Electrophoretic assembly of colloidal crystals with optically tunable micropatterns

The production of materials with micrometre- and submicrometre-scale patter ns is of importance in a range of applications, such as photonic materials( 1,2), high-density magnetic data storage devices: microchip reactors(4) and biosensors(5). One method of preparing such structures is through the asse mbly of colloidal particles(5-10). Micropatterned colloidal assemblies Have been produced with lithographically patterned electrodes(5,11) or micromou lds(12). Here we describe a different method that combines the well-known p hotochemical sensitivity of semiconductors(13,14) with electric-field-induc ed assembly(15,16) to create ordered arrays of micrometre-sized colloidal p articles with tunable patterns. We show that light affects the assembly pro cesses, and demonstrate how to produce patterns using electrophoretic depos ition in the presence of an ultraviolet (UV) illumination moth. The distrib ution of current across an indium tin oxide (ITO) electrode can be altered by varying the illumination intensity: during the deposition process, this causes colloidal particles to be swept from darkened areas into lighted reg ions. Illumination also assists in immobilizing the particles on the electr ode surface. Although the details of these processes are not well understoo d, the patterning effects of the UV light are discussed in terms of alterat ions in the current density(15,17) that affects particle assembly on an ITO electrode.