## 3 Results and Discussion
### Photoluminescence Measurements  
The photoluminescence emission spectrum for pure ZnSe is shown in Figure 5. Two bands A and B are seen in the figure. The B band was centred around 610 nm at low temperatures and showed a slight shift to the lower wavelengths decreasing with temperature, as shown in Table 3. The band A was centred at 637 nm at low temperature. The theoretical fitting of these curves as a function of temperature for the two bands gives the following equations, respectively:  
\[\lambda_{A}=610.6-0.0247T\ (\mathrm{nm}) \tag{3}\]  
\[\lambda_{B}=636+0.09T-0.001T^{2}\ (\mathrm{nm})\] \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample** & \(E_{0}\)**(0), eV** & \(\gamma\) (\(\times 10^{-6}\)), eV/K & \(\beta\), K \\ \hline ZnSe:Eu\({}^{3+}\) & 2.25 & 264 & 160.89 \\ Zn:Se:Eu\({}^{3+}\)/Yb\({}^{3+}\) & 2.56 & 260 & 82.75 \\ \hline \end{tabular}
\end{table} 
Table 2: Values of the Varshni parameters obtained from the fitting of Equation (2). Figure 4: Energy band gap variation as a function of temperature for ZnSe:Eu\({}^{3+}\) (green squares) and ZnSe:Eu\({}^{3+}\)/Yb\({}^{3+}\) (red dots). The line represents a theoretical fitting with Equation (2). Figure 6 shows the photoluminescence emission when the ZnSe semiconductor was doped with a single rare earth Eu\({}^{3+}\). The emission spectrum was the same as pure ZnSe (Figure 5), but band B was more intense. Moreover, the temperature at which the photoluminescence was detectable was higher (250 K), as shown in Table 4. This is an expected result because of the additional photon energy supplied by donor-acceptor levels created with the inclusion of Eu\({}^{3+}\) ions in the crystal, which enables more efficient pumping. Figure 5: Photoluminescence spectrum of pure ZnSe. A, B are the emission bands, the blue line represents the sum of bands A plus B. The excitation was performed at 457.9 nm. Figure 6: Photoluminescence bands of ZnSe doped with Eu\({}^{3+}\). A, B are the emission bands, the blue line represents the sum of bands A and B.  
Eu shows two valence states, trivalent (Eu\({}^{3+}\)) and divalent (Eu\({}^{2+}\)), in certain compounds. However, the co-existence of (Eu\({}^{3+}\)) and (Eu\({}^{2+}\)) is hard to deduce from the absorption measurements directly. In fact, the conduction band absorption edge was much more intense than the Eu absorption peaks, which makes the identification of Eu bands a very difficult task. Nevertheless, the variation in the valence states of the Eu ions tends to influence the emission properties. In this regard, the photoluminescence spectrum carries useful information on the valence state of Eu ions. In general, the emission spectrum should include a broad emission band and several sharp emission lines throughout the visible light region if the sample is excited at 310 nm. We observed only the sharp emission lines that peaked around 622 nm, which were attributed to the \({}^{5}\)D\({}_{0}\rightarrow\)\({}^{7}\)F\({}_{2}\) transition emissions of Eu\({}^{3}\) ions incorporated in the ZnSe lattice. The samples were excited at 488 nm, and because of that, we did not observe the broad emission band positioned at 445 nm, which can be assigned to the 5d-4\(t\) transition emission of Eu\({}^{2+}\) ions. Therefore, we cannot completely discard the eventual presence of Eu\({}^{2+}\) in addition to Eu\({}^{3+}\). Interestingly, when this semiconductor was co-doped with the two rare earths (Eu\({}^{3+}\) and Yb\({}^{3+}\)), the spectrum presented new narrow emissions (b, c, d) and more intense lines, as shown in Figures 7-9. The emission spectrum of ZnSe co-doped Eu\({}^{3+}\)/Yb\({}^{3}\) at 20 K is shown in Figure 7, which had characteristic bands belonging to the ZnSe and superimposed the narrow emission bands of the rare earths. Eliminating the ZnSe bands from the emission spectrum, the pure emission spectra of co-doped rare earths at the temperature of 20 K is shown in Figure 8 and at 200 K in Figure 9, respectively. The temperature dependence of the photoluminescence peak emission is shown in Table 5.