Highly oriented 3D-hexagonal silica thin films produced with cetyltrimethylammonium bromide

Mesoporous silica thin films have been produced by sol-gel chemistry in the presence of cetyltrimethylammonium bromide (CTAB) template. The films were deposited on silicon or glass substrates by dip-coating and underwent different treatments to eliminate the CTAB and create porosity. As-prepared and treated coatings exhibit good optical quality. Their structures were fully characterised by transmission electron microscopy (TEM) performed on film cross-sections and by X-ray diffraction (XRD) in θ-2θ scan mode, as well as in transmission mode using two different scattering geometries. The films exhibit large and homogeneous domains organised in a 3D-hexagonal (P6₃/mmc) structure with the c axis normal to the surface throughout their whole thickness. Numerical analysis of the TEM pictures confirms the space group deduced from the XRD measurements. To our knowledge, these are the first reported thin films obtained by dip-coating in the presence of CTAB which show such extended and highly mono-oriented 3D-hexagonal (P6₃/mmc) domains. The film thickness, porosity and refractive index were evaluated by ellipsometry for the various treated films.