Synthesis of cadmium-based and cadmium-free colloidal nanocrystals for efficient lighting, display and biomedical applications

Colloidal semiconductor nanocrystals have gained significant interest in recent years as promising materials for next-generation optoelectronic and biomedical technologies, owing to their tunable optical properties, high photoluminescence efficiency, and compatibility with solution-processable synthesis routes. Their potential in solid-state lighting, display technologies, and neural interfaces continues to expand as performance demands increase. This dissertation presents a detailed investigation into the synthesis, structural tuning, and integration of both cadmium-based and cadmium-free colloidal nanocrystals. Alongside application-focused studies, it also introduces the development of a new core/shell nanocrystal system, investigated primarily from a materials chemistry perspective. In the first part of this thesis, cadmium-based quantum dots (QDs) were developed with optimized optical properties for high-performance white light-emitting diodes (WLEDs). The ZnCdSe/ZnSe core/shell QDs, synthesized via low-temperature nucleation followed by high-temperature shell growth, exhibited photoluminescence quantum yields (PLQYs) up to 94%. When integrated into a liquid-type LED configuration, the total cadmium content in the final device remained below 100 ppm, in compliance with current RoHS regulations. This integration led to luminous efficiency values exceeding 170 lm/W, and simulations suggested that efficiencies above 230 lm/W could be achieved using ultra-efficient blue LED pumps. In the second part of this thesis, highly emissive green CdZnSeS/ZnS and red-emitting giant CdSe/CdS QDs were synthesized to enhance display backlighting performance, yielding near-unity PLQYs and narrow emission bandwidths. When incorporated into a liquid matrix over blue LEDs, these QDs enabled devices with external quantum efficiencies (EQEs) reaching up to 39.8% and a color gamut coverage of 133.3% NTSC. These results indicate that QD-based LEDs can outperform conventional color enhancement films while significantly reducing the amount of material required. In the third part of this thesis, further improvements were achieved by tailoring the morphology of nanocrystals. A dot-to-rod transition in CdSe/CdS QDs introduced a large Stokes shift (~780 meV), effectively minimizing reabsorption losses. When combined with green-emitting ZnCdSe/ZnSe QDs, the resulting white LED devices achieved an EQE of 42.9%, representing more than a 10% improvement compared to systems utilizing QDs alone. In the fourth part of this thesis, cadmium-free AgBiS2 nanocrystals were developed as an environmentally friendly alternative to address sustainability concerns. For the first time, a novel wet-chemical method was established to grow ZnS shells on high-quality AgBiS2 cores, resulting in the formation of AgBiS2/ZnS core/shell nanocrystals. This approach led to the first reported photoluminescence from AgBiS2 nanocrystals, with emission centered at 764 nm and a PLQY of 15.3%. Their strong near-infrared (NIR) absorption and composition of earth-abundant, non-toxic elements make these nanocrystals promising candidates for future applications in lighting, bioimaging, and NIR optoelectronics. In the fifth and final part of this thesis, AgBiS2 nanocrystals were implemented in neural interface devices. A thin (24 nm) nanocrystal film generated a stable photocurrent of 2.3 mA·cm−2 and achieved charge injection levels exceeding 10 μC·cm−2 in artificial cerebrospinal fluid. No cytotoxicity was observed in neuronal cultures, and patch-clamp recordings confirmed reliable activation of hippocampal neurons under biologically safe near-infrared (NIR) light conditions. The device also exhibited a projected operational lifetime of over 12 years, demonstrating its strong potential for future use in minimally invasive retinal prosthetic systems. In summary, this thesis presents useful findings and developments in colloidal nanocrystal research, helping to build a solid base for sustainable and efficient materials in future lighting, display, and neural technologies.