The Fabrication and Bandgap Engineering of Photonic Multilayers

This communication describes a method to fabricate multilayer colloidal crystals formed by the layer-by-layer deposition of silica beads on a glass substrate. Each layer of the crystal consists of a three-dimensionally ordered array of close-packed colloids. These multilayer samples are amenable to templating methods for tuning the dielectric contrast of the material. The resulting photonic crystal structures exhibit optical properties which resemble the superposition of the properties of each individual crystal, with additional structure that suggests the onset of superlattice-type miniband formation. These multilayer structures thus afford new opportunities for engineered photonic behavior. Traditionally colloidal crystals are three dimensional periodic structures formed from monodisperse colloids. Because of their diffractive optical properties they are a type of photonic crystal and may have applications as optical filters and switches, high density magnetic data storage devices, chemical and biochemical sensors, or as removable scaffolds for the formation of highly ordered, macroporous materials. They are also useful as model systems for fundamental studies of crystal melting and phase transition behavior. The process of colloidal crystallization has been extensively studied, leading to the development of several methods to make high quality colloidal crystals with few crystalline defects. These techniques include electrostatically induced crystallization, gravity sedimentation, electro-hydrodynamic deposition, colloidal epitaxy, physical confinement and convective self-assembly. Bimodal AB2 and AB13 colloidal crystals with complex structures have also been observed in binary mixtures of hard-sphere colloids with specific radii ratios. Here we report a method to make a new form of colloidal crystal, a multilayer crystal, using successive deposition of crystals of colloids of arbitrary sizes. The multilayer colloidal crystal is schematically represented in Figure 1A. Spheres of different colors represent submicrometer silica or polystyrene colloids of different sizes. Each layer of the crystal is a close-packed array of colloids, and the overall structure consists of successively stacked crystals, formed of colloids of arbitrary diameters. The preparation of these structures is described in the experimental section. The high uniformity of the resulting crystals can be illustrated by the transmission (Fig. 1B) and reflection (Fig. 1C and D from different angles) photographs of a threelayer crystal. This sample is formed by consecutive deposition of 13 layers of 430 nm silica spheres, followed by 16 layers of 253 nm silica spheres, followed by 10 layers of 338 nm silica spheres. We describe the multilayer colloidal crystal pattern by listing the sphere size from bottom to top. For example, the sample in Figure 1 is referred to as 430 nm/253 nm/338 nm. The reflected colors are caused by Bragg diffraction of visible light by the three-dimensionally ordered arrays of submicrometer colloids. When two overlapping layers are made from colloids with extremely different sizes, most of the reflected light from the bottom layer will transmit through the upper layer, resulting in the transparent appearance of the second layer in Figure 1C (430 nm/253 nm). Crystalline quality is among the most important parameters in determining the performance of colloidal crystals in optical applications. Figure 2 shows the typical top-view and crosssectional scanning electron microscopy (SEM) images of each astepo of the sample shown in Figure 1. In Figure 2A, the hexagonal close-packed (hcp) arrangement of the top 430 nm layer is evident. The sharp peaks in the two-dimensional fast Fourier transform (FFT, inset) of a low-magnification image confirm the presence of long-range crystalline order extending over the largest length scales (40  40 lm) accessible in a single image. The stacking of close-packed layers shown in Figure 2B demonstrates the high degree of order along the (111) crystallographic axis, perpendicular to the substrate.