2.3.1. Single Wavelength Ellipsometry Experiments

A single wavelength (632.8 nm) Multiskop (Optrel GbR, Sinzing, Germany) ellipsometer was used to study the change in the optical refractive index, *n*, and thickness, *d*, of the TEOS-LTL and TEOS-only films during exposure to Cu2<sup>+</sup> water solutions with concentrations of 0, 2 and 4 mM. A custom-built liquid cell with quartz windows allowed for the transmission of the 632.8 nm ellipsometer probe beam with minimal losses. A similar approach was successfully used by Pristinski et al. [14] for the study of thin film swelling in liquid environments. Each test film was placed in the cell, which was then filled with 90 mL of deionized water. The measurement was commenced as soon as the probe beam was correctly aligned with both the sample and detector. A value for the Δ and ψ parameters was captured every 10 s for 700 s. After 200 s, 10 mL of a concentrated Cu2<sup>+</sup> solution was then added to the cell, ensuring not to disturb the test film. Due to dilution, the final overall concentrations in the cell were 0, 2 and 4 mM. The experiment was carried out in this fashion in order to determine if the change observed in the Δ and ψ parameters of the test films is due simply to water or if the metal solution has a separate effect.

Following data collection, a four-media model within the Optrel Elli v3.2 software was used to calculate values for the *d* and *n* of the test films as function of time. These four media from top to bottom are: water (*n* = 1.33), the test film, SiO2 (*n* = 1.4585, *d* = 3 nm) and Si (*n* = 3.8858, *k* = 0.018 [15]). The model was additionally supplied with upper and lower limits *n* and *d* of the test film: *n* = 1.35–1.60 and *d* = 140–260 nm.
