The effect of the pH and the ionic strength on the self-assembly of colloidal silica spheres have been studied. The optical properties of the obtained opaline structures have been investigated. The deposition process of colloidial silica spheres and the. ne structure of opal films have been examined by SEM. It was shown that the surface charge of particles, modified or screened by pH and counterions, has a crucial role in the formation of homogeneous and well ordered opal films. The present work supplied an essential comprehension of the self-assembly of SiO2 spheres towards a better control of vertical deposition method leading to high quality opal-based photonic crystals. (c) 2008 Elsevier B. V. All rights reserved.