Systematic positron study of hydrophilicity of the internal pore surface in ordered low-k silica thin films

Non-destructive Doppler Broadening (DB), Positronium fraction (f-Ps) and Two Dimensional Angular Correlation of Annihilation Radiation (2D-ACAR), Positron Beam Analysis (PBA) techniques have been used to study well-ordered mesoporous silica thin films with hydrophobic or hydrophilic character. The DB results, characterized by the S and W parameters, are related to both the open volume and the chemical environment at the positron annihilation site. The f-Ps and 2D-ACAR techniques are very sensitive probes to determine the type of porosity (open or closed in terms of positronium escape) in thin film materials. Samples with varying pore size (well-controlled at 2, 3 and 4.5 nm, or incorporating microporosity in the silica), pore fraction (from 4 to 57%) and extent of hydrophilicity have been studied. In the hydrophilic samples with small pore size the S-parameter increases with the porosity while the Ps-fraction remains almost unchanged. Increasing the pore size leads to smaller changes in S with increasing porosity. However, an abrupt change in the Ps-fraction is observed for samples with porosity higher than 45%, indicating a positronium percolation threshold for samples with 2D ordering structure. On the other hand, for hydrophobic samples with high porosity (57%) the highest S parameter and Ps-fraction were obtained for the three pore sizes studied. 2D-ACAR is used to determine the fraction and velocity of Ps escaping from some of the latter samples exhibiting positronium percolation. The relationship between these observations and porosity will be discussed in terms of branching of the positron annihilation channels inside the mesoporous films