Lattice variation upon water adsorption in silica opals measured by <i>in situ</i> atomic force microscopy

Colloidal photonic crystals, or artificial opals, derive their unique optical properties from the periodic arrangement of their constituent particles. Central to the functionality of these materials is the precise control over their lattice parameter, which directly determines the photonic bandgap. Adsorption of environmental vapors may have a significant impact on the photonic response, most of which has been attributed to lattice variations. Here we aim to directly measure the arrangement and lattice parameter in opals made of silica spheres by using <i>in situ</i> atomic force microscopy (AFM) under changing ambient conditions. The periodicity of the opal structure offers a decisive advantage for such study, enabling the use of correlation tools for image processing. Opals having high mechanical stability (avoiding lattice distortions or sphere detachment during scanning) allow reliable AFM data and good analytical resolution. We show direct evidence of reversible lattice increments upon water adsorption, up to several nanometres in agreement with prior indirect estimates, which is compatible with swelling of the spheres due to filling of silica micropores.