Specular X-ray reflectivity and small angle neutron scattering for structure determination of ordered mesoporous...

Specular X-ray reflectivity (SXR) and small-angle neutron scattering (SANS) are used to characterize the structure of a thin film containing cylindrical mesopores. The 3-D structure of the mesoporous film was determined from SANS measurements taken at multiple rotation angles between the incident beam and the film. The film was found to be composed of a randomly packed core and surface layers within which the hollow cylinders were regularly packed and oriented along the surface. The packing of the cylindrical mesopores was not hexagonal but rather rectangular with a conical angle of 55.7 degrees instead of 60 degrees expected for hexagonal packing. The extent of the planar orientation of the cylindrical mesopores within the surface layers was estimated from the width of the Bragg reflection in the SXR result to be about 25 repeating layers at both interfaces. This was further confirmed from cross-section transmission electron microscopy (TEM) results. The SXR results of this film exhibit an anomalous decrease in reflected intensity after each Bragg reflection. This anomaly in SXR can be modeled in a Parratt formulism using a depth profile composed of two characteristic lengths, the repeating distance among layers and the curvature of the density profile of each layer.