Metal halide perovskite nanoparticles present unique and attractive optical and electronic properties, leading to promising application in optoelectronic devices such as light-emitting diodes and solar cells. The optical properties of metal halide perovskite nanoparticles are tuneable by size, therefore, synthesizing metal halide perovskite nanoparticles with size control, especially the quasi-2D nanoplatelets, are of great interest. However, the lack of deep understanding of the synthesis reaction hinders further development of the quasi-2D metal halide perovskite nanoplatelets. The lack of size control and thickness tuning impedes the colour-pure and tuneable emission obtainable from monodisperse metal halide perovskite nanoplatelets. In addition, the stability of these materials is relatively low, hence requiring further development.

This thesis focuses on the controllable synthesis of caesium lead halide (CsPbX₃) nanoplatelets (NPls) with size tunability and phase purity using continuous flow reactors and the stability improvement of CsPbX₃ nanocrystals (NCs). The controllable synthesis is achieved by reactor engineering, and the importance of mass transport in the synthetic processes is revealed. The stability improvement is attained by encapsulation approach with a series of materials.

First, the importance of transport phenomena on the synthesis of metal halide perovskite nanoplatelets is illustrated with CsPbBr₃ NPl synthesis reaction in batch and flow reactors. The need for fast and homogeneous mixing is demonstrated to achieve monodispersed crystals, in this case CsPbBr₃ NPls with sharp blue emission. The modularity of the synthesis method, combining fluid dynamic simulations in a collaboration, bespoke 3D flow reactors, and on-line measurements (UV-vis absorbance and photoluminescence) reveals fundamental understanding about the growth of these perovskite nanomaterials, which in turn leads to superlative size reducibility (2.2 ± 0.3 nm) and properties (sharp blue emission at 472 nm wavelength, and PLQY 24.5±0.4%). Understanding the dynamics of the synthesis in the very early stages (within the first 100-300 milliseconds) shows that mixing of the precursors plays a key role not only during the nucleation step, as previously believed, but also during the growth stage. This work also demonstrates that the flow reactors can provide a consistent and steady output capable of fulfilling the large demand of blue light emitters for a variety of applications.

Second, the phase pure CsPbI₃ NPls are selectively synthesized in flow reactors, obtaining monodisperse thicknesses of 3 and 4 monolayers, of which the latter has the most suitable emission wavelength for red emitters in display applications. The synthesis study using two different reactors reveals that efficient mixing is critical for obtaining both phase pure CsPbI₃ NPls, while the fast or very fast mixing critically controls the synthesis by manipulating monomer formation to achieve selective synthesis. More importantly, the comparative studies illustrated fast growth of CsPbI₃ NPls under room temperature synthesis. This work shows that different strategies are required for different phases and demonstrates the advantage of flow systems to achieve highly reproducible large-scale synthesis of materials with fast reaction rates.

Last, a series of strategies of encapsulating CsPbBr₃ NCs with metal oxide are studied aiming at improving the stability of CsPbBr₃ NCs in various scenarios. The post-synthesis treatment approach targeting at SiO₂, TiO₂, AlO_x, and NiO_x shells are studied with separate reaction steps, and their respective effectiveness are evaluated against water, solvents, irradiation, and ion exchange. Progress and challenges are presented with each shell material, and the results and discussions are useful as guidance for future studies.

This thesis contributes to the metal halide perovskite field of research with controllable synthesis and tuneable optical properties to enable their potential application in optoelectronic devices. The 3D reactors demonstrate unparalleled potential in continuous synthesis of metal halide perovskite materials in large scale.