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

卤化铅钙钛矿晶体由于其明亮的光致发光(PL)和窄的光电子发射带宽(发射光的颜色)而成为高性能光电子学应用中有前途的和负担得起的材料。当晶体尺寸缩小到20纳米以下时，它们的发光活性是由量子限制效应引起的。然而，这些小晶体在环境条件下被发现是不稳定的，因为它们在受光和暴露在潮湿或氧气中时非常活跃。另一方面，由于晶体缺陷的产生和量子限制效应的减弱，大晶体通常表现出相对较低的发光活性。如果稳定性问题的解决方案能够得到解决，卤化铅钙钛矿将在许多商业光电设备中带来变革性的技术，包括LED、激光、太阳能电池和显示面板。在这个项目中，将使用非传统的结晶方法生长出更大、更稳定、具有可调光电性能的晶体。这种方法被称为定向附着，它的工作原理是将纳米晶体组装成更大的晶体，同时通过组装的纳米晶体之间的界面保持所需的量子限制特性。由于大量的材料和工艺选择，以及对定向组装过程的缺乏了解，本研究计划将建模和实验相结合，揭示组装过程背后的物理和化学机理。该项目将为本科生和研究生以及代表性不足的学生提供培训机会。还计划让K-12科学教师参与该项目，以扩大他们对下一代科学家的培训。拟议研究的目的是了解CsPbBr3卤化铅钙钛矿纳米晶体的定向附着结晶机制，以控制Ruddlesden-Popper断层和晶界的缺陷，以及二维纳米板或三维纳米立方体的晶体形态。这项研究的动机是初步实验结果表明，卤化铅钙钛矿纳米晶体的组装可以产生平面缺陷，导致较大(但仍是纳米级)组装晶体的量子限制。虽然组装的晶体形态、尺寸和缺陷结构可以用来优化其稳定性和量子产率，但如果没有对定向附着晶体生长机制的基本了解，特别是组装过程中纳米晶体的平移和旋转运动，这些属性就不能以系统的方式进行控制。通过建模/实验合作，研究小组将开发非球形卤化铅钙钛矿前驱体纳米颗粒之间相互作用力/扭矩的机制模型。这个模型将被用来研究结晶机制以及它们如何依赖于附着在纳米晶体表面的配体的分子水平细节。这个纳米晶体相互作用模型将作为速率表达式并入布居平衡模型中，以研究定向附着结晶过程的动力学，以预测组装晶体的形态、维度和缺陷密度的时间演化。一个新的高级本科生/研究生课程将基于从拟议的研究计划中获得的基本知识而开发。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

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.