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

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 cannot

propagate 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 quantities

of 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 manufacturing

process 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 needed

to 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.