Single-crystal colloidal multilayers of controlled thickness

Materials whose dielectric constant varies spatially with submicrometer per iodicity exhibit diffractive optical properties which are potentially valua ble in a number of existing and emerging applications. Here, such systems a re fabricated by exploiting the spontaneous crystallization of monodisperse silica spheres into close-packed arrays. By reliance on a vertical deposit ion technique to pack the spherical colloids into close-packed silica-air a rrays, high quality samples can be prepared with thicknesses up to 50 mu m. These samples are planar and thus suitable for optical characterization. S canning electron microscopy (SEM) of these materials illustrates the close- packed ordering of the spherical colloids in planes parallel to the substra te; cross-sectional SEM micrographs of the arrays as well as optical method s are used to measure sample thickness and uniformity. Normal-incidence tra nsmission spectra in the visible and near-infrared regions show distinct pe aks due to diffraction from the colloidal layers. While these basic optical characteristics are similar to thicker and polycrystalline gravity-sedimen ted colloidal crystals, the systematic control over the number of colloidal layers allows the effect of sample thickness on the optical spectrum to be studied for the first time.