# Solar Explosive Evaporation Growth of ZnO Nanostructures
## 3 Results
The presence of the \(qA_{1}{}^{Lo}\) mode is related to impurities and defects, or free carriers in ZnO nanoneedles [16]. It is very interesting that an additional mode at 275 cm\({}^{-1}\) was found in the Raman spectra (Figure 5). The nature of this mode is still thoroughly unknown. The mode at 275 cm\({}^{-1}\) was found in Raman spectra of doped ZnO by nitrogen [17; 18; 19], by Al-N [20], and by Fe, Sb, Al, Ga, and Li [21]. This mode also appears in the Raman spectra of undoped ZnO films [22]. [PERSON] et al. attributed Raman mode at 275 cm\({}^{-1}\) to silent mode (\(B_{1}{}^{Low}\)) of wurtzite-ZnO allowed by the breakdown of the translational crystal symmetry caused by defects or impurities [23]. The fact that EDX analysis of ZnO needles (Figure S1) did not reveal the presence of nitrogen and other above-mentioned impurities indicates that the mode at 275 cm\({}^{-1}\) is silent mode \(B_{1}{}^{Low}\) caused by the presence of defects in the ZnO lattice. Room temperature PL investigations were carried out to examine the optical quality and presence of defects in ZnO nanostructures deposited by MEE. PL spectra of diverse types of ZnO nanostructures (Figure 6) have two typical regions: near band edge (NBE) excitonic emission and emission in the visible range (i.e., deep level emission--DLE) [7]. To carry out a qualitative comparison of the PL for our ZnO nanostructures, the intensities of PL were plotted on a logarithmic scale. Therefore, ZnO nanoneedles have intense NBE emission at 3.29 eV with FWHM about 86 meV and negligible DLE in the visible range with maximum about 2.1 eV, while DLE from ZnO nanohexagons and ZnO nanospheres were greater than their NBE. To estimate the optical quality of nanostructures, the ratio between the spectral integral intensity of NBE and DLE bands [8] was calculated. This ratio was 10.14 for ZnO nanoneedles, 0.03 for ZnO nanospheres, and 0.01 for ZnO nanoneedles. Hence,  
Figure 5: The room temperature Raman spectra of ZnO nanoneedles grown on the Si substrate. ZnO nanoneedles have the best optical quality compared to other types of nanostructures deposited by MEE. The deconvolution of DLE on different peaks by Lorentz approximation allows us to estimate the type of defects that are present in ZnO nanostructures. Figure 6 shows peaks at 2.59 eV (DLE1), 2.3 eV (DLE2), and 2.1 eV (DLE3) in visible range. We suggest that the nature of defects caused DLE is the same as that described in [8], because our ZnO nanostructures were grown under similar conditions. Therefore, DLE1 band at 2.59 eV is related to the presence of oxygen vacancies (Vo) and zinc interstitial (Zn\({}_{\text{i}}\)). The DLE2 band at 2.3 eV is caused by transition between electron near conduction band and a hole trapped at V\({}_{\text{v}}\)\({}^{**}\) level in the bulk of the particle, with the surface trapping of the photogenerated hole being the first step in the process [24]. DLE3 at 2.1 eV can be attributed to such point defects as oxygen interstitial (O\({}_{\text{i}}\)) [25; 26]. It should be noted that all defect-related DLE bands (DLE1, DLE2, and DLE3) were observed on PL spectra of all ZnO nanostructures (Figure 6). ZnO nanoneedles have smaller defect concentrations compared to other nanostructures.