Synthesis of Silicon Based Opal by Chemical Reduction of Silica Opal
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Synthesis of Silicon Based Opal by Chemical Reduction of Silica Opal. A. E. Aliev, M. A. O. Royer, A. A. Zakhidov, and R. H. Baughman NanoTech Institute, University of Texas at Dallas, Richardson, TX, 75083 ABSTRACT By reduction of SiO2 opal in the atmosphere of nitrogen (80%)-helium (15%)-hydrogen (5%) gas mixture, at various temperatures we tuned the dielectric contrast ratio and band gap of photonic crystal (PC) up to 10 %. The SEM images do not show any detectible change in lattice parameters, whereas the EDS analysis shows remarkable decrease of oxygen content. The shift of reflectance spectra towards long wavelength and increase of transmission in the infrared region confirm the reduction of silica and partial transformation to the silicon. INTRODUCTION Photonic crystal (PC) is a periodically structured electromagnetic media possessing photonic band gaps: ranges of frequency in which light cannot propagate through the structure [1,2]. It has been shown that using strong dispersion of light in PC it is possible to obtain a negative refraction behavior at optical wavelengths [3-13], i.e., a refracted beam going to the “wrong direction” compared to the direction expected on the basis of classical refraction laws. To obtain effective negative phase refractive index in the vicinity of gap opening the strong modulation (high dielectric contrast ratio) is a crucial requirement. Moreover the negative refraction is a direct consequence of the gap opening dominated on the overall equifrequency surface structure [4]. There are several attempts to create the PC structures with high contrast ratio. The most common method of fabricating them is the micro-machining of a bulk materials (Si, Ge) by lithographic methods, but this approach is so expensive and time consuming that only the creation of PC of a few monolayers are technically achievable at present [14,15]. It is also difficult to obtain a complete gap in PC in a particular spectral region. An alternative route involves self-assembly methods to achieve opal-like structures [16,17]. Opals are gemstones formed through the settlement of monodispersed submicrometer silica particles. As a consequence of periodicity they show opalescence: reflected colors that come from the Bragg diffractions in the periodic distribution of particles. However the dielectric contrast ratio of silica opal is far below of that required to obtain the full band gap. The common technique used to tune the effective refractive index of opal-like structures is an impregnation of porous opal structure with materials possessing high refractive index and transparent in some particular region [17]. The further extraction of host matrix of silica spheres by chemical etching (10% HF) gives even higher contrast ratio. The described colloidal crystal templating approach is very general and can be applied to sol–gel, salt solution, CVD, electro-chemical, nanocrystalline, and other precursors to produce 3D PCs. However, taken into account the glass transition point of silica it is become clear, t
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