Templating Colloidal Crystal Growth Using Chirped Surface Relief Gratings.

We report a method for controlling the lattice geometry of monodisperse colloidal crystals formed by confined convective self-assembly on a substrate patterned with a chirped surface relief grating. Chirped gratings were fabricated using laser interference lithography and a curved mirror reflector to create photoresist patterns with pitch values ranging from ∼500 to >10 000 nm spread over a planar surface. These surface nanostructures are shown to guide the formation of various lattice geometries not normally found via colloidal assembly on planar surfaces. It is shown that when the pitch of the grating is much larger than the diameter of the colloidal particles, the grating trenches serve as compartments for deposition and the particles form close-packed, linear chains. Various ordered structures are observed as the dimensions of the grating pitch decrease and approach the diameter of the particles. The grating nanostructures guide the formation of various lattice geometries due to specific particle-surface and particle-particle interactions. Observed crystal lattices include square, hexagonal, and rhombic structures. The formation of these structures is explained in terms of the geometrical constraints imposed by the surface pattern and the particle diameter. These crystal lattices can be translated into large area samples when using corresponding single-pitch grating substrates. The initial monolayer lattice can also serve as a template for the growth of unique, bilayer structures that include rectangular lattices, chains of particle pairs or triplets, and graphitelike structured lattices. In addition, when coated with a thin silver layer, these various lattice configurations are shown to produce optical reflection features that are precisely controlled by the underlying structure as it varies from widely spaced particle chains to close-packed lattice geometries.