Model 2D Ordering: Structure and Dynamics of Nanoparticles and Their Mixtures at Liquid Interfaces

非技术摘要：纳米粒子太小，即使是最好的光学显微镜也看不见，只比单个分子大一点，但它们可以附着在液体表面，并以越来越大的密度堆积在那里，形成只有一个粒子厚的二维层。为了防止颗粒聚集，它们通常被一层溶剂膨胀的聚合物包裹，以防止颗粒相互物理接触。单个纳米颗粒形成具有磁性、导电性、光学性或筛分性的层。这些纳米粒子组件对新兴技术越来越重要，从显示器到存储介质再到柔性电子产品。在这个项目中，一种新的电子显微镜方法可以对纳米颗粒在液体表面重新排列、结晶或堵塞时进行现场实时成像。在单颗粒分辨率下可见的现象包括不同纳米颗粒的混合/分离，通过改变聚合物化学来控制堆积，以及棒状纳米颗粒的相互组织/取向。视频及时记录了纳米颗粒是如何组织起来的。一个进一步的项目目标是建立电子显微镜设备，机械地对充满纳米颗粒的液体表面施加压力，迫使它们重新组织，开辟更完美的包装路线，甚至在使用两种或两种以上不同类型的纳米颗粒时形成表面图案。在开发和应用这种新的成像方法的过程中，一名博士生和一名博士后在聚合物、纳米颗粒、显微镜和界面的学科交叉方面获得了技能。收集的纳米尺度过程的图像和电影提供了一个视觉上吸引人的纳米技术和软物质研究的介绍，整合在K12和公共宣传活动中。技术摘要：纳米颗粒作为吉布斯单层附着在液体界面上是许多技术的关键，更一般地说，这些系统为探测二维（2D）组装过程提供了良好的模型。然而，纳米颗粒太小，无法在光学显微镜下分辨，而且人们对它们在液体界面上的行为知之甚少。与大颗粒不同，稳定纳米颗粒的柔性聚合物配体通常接近甚至超过颗粒大小；这些配体是如何在液体表面促进和稳定纳米颗粒的，目前还很少有人研究。一种新的原位扫描电子显微镜（SEM）方法可以在单粒子水平上实时观察密集堆积的界面纳米颗粒的微观结构和动力学，甚至可以观察那些正在经历二维结晶和干扰的纳米颗粒。在这里，该方法解决了界面纳米颗粒组装中的几个突出问题，包括配体在调节界面纳米颗粒相互作用中的作用，纳米颗粒形状对单层组装的影响，纳米颗粒界面混合物的相行为，以及用于原位操纵纳米颗粒装饰液体界面的机电设备的开发。虽然扫描电镜方法的基础实验依赖于离子液体的独特性质，但使用可变压力扫描电镜可以解除对这种特殊液体的需求，极大地扩大了纳米颗粒、液体和配体的成像范围。随着这项研究的深入，界面纳米颗粒技术的可能性得到了极大的扩展，其微观结构、动力学和物理性质首次得以预测。此外，这些改进的理解将导致新的行为，从而在3D打印、选择性分子筛分和纳米级表面图案等领域创造新技术。该奖项反映了美国国家科学基金会的法定使命，并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。

Non-Technical Abstract:Nanoparticles, too small to see with even the best optical microscopes and just larger than individual molecules, can attach to a liquid surface and pack there at increasing density to create a two-dimensional layer only one particle thick. To keep the particles from aggregating, they typically are coated with a solvent-swollen polymer that prevents the particles from physically touching each other. The individual nanoparticles form layers that possess magnetic, conductive, optical or sieving properties. These nanoparticle assemblies are of increasing importance for emerging technologies, ranging from displays to storage media to flexible electronics. In this project, a new electron microscopy method enables the in situ, real-time imaging of nanoparticles as they rearrange and possibly crystallize or jam at a liquid surface. Among the phenomena visualized at single-particle resolution are mixing/separation of dissimilar nanoparticles, control of packing through variation of polymer chemistry, and mutual organization/orientation of rod-like nanoparticles. Videos document in time how the nanoparticles organize themselves. A further project goal is to build electron microscope devices that mechanically stress a liquid surface laden with nanoparticles, forcing them to re-organize, opening routes to more perfect packings or even the formation of surface patterns when two or more different types of nanoparticles are used. In developing and applying the new imaging method, a Ph.D. student and a post-doctoral student are gaining skills at the disciplinary intersection of polymers, nanoparticles, microscopy, and interfaces. The collected images and movies of nanoscale processes offer a visually appealing introduction to nanotechnology and soft matter research that is integrated in K12 and public outreach activities.Technical Abstract:Nanoparticles attached to liquid interfaces as Gibbs monolayers are key to numerous technologies, and more generally, these systems provide excellent models for probing two-dimensional (2D) assembly processes. However, nanoparticles are too small to resolve in an optical microscope, and little is known about their behavior at liquid interfaces. Different than larger particles, the flexible polymeric ligands that stabilize nanoparticles often approach or even exceed the particle size; how these ligands promote and stabilize nanoparticles on a liquid surface has scarcely been explored. A new in situ scanning electron microscopy (SEM) method can visualize, to the single particle level and in real-time, the microstructures and dynamics of densely packed interfacial nanoparticles, even those that are undergoing 2D crystallization and jamming. Here, this method addresses several outstanding problems in interfacial nanoparticle assembly, including the role of ligands in modulating interfacial nanoparticle interactions, the influence of nanoparticle shape on monolayer assembly, the phase behavior of nanoparticle interfacial mixtures, and the development of electromechanical devices for in-situ manipulation of nanoparticle-decorated liquid interfaces. While the SEM method’s foundational experiments relied on the unique properties of ionic liquids, using a variable pressure SEM the need for such special liquids can be lifted, vastly expanding the range of nanoparticles, liquids, and ligands accessible to imaging. With the insights gained in this study, possibilities for interfacial nanoparticle technologies are being greatly expanded, with microstructures, dynamics, and physical properties made predictable for the first time. Further, the improved understandings are leading to novel behaviors that will create new technologies in areas such as 3D printing, selective molecular sieving, and nanoscale surface patterning.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.