# Solar Explosive Evaporation Growth of ZnO Nanostructures
## 3 Results
It should be also noted that the hexagons and the needles have similar crystal-like nature: the hexagons are oriented perpendicular to plane of the substrates, while the needles are hexagon-like crystals that are ended by needles oriented at different angles to the substrate due to heterogeneity of the surface of unpolished Si substrates. In the case of the spheres, XRD peaks of (002), (103), (112) have similar integral intensities. This may be due to their sphere-like morphology. ZnO nanoneedles demonstrated the largest integral intensity of XRD peaks, suggesting their crystalline quality was best among the others. FTIR spectroscopy was used for investigation of the aforementioned ZnO nanostructures. Generally, it is possible to study the chemisorbed contamination in nanostructures as well as the distribution of the latest ones over the substrate surface. Figure 4 shows typical reflectance spectra for ZnO nanostructures deposited on Au/Si (curve 1), Ag/Si (curve 2), and unpolished Si (curve 3) substrates. It was not possible to investigate samples in transmittance mode, because Au/Si, Ag/Si, and Si substrates were not transparent. FTIR measurements of the given spectral range will not allow us to establish the fact of the formation of ZnO, because the Zn-O vibration bond at 482 cm\({}^{-1}\)[11] is located outside of the spectral range of the measurements. The typical reflectance spectra of ZnO nanostructures reveal the absence of good resolved peaks associated with chemisorbed contaminations on the surface of ZnO nanostructures (Figure 4). \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**ZnO Morphology**} & **Crystal Planes** & \multirow{2}{*}{**20(h k l)**} & \multirow{2}{*}{**XRD FWHM (deg.)**} & \multicolumn{2}{c}{**Integral Intensity of XRD**} \\  & **(h k l)** & & & **(h k l) (a.u.)** \\ \hline \multirow{4}{*}{Needles} & (002) & 34.36 & 0.18 & 157.2 \\  & (101) & 36.21 & 0.36 & 3.7 \\  & (102) & 47.51 & 0.24 & 5.1 \\  & (103) & 62.87 & 0.33 & 24.6 \\ \hline \multirow{4}{*}{Spheres} & (002) & 34.09 & 0.48 & 18.9 \\  & (103) & 63.12 & 0.9 & 23.9 \\  & (112) & 67.63 & 0.99 & 18.71 \\ \hline Hexagons & (002) & 34.17 & 0.12 & 0.45 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The results of XRD analysis for ZnO nanostructures. Figure 4: The typical Fourier transform infrared (FTIR) spectra of ZnO nanostructures grown on **(1)** Au/Si; **(2)** Ag/Si; **(3)** unpolished Si substrates. It is significant to note that Raman scattering studies are very sensitive for determining crystal perfection and structural defects. Figure 5 presents ZnO Raman spectrum only for ZnO nanoneedles due to their homogeneous dense bulk arrangement on the surface of Si substrate and due to the confocal nature of Raman scattering measurements. Raman signals were unable to be obtained for uniformly grown ZnO hexagons and ZnO spheres that have small bulks and because of the presence of carbon on the surfaces of nanostructures (Figure S1a,b). In the Raman spectra of ZnO nanoneedles (Figure 5), there is a well-known intense \(E_{2}{}^{low}\) phonon band at 98.9 cm\({}^{-1}\) (FWHM \(\approx\) 5.3 cm\({}^{-1}\)) and a \(E_{2}{}^{high}\) phonon band at 437 cm\({}^{-1}\) (FWHM \(\approx\) 9.5 cm\({}^{-1}\)) associated with the motion of Zn and O sublattices, respectively [12]. These small FWHM magnitudes of \(E_{2}{}^{Low}\) and \(E_{2}{}^{High}\) modes confirm the wurtzite ZnO structure for nanoneedles deposited by MEE using concentrated solar radiation [13; 14]. The position of the \(E_{2}{}^{high}\) band at 437 cm\({}^{-1}\) corresponds to the phonons of a bulk ZnO crystal [15], indicating a strain-free state of the nanoneedles. The weak band at 331 cm\({}^{-1}\) is assigned to \(E_{2}{}^{high}\)-\(E_{2}{}^{low}\) which is a second-order mode caused by multi-phonon processes. The wide phonon band at about 578 cm\({}^{-1}\) is attributed to the quasi-\(A_{1}{}^{Lo}\)(\(E_{1}{}^{Lo}\)) mode reflecting the random orientation of ZnO nanoneedles.