Employing Convective Assembly for Micro-/Nano-Fabrication of Colloidal Crystals

弱相互作用的胶体颗粒大小均匀，从几纳米到微米不等，可以从浓缩的液体悬浮液中自发地组织成紧密排列的晶体。因为它们提供了简单有序的结构，具有良好控制和均匀的孔隙度，这些材料已经被研究用于许多重要的应用，包括传感、分离、微过滤和电池。胶体晶体的一个特别有趣和有前途的应用是它们在制造光子晶体中的作用。这些晶体表现出光子的带隙，即在材料内部存在一个光子频率范围，在这个范围内光不能向任何方向传播。这种特性可以用于操纵光子，用于新型光学电路、生物和化学传感器以及高效的热发射源。为了推进所有这些应用，需要一种高效、低成本的方法来制造大量高质量的胶体晶体。胶体晶体传统上是通过液体悬浮液中球体的温和沉降制成的。这种技术不适合作为制造过程，因为沉淀速度非常慢，需要几个月的时间。如果匆忙，所得到的晶体通常会由于大量的无序而有缺陷。一种被称为对流自组装的过程可以在几小时内迅速地将胶体颗粒沉积在浸入蒸发液体悬浮液的倾斜板上。令人惊讶的是，这些蓬勃生长的层具有近乎完美的面心立方（fcc）晶体结构，这是该系统的平衡填料。快速的生长速度和高质量的材料使对流组装成为一种有吸引力的候选胶体晶体制造工艺。本研究将计算模型和实验相结合，以了解流体流动和毛细作用在纳米级胶体颗粒对流组装过程中形成晶体结构的作用。与传统的颗粒沉降方法相比，对流组装工艺证明了更高的生产率和更高的材料质量。从这个意义上说，毛细作用和流体运动协调了粒子相互作用的大规模并行化，以实现产量和质量的提高；然而，要利用这一过程实现工业规模的生产、可靠性、稳健性、产量、效率和成本的衡量，还需要在理解上取得重大进展。这种理解对于大规模纳米制造工艺的发展至关重要。这项工作的社会效益将包括纳米制造新方法的发展，以及纳米颗粒基晶体材料的可用性所带来的长期效益，这些材料将影响环境、能源和信息技术的应用。更广泛的活动包括对本科生和研究生的纳米技术教育，以及涉及明尼苏达科学博物馆的面向公众的推广计划。

Weakly interacting colloidal particles, with uniform sizes ranging from several nanometers to microns, can spontaneously organize into close-packed crystals from concentrated liquid suspensions. Because they provide a simple, ordered structure with well-controlled and homogeneous porosity, these materials have been studied for many important applications, including sensing, separations,microfiltration, and batteries. A particularly interesting and promising application for colloidal crystals is their role for fabricating photonic crystals. These crystals exhibit a band gap for photons, namely there exists a range of photon frequencies inside the material for which light cannotpropagate in any direction. This property could be utilized to manipulate photons for novel optical circuits, biological and chemical sensors, and efficient thermal emission sources. To advance all of these applications, there is a need for an efficient, low-cost means to manufacture large quantitiesof high-quality colloidal crystals. Colloidal crystals have traditionally been made via the gentle sedimentation of spheres in a liquid suspension. This technique is ill-suited as a manufacturing process, since the settling rate is very slow, requiring months. If rushed, the resultant crystal is typically flawed by a significant amount of disorder. A process known as convective self-assembly can quickly, within hours, deposit colloidal particles into layers onto an inclined plate immersed within an evaporating liquid suspension.Surprisingly, these vigorously growing layers are characterized by a nearly perfect, face-centered cubic (fcc) crystalline structure, the equilibrium packing for this system. The fast growth rate and high material quality make convective assembly an attractive candidate for a manufacturingprocess for colloidal crystals. This research combines programs of computational modeling and experiments to understand the role of fluid flow and capillarity during the convective assembly of nanoscale, colloidal particles to form crystalline structures. Convective assembly processes have demonstrated greater production rates and higher material quality than achieved by classical particle settling methods. In this sense, capillarity and fluid motion coordinate a massive parallelization of particle interactions to achieve increases in production and quality; however, significant advances in understanding are neededto harness this process to achieve industrial-scale measures of production, reliability, robustness, yield, efficiency and cost. This understanding will be critical for the development of large-scale, nanomanufacturing processes.The societal benefits of this work will include the development of new approaches to nanomanufacturing, with longer-term benefits promised by the availability of nanoparticle-based crystalline materials that will impact applications for the environment, energy, and information technology.Broader activities include the education of undergraduate and graduate students in nanotechnology, as well as an outreach program for the general public involving the Science Museum of Minnesota.