*3.2. Single Wavelength Ellipsometric Studies*

Figure 3 shows the results from the single wavelength ellipsometric measurements carried out using both undoped and doped (i.e., TEOS-only and TEOS-LTL) samples that have been exposed to water solutions of Cu2<sup>+</sup> ions with concentrations of 0, 2 and 4 mM. All samples were thermally treated in a vacuum oven at 170 ◦C for 3 h in order to remove any residual water from the samples as a result of ambient humidity. A significant effort was made to minimize the period required for ellipsometer beam realignment between the addition of water and the commencement of the measurement (i.e., *t* = 0 s in Figure 3). This period was typically 50 ± 10 s. While some swelling of the film is expected during this period, the initial absolute values for *n* and *d* were largely consistent, implying that changes occurring during this period were minimal and consistent across all samples. Specifically, for the TEOS-LTL data shown in Figure 3, the average *n* at *t* = 0 s was 1.4412 ± 0.0029; the average *d* was 218.83 ± 4.00 nm. For the TEOS-only samples, the average *n* at *t* = 0 s was 1.4198 ± 0.0024; the average *d* was 227.63 ± 7.05 nm.
