Electrostatic self-assembly of polystyrene microspheres by using chemically directed contact electrification.

Herein we describe a process—based on contact electrification and electrostatic interactions—that directs the selfassembly of chemically modified polystyrene microspheres to form three-dimensional microstructures. When two solid surfaces are brought into contact and separated, charge is often transferred from one surface to the other in a process known as contact electrification. 2] We can predictably and rationally control the contact electrification of polystyrene microspheres, and use the resulting charged materials for electrostatic self-assembly. We control the contact electrification of these microspheres by introducing immobilized ions and mobile counterions: the choice of these ions determines the electrostatic charges that these beads acquire through contact before and during the assembly process. Oppositely charged microspheres assemble into uniform spherical microstructures under the influence of electrostatic forces. Sequential steps of self-assembly can create multilayered microstructures. There are many examples of electrostatic self-assembly of charged ions, polyelectrolytes, and colloids in solution. Xerography, an example of dry electrostatic self-assembly, uses corona discharge from a high-voltage electrode to create a charge on the imaging drum, and contact electrification to create an opposite charge on the toner particles. Contact electrification can also direct the self-assembly of millimetersized spheres into ordered two-dimensional lattices. (That process used the inherent differences in contact electrification of various polymers, in contrast to the rational, chemically directed contact electrification we describe herein.) Patterns of charge, created by electron-beam writing, an atomic force microscopy (AFM) tip, or electrical microcontact printing on a dielectric surface, can guide the self-assembly of microor nanoparticles with sub-100 nm lateral resolution. Although contact electrification is a familiar phenomenon, the detailed mechanisms of contact electrification are not known, and it is likely that different mechanisms may be involved depending on the specific materials and environmental conditions. There is one class of materials that exhibits predictable contact electrification and for which Diaz and coworkers have proposed a plausible mechanism of chargetransfer: the contact charging of ionomers (polymers with covalently bound ionic functional groups) is believed to result from the transfer of mobile ions from the ionomer to another material (Figure 1).