## 3 Results and Discussion
PL spectra were also developed in the structures both exciting with the UV laser and with the red. Let us start by describing the results obtained under excitation with the red laser (Figure 11a). In these conditions, we did not excite the luminescence of the ZnO, but it is possible to excite emissions from the Eu. The spectra were performed on clusters of various structures because the signal from a single structure was not very large. In all cases, emission lines were observed that may correspond to intraionic emissions of the Eu\({}^{3+}\); however, there were significant differences between the spectra taken in the structures to which Li was not added and those of the structures that did have Li. In the first case (black line), three sets of lines were observed, an emission in 1.8 eV (687 nm), a double in 1.79 and 1.784 eV (692 and 694 nm) in which the emission of greater intensity was the one with the highest energy and a multiplet with emissions in 1.765, 1.755, 1.752, 1.749 and 1.745 eV (702, 706, 708, 709 and 711 nm). The last multiplet was already observed in the CL spectra and corresponded to the transitions \({}^{5}\)D\({}_{0}\)\(\rightarrow\)\({}^{7}\)F\({}_{3}\); however, the emission at 1.8 eV and the multiplet, not observed in CL spectra, would correspond to the transitions \({}^{5}\)D\({}_{0}\)\(\rightarrow\)\({}^{7}\)F\({}_{4}\). When introducing the Li in the structures, the emissions of the multiplet disappeared completely, and we only observed the lines of the double that we mentioned previously peaked at 1.79 and 1.784 eV (692 and 694 nm). In addition, in this case, the relative intensities of both emissions were the opposite of the previous case; the component of lower energy was more intense. Finally, a small band at 1.794 eV (691 nm) can be seen in the spectra taken on the structures of 5 and 10% Li that was not seen in the other cases. In the case of spectra made by excitation with the UV laser, most of the time, what is observed is a large number of modulations, both in the emission of the band edge and in the band of defects when it appears. By selecting any of the spectra in which the modulations are not so pronounced, differences can be observed, especially in the emission of the band edge (Figure 11b). In the spectrum taken on structures in which there is no Li (black line), two emissions in that range centered on 3.12 and 3.25 eV can be clearly seen. In this case, the most intense emission corresponds to the component of lower energy. In addition, the relative intensity of the band edge emission is less than the band defects that are clearly observed. By adding Li (red, green and blue lines), the overall relative intensity of the band edge increases and, if we look at the two components that are resolved, it can be clearly seen that the component of 3.25 eV increases while that of 3.12 eV decreases until it is almost not resolved in the sample with the highest content in Li (blue line); it may be masked by the existence of modulations in that area. Figure 12 show a summary of the electronic levels involved in the transitions mentioned above [21; 23; 45; 46]. Figure 3:0 PL spectra performed at room temperature on individual structures grown on the different samples using a laser with (**a**) \(\lambda\) = 633 nm and (**b**) \(\lambda\) = 325 nm as excitation sources. As we have mentioned, the appearance of modulations was clear in almost all the structures investigated of all the samples containing Li. For the reference sample without Li, the structures were smaller (both in length and diameter), and it was not possible to measure individual structures in which the effect was appreciated. Figure 13 show two examples of structures acting as optical resonators; part a corresponds to a tape-like structure in the sample with 5% Li\({}_{2}\)O, and in part b, a rectangular plate of the sample with 10% Li\({}_{2}\)O can be observed. In both cases, there are very clear modulations in the area of the lower energy band edge.