Fabrication of 3-dimensional Si, Si-Ti or Ti photonic crystals by the direct metallothermic reduction of oxide precursors; A feasibility study

Abstract

Photonic crystals are well ordered materials characterised by a regularly modulated refractive index in 1, 2 or 3 dimensions, with a period typically near the length scale of the visible spectrum of light. Periodicity gives rise to photonic band gaps; a narrow range of frequencies for which light wave propagation within the photonic crystal is forbidden. Photonic Band Gap materials offer a multitude of opportunities for the manipulation of light, and hence they are finding increasingly widespread use in optical and optoelectronic devices. Currently there is an enormous interest in the fabrication of 3 dimensional Si photonic crystals. This Project aims to explore the feasibility of direct magnesiothermic reduction of SiO₂ colloidal crystals (synthetic opals) as a means to fabricate 3 dimensional Si photonic crystal materials. Initially, batches of monodisperse, sub micron sized silica (SiO₂) colloids were prepared by the ammonia catalysed hydrolysis of tetraethyl orthosilicate (TEOS) using the well known Stöber method. By varying the reaction temperature (20-65°C) and keeping all other reaction parameters constant ([TEOS] = 0.4M, [NH₃] = 0.5M, [H₂O] = 10M, solvent ethanol), batches of monodisperse silica colloids with diameters of 120-400 nm were synthesised successfully. The silica colloids readily self-assembled to form 3-dimensional silica colloidal crystals (synthetic opals). Silica colloidal crystal powders were obtained by gravitational sedimentation of silica colloids in ethanol, followed by removal of the ethanol and air drying. Silica colloidal crystal thin films on glass or silicon substrates were obtained immersing the planar substrate vertically in dilute ethanolic dispersions of SiO₂ colloids, and allowing the ethanol to slowly evaporate at room temperature. SiO₂ colloidal crystals formed at the falling meniscus. The obtained silica colloidal crystals displayed angle dependent structural colour similar to natural opal and were structurally and optically characterised by SEM, TEM, XRD, UV-Vis transmission and UV-Vis reflectance measurements. The optical characteristics of the silica colloidal crystals obeyed a modified Bragg's law expression which takes account of both refraction and diffraction of light in the silica colloidal crystal. Magnesiothermic reduction of SiO₂ colloidal crystal powders and thin films (on Si substrate) was investigated over the temperature range 650-800°C and at SiO₂: Mg ratios of 1:2-1:2.5. The experiments were carried out in sealed stainless steel Swagelok reactors. In agreement with the thermodynamic predictions Mg reduced SiO₂ to Si effectively in this temperature range, yielding MgO as a by-product. After digestion of the MgO in dilute hydrochloric acid, the obtained 3-dimensional Si colloidal crystals were successfully characterised by SEM, TEM, EDAX, XRD, N2 physisorption and XPS. The data confirmed the transformation of dense amorphous SiO₂ colloids to porous nanocrystalline Si, whilst preserving the overall structure of the 3-dimensional Si colloidal crystal. The Si solid volume fraction in the obtained product was ~30-35% compared to 74% in the SiO₂ colloidal crystal precursors. Attempts to prepare Si colloidal crystals by the calciothermic reduction of SiO₂ colloidal crystals at 850°C were ineffective, and yielded CaSiO₃ as the main product. Magnesiothermic reduction of TiO₂ powders at 675-850°C yielded TiO as the main titanium containing phases. By comparison, calciothermic reduction of TiO₂ yielded alpha-Ti as the main phase; furthermore magnesiothermic reduction of SiO₂-TiO₂ yielded Si-Ti alloy phases. All of which is indicative towards the potential of metallothermic reduction of TiO₂ containing phases.