsPbBr3) perovskite nanocrystal films. While hole injection into the valence band is achieved, the injected charge proves unstable due to chemical reactions within the material. Computational analysis confirms that stable p-doping is theoretically possible.

Chapter 6 further examines the instability of electrochemical n-doping in CsPbBr3 nanocrystals. Spectroelectrochemical experiments reveal that degradation is largely determined by the solubility of surface ligands in different electrolytes, which limits the electrochemical stability window of these nanocrystals.

Overall, the research demonstrates that surface effects dominate the behaviour of nanoscale optical materials. Surface passivation, particularly through inorganic shell growth, is shown to be an effective strategy to improve optical performance and stability, although challenges remain for materials without suitable shell structures.