Mechanistic Understanding of Oriented Attachment Crystallization Processes for Lead-Halide Perovskite Nanocrystals

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Lead-halide perovskite crystals have emerged as promising and affordable materials for high-performance optoelectronics applications because of their bright photoluminescence (PL) and narrow photoemission bandwidth (the color of the emitted light). Their PL activity arises from quantum confinement effects that occur when the crystal dimensions shrink below 20 nanometers. These small crystals, however, have been found to be unstable under ambient conditions since they are extremely reactive when illuminated and exposed to moisture or oxygen. On the other hand, large crystals typically demonstrate a relatively low PL activity due to undesirable crystal defect generation and weakened quantum confinement effects. If a solution to the stability problems could be resolved, lead-halide perovskites would lead to transformative technologies in many commercial optoelectronic devices, including LEDs, lasers, solar cells, and display panels. In this project, larger and more stable crystals with tunable optoelectronic properties will be grown using an unconventional crystallization approach. Called oriented attachment, the liquid-phase process works by assembling the nanocrystals into larger crystals while retaining the desired quantum confinement properties by the resulting interfaces between the assembled nanocrystals. Because of the large number material and processing choices and the lack of understanding of the oriented assembly process, this research program will combine modeling and experimentation to uncover the physical and chemical mechanisms behind the assembly process. The project will provide training opportunities for undergraduate and graduate students, as well as underrepresented students. Plans also are in place to involve K-12 science teachers in the project, to expand the outreach to their training of the next-generation scientists.The objective of the proposed research is to understand the oriented attachment crystallization mechanisms of CsPbBr3 lead-halide perovskite nanocrystals to control the defects of Ruddlesden-Popper faults and grain boundaries as well as the crystal morphology of 2-dimensional nanoplates or 3-dimensional nanocubes. The research is motivated by preliminary experimental results demonstrating that assemblies of lead-halide perovskite nanocrystals can generate planar defects that lead to quantum confinement in the larger (but still nanoscale) assembled crystals. While assembled crystal morphology, dimensionality, and defect structures can be used to optimize its stability and quantum yield, these attributes cannot be controlled in a systematic manner without a fundamental understanding of oriented attachment crystal growth mechanisms, particularly translational and rotational motions of the nanocrystals during the assembly process. Through a modeling/experimental partnership, the research team will develop mechanistic models of the interaction force/torque between non-spherical lead-halide perovskite precursor nanoparticles. This model will be used to investigate the crystallization mechanisms and how they depend on the molecular-level details of the ligands attached to the nanocrystal surfaces. This nanocrystal interaction model then will be incorporated as a rate expression in a population balance model to study the dynamics of the oriented attachment crystallization process to predict the time-evolution of assembled crystal morphology, dimensionality, and defect density. A new upper-level undergraduate/graduate course will be developed based on the fundamental knowledge obtained from the proposed research program.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.