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）和窄的光致发射带宽（发射光的颜色）而成为高性能光电子应用的有前途和负担得起的材料。它们的PL活性来自于当晶体尺寸缩小到20纳米以下时发生的量子限制效应。然而，已经发现这些小晶体在环境条件下是不稳定的，因为它们在被照射并暴露于湿气或氧气时具有极强的反应性。另一方面，由于不期望的晶体缺陷产生和减弱的量子限制效应，大晶体通常表现出相对低的PL活性。如果能够解决稳定性问题，铅卤化物钙钛矿将在许多商业光电设备中带来变革性技术，包括LED，激光器，太阳能电池和显示面板。在该项目中，将使用非常规的结晶方法生长具有可调光电特性的更大，更稳定的晶体。称为定向附着，液相过程通过将纳米晶体组装成更大的晶体而起作用，同时通过组装的纳米晶体之间的所得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和缺乏了解的面向装配过程中，本研究计划将结合联合收割机建模和实验，揭示背后的物理和化学机制的装配过程。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，该研究的目的是了解CsPbBr 3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片或三维纳米片的晶体形态。三维纳米立方体这项研究的动机是初步的实验结果表明，铅卤化物钙钛矿纳米晶体的组件可以产生平面缺陷，导致量子限制在更大的（但仍然是纳米级）组装晶体。虽然组装的晶体形态、维度和缺陷结构可用于优化其稳定性和量子产率，但在没有对定向附着晶体生长机制（特别是组装过程中纳米晶体的平移和旋转运动）的基本理解的情况下，不能以系统的方式控制这些属性。通过建模/实验合作，研究团队将开发非球形卤化铅钙钛矿前体纳米颗粒之间相互作用力/扭矩的机械模型。该模型将被用来研究结晶机制，以及它们如何依赖于分子水平上的细节连接到表面的配体。然后，将该相互作用模型作为速率表达式纳入到群体平衡模型中，以研究取向附着结晶过程的动力学，从而预测组装晶体形态、维度和缺陷密度的时间演化。该奖项反映了NSF的法定使命，通过使用基金会的知识价值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.