Visualizing Nanoparticle Packing at Liquid Interfaces

非技术摘要纳米粒子，比分子大不了多少的物体，在二维和三维空间中组装成具有独特和技术重要性的材料。 直到最近，由于纳米颗粒的尺寸非常小，这种组装无法直接看到。 对于这一提议，已经开发了一种新的成像方法，可以解析组装在液体界面上的单个纳米颗粒的结构和运动。 该方法结合了高分辨率的电子显微镜，约100-1000倍大于光学显微镜，与非挥发性的离子液体，基本上是液体盐，探测纳米粒子的二维组装作为纳米粒子的尺寸，几何形状，本体化学，表面化学，和液体润湿程度的函数。 与较大的颗粒不同，纳米颗粒可以在与颗粒本身一样大的距离上相互作用，这一特征有助于组装成有序结构。与较大的粒子不同，这些相互作用很弱，允许粒子“抖动”以找到最佳排列。在许多实验中，不同种类的纳米粒子被混合，增加了有序结构的多样性，或者在其他情况下，创造了一个冻结的无序“堵塞”状态。 确定纳米颗粒组织的条件，了解这些变化如何影响组织，以及揭示制造完全无序系统的方法是该项目的关键目标。 这项研究是由本科生，研究生和博士后研究员进行的，结果往往是每个人都容易理解的惊人图像或电影。 从长远来看，从这项研究中出现的工具和技术应该很容易地应用于其他技术上重要的材料，如凝胶，乳液，液晶和悬浮液，所有这些都显示出由纳米尺度特征和事件决定的整体性质。技术摘要尽管需要施加真空，扫描电子显微镜可以在开放的液体样品上进行，当组分被润湿或分散在，非挥发性离子液体。 在3-5 nm的成像分辨率下，可以在无限的时间内每秒采集几帧。 初步实验建立了扫描电子显微镜的可行性，单颗粒跟踪，即使是密集的纳米颗粒包装和非球形颗粒形状。该方法的功能很像光学视频显微镜，但放大倍数为10- 100倍。 离子液体表面上的二维纳米颗粒有序化现在被全面和定量地追求，检查纳米颗粒尺寸、几何形状、本体化学、表面化学（即，聚合物配体类型）和液体润湿程度。 此外，混合不同类型的纳米颗粒。与较大的胶体颗粒不同，纳米颗粒相互作用通常较弱，仅在布朗运动下允许平衡组装。将针对所有列出的参数评价相互作用势，最关注配体类型，其控制相互作用长度尺度，达到或超过粒度。配体还影响接触角，这决定了界面捕获的颗粒伸入底层液体的深度，反过来，这个深度影响颗粒动力学。一种新的透射电子显微镜方法纳米粒子接触角正在评估，并通过新的扫描电子显微镜跟踪方法，相关的界面纳米粒子的动力学正在量化。正在研究有序和无序的致密纳米颗粒填料，后者通过混合不同尺寸/形状的颗粒来促进。为了控制纳米颗粒的面密度，将用于控制界面面积的独特装置放置在显微镜内，在收集图像序列时原位进行面积调整。 通过计算平移和取向参数来分析组装成致密结构。 最后，一个有趣的和无法解释的耦合之间的显微镜的电子束和固体金属或金属涂层的纳米粒子正在追求创造精确的纳米粒子图案在液体表面上。这个奖项反映了NSF的法定使命，并已被认为是值得支持的评估使用基金会的智力价值和更广泛的影响审查标准。

NON-TECHNICAL ABSTRACTNanoparticles, objects not much larger than molecules, assemble in two- and three-dimensions into materials with unique and technologically important properties. Until recently, this assembly could not be directly seen because of the very small size of nanoparticles. For this proposal, a new imaging method has been developed that can resolve the structure and motions of individual nanoparticles assembled on a liquid interface. The method combines the high resolution of electron microscopy, about 100-1000 times greater than optical microscopy, with the nonvolatility of ionic liquids, essentially liquid salts, to probe the two-dimensional assembly of nanoparticles as a function of nanoparticle size, geometry, bulk chemistry, surface chemistry, and degree of liquid wetting. Unlike larger particles, nanoparticles can interact with each other over distances as large as the particles themselves, a feature that facilitates assembly into well-ordered structures. Also unlike larger particles, these interactions are weak, allowing the particles to "jiggle around" so as to find the best arrangement. In many experiments, different sorts of nanoparticles are mixed, increasing the diversity of the ordered structures, or in other cases, creating a frozen disordered "jammed" state. Determining conditions under which nanoparticle particles organize, understanding how these changes affect organization, and uncovering ways to make completely disordered systems are the key project objectives. This research is performed by undergraduate, graduate, and post-doctoral fellows, and the outcomes are often striking images or movies easily understandable by everyone. In the long term, the tools and techniques emerging from this research should apply easily to other technologically important materials such as gels, emulsions, liquid crystals, and suspensions, all of which display bulk properties determined by nanoscale features and events.TECHNICAL ABSTRACTDespite a need to impose vacuum, scanning electron microscopy can be performed on open liquid specimens when the components are wetted by, or dispersed in, a nonvolatile ionic liquid. At an imaging resolution of 3-5 nm, several frames per second can be acquired over an unlimited time. Preliminary experiments established the feasibility of scanning-electron-microscope, single-particle tracking even for dense nanoparticle packings and non-spherical particle shapes. The method functions much like optical video microscopy but at 10-100X greater magnification. Two-dimensional nanoparticle ordering on ionic liquid surfaces is now pursued comprehensively and quantitatively, examining effects of nanoparticle size, geometry, bulk chemistry, surface chemistry (i.e., polymer ligand type), and degree of liquid wetting. In addition, different types of nanoparticles are mixed. Unlike larger colloidal particles, nanoparticle interactions are typically weak, allowing equilibrium assembly under Brownian motion alone. The interaction potential will be evaluated for all listed parameters, with greatest attention on ligand type, which controls the interaction length scale, reaching or exceeding the particle size. Ligands also affect the contact angle, which dictates how deeply the interface-trapped particle protrudes into the underlying liquid, and, in turn, this depth influences particle dynamics. A new transmission-electron-microscopy approach to nanoparticle contact angle is being assessed, and by the new scanning-electron-microscopy tracking method, the associated interfacial nanoparticle dynamics are being quantified. Both ordered and disordered dense nanoparticle packings are being studied, with the latter facilitated by mixing particles of different size/shape. To control nanoparticle areal density, a unique device for control of interfacial area is placed inside the microscope, with areal adjustments made in situ as image sequences are collected. Assembly into dense structures is analyzed by calculating translational and orientational parameters. Lastly, an interesting and unexplained coupling between the electron beam of the microscope and solid metallic or metal-coated nanoparticles is being pursued to create precise nanoparticle patterns on liquid surfaces.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.