Formation of nanoscopic CaF2via a fluorolytic sol–gel process for antireflective coatings

Abstract

The synthesis of nanoscopic calcium fluoride was performed by the fluorolytic sol–gel process. Antireflective coatings of CaF₂ were prepared from sols obtained by the reaction of CaCl₂ with HF and subsequent dip coating. The addition of tetramethyl orthosilicate (TMOS) or tetraethyl orthosilicate (TEOS) after fluorination promotes the formation of transparent sols. The formation and crystallisation of CaF₂ nanoparticles was studied by ¹⁹F liquid and solid state NMR spectroscopy, dynamic light scattering (DLS) and X-ray powder diffraction (XRD). The morphology of a CaF₂-film was analysed by high resolution scanning electron microscopy (HR-SEM) and the mechanical stability of a CaF₂-film was evaluated by the Crockmeter test using both felt and steel wool. The refractive index for a CaF₂-film was measured by ellipsometry. The synthesis of CaF₂ nanoparticles derived from CaCl₂ is a good way to achieve porous antireflective coating layers.