Layer-specific porosity analysis in sub-nanoporous silica thin films using variable-energy positron annihilation lifetime technique, ellipsometric porosimetry, and attenuated total reflection infrared spectroscopy

We present a hybrid analytical method that combines the variable-energy positron annihilation lifetime (VEPAL) technique with in situ attenuated total reflection infrared (ATR-IR) spectroscopy to examine the depth-resolved open porosity (surface-connected pore fraction relevant to adsorbate-accessible open pores) of sub-nanoporous silica thin films. Model samples with low-, high-, and bilayer-porosity architectures were fabricated by plasma-enhanced chemical vapor deposition and characterized using VEPAL, ellipsometric porosimetry (EP), and ATR-IR. The depth-dependent ortho-positronium lifetimes obtained by VEPAL revealed distinct porous sublayers in the bilayer films, consistent with their designed architecture. Open porosity values derived from EP correlated well with ATR-IR absorbance under controlled humidity, validating the semi-quantitative use of IR spectroscopy for probing open pores near the surface. The cross-validation of the three techniques enabled nondestructive, layer-specific quantification of open porosity, offering a versatile platform for optimizing the structure and function of porous materials in sensing, dielectric, and membrane applications.