The beauty of opals results from a densely packed, highly ordered arrangement of silica spheres with a diameter of several hundred nanometers. Such ordered nanostructures are typical examples of materials called photonic crystals, which can be formed by known microstructuring methods and by self-assembly. Opals represent a self-assembly approach to these structured media; such an approach can lead to novel materials for photonics, photocatalysis, and other areas. Although self-assembly leads to many types of defects, resulting in the surprising and very individual appearance of natural opals, it causes also difficulties in technological applications of opal systems.
We use tapered silica fibers to inject laser light into ZnO nanowires with diameters around 250 nm to study their waveguiding properties. We find that high-order waveguide modes are frequently excited and carry significant intensity at the wire surface. Numerical simulations reproduce the experimental observations and indicate a coupling efficiency between silica and ZnO nanowires of 50%. Experimentally, we find an emission angle from the ZnO nanowires of about 90 degrees , which is in agreement with the simulations.
Powder diffraction patterns of ordered mesoporous materials are simulated with a newly developed program, which allows investigation of the influence of any desired matter distribution in the unit cell on the diffraction pattern. The simulation process can be subdivided into two major steps. First, a unit cell is generated from SiO2 and, optionally, other building units. A weighted random placement of atoms is used to simulate the distribution of different atoms in different parts of the unit cell. This is done by a Fermi-type function, by which the probability of finding an atom on a site depends on the distance of a point from the center of a pore, leading to a smooth, continuous transition from wall to pore. Secondly, structure factors and then intensities of reflections are calculated, using the Lorentz correction and a geometric correction for powder data. The use of this program is demonstrated by the simulation of diffraction patterns, mainly for unmodified and modified SBA-15 as well as for MCM-41. Good agreement of simulated and experimental data is observed
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