Growth Mechanisms and Shape Control of Light-emitting Lead Halide Perovskite Nanocrystals

Abstract

Metal halide perovskite nanocrystals, as a new class of light-harvesting and light-emitting materials, have recently attracted a lot of attention for an impressive variety of optoelectronic applications. Some advantages of perovskite nanocrystals include their low-cost, easy-to-perform synthetic routes, convenient solution processability, precise bandgap tunability over the entire visible spectral range, and exceptionally high photoluminescence quantum yields. In this thesis, we develop strategies to get control over shape and composition of perovskite nanocrystals and study the growth mechanisms and the optical properties of these nanomaterials.

The first Chapter is a literature review on recent advances of perovskite nanocrystals with an emphasis on the synthetic methods, surface treatment, optical properties, growth mechanisms, and related applications. The focus is placed on emerging new results in terms of the increasing diversity in the synthetic methodologies, ability to control nanoparticle shapes, stability enhancement strategies (including direct syntheses in water), and on the particle formation mechanisms. The basic design principles and up-to-date performance of optoelectronic devices based on perovskite nanocrystals are considered, with a main focus on light-emitting diodes, but also touching upon solar cells, photodetectors, and lasers.

The most widely employed method for perovskite nanocrystals by far, hot-injection synthesis, makes it rather difficult to study their formation mechanism in detail, as the nanocrystals are obtained literally within a few seconds. In Chapter 2 we present a microwave-assisted slowed-down synthesis of CsPbBr₃ perovskite nanocrystals, which retards the reaction and allows us to gather useful insights into the formation mechanism of these nanoparticles, by examining the intermediate stages of their growth. The results show the change of the crystal structure of CsPbBr₃ nanocrystals from a deficient and easier to be destroyed lattice to a well crystallized one. Conversely the shift in the ease of degradation sheds light on the formation mechanism, indicating first the formation of a bromoplumbate ionic scaffold, with Cs-ion infilling lagging a little behind. A model was proposed to illustrate the growth evolution from a Cs-deficient towards the complete and more robust crystal structure of CsPbBr₃ NCs.

After the formation of CsPbBr₃ NCs, shape and composition of CsPbBr₃ could still be manipulated by the introduction of extra ligands. The compositional and structural complexity of perovskite NCs beyond their classical ABX₃ structure has been also explored. Chapter 3 presents a reversible transformation between CsPbBr₃ and Cs₄PbBr₆ NCs achieved through simple treatments with extra OA/OLA ligands or PbBr₂, which is governed by an extraction or insertion (for the reverse direction) of PbBr₂. These transformations are so far the first report of direct reversibility, without a net change in morphology, between CsPbBr₃ and Cs₄PbBr₆ and then back to CsPbBr₃ nanocrystals, which provides a mechanism for flexible reorganization of the lead halide perovskite lattice. This study not only offered understanding of the detailed structural characteristics of CsPbBr₃ NCs and Cs₄PbBr₆ NCs, but also allowed for the flexible reorganization of lattices in perovskite systems.

Given that the surprising results on postsynthetic treatment induced shape and compositional changes, there will be a naturally raised expectation on the direct synthesis of perovskite NCs beyond cubic shape from precursor. It is widely accepted that particle geometry has a substantial influence on the optical properties of quantum-sized semiconductor materials. For example, semiconductor nanorods offer the property of linearly polarized light emission, which can greatly improve the power efficiency of displays. Chapter 4 presents a direct chemical synthesis of CsPbBr₃ nanorods with an average width of around 5 nm and average lengths of 10.8 and 23.2 nm, respectively, in two samples, which show a high photoluminescence quantum yield of 60−76% and reasonably high emission anisotropy of about 0.2 for longer rods. Remarkably, the synthesis of these nanorods has been achieved in polar alcohols, a class of solvents not usually found to be beneficial in classical perovskite nanoparticle synthesis. This study not only offered the possibility to control the shape of chemically synthesized perovskite nanocrystals, but also constituted the hitherto less common strategy of synthesizing perovskite nanoparticles in polar rather than nonpolar or only weakly polar solvents.

Lastly, optical properties of perovskite nanocrystals were used in a digital microfluidic device to perform for spatially-resolved temperature measurements during droplet movements. In Chapter 5, a composite film material which combines CsPbBr₃ perovskite nanocrystals with Hyflon AD 60 fluoropolymer was developed and utilized for high resolution optical temperature imaging. It exhibited bright luminescence and, most importantly, long-term stability in aqueous medium. The CsPbBr₃ NCs-Hyflon films showed uniform surface morphology, strong water resistance for at least four months, excellent photostability at 20 and 80 °C, and desirable performance as temperature sensor with high sensitivity and stability.

To summarize, Chapter 6 highlights the general conclusions of my study, emphasizes the originality and topicality of the issues addressed and summarizing the conclusions of each chapter. As the end, we provide an outlook into some open issues and future perspectives of this versatile, still rapidly developing field.

Date of Award	31 Jul 2020
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Andrey ROGACH (Supervisor)