In Situ Investigation of Large-Area Uniform Colloidal Crystal Fabrication

Colloidal crystals, with their highly ordered structure and tunable properties, are revolutionizing industries from energy storage and solar technology to catalysis and biosensors. These materials are synthesized by processing colloidal dispersions under conditions that promote self-assembly. As the demand for advanced materials grows across the above industries, a fundamental understanding of how to fabricate these materials at scale becomes essential for optimizing their performance.

The solution-based approach of manufacturing colloidal crystals inherently requires a drying step, as the dry colloidal crystal is the form implemented into functional devices. This drying process is critical; it shapes the final morphology and can disrupt the long-range order of the colloidal crystal. Despite many researchers reporting uniform large scale deposition of colloidal crystals, closer analysis of their results often reveals significant sample heterogeneity. Additionally, reliance on single optical or scanning electron microscope images to validate large-scale uniformity can be misleading. Therefore, addressing the drying-induced heterogeneity is key to unlocking the full potential of colloidal crystals in scalable, economical devices.

Our research investigates the fabrication of large-area, uniform colloidal crystals using gravitational sedimentation self-assembly. This self-assembly method has the advantage of enabling the colloidal self-assembly and drying to occur separately. This facilitates independent study of these two important processes. This self-assembly method can also be readily applied to roll-to-roll slot die coating, a manufacturing technique ubiquitous in battery, solar panel and liquid crystal display (LCD) manufacturing. Our in-house developed roll-to-roll slot die coater observes the self-assembly and drying of large-area colloidal crystals insitu, using optical microscopy and transmission spectroscopy. We investigate the effects of solution pH, drying rate and addition of free polymers. Critical drying phenomena such as the meniscus receding, the coffee ring effect and Marangoni flows are observed on our semi-production equipment. In addition to exploring known disruptions, our study uncovers new previously unreported phenomena that appear intrinsic to the drying of colloidal crystals, with the root cause of these new disruptions remaining unclear. These novel insights offer guidance for the design of colloidal crystals and strategies to minimize disruption during drying, thereby enhancing the uniformity and quality of dried colloidal crystal samples.

To complement these findings, we conduct further insitu studies using smaller scale, confined volume experiments. These targeted experiments allow us to explore in more detail how variables such as substrate surface, solution pH, and colloid volume fraction impact crystallization behaviour. Our real-time crystallization observations link the “wet” or solution-based colloidal crystal to the final colloidal crystal structure and meso-scale drying pattern. For example, we observe that as the pH of the colloidal dispersion decreases, inter-colloid spacing increases and long-range order diminishes. However, this trend is not retained in the dry material; lower pH results in smaller inter-colloid spacing and improved long-range order in the dry form, leading to more uniform samples. These findings facilitate precise control over the resultant colloidal crystal structure.

Our findings provide valuable insights and practical solutions for a diverse audience from research scientists to large-scale manufacturers. Beyond its practical contributions, our work offers a deeper understanding of the formation mechanisms of the final colloidal crystal and factors which disrupt or enhance their structure. Therefore, this study paves the way for scalable and uniform production of advanced self-assembled materials, essential for their industrial implementation.