Ellipsometry Studies of Inorganic and Hybrid Multi-Layer Membranes

Spectroscopic Ellipsometry is a contactless and hence non-destructive, optical technique that can be used to obtain the thickness and complex refractive index (n,k) of stacked layers. A polarized light beam is focused on the surface, and changes in the amplitude and phase of light after reflection are measured vs wavelength. This information, in turn, is used to obtain individual layer properties by curve fitting to pre-defined multi-layer models. Information about membrane composition and gas sorption can then be obtained by analysis of n and k. The technique was originally developed for thin films on flat glass or silicon wafers. However, more recently it was demonstrated that it can be used very well for supported membrane layers, even with substantial interfacial of surface roughness, or curvature.

We present characterization results for a variety of single- and multi-layered gas separation membranes on polymer supports, and flat and tubular ceramic membrane supports. Examples are α‑ and γ‑alumina on polysulfone and polyethersulfone supports, Ce_0.9Gd_0.1O_1.95 on ceramic tubes, γ‑alumina on two-layer α‑alumina supports, and layers of amorphous microporous silica or polydimethylsiloxane (PDMS) on mesoporous intermediate layers. Best practices will be provided for measurement and modeling procedures, in addition to a discussion of the statistical relevance.