# Growth of Zr/ZrO\({}_{2}\) Core-Shell Structures by Fast Thermal Oxidation
## 3 Results and Discussion
However, T-centers composed of two oxygen vacancies and a Zr cation (V\({}_{\rm O}\)-Zr-V\({}_{\rm O}\)), hole centers such as oxygen with a trapped hole (O\({}^{-}\)) or V-type defects are also present [29]. The  
Figure 8: Schematic representation of the different F centers expected in this system [9; 10; 11; 12] (upper) and the corresponding recombination paths (lower). (**a**) Reactions 1, 3 and 4; (**b**) reactions 5 and 6; (**c**) reactions 7 and 8. The asterisk (*) means excited state. Figure 7: (**a**) Cross-section of the wire; (**b**) Zr map (upper) and profile (lower), showing the uniformity of Zr distribution; (**c**) O map (upper) and profile (lower), the oxygen signal is significant only at the outer surface of the wire (the lack of O signal at the right border is a shadow effect due to the topography of the sample and its orientation respect to the detector); (**d**) EDX spectrum recorded with an accelerating voltage of 10 keV on the surface of the wire. The inset shows an estimation of the intensity of the Zr K\({}_{\rm\alpha}\) line relative to the peaks shown in the main graph. mechanisms behind the luminescence in ZrO\({}_{2}\) are not well understood yet. Even so, most of the previous works rely on the relation of the visible emission with oxygen vacancies and complexes V\({}_{\text{O}}\)-M, where M is a cationic impurity that would occupy Zr sites [18; 27; 28] (see the references therein). In this case, we refer to F, F\({}_{\text{A}}\) and F\({}_{\text{AA}}\) centers as the main ones responsible for the visible emission bands. Figure 8 shows a sketch of the three types of F centers present in the material. The F center (part a) is formed when an oxygen vacancy adjacent to two Zr ions traps two electrons [Zr- V\({}_{\text{O}}\)- Zr]. The centers shown in parts b and c are extrinsic since they involve the cationic impurity. In the first case, (Figure 8b) Zr and the impurity cations are the nearest neighbors of the oxygen vacancy [M- V\({}_{\text{O}}\)- Zr]. Finally, part c shows the defect formed when the two nearest neighbors are cationic impurities [M- V\({}_{\text{O}}\)- M]. The most frequently reported impurities are Ti\({}^{+4}\) (or Ti\({}^{+3}\)), as residual impurities that cannot be eliminated, and Y\({}^{+3}\), since yttria is one of the compounds used to stabilize the zirconia. According to the specifications of the provider, the main impurity present in the Zr wire is hafnium, which is also mentioned as a candidate for cationic impurity [30]. The defect reactions that would give rise to the visible emissions according to [31] are written below. The first step is the excitation of electrons through the band gap  
\[\text{ZrO}_{2}+h\
u\ (5.82\ \text{eV})\to e^{-}+h^{+} \tag{1}\]  
Once the electron-hole pairs are created, several recombination paths could be activated. In our system, the electrons promoted to the conduction band could be easily trapped by the singly ionized oxygen vacancies (\(F^{+}\) centers)  
\[F^{+}+e^{-}\to F \tag{2}\]  
The trapping of a hole by the F center would create an excited \(F^{+}\) level  
Figure 2:0  
When the ground state is recovered  
Figure 2:1  
The defect reactions for the extrinsic \(F\) centers are written as [18; 31; 32]  
Figure 2:2  
Figure 2:3  
Figure 2:4  
Figure 2:5  
The typical photoluminescence (PL) spectrum recorded under excitation with a 325 nm He-Cd laser (Figure 9a) exhibits a broad band centered around 2.4 eV. A similar band that peaked around 2.4-2.5 eV has been previously reported [33; 34; 35], and it is attributed to oxygen-vacancy-related centers. The deconvolution of this band in three components fits quite well with the transitions involving the F centers mentioned above [32; 36]. It is worthy to note that the position of the electronic levels associated with defects are very sensitive to the surroundings and lattice relaxation states, which could play an important role in our case due to the low oxygen stoichiometry.