## 3 Results and Discussion
### Photoluminescence  
The excitation spectra of BBO(50:50) + Eu, BBO(60:40) + Eu and BBO(70:30) + Eu as-prepared samples monitored at 615 nm (Figure 8a-c, respectively) reveal several bands associated with the 4f-4f transitions of Eu\({}^{3+}\). They can be assigned to \({}^{7}\)Fo\({}_{-}\)5D\({}_{4}\) (362 nm), \({}^{7}\)Fo\({}_{-}\)5G\({}_{2}\) (382 nm), \({}^{7}\)F\({}_{0}\)-5I\({}_{6}\) (395 nm), \({}^{7}\)F\({}_{1}\)-\({}^{5}\)D\({}_{3}\) (415 nm) and \({}^{7}\)F\({}_{0}\)-5D\({}_{2}\) (465 nm) transitions [35; 36]. In the case of two glass compositions, two bands at 395 and 465 nm are characterized  
Figure 6: Ag3d photoelectron peaks (**a**), Auger lines (**b**) and survey spectrum (**c**) of the BBO(50:50) + Ag sample annealed at 480 \({}^{\circ}\)C for 5 h.  
Figure 7: TEM image of the BBO(50:50) + Ag sample annealed at 480 \({}^{\circ}\)C for 5 h.  
by the highest intensity. The 454 nm wavelength line was selected in order to observe the emission spectra. The emission spectra of the as-prepared BBO(50:50) + Ag + Eu, BBO(60:40) + Ag + Eu and BBO(70:30) + Ag + Eu glasses and glasses annealed at 480 \({}^{\circ}\)C for 5 h are shown in Figure 8a-c, respectively. In all of them, several bands corresponding to Eu\({}^{3+}\) radiative transitions are present: \({}^{5}\)D\({}_{0}\)\(\rightarrow\)\({}^{7}\)F\({}_{0}\) (578 nm), \({}^{5}\)D\({}_{0}\)\(\rightarrow\)\({}^{7}\)F\({}_{1}\) (591 nm), \({}^{5}\)D\({}_{0}\)\(\rightarrow\)\({}^{7}\)F\({}_{2}\) (615 nm), \({}^{5}\)D\({}_{0}\)\(\rightarrow\)\({}^{7}\)F\({}_{3}\) (652 nm) and \({}^{5}\)D\({}_{0}\)\(\rightarrow\)\({}^{7}\)F\({}_{4}\) (700 nm). It was found that the electric dipole transition \({}^{5}\)D\({}_{0}\)\(\rightarrow\)\({}^{7}\)F\({}_{2}\) was the transition with the highest intensity for all samples.  
However, the positive effect of Ag nanoparticles on the emission of Eu\({}^{3+}\) ions was not observed for all samples. Metallic NPs embedded in a glassy matrix near rare-earth ions can significantly influence the excitation and emission rates of these ions [37; 38]. This effect is due to the enhancement of the local electromagnetic field around the nanoparticles caused by their localized surface plasmon resonance. As a result, energy transfer between the NPs and rare-earth ions can occur. However, the energy transfer can proceed in two directions: from metallic NPs to rare-earth ions, leading to an enhancement of luminescence intensity, and/or from rare-earth ions to NPs, resulting in luminescence quenching. The direction of the process depends on the concentration of metallic nanoparticles in the glassy matrix, or more precisely, on the ratio of the concentration of nanoparticles to Re ions [39; 40]. If this ratio is greater than one, a reverse transfer from the ion to the nanoparticle will most likely occur, resulting in a decrease in luminescence intensity. Annealing had a positive effect on emission intensity only for the BBO(70:30) + Ag + Eu sample. It is possible that in this particular sample, where neither UV-Vis spectrum peak indicative of LSPR nor any color change of the sample after annealing were observed, a very small amount of nanoparticles was formed, and energy transfer from the nanoparticles to Eu\({}^{3+}\) ions occurred, leading to an enhancement of luminescence. Meanwhile, in the other samples, a high concentration of nanoparticles had a negative effect, causing luminescence quenching due to reverse energy transfer from Eu\({}^{3+}\) ions to Ag nanoparticles. Of course, further research would  
Figure 8: Excitation and emission spectra of as-prepared and annealed samples: (**a**) BBO(50:50) + Ag + Eu; (**b**) BBO(60:40) + Ag + Eu; (**c**) BBO(70:30) + Ag + Eu.  
be necessary to confirm the validity of the proposed explanation. Further research is also required in order to select the optimal glass composition at which the intensity of rare-earth ion luminescence would be maximal. Nevertheless, the obtained results show that borate-bismuth glasses are very promising materials for the manufacturing of silver nanostructures in them.