Photonic crystals self-assembled against templates

Self-assembly of monodispersed spherical colloids has been demonstrated as an effective strategy to fabricate three-dimensional photonic bandgap crystals. The major challenge in this field is to control the order, thickness, domain size, crystal orientation, defects, and registry of colloidal crystals. In this paper we describe a Template-Assisted Self-Assembly (TASA) approach to control the orientation of the photonic crystals. The self-assembled crystalline lattice usually has a face-center-cubic (fcc) structure with its (111) planes parallel to the surface of the solid support. In TASA process, we used an array of pyramid-shaped pits etched in a Si (100) wafer as the templates. The pits were fabricated by photolithographic patterning and anisotropic etching. Owing to the 70.6° angular geometry of the pyramid-shaped pits, monodispersed colloids nucleated and grew in a vectorial fashion exclusively within the pits to forma pattern of fcc colloidal crystals with (100) layer planes parallel to the (100) face of the single crystalline Si wafer. The small crystals in silicon pits then served as seeds to define and direct the further growth of the crystal along the direction perpendicular to the substrate. A large, (100)-oriented single crystal of colloids with well-controlled thickness was obtained if the diameter of the colloids matched the separation between adjacent pits, and if the raised edge between adjacent pits was small enough.