# Towards a Glass New World: The Role of Ion-Exchange in Modern Technology
## 2 Glass Strengthening: The Ion-Exchange Contribution
### Chemical Strengthening: Applications
Actually, after introducing silver in the glass, typical emission of both Eu\({}^{2+}\) and Eu\({}^{3+}\) ions increased, but excitation would switch from the 270 nm band of Ag\({}^{+}\) ions to the 350 nm ascribed to Ag clusters, which get formed at expense of Ag\({}^{+}\) isolated ions and smaller aggregations by increasing ion-exchange time. Using 350 nm excitation, an otherwise weak band assigned to the 4\({}^{6}\) 5d \(\rightarrow\) 4\({}^{7}\) transition of Eu\({}^{2+}\) ions was also enhanced. Therefore, it is evident how in this case both the optimal excitation and the emission depend on the valence and aggregation of silver, which can be controlled by the ion-exchange process parameters. Similarly, [PERSON] et al. studied the effects of silver doping by ion-exchange on the photoluminescence emission in Eu\({}^{3+}\) sodium-aluminosilicate glass [60]. As shown in Figure 3, the inclusion of silver aggregates produced a broad emission band in the 400-700 nm interval overlapping the Eu\({}^{2+}\) 5d-4f emission, and a substantial increase of the 615 nm line of Eu\({}^{3+}\) when excited at 350 nm as the silver content was increased. The effect was ascribed by the authors solely to energy transfer from silver ions to Eu\({}^{3+}\) ions. The combined emission by silver, when suitable ion-exchange parameters were chosen, and Eu\({}^{3+}\) ions produced white light. Therefore, silver contributed both by enhancing Eu\({}^{3+}\) emission in the red and by its own emission in the blue-green region. The same group of authors has obtained very similar results with Sm\({}^{3+}\) and Tb\({}^{3+}\) in the same glass [59; 145] and in borosilicate glass [64]. [PERSON] and [PERSON] demonstrated energy transfer from silver ions/dymers to erbium ions implanted in BK7 after Ag\({}^{+}\)/K\({}^{+}\) ion-exchange [56]. Our research group has characterised very thin wafers of Er\({}^{3+}\)-doped silicate glasses entirely ion-exchanged in an Ag/NaNO\({}_{3}\) melt. [57, 58]. We tested several post-exchange annealing processes evidencing the effect on absorption and emission spectra. In particular, since Er\({}^{3+}\) resonant excitation was not affected by the silver presence, whereas non-resonant was enhanced (up to 100 times), we concluded that the effect should be ascribed to energy transfer from the silver nanoparticles to the Er\({}^{3+}\) ions, rather than local field enhancement. Similar results have been demonstrated in different Er\({}^{3+}\)/Yb\({}^{3+}\) glass formulations, such as silicate and titaniumobophosphate glasses, where planar waveguides have been fabricated by Ag\({}^{+}\)/Na\({}^{+}\) ion-exchange [62, 146]. Photoluminescence enhancement of Dy\({}^{3+}\)/Eu\({}^{3+}\) co-doped tellurite glasses was reported by [PERSON] and co-workers [78]. In this case, the quantum efficiency improvement (several % units, depending on excitation wavelength, ion-exchange conditions and glass doping) was attributed mainly to the local plasmon resonance field enhancement produced by the silver nanoparticles. Also in this case, different ion-exchange parameters produce glasses capable of emitting with different CIE (Commission Internationale de l'Eclairage) coordinates, and therefore emission tailoring is readily possible. Recently, the Yb\({}^{3+}\)/Ho\({}^{3+}\) doping system, interesting for the fabrication of laser sources for medical applications operating at 2 \(\mu\)m, has been studied by [PERSON]\(\check{\text{a}}\)[PERSON] et al. in zinc-silicate glasses [147]. In this case, Ag\({}^{+}\)/Na\({}^{+}\) and Cu\({}^{2+}\)/Na\({}^{+}\) ion-exchanges have been tested, in order to introduce either silver or copper nanoparticles inside the glass. In both cases, planar waveguides were obtained and characterised. Regarding photoluminescence, unfortunately the effect of copper was, even if only slightly, negative, since the emission decreased after ion-exchange, and a subsequent annealing step did not improve it.