## 3 Results and Discussion
### Luminescence Study  
The emission of the elaborated NCs (Figure 8) was studied under an excitation wavelength of 366 nm from a Xenon lamp using an optical filter which cuts up to 410 nm. As a result, the exciton band of ZnO does not appear in the spectra. This is normally centered at 370 nm and the ratio of its intensity to that of visible band characterizes the ZnO crystallinity [15]. Moreover, no PL bands relative to Eu\({}^{3+}\) appear because the excitation wavelength is not resonant for any energy transition in Eu\({}^{3+}\). Figure 8 shows two kinds of emission for ZnO NCs. The first type, revealed by the intense band in the blue-violet range (-415 nm), results from the transition of the level defects of Zinc interstitial (Zn\({}_{\mathrm{i}}\)) to the valence band or also from the conduction band to the level of zinc vacancies (V\({}_{\mathrm{Zn}}\)) which is located slightly above the valence band. The intensity of this band increases slightly with Na\({}^{+}\) content in ZnO. As it has been reported previously,  
Figure 8: PL spectra of undoped and Eu, Na codoped ZnO NCs for \(\lambda_{\mathrm{exc}}\) = 366 nm. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Samples** & **ZnO** & **ZnEu** & **ZnEuNa1** & **ZnEuNa2** & **ZnEuNa3** & **ZnEuNa4** & **ZnEuNa5** & **Zn5 Na** & **ZnEu** \\  & & & & & & & & & \\ \hline \(E_{g}\) (eV) & 3.27 & 3.28 & 3.27 & 3.25 & 3.24 & 3.23 & 3.26 & 3.38 & 3.31 \\ \hline \(E_{u}\) (eV) & 0.092 & 0.074 & 0.094 & 0.102 & 0.108 & 0.103 & 0.100 & 0.108 & - \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Variation of \(E_{g}\) and \(E_{u}\) with the doping concentration. Na\({}^{+}\) ion can occupy a Zn site to form the acceptor complex Na\({}_{\rm Zn}\) whose energy level is located near the valence band [39]. Thus, the formed acceptor complex contributes to the blue-green emission from ZnO nanocrystals. The second type of emission is presented by the wide band located in the visible region It originates from various oxygen defects in ZnO such as oxygen vacancies (V\({}_{\rm O}\)), oxygen interstitials (O\({}_{\rm i}\)) and oxygen anti-sites [40]. For undoped ZnO NCs, the large PL band suggests the contribution of all types of oxygen defects. However, the disappearance of this band is noticed after doping with Na\({}^{+}\) ion. In order to show the emission peaks related to Eu\({}^{3+}\) ions, we have chosen to excite the samples by the 405 nm wavelength. Figure 9 shows the well-known emission peaks assigned to the intra-4f shell transition \({}^{5}\)D\({}_{0}\)\(\rightarrow\)\({}^{7}\)\(F_{j}\) (j = 1-4) in Eu\({}^{3+}\) ion [41]. The observed peaks correspond to \({}^{5}\)D\({}_{0}\)\(\rightarrow\)\({}^{7}\)\(F1\) (at 591 nm), \({}^{5}\)D\({}_{0}\)\(\rightarrow\)\({}^{7}\)\(F2\) (at 615 nm), \({}^{5}\)D\({}_{0}\)\(\rightarrow\)\({}^{7}\)\(F3\) (at 650 nm) and \({}^{5}\)\(D_{0}\)\(\rightarrow\)\({}^{7}\)\(F4\) (at 695 nm) [42]. The most intense emission is related to \({}^{5}\)D\({}_{0}\)\(\rightarrow\)\({}^{7}\)\(F2\) which arises from the electric-dipole transition, allowed when the Eu\({}^{3+}\) ion is found on a site of low symmetry. We cannot exclude that the emission arises from a fraction of Eu\({}^{3+}\) located at the surface of the ZnO nanocrystal [43]. In addition, the red emission is observed for all NCs codoped (Eu\({}^{3+}\)/Na\({}^{+}\)) which shows that the Eu\({}^{3+}\) ions are active in the ZnO NCs despite the thermal annealing not being performed at high temperature. The PL intensity is improved by Na\({}^{+}\) addition till 4 at.% and then decreases. The Eu\({}^{3+}\) emission is governed from an excitation transfer process from Na\({}^{+}\) ions which act as sensitizers for Eu\({}^{3+}\)[44]. In addition, the disappearance of the visible band of ZnO after doping with Eu\({}^{3+}\) ions is a sign of excitation transfer from deep levels to Eu\({}^{3+}\) ions. The PL decays, measured for the emission relative the \({}^{5}\)\(D_{0}\)\(\rightarrow\)\({}^{7}\)\(F2\) transition, confirm the excitation process of Eu\({}^{3+}\) ions (Figure 10).