3.3.1. Studies of Film *n* and *d* due to Exposure to Water

Initially, we studied the optical behaviour of samples in pure water. Figure 4 presents the dispersion curves of the calculated refractive index, *n*, of both sets of samples measured prior to and after the immersion in water, while the calculated thickness values are displayed in Table 1. It is seen from Figure 4 that the refractive index of the TEOS-LTL composites annealed at both temperatures was substantially lower compared to the TEOS-only layers. The reason is that the refractive index of the LTL zeolites (~1.37) [16] is less than the refractive index of the TEOS matrix, and thus LTL addition reduces the effective refractive index of the composite. Furthermore, the annealing at higher temperature also led to a reduction in the refractive index; this effect was most pronounced for doped samples. The most probable reason is that after 170 ◦C annealing in air, water was still present in the pores of zeolite particles and the TEOS matrix but was then removed after the higher temperature treatment at 320 ◦C.

**Figure 4.** Dispersion curves of refractive index of TEOS-only films annealed at 170 ◦C (**a**) and 320 ◦C (**b**) and TEOS-LTL films annealed at 170 ◦C (**c**) and 320 ◦C (**d**) immersed in water for the denoted duration. The reference curve of refractive index of the layers prior to immersion is also plotted (solid black line).

**Table 1.** Calculated thickness values (in nm) and thickness change Δ*d* = (*d*1000s − *d*90s)/*d*90s of undoped (TEOS-only) and doped (TEOS-LTL) films annealed at 170 and 320 ◦C after immersion in water for 90, 500 and 1000 s.


For all samples, the immersion in water led to an increase in the refractive index; this is expected considering the presence of porosity in the layers. Water, with a higher refractive index (*n* ~ 1.33) than air (*n* = 1), penetrates the pores, thus increasing the effective refractive index. It may be expected that with increasing the immersion time this trend will continue. However, it is interesting to note that for all samples except TEOS-LTL annealed at 320 ◦C, *n* was observed to decrease with increasing time of immersion (Figure 5). The most pronounced decrease was for samples annealed at 170 ◦C: decrease in n from 7 <sup>×</sup> 10−<sup>4</sup> at 500 s to 11 <sup>×</sup> 10−<sup>4</sup> at 1000 s was observed. The reason is the swelling of the TEOS matrix with the time of immersion led to a decrease in the overall film density, thus decreasing the refractive index. From Table 1, it can be seen that the thickness of samples annealed at 170 ◦C increased by 0.57% and 0.33% for the undoped (TEOS-only) and doped (TEOS-LTL) films, respectively. These results suggest that the addition of LTL zeolite particles in the TEOS matrix positively influences its mechanical stability, and dimensional changes due to immersion in water are less pronounced. Additionally, the calculated thickness changes (Table 1) show that the TEOS matrix annealed at 320 ◦C is more rigid compared to that annealed at 170 ◦C; the increase in *d* was only 0.19% and this explains the weaker decrease in *<sup>n</sup>* in this case: 1 <sup>×</sup> <sup>10</sup>−<sup>4</sup> at 500 s to 1.4 <sup>×</sup> <sup>10</sup>−<sup>4</sup> at 1000 s. Furthermore, the addition of zeolites in the TEOS matrix and annealing at higher temperature contribute to an increase in the mechanical strength of TEOS-LTL samples, thus minimizing its swelling when immersed in water (thickness change is 0.15% only). In this case, an expected increase of *n* was observed with the time of immersion due to the constant penetration of water in the pores: 1.8 <sup>×</sup> 10−<sup>4</sup> at 500 s to 3.6 <sup>×</sup> 10−<sup>4</sup> at 1000 s.

**Figure 5.** Relative change of refractive index (at wavelength of 800 nm) as a function of immersion time in water.

We should note that for samples annealed at 170 ◦C, especially these of TEOS-only, the spectroscopic ellipsometry yielded higher changes in their *d* and *n* compared to the results obtained from single-wavelength ellipsometry. The reason is that the samples are not fully identical; the first set is annealed in air, while the second set is in vacuum. The conclusion is that annealing at a low temperature (170 ◦C in this case) in air is not effective enough for the TEOS matrix to become sufficiently rigid, and changes related to removing organic residues and water still take place. Considering all of the above, we decided to use only samples annealed at 320 ◦C for further experiments.

## 3.3.2. Studies of Film *n* and *d* due to Exposure to Copper Ions in Water

The relative changes of *n* (at wavelength of 800 nm) as a function of immersion time in pure water and Cu2+-containing water solution with a concentration of 4 mM for films annealed at 320 ◦C are presented in Figure 6. It was seen that, in both cases (water and Cu2<sup>+</sup> ions), the influence of immersion was more pronounced for the TEOS-LTL samples. From the results presented above for water immersion, it has already been determined that the thickness changes in samples annealed at 320 ◦C are negligible and the refractive index changes are mainly due to the penetration of water inside the pores. As noted above, this is the reason for the continuous change of TEOS-LTL refractive index with immersion time, as seen in the earlier studies (Figure 5). It is also seen from Figure 6 that the changes in sample *n* when immersed in the copper ion solution are weaker compared to the case of water immersion. Considering that the calculated thickness changes in this case are less than 0.1%, we may conclude that the immersion of samples in Cu2+-containing solution leads to a decrease in the films' refractive index for two reasons. The first reason is that the refractive index of copper solution is smaller than water [17], which leads to decrease of the effective refractive index of the films when the Cu2<sup>+</sup> water solution penetrates the pores. The second reason is the decrease in the hydrophilicity of the zeolites by introducing copper; more Cu in the zeolites decreases the water content, leading to a decrease in the effective refractive index of the film. This was verified by contact angle measurement using a First Ten Angstroms (FTA200, USA) surface energy analyser (details of the measurement technique can be found in the Supplementary Material). The contact angles of the TEOS-only films and TEOS-LTL films were measured to be 62.55◦ and 70.50◦, respectively. The increase in the contact angle value verifies the decrease in the hydrophilicity of the TEOS matrix due to the incorporation of LTL zeolites. The further increase in the contact angle of the TEOS-LTL films to 106.56◦ after exposure to Cu2<sup>+</sup> ions in solution supports the claim that the hydrophilicity of the zeolite-doped film decreases due to the adsorption of copper (see Figure S1). Because for the TEOS-only films there is no change of hydrophilicity (no LTL zeolites), the decrease in *n* is smaller compared to the case of the TEOS-LTL films where both factors contribute to the decrease in *n*. The further increase in refractive index with time is due to the penetration of Cu2<sup>+</sup> water solution inside the pores. For the TEOS-LTL samples, the difference in *<sup>n</sup>* of films in water and copper solution is almost the same at 500 and 1000 s—2.8 <sup>×</sup> <sup>10</sup>−<sup>4</sup> and 3.1 <sup>×</sup> 10<sup>−</sup>4, respectively. The same trend is observed for the TEOS-only samples, but the changes are weaker (0.9 <sup>×</sup> <sup>10</sup>−<sup>4</sup> and 1 <sup>×</sup> <sup>10</sup><sup>−</sup>4, respectively).

**Figure 6.** Relative change in refractive index, *n*, (at wavelength of 800 nm) as a function of immersion time in water (solid symbols) and the solution of Cu2<sup>+</sup> with a concentration of 4 mM (open symbols) for films of TEOS-only (circles) and TEOS-LTL (triangles) annealed at 320 ◦C.
