## 4 Conclusions  
In this work, we demonstrated the influence of Li co-doping on the morphology and optical properties of the ZnO:Eu nano and microstructures. By varying the co-dopant amount, we were able to observe the different positions that Li ions can occupy and their effect on the defect structure. On the other hand, the incorporation of Li considerably improves the behavior of the structures as optical resonant cavities compared to the structures that only contain Eu.  
A large variety of nano- and microstructures of ZnO co-doped with Eu and Li were obtained by a vapor solid growth method. The changes in the energies of the crystallographic and, thus, in the growth directions due to the incorporation of the dopants lead to different morphologies depending on the initial Li content. While hexagonal rods which grow on the pellets were obtained in the samples where no Li was added, ribbons, sword-like and comb-like structures were more likely to appear as the Li amount increases. In some cases, secondary growths on the lateral faces of the comb-like structures were observed. The EBSD analysis performed in this type of structure confirmed that the growth direction changes between the plate and the prongs. XRD patterns and Raman spectroscopy indicated that the structures have good crystalline quality and grow in the ZnO wurtzite phase. Luminescence investigations showde both the well-known UV and green emission bands of ZnO; however, differences in the relative intensity as well as in the peak position were observed due to changes in the concentration of defects related to variations in the incorporation of dopants within the structures. In addition, emission lines associated with Eu\({}^{3+}\) intrashell transitions were detected in all samples, although differences in the levels involved were observed between the samples with or without Li. Finally, the waveguiding behavior and the establishment of FP optical resonant modes inside the structures were demonstrated. Quality factors around 1000 in the UV region were calculated, which opens the possibility of using these structures in photonic nano- and microdevices.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Wavelength (nm)** & **Quality Factor (Q)** & **Fineness (F)** \\ \hline
384.5 & 1022.88 & 3.09 \\
385.66 & 959.78 & 3.26 \\
386.97 & 860.70 & 3.16 \\
388.39 & 788.93 & 3.39 \\
390.06 & 723.67 & 3.41 \\
391.9 & 663.90 & 3.47 \\
393.95 & 602.74 & 3.44 \\
396.2 & 555.52 & 3.53 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Calculation of the quality factor (Q) and finesse (F) for the structure shown in Figure 13a.  
**Author Contributions:** Validation, F.P., A.U. and P.F.; Formal analysis, F.P., A.U. and P.F.; Investigation, F.P., A.U. and P.F.; Writing--original draft preparation, F.P., A.U. and P.F.; Visualization, F.P., A.U. and P.F.; Conceptualization, A.U. and P.F.; Methodology, A.U. and P.F.; Writing--review & editing, A.U. and P.F.; Supervision, A.U. and P.F.; Resources, P.F.; Project administration, P.F.; Funding acquisition, P.F. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This work was founded by the Complutense University of Madrid--Banco Santander via project UCM-Santander 2019 (PR87/19-22613) and Complutense University--Comunidad de Madrid via project PR65/19-22464.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Conflicts of Interest:** The authors declare no conflict of interest.