Comparing methods for measuring thickness, refractive index, and porosity of mesoporous thin films

This study compares systematically contact profilometry, interferometry, ellipsometry, ellipsometric porosimetry, and nitrogen porosimetry for measuring thickness, effective refractive index, and/or porosity of mesoporous thin films. Indeed, such measurements are crucial in elucidating the structure-property relationships of mesoporous materials. Here, sol-gel and nanoparticle-based mesoporous silica and silica-titania thin films were synthesized and characterized using the aforementioned methods. The films were made by evaporation-induced self-assembly process using (i) tetraethyl orthosilicate and titanium isopropoxide as the silica and titania precursors or (ii) silica nanoparticles with a mean diameter of 9 nm as building blocks of the silica framework along with either Pluronic F127 or P123 block copolymers as structure directing agents. The synthesis recipe and deposition conditions were varied to achieve a wide range of compositions (silica:titania molar ratio from 100:0 to 70:30), thickness (80 nm ≤ L ≤ 630 nm), effective refractive index (1.11 ≤ neff,λ ≤ 1.75), porosity (0% ≤ ϕ ≤ 70%), and peak pore diameter (2.5 nm ≤ dp ≤ 20 nm). Overall, the thickness, effective refractive index, and/or porosity obtained from contact profilometry, interferometry, ellipsometry, and ellipsometric porosimetry agreed very well. However, porosity and pore size distribution obtained from nitrogen porosimetry on powder samples differed significantly from those of the equivalent thin films.