Materials whose dielectric constant varies spatially with submicrometer periodicity exhibit diffractive optical properties which are potentially valuable in a number of existing and emerging applications. Here, such systems are fabricated by exploiting the spontaneous crystallization of monodisperse silica spheres into close-packed arrays. By reliance on a vertical deposition technique to pack the spherical colloids into close-packed silica-air arrays, high quality samples can be prepared with thicknesses up to 50 µm. These samples are planar and thus suitable for optical characterization. Scanning electron microscopy (SEM) of these materials illustrates the close-packed ordering of the spherical colloids in planes parallel to the substrate; cross-sectional SEM micrographs of the arrays as well as optical methods are used to measure sample thickness and uniformity. Normal-incidence transmission spectra in the visible and near-infrared regions show distinct peaks due to diffraction from the colloidal layers. While these basic optical characteristics are similar to thicker and polycrystalline gravity-sedimented 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.

We report a nanoscale "lost-wax" method for forming colloids with size distributions around 5% and their corresponding colloidal crystals. Macroporous polymer templates are first prepared from a silica colloidal crystal. We then use the uniform and interconnected voids of the porous polymer to generate a wide variety of highly monodisperse inorganic, polymeric, and metallic solid and core-shell colloids, as well as hollow colloids with controllable shell thickness, as colloidal crystals. We can also uniformly deform the polymer template to alter colloidal shape and demonstrate the formation of elliptical particles with precisely controlled aspect ratios.

No abstract

No abstract

The fabrication of polymeric materials with ordered submicron-sized void structures is potentially valuable for many separation technologies as well as for emerging optical applications. This paper reports the preparation of macroporous polymer membranes with regular voids and the characterization of their diffractive optical properties. These materials are made using a colloidal crystal template of silica microspheres; the air between the spheres can be replaced by monomers that can be subsequently polymerized. The use of silica microspheres as templates makes it possible to employ chemical rather than thermal methods for template removal. For this reason, polymers as diverse as polyurethane and polystyrene can be used to create free-standing macroporous films, with thickness ranging from 0.5 to 50 μm. Scanning electron microscopy of these samples indicates a well-formed porous structure consisting of voids ranging in diameter from 200 to 400 nm. These large cavities are not isolated, but rather interconnected by a network of monodisperse smaller pores (d = 50−130 nm) whose size can be controlled by varying the polymerization temperature. These membranes exhibit striking optical properties due to the periodic arrangement of air spheres in the polymer medium. Normal-incidence transmission measurements of these samples are compared to a theoretical model based on a scalar wave approximation. This model assumes an ordered structure of close-packed, three-dimensional air spheres. The good agreement between theory and experiment provides additional evidence of the long-range order of these samples.

We report observations of the optical stop band of periodic planar arrays of submicron silica spheres, and of macroporous polymers grown from these silica templates. The stop-band width and peak attenuation depend on the number of layers and on the dielectric contrast between the spheres and the interstitial regions, both of which are experimentally controlled. The results are compared to the predictions of the scalar wave approximation. This is the first systematic study of the thickness dependence of the stop band in colloidal photonic band gap structures.The optical properties of photonic band gap materials have been the subject of many experimental and theoretical studies in recent years [1]. In these systems the dielectric function is spatially periodic in one or more dimensions, and as a result their optical properties are dominated by strong diffraction effects. The propagation of light is strongly inhibited over a narrow range of frequencies; this produces a dip in the transmission spectrum known as a stop band. In order to construct structures whose stop bands are centered in the visible region, the length scale of the periodicity must shrink accordingly. As a result, lithographic fabrication of materials with visible stop bands is quite challenging.One class of materials which offers a unique solution to this problem is colloidal crystals [2][3][4][5]. Here, one relies on the tendency of submicron dielectric spheres to spontaneously self-assemble into ordered arrays. This approach has several advantages over the conventional "top-down" fabrication techniques, including a wider versatility with respect to the choice of materials, the relative ease of casting high quality planar films as optical coatings, and the low cost of implementation. Further, colloids as small as 50 nm have been crystallized [6]; such systems could extend the applicability of photonic band gap materials into the soft-x-ray range. Finally, unlike lithographic and machining techniques, which generally limit sample thickness to only a few repeating layers, colloidal crystals can be fabricated with controlled thickness, up to hundreds of layers [7]. This may prove to be a significant advantage if control over sample thickness provides an avenue for tuning the optical properties.Thus, an understanding of the evolution of the optical properties of these materials with an increasing number of layers is an important ingredient in the study of these strongly diffracting systems. Yet, a systematic investigation of this thickness dependence has been lacking. This is due to the difficulty in controlling the thickness of colloidal samples, and also in producing samples with sufficient uniformity and optical quality for such measurements. Although the thickness dependence of the optical properties of periodic dielectric media has been studied in the microwave and submillimeter range [8,9], these systems do not mimic the spherical close-packed arrangement common to most colloidal crystals.In this work, we address this issue using colloidal crysta...

We present a quantitative comparison between two analytic theories for the propagation of electromagnetic waves in periodic dielectric structures. These theories have both been used extensively in the modeling of optical spectra of colloidal crystals exhibiting photonic band gap behavior. We demonstrate that dynamical diffraction theory is equivalent to the scalar wave approximation, in the limit of small dielectric contrast. This equivalence allows us to place quantitative limits on the validity of dynamical diffraction, relative to the predictions of the more accurate scalar wave theory. We also note that dynamical diffraction is often applied with boundary conditions which neglect the strong interference between the incident and diffracted waves within the periodic medium. These boundary conditions lead to expressions for the transmission spectrum which cannot be generalized to the case of normal-incidence propagation. We provide a corrected form for these expressions, and use them in comparisons with experimental spectra. Excellent agreement between theory and experiment is obtained for the widths of optical stop bands, for both positive and negative values of the dielectric contrast. These are among the first quantitative comparisons between theoretical and experimental optical spectra of colloidal photonic crystals.

This paper describes for the first time a chemical method for the preparation for nanocrystalline quartz. Submicron quartz powders are initially produced in hydrothermal reactions where soluble silica precursors precipitate as pure crystalline silica. To yield nanocrystalline material these particles can be purified and size selected by dialysis, filtration, and centrifugation. Transmission electron microscopy and X-ray diffraction illustrate that the product is pure phase r-quartz, consisting of isolated (i.e., nonaggregated) nanocrystals. Depending on the size selection method, crystallites with average sizes of 10 to 100 nanometers can be recovered.