## 3 Results and Discussion
### Luminescent Properties of Silver Molecular Clusters Formed in Ce- and Sb-Doped Glasses  
Silver molecular clusters (SMCs) are subnanometer silver aggregates that consist of several atoms/ions. SMCs are characterized by photoluminescence and do not possess plasmon resonance. Stabilized in different matrices, including liquids, polymers, glasses, and zeolites, SMCs are used for developing white LEDs [49], sensors [50], spectral converters [51; 52], and data storage devices [53]. Glasses doped with SMCs attract particular attention due to transparency in a wide spectral range, high thermal stability, and excellent chemical durability. A PTR glass matrix transparent in the UV range opens the possibility to study the spectral properties of SMCs in details. To promote the growth of SMCs in the Na+-Ag+ ion-exchanged PTR glass, silver ions should be reduced to an atomic state. Cerium and antimony are polyvalent ions that can be in different charge states (Ce3+/Ce4+ and Sb3+/Sb5+), which determines their spectroscopic manifestation in glass  
Figure 11: Coloring and bleaching kinetics for PTR-Br glass sample ion exchanged for different time and heat-treated at 500 \({}^{\circ}\)C for 9 h. Inset: photo of the samples after irradiation by the UV pulsed laser during 1000 s.  
matrices [54; 55]. These ions are typically used in PTR glasses to provide photosensitivity of the glass [3; 4]. Possible redox reactions involving silver and cerium/antimony ions are as follows:  
\[\text{Ag}^{+}+\text{Ce}^{3+}\leftrightarrow\text{Ag}^{0}+\text{Ce}^{4+}, \tag{4}\]  
\[2\text{Ag}^{+}+\text{Sb}^{3+}\leftrightarrow 2\text{Ag}^{0}+\text{Sb}^{5+}. \tag{5}\]  
To determine the direction of a redox reaction, one needs to know the value of the standard electrode potential. For example, the reaction in Equation (4) proceeds in standard conditions from right to left. Adding silver into the PTR glass matrix was earlier shown to result in a decrease in the Ce\({}^{4+}\)-related absorption band, proving that silver plays the role of a reducing agent for cerium ions in the PTR glass melt [56]. However, the concentration of Ag\({}^{+}\) ions in the ion-exchanged PTR glass layer exceeds by orders the equilibrium value that can be reached via glass synthesis, whereby the equilibria in Equations (4) and (5) shift to the right-hand side despite the direction of the reaction in standard conditions. Neutral silver atoms obtained as a result of the redox reactions can aggregate further in SMCs or  
Figure 12: In situ absorbance spectra measured at different temperatures of PTR-Br glass sample ion-exchanged for 15 min and heat-treated at 500 \({}^{\circ}\)C for 9 h. Inset: photos of the sample at room temperature (**top image**) and after pulling it out from a muffle furnace heated to 500 \({}^{\circ}\)C (**bottom image**). Figure 13: (**a**) Location of UV absorption edge and (**b**) absorbance increment at a fixed wavelength vs. temperature for PTR-Br glass samples ion-exchanged for different time and heat-treated at 500 \({}^{\circ}\)C for 9 h.  
NPs depending on their size. As shown below, the spectral luminescent properties of PTR Na+-Ag+ ion-exchanged glass can be tuned in a wide range by varying the concentration of glass dopants, as well as the IE and subsequent heat treatment parameters. The concentration of polyvalent ions determines the rate of the redox reaction with silver. Thus, a higher content of polyvalent ions allows a higher concentration of SMCs and SNPs to be obtained in glass at fixed parameters of the IE and heat treatment processes (Figure 14). It is worth noting that, for developing efficient phosphors or spectral converters based on a glass doped with SMCs, the concentration of polyvalent ions should be very low, so as to increase the possibility of SMCS growth consisting only of few atoms. In the other case, the growth of large clusters and/or SNPs results in luminescence quenching and a significant lowering of the photoluminescence quantum yield (PLQY).