Colloidal Semiconductor Nanocrystals: from synthesis to applications

My research activities during the PhD focused on the study of colloidal semiconductor nanocrystals (NCs) in the context of their application in light-emitting diodes (LED). In fact, their attractive optoelectronic properties make them particularly suitable for lighting and display technology. It is common knowledge that nanocrystals-based LEDs are now regarded as one of the most promising emitting materials in terms of light-emitting efficiency, wavelength tunability and cost. Indeed, future applications using NC-LEDs could range from wide colour gamut and large displays to virtual reality, or flexible and transparent screens requiring high performance. In particular, I focused on two types of colloidal NCs: green-emitting CsPbBr3 having a perovskite structure and blue-emitting core-crown CdSe/CdS nanoplatelets (NPLs). Regarding CsPbBr3 NCs, I studied different ligand-exchange processes: in solution state, but especially in the solid state, whose fewer literature studies are available. Solid-state ligand treatment was carried out after the fabrications of NC thin films: a relevant improvement of the photoluminescence quantum yield (PLQY) of the film itself has been reached, but also a change of solubility of its surface, allowing multiple deposition of nanocrystal layers. A greater thickness of the emitting film can indeed ensure an abundant recombination of injected charges. Afterwards, and precisely thanks to the change in solubility, I developed a layer-by-layer (LbL) assembly methodology that results in high quality films with finely controllable thickness. Concerning instead CdSe/CdS NPLs, I worked on 3,5 monolayer (ML) core synthesis in order to improve their PLQY and aspect ratio (length/width). To do this, I exploited Experimental Design techniques, which allowed me to study the variability of the system in greater depth. I then crowned CdSe core nanoplatelets with a layer of CdS to achieve better stability and luminescence efficiency. Specifically, I obtained a PLQY of 66% and with such NCs it was possible to fabricate LEDs, carefully studying different type of structures. With an External Quantum Efficiency (EQE) of 1.8%, I obtained the highest value ever reported for blue core/crown CdSe-based nanoplatelets in LEDs.