## 3 Results and Discussion
The optical properties of these Ag-aggregates can be better investigated by 330 nm excitation, which is reported in Figure 3. Additionally, there is also higher interest in analyzing near UV and blue excitation because sources like lasers, LEDs, or lamps for these wavelengths are much more available and cheaper than deeper UV light sources. As expected, the PL spectra of Ag-exchanged GC samples under 330 nm excitation, before (GC0A) and after annealing at 380 \({}^{\circ}\)C (GC0B) and 430 \({}^{\circ}\)C (GC0C), show a broadband emission related to multimers and aggregates, covering up to 750 nm. As previously observed [32], the intensity of this emission increases by annealing, as a possible consequence of matrix recovery with the  
Figure 2: XRD comparison between undoped (GC0) and Yb-doped (GC4) silica-zirconia samples. The inset reports a selected region of the diffraction pattern around 20 \(\approx\) 35\({}^{\circ}\), attesting to different crystal structures of the two samples: tetragonal-phase (diffraction standard reference ICSD #85322) zirconia nanocrystals in undoped GC0 and cubic-phase (diffraction standard reference PDF #49-1642) zirconia nanocrystals in Yb-doped GC4. removal of non-radiative defects, and of the increase of the number of multimers at the expense of single ions and dimers, at least for 380 \({}^{\circ}\)C annealing. A further thermal treatment at 430 \({}^{\circ}\)C seems detrimental with respect to 380 \({}^{\circ}\)C annealing. The reduction of PL intensity suggests a decrease in the effective number of emitting species, which is corroborated by the following PL analysis in the NIR spectral range. The NIR PL emission under 330 nm excitation is reported in Figure 4b for Yb\({}^{3+}\)-doped GC samples before Ag-exchange (GC4), after exchange (GC4A), and after annealing at 380 \({}^{\circ}\)C (GC4B) and 430 \({}^{\circ}\)C (GC4C). The direct excitation of Yb\({}^{3+}\) ions by 330 nm light is completely absent. After Ag ion exchange, a strong Yb\({}^{3+}\) PL emission peak at 975 nm is detected, possibly due to dimers, multimers or small aggregates formed during the ion exchange process, which act as efficient sensitizers. In agreement with the previous PL analysis of Ag aggregates, annealing is expected to improve the quality of the matrix, which should increase the PL intensity. However, annealing also decreases the number of sensitizers due to the formation of multimers and bigger aggregates, and this additionally increases the average distance between them and Yb\({}^{3+}\) ions, reducing the efficiency of energy transfer. The overall combination of these positive and negative effects is almost balanced at 380 \({}^{\circ}\)C, while it results in a significant decrease in the NIR PL signal at 430 \({}^{\circ}\)C. Therefore, Ag as-exchanged samples are actually the best performing NIR emitters. Figure 4: PL excitation (**a**) and near-infrared (NIR) emission (**b**) of Yb-doped samples before Ag-exchange (GC4), after exchange (GC4A), and after annealing at 380 \({}^{\circ}\)C (GC4B) and 430 \({}^{\circ}\)C (GC4C). A broadband excitation up to about 500 nm can be observed after Ag-exchange. Moreover, 330 nm wavelength is not absorbed by Yb\({}^{3+}\) ions, while it provides a strong PL emission in Ag-exchanged samples. Figure 3: Photoluminescent (PL) emission by 330 nm excitation of undoped samples before Ag-exchange (GC0), after exchange (GC0A), and after annealing at 380 \({}^{\circ}\)C (GC0B) and 430 \({}^{\circ}\)C (GC0C). The broad emission, up to 750 nm and more, is reasonably attributed to Ag multimers and small aggregates. Deeper investigation of the excitation behavior of Yb\({}^{3+}\)-doped GC samples, before and after Ag-exchange and annealing, is reported in Figure 4a. The intensity of the PLE curves agrees well with the PL analysis described above, with a further remarkable feature: the possibility for efficient and broadband excitation on Ag-exchanged samples, covering the whole spectral range from UV to blue, up to about 500 nm.