From atoms to photons
Authors:   , Prins Paul Timotheüs 
Published: November 2023

Light, radiation in the form of photons, is crucial for life on Earth, serving a multitude of functions, including providing warmth, supporting plant growth, regulating our day-night cycle, and enabling vision. The sun has long been our primary light source, but in the modern world, artificial light is indispensable for illuminating the night, communication, and various activities. This transformation was made possible by advances in material synthesis, enabling the creation of artificial light sources using atoms as building blocks for the synthesis of photoactive materials.
Photoactive nanomaterials, with dimensions ranging from 1 to 100 nanometers, exhibit unique photophysical properties. For example, nanocrystalline quantum dots offer a tunable optical transition by simply changing the nanocrystal size, revolutionizing technology like QLED TVs. Initially, the synthesis of these materials was often a matter of trial and error or good fortune. By combining the right atoms under the right physicochemical conditions, nanocrystals can by optimized for light emission. Our understanding of nanocrystal growth is ever expanding, enabling us to design and synthesize materials on a more rational basis.
This thesis investigates the formation and properties of photoactive nanomaterials across several chapters. In Chapter 2, colloidal CdSe quantum dots are studied using in situ X-ray scattering, revealing extended nucleation and reaction-limited growth. The growth rate decreases with increasing nanocrystal radius, allowing to control the size of the nanocrystals. Chapter 3 focuses on anisotropic colloidal CdSe nanocrystals, known as nanoplatelets, with well-defined thickness resulting in remarkably monochromatic emission controllable by the thickness of the platelets. Quantitative approaches are used to understand size and concentration variations during synthesis, shedding light on the origin of the anisotropic shape and the presence of spherical nanocrystal by-products. Chapter 4 explores insulator NaYF4 nanocrystals doped with photoactive lanthanide ions, known for their narrower emission lines compared to semiconductors and their role in upconversion. The synthesis of hexagonal-phase NaYF4:Er3+,Yb3+ nanocrystals from cubic-phase precursors is studied, revealing bimodal size distributions with the composition of the cubic-phase nanoparticles as a crucial factor. In Chapter 5, the fundamentals of photon absorption by semiconductor nanomaterials are discussed. An intriguing relationship is unveiled between photon absorption probability and exciton size. The potential of quantum dots for enhanced light absorption is discussed, particularly by embedding them in high dielectric constant mediums. Chapter 6 investigates rapid relaxation processes after photon absorption by semiconductor nanocrystals, focusing on carrier cooling in Cu-doped InP nanocrystals. Electron cooling rates are found to be faster than hole cooling. As the electron cooling occurs mainly by Auger interaction with the hole, our results indicate that hole localization on copper does not hinder Auger cooling of the electron.
In summary, this thesis provides a comprehensive exploration of the formation and properties of photoactive nanomaterials, shedding light on their unique characteristics and potential applications in various fields.