## 3 Results and Discussion
### Physicochemical Properties of the NFs
In the former case, all detected peaks were ascribable to the reflections from crystallographic planes of rhombohedral hematite (\(\alpha\)-Fe\({}_{2}\)O\({}_{3}\), JCPDS card no. 33-0664. ), indicating the formation of a single crystalline phase, as expected on the basis of previous studies [36; 39; 42] and literature [46; 47; 49; 51; 52; 53; 57] as well. The average crystallite size (30.4 nm) was estimated from the FWHM (full width at half-maximum, \(\beta\)) of the most intense (104) peak, via the Scherrer formula, \(d\) = K\(\Delta\)g\(\cos\)8, with K = 0.9 and \(\lambda\) = 0.1541 nm. In the X-ray diffractogram of the germanium-containing NFs, the peaks indexed to \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) were weaker and broader. In addition, the diffraction peaks typical of \(\gamma\)-phase of the iron (III) oxide (maghemite, JCPDS No.39-1346) were detected [58; 59; 60]. The estimation of the average crystallite size from the (104) peak FWHM gave a smaller value (18.1 nm) with respect to the pure oxide, in good agreement with the TEM analysis and literature reports on doped Fe\({}_{2}\)O\({}_{3}\)[61; 62; 63] as well. Finally, the detection of a very broad background in the pattern of Ge-containing NFs suggested the presence of an amorphous component. Figure 2 compares the micro-Raman spectra of Fe\({}_{2}\)O\({}_{3}\) and Fe\({}_{2}\)O\({}_{3}\):Ge NFs, as resulting from the average on different random locations in each specimen. The group theory predicts seven normal phonon modes for \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\)[64; 65; 66], namely two \"external\" modes involving the translations and rotations of entire Fe\({}_{2}\)O\({}_{3}\) units (\(E_{\text{g}}\)(1) and \(E_{\text{g}}\)(2) at 245 and 294 cm\({}^{-1}\), respectively) and five \"internal\" modes associated to the motion within a single Fe(O)\({}_{6}\) octahedral unit (\(A_{\text{1g}}\)(1), \(E_{\text{g}}\)(3), \(E_{\text{g}}\)(4), \(A_{\text{1g}}\)(2) and \(E_{\text{g}}\)(5) at 225, 298, 412, 500, and 612 cm\({}^{-1}\), respectively) [67; 68; 69]. In addition, the peaks originating from these modes, a strong asymmetrical band peaking at \(\sim\)1320 cm\({}^{-1}\) and two weak features at \(\sim\)660 and \(\sim\)830 cm\({}^{-1}\) were detected in the spectrum of the Fe\({}_{2}\)O\({}_{3}\) NFs. The peak at \(\sim\)660 cm\({}^{-1}\) is associated to the IR-active longitudinal optical (LO) \(E_{\text{u}}\) mode, activated by disorder (grain size, defects, heteroatoms) in the hematite lattice [36; 42; 56; 66; 69; 70], while the asymmetrical band has been recently ascribed to its overtone [66; 67; 69]. The peak at \(\sim\)830 cm\({}^{-1}\) is assigned to the photo-excitation of a spin-wave (one-magnon mode, 1M) in the canted antiferromagnetic structure of \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\)[66; 71]. Spectra collected from different random locations in the reference sample (not shown) did not significantly differ from each other, evidencing the spatially uniform formation of hematite. Five Raman-allowed phonon modes (\(A_{\text{1g}}+E_{\text{g}}+3T_{\text{2g}}\)) are predicted for the \(\gamma\)-polymorph of iron (III) oxide having a spinel crystalline structure [65; 72], similarly to magnetite (Fe\({}_{3}\)O\({}_{4}\)). Nonetheless, in maghemite, a Fe-deficient form of Fe\({}_{3}\)O\({}_{4}\) (Fe\({}^{3+}_{\text{1g}}\) [Fe\({}_{5/3}\)\({}^{3+}_{\text{1g}}\)\(\square\)\(\square\)\({}_{1/3}\)\(\square\)\({}_{1}\)O\({}_{4}\)\({}^{-2}\), with \(\square\) denoting a Fe-vacancy and \(th\) and \(oh\) indicating tetrahedral and octahedral sites, respectively), the peaks are broader [72]. Indeed, three peaks are usually detected (\(T_{\text{1g}}\), \(E_{\text{g}}\) and \(A_{\text{1g}}\) at \(\sim\)360, \(\sim\)510 and \(\sim\)705 cm\({}^{-1}\), respectively) in \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\), along with the band originating from 1M photo-excitation at \(\sim\)1430 cm\({}^{-1}\)[73; 74].