# Growth of Zr/ZrO\({}_{2}\) Core-Shell Structures by Fast Thermal Oxidation
## 3 Results and Discussion
At low magnification (Figure 5a,b) a form of columnar growth is observed: this morphology reflects the drawing tracks from the metal wire. This pattern is similar in all the samples, although in those grown at higher intensities (b), the oxide layer is thicker, and therefore, the grain structure is better defined. Part c of this figure shows a detail of the oxide layer, where the formed grains are clearly visible. The charge effects observed in this image also hint to the thickness of the oxide layer. Figure 4: Typical Raman spectrum obtained from the samples. Colored labels: red, O-O bonds; green, Zr-Zr bonds; orange, Zr-O bonds. In the second set of samples, we focused on the growth of microstructures and their evolution with treatment time. A current of 2.2 A was fixed in all cases; the difference resides in the time profile. In the samples grown for the shortest times, (time profiles shown in Figure 2a-c for 30 s), only the first stages of microstructure growth are observed. As shown in Figure 6, small needles grew between the grooves associated to the drawing lines. The X-ray microanalysis shows a homogeneous distribution of Zr and O across the shell. Figure 7a shows a cross-section of the wire. Figure 7b,c show the maps and line profiles corresponding to O and Zr, respectively. The maps evidence that oxygen was only detected from the outer part corresponding to the oxidized layer. As aforementioned, the thickness of the shell (about 10 \(\upmu\)m) was taken as the average of some of the circular crown radii of the oxide layer. The profile line shown in Figure 8c strengthens the prior estimation. The spectrum shown in Figure 7d was recorded only from the shell (not made in cross-section) and must be carefully considered. The spectra were recorded at an accelerating voltage of 10 keV to minimize the contribution of the zirconium core to the signal. However, the K\({}_{\alpha}\) line from Zr (15.74 keV) is not perceptible in these conditions, and the characteristic line used for Zr is L\({}_{\alpha}\), with an energy of 2.04 keV, albeit its intensity was 25 times lower [25]. This estimation is based on the ratio of average intensity fluorescence coefficients of K\({}_{\alpha}\) and L\({}_{\alpha}\) shells [25]. That is the reason why the most intense signals appear to come from Si and S, whose K\({}_{\alpha}\) lines are placed at 1.739 and 2.307 keV, respectively. According to the provider, the main impurities are Hf, O and C. Nevertheless, as shown in Figure 7d, Si and S can also be observed, and these were possibly added to Zr in the purification process from mineral ore. The carbon peak is due to graphite tape, which was used to stick the sample to the holder. The inset of Figure 7d shows an estimation of the relative intensities of both Zr lines. Figure 5: SEM images obtained from simples grown at a current of: (**a**) 2.2 A; (**b**) 2.8 A; (**c**) detail of the sample shown in part (**b**). Figure 6: (**a,b**): SEM images of the nanostructures obtained at a current of 2.2 A for 30 s which shows needles between the grooves associated to the drawing lines. Luminescence properties were studied by means of photoluminescence (PL) and cathodoluminescence (CL). Depending on the excitation mechanism (PL/CL), different routes for valence electrons promoted to the conduction band could be enhanced, so complementary information about the recombination paths and defect structure can be obtained using both techniques. In both cases, the spectra consist of a broad visible band extending from blue to green, although the relative intensity of the different components depends strongly on the excitation source. The origin of these emissions is not clear [26], but there is a consensus view about the role of defects and residual impurities on them [17; 27; 28]. The majority of the defects found on oxides are color centers. The most common ones are F- or F\({}_{2}\) -type centers, oxygen vacancies or divacancies with trapped electrons, respectively.