The promise of nanotechnology lies in how new nanostructured materials react to and guide heat, light, charge, or molecular species. The design of nanomaterials with tailored transport properties—even unusual ones not found in nature, such as "metamaterials" that have negative refractive indices—are the foundation of new technologies for a broad range of applications, including energy production and storage, separations, and catalysis.
Sophisticated nanoscale structures can be manufactured from the "top down." The fabrication processes of repeated etching and deposition of semiconductors, metals and dielectrics gives us complex and powerful integrated circuits. But nanoscale structures can also be created from the "bottom-up", wherein a milieu of building blocks in a dispersed, homogeneous state is cast, printed, or deposited. From this precursor jumble, functional nanomaterials spontaneously form by self-assembly as the building blocks arrange into coherent nanoscale structures dictated by their thermodynamics. This bottom-up approach to nanomanufacture promises low-cost and high-rate continuous processes.
In this talk, I will focus on colloidal and nanoparticle self-assembly using directing fields. While thermodynamics dictates the structures that form by self-assembly, the kinetics of assembly often trap structures into arrested states, such as glasses and gels. Kinetic bottlenecks are circumvented using electric and magnetic fields to guide self-assembly. We show that by toggling an applied field on and off, suspensions of polarizable colloids can be annealed into a crystalline state. A key advantage of directed self-assembly in toggled fields is the relatively large range of field-strengths (effective temperatures) that lead to phase separation. These results demonstrate how kinetic barriers to a colloidal phase transition are subverted through measured, periodic variation of driving forces while retaining the strengths of a “bottom-up” self-assembly process.