## 3 Results and Discussion
Figure 9: (**a**) Raman spectra recorded on individual rod-like structures grown on the different samples, (**b**) zoom-in of the E\({}_{2}\)high Raman mode (438 cm\({}^{-1}\)) showing the shift of the frequency as the Li content varies. Spectrum from pure ZnO is also shown for comparison. the normalized spectra recorded on structures. The spectrum is, in general, typical of ZnO with two clear bands, the first around 3.1 eV, which corresponds to the band edge, and the second wide and with the maximum around 2.4 eV, which corresponds to the band of deep centers. The origin of these emissions is still under revision due to the complex character of the band. The main native defects in ZnO are oxygen and zinc vacancies (V\({}_{\rm O}\) and V\({}_{\rm Zn}\)) and their different charge states; oxygen and zinc interstitials (O\({}_{\rm i}\), Zn\({}_{\rm i}\)) and zinc antisites (O\({}_{\rm Zn}\)) [21, 23, 37, 38, 39, 40, 41]. However, there are significant differences between the spectra taken in each sample. The first remark is that the emission of the band edge can hardly be seen in the structures of the sample without Li (Eu5) (black line); in this spectrum, the highest intensity corresponds to the deep level band [20]. Some narrow emissions that correspond to intraionic transitions of the Eu\({}^{3+}\) centered on 2.02, 1.96 and 1.75 eV (612, 628 and 707 nm, respectively), can be clearly observed, overlapping with the deep level band [23, 26, 42, 43]. The first two lines correspond to the multiplet \({}^{5}\)D\({}_{0}\)\(\rightarrow\)\({}^{7}\)F\({}_{2}\), and the third one corresponds to the transition \({}^{5}\)D\({}_{0}\)\(\rightarrow\)\({}^{7}\)F\({}_{3}\). On the other hand, by adding Li to the samples, the relative intensity of the bands changes completely, becoming the most intense emission of the band edge. The lines of the Eu are no longer clearly seen (the 612 nm is seen in the sample with less Li) although a shoulder is observed around the same energy values that could correspond to the emission of the ion that it is not completely resolved. On the other hand, the position of the band edge emission shifts slightly between the samples, has the maximum at 3.04 eV for the sample with less Li (red line), at 3.13 eV for the sample with intermediate content (green line) and at 3.11 eV for the sample with the highest amount of Li (blue line). As with other dopants, there are several emissions involved in the band whose relative weight causes the maximum to shift slightly [9]. In fact, in the sample with the lowest Li content (red line), the asymmetry of the band that has a wide tail in the low energy zone is clearly observed. Regarding the spectra made on the surface of the pellet once the structures have been removed (Figure 10b), it should be noted that in all of them, several sets of narrow emission lines that correspond to the multiplets of the Eu\({}^{3+}\) that were seen in the spectra made on the structures, although in this case more lines are seen and better resolved. There are more differences between the spectra. In the sample without Li (black line), there are no additional emissions to those mentioned in the Eu. However, after adding Li, more emissions are observed. In the sample with the lowest Li content (red line), Eu emissions overlap with a broadband whose maximum is 2.45 eV and which could correspond to the green band of the ZnO. On the contrary, in the two samples with more Li (green and blue lines), this band is not observed. In its place appears an intense emission around 3.6 [44], which could be due to the band-band emission of the ZnS that is used as a precursor and that remains in the pellet, and also another little intense band around 3.1 eV that could be due to the presence of a little ZnO existing in the pellet that has been formed during the thermal treatment. Figure 10: CL spectra performed at room temperature on (**a**) individual structures grown on the different samples and (**b**) the surface of the pellet after the thermal treatments.