Template-directed syntheses have been broadly applied to the creation of both meso-and macroporou... more Template-directed syntheses have been broadly applied to the creation of both meso-and macroporous ceramics 1-4 and polymers. 5 The extension of such methods to metals is of particular interest. Monolithic metals containing large internal surface areas could be used in numerous applications, ranging from electrochemical sensors 6 to catalytic converters. 7 Moreover, recent work on lithographically prepared metallodielectric structures suggests metals with well-ordered porous networks would exhibit interesting photonic properties. 8 The adaptation of template methods to the formation of porous metals has been relatively limited. [9][10][11][12] Lyotropic liquid crystalline 12 and anodic alumina 10 have been used as templates to produce mesoporous (voids <50 nm) metal films with parallel and cylindrical pores; however, techniques to make macroporous (voids >50 nm) and high void volume porous films have not yet been reported. In this work we describe a strategy for extending the general methodology of template-directed synthesis to the formation of porous metals. These samples are three-dimensional highly ordered, free-standing macroporous metal films with interconnected spherical voids (200-400 nm). We use as templates single-crystal colloidal multilayers made by a vertical deposition technique. 13 The uniform deposition of metals into colloidal arrays is challenging for several reasons. Existing examples of metal deposition into lyotropic liquid crystal 12 and anodic alumina 10 templates exploit flat and conductive surfaces to catalyze metal formation. Such surfaces are not available in the interstitial regions of colloidal crystals which are narrow and relatively inaccessible. Also, these templates as made are relatively fragile and easily disrupted by agitation or gas evolution during deposition. To overcome these obstacles, a metal nanocrystal catalyzed electroless deposition technique 14,15 has been developed.
We report a nanoscale &quot;lost-wax&quot; method for forming colloids with size distribu... more We report a nanoscale &quot;lost-wax&quot; 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.
Materials whose dielectric constant varies spatially with submicrometer periodicity exhibit diffr... more 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.
This article reviews the synthesis, characterization, and potential applications of macroporous m... more This article reviews the synthesis, characterization, and potential applications of macroporous metals created from colloidal crystal templates. Porous metals formed in this way are over 70% airÐresulting in a net surface area of several square meters for a thin film only one ...
We report observations of the optical stop band of periodic planar arrays of submicron silica sph... more 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.
This communication describes a method to fabricate multilayer colloidal crystals formed by the la... more 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.
This article provides a review of the recent literature on microfinance in developing countries a... more This article provides a review of the recent literature on microfinance in developing countries and a critical assessment of its effectiveness. It examines the experience of India, which has one of the largest microfinance sectors in the world, and particularly the unfolding of the microfinance crisis in Andhra Pradesh. It concludes that microfinance cannot be seen as a silver bullet for development and that profit-oriented microfinance institutions are problematic. To fulfil even some of its progressive goals, it must be regulated and subsidised, and other strategies for viable financial inclusion of the poor and of small producers must be more actively pursued.
Template-directed syntheses have been broadly applied to the creation of both meso-and macroporou... more Template-directed syntheses have been broadly applied to the creation of both meso-and macroporous ceramics 1-4 and polymers. 5 The extension of such methods to metals is of particular interest. Monolithic metals containing large internal surface areas could be used in numerous applications, ranging from electrochemical sensors 6 to catalytic converters. 7 Moreover, recent work on lithographically prepared metallodielectric structures suggests metals with well-ordered porous networks would exhibit interesting photonic properties. 8 The adaptation of template methods to the formation of porous metals has been relatively limited. [9][10][11][12] Lyotropic liquid crystalline 12 and anodic alumina 10 have been used as templates to produce mesoporous (voids <50 nm) metal films with parallel and cylindrical pores; however, techniques to make macroporous (voids >50 nm) and high void volume porous films have not yet been reported. In this work we describe a strategy for extending the general methodology of template-directed synthesis to the formation of porous metals. These samples are three-dimensional highly ordered, free-standing macroporous metal films with interconnected spherical voids (200-400 nm). We use as templates single-crystal colloidal multilayers made by a vertical deposition technique. 13 The uniform deposition of metals into colloidal arrays is challenging for several reasons. Existing examples of metal deposition into lyotropic liquid crystal 12 and anodic alumina 10 templates exploit flat and conductive surfaces to catalyze metal formation. Such surfaces are not available in the interstitial regions of colloidal crystals which are narrow and relatively inaccessible. Also, these templates as made are relatively fragile and easily disrupted by agitation or gas evolution during deposition. To overcome these obstacles, a metal nanocrystal catalyzed electroless deposition technique 14,15 has been developed.
We report a nanoscale &quot;lost-wax&quot; method for forming colloids with size distribu... more We report a nanoscale &quot;lost-wax&quot; 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.
Materials whose dielectric constant varies spatially with submicrometer periodicity exhibit diffr... more 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.
This article reviews the synthesis, characterization, and potential applications of macroporous m... more This article reviews the synthesis, characterization, and potential applications of macroporous metals created from colloidal crystal templates. Porous metals formed in this way are over 70% airÐresulting in a net surface area of several square meters for a thin film only one ...
We report observations of the optical stop band of periodic planar arrays of submicron silica sph... more 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.
This communication describes a method to fabricate multilayer colloidal crystals formed by the la... more 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.
This article provides a review of the recent literature on microfinance in developing countries a... more This article provides a review of the recent literature on microfinance in developing countries and a critical assessment of its effectiveness. It examines the experience of India, which has one of the largest microfinance sectors in the world, and particularly the unfolding of the microfinance crisis in Andhra Pradesh. It concludes that microfinance cannot be seen as a silver bullet for development and that profit-oriented microfinance institutions are problematic. To fulfil even some of its progressive goals, it must be regulated and subsidised, and other strategies for viable financial inclusion of the poor and of small producers must be more actively pursued.
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