Germanium inverse opals with a full photonic bandgap in the NIR region are accessible by CVD. Deposition of digermane on sintered opals made of silica microspheres, followed by removal of the silica by etching, yields inverted Ge opals (see Figure for an SEM image of a cleaved edge, revealing the Ge layer) whose lattice parameters, network topology, and Ge coating thickness determine the optical properties of the inverse Ge opal.
In the past, helical shapes in nature have inspired inventions such as the water screw for agriculture, the retaining screw for wine presses, and architectural designs for spiral staircases. [1] Similarly, these days helix-shaped DNA, proteins and carbon nanotubes evoke great interest in biotechnology and nanotechnology. [2±4] Also biomimetic synthesis of helical morphologies of calcium carbonate, barium sulfate, and silica provides insight into morphogenesis of mineralized spiral forms in biology and ideas for new opportunities in materials science. [5±9] Herein we describe the synthesis of hollow helicoids made of hexagonal mesoporous silica, a remarkable topology in the materials world. They have a hierarchical architecture comprised of 5 nm diameter channels that coil in the form of a micrometerscale tubular spiral. A population analysis of helicoid shapes defines a surprisingly narrow distribution of pitch and flute widths, pitch angles, inside and outside diameters, and significantly an equal number of left-and right-handed forms. Evidence is presented that morphogenesis involves polymerization-induced differential contraction of a patch of hexagonal silicate liquid-crystal film formed at the air± water interface, which can fold into a hollow helicoid. A supramolecular Origami theoretical model explains the creation and observed narrow distribution of mesoporous silica, hollow helicoid shapes.Mesoporous silica hollow helicoids were prepared by using cetyltrimethylammonium chloride (CTACl) as the surfactant micellar template and tetraethylorthosilicate (TEOS) as the silica precursor. An aqueous solution of CTACl, hydrochloric acid and formamide was aged for 48 h before adding TEOS, and the material was formed after 3 days in a quiescent state. The use of formamide in the synthesis is intentional because upon acid hydrolysis it yields ammonium chloride and formic acid to give an ultimate solution ca. pH 1.9 and an ionic strength that favors hollow helicoid formation. This solution pH is notably higher than the one used in the synthesis of mesoporous silica curved shapes. [10] Control experiments demonstrate that a high concentration of ammonium and formate ions is essential for the formation of mesoporous silica at a pH close to two, which borders on the isoelectric point of aqueous silica. We believe that a low acidity and high ionic strength medium favor a slow rate of silicification, and hence polymerization-induced differential contraction of silicate micelle rods in a patch of silicate liquid-crystal film formed at the air±water interface becomes influential in hollow helicoid formation. [11] Powder X-ray diffraction (PXRD) patterns in Figure 1 clearly define as-synthesized and calcined materials as the MCM-41 hexagonal mesoporous silica. [12] Scanning electron microscopy (SEM) images reveal morphologies consisting of extraordinary hollow helicoid shapes that resemble a screw thread, Figure 2a±c. Transmission electron microscopy (TEM) images, Figure 3a, on whole-mounted helicoids reveal that they are h...
This article focuses attention on recent research on the silicon inverse opal, the first self‐assembled or bottom–up synthetic photonic crystal to exhibit a complete photonic bandgap (PBG) at 1.5 μm[1] in accordance with theoretical predictions.[2] The silicon inverse opal has since proven to be a useful platform for assembling on‐chip films[3] and in‐chip patterns,[4] engineering extrinsic defects,[5] mapping photon density of states,[6] switching light with light, and inhibiting spontaneous emission.[7] Also, new and exciting colloidal‐crystal‐based structures are being developed based on experimental and theoretical knowledge acquired for the synthesis of inverted silicon photonic crystals.[8–10] It has also inspired the idea of the silicon inverse opal heterostructure, a theoretical construct that could enable an all‐optical microchip for single mode diffractionless waveguiding of light in air throughout a bandwidth of more than 70 nm at 1.5 μm.[11]
The cocondensation reactions of nickel, palladium, and platinum atoms with gaseous oxygen or dilute oxygen-argon mixtures at 4.2-10 "K give rise to binary transition metal dioxygen complexes. Variable concentration and diffusion controlled warm up studies, 160/'80 isotopic substitution studies, normal coordinate and isotope intensity calculations establish the complexes to be of the types M(0,) and (Oz)M(02). Both the mono-and bisdioxygen complexes contain the dioxygen molecules coordinated to the metal atom in a side-on fashion, with a unique Dzd "spire"-type structure favored for the biscomplexes. Frequency and bond stretching force constant trends within the series of complexes are examined and suggest that the 0-0 and M-0 values increase with increasing coordination number, and thus with an increase in partial positive charge on the metal. This trend in 0-0 stretching frequencies is not in accord with the evaluation of metal-dioxygen bond strengths as based on the degree of reversibility of dioxygen coordination.
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