UC Berkeley

Colloidal quantum dots are semiconductors with dimensions on the nanoscale and they have remarkable size-tunable optical properties. Colloidal quantum dots have the potential to combine the scalable synthesis and processing conditions of molecular luminophore dyes with the high photoluminescence quantum yield and broad absorption of highly optimized bulk semiconductors. Quantum dots have been used in several optoelectronic applications, including light-emitting diodes and display technologies, in which the performance of the luminophore critically determines the overall efficiency. In addition, there are on-going efforts to develop luminescent concentrators and other optical cavity-based applications that are theorized to experience divergent efficiencies as the quantum yield of the luminophore approaches nearer to unity. While the performance of colloidal quantum dots has continued to improve over the years, there is a tremendous opportunity to optimize the performance of materials operating at near-unity efficiencies. To improve the performance of these materials, strategies must be developed and employed to robustly mitigate the optoelectronic consequences of optical defect states.

This dissertation presents efforts to synthesize, optimize, and characterize high quantum yield colloidal quantum dots. An emerging class of nanoscale luminophores are the cesium lead halide (CsPbX3; X = Cl, Br, I) nanoparticles due to an impressive combination of optical properties and tolerance to material defects. By understanding the underlying mechanism and kinetics of anion exchange reactions, high quality CsPbX3 nanocrystals of any halide composition and visible photoluminescence band gap can be reliably accessed, these ideas are developed in Chapter 2. However, the optical efficiencies of the CsPbX3 nanocrystals are still hindered by non-radiative losses from optical defect states. Post-synthetic selective chemical etching can eliminate the presence of shallow electron traps from the surface of CsPbBr3 nanocrystals and improve the optical performance of these materials to near-unity. The selective chemical etching of CsPbBr3 nanocrystals and the mechanistic understanding of the etching reaction are developed and presented in Chapter 3. In Chapter 4, the optimization of synthetic protocols to robustly access near-unity quantum yield CdSe/CdS core/shell quantum dots is presented. Additionally, the optical characterization of the CdSe/CdS quantum dots and development of a quantum yield measurement with two orders of magnitude less uncertainty than traditional techniques are presented in Chapter 4. These investigations lay the ground work for further optimization of near-lossless materials and development of applications that employ photons as a working fluid.