**3. Conclusions and outlook**

This chapter gave a look on the potential of Raman spectroscopy when applied to nanosized zinc ferrites. The nanostructured spinel ZnFe<sup>2</sup> O4 display a peculiar mixing of high-quality functional properties, and it is used in magnetic storage, ferrofluids, catalysis, and biomedical applications, as for instance theranostics and hyperthermia. For the latter applications, a strong requirement is the superparamagnetic behavior at RT, which take place in zinc ferrites thanks to the nanometric scaling. This behavior has been revealed even in pure ZFO nanoparticles due to the inversion between divalent and trivalent cations on the tetrahedral and octahedral sites of the spinel-type structure. In this frame, thanks to its intrinsic sensitivity to chemical and structural composition, Raman spectroscopy has been widely used to monitor cations distribution in pure and doped zinc ferrites. In the first part of the chapter, the state of the art concerning the results from Raman scattering in pure and doped zinc ferrites has been presented in connection to the different synthesis methods employed to obtain high quality nanostructured ferrites. It appears clear how detailed analyses on Raman data allow to evaluate the influence of nanometric scaling on Raman lines features—peak energies, widths and shape, to obtain information on the inversion degree, to reveal the presence of extrinsic iron oxides phases like hematite or magnetite and to monitor stability of spinel ferrites vs. light exposure or thermal treatment. This information is essential to assess the functional quality of the materials and it is essential to study and understand the close correlation between functional parameters and morphology, gran size, intentional doping, cation distribution, and nanoparticle architecture. Indeed magnetic, structural and chemical purity and homogeneity of nanoparticles must be thoroughly verified because even low amount of extrinsic phases can alter the functional properties. Indeed, low amount of hematite or maghemite can alter the long-term stability of nanoparticles especially in biological environment, favoring aggregation.

In the second part of the chapter, we presented original results from room temperature micro-Raman studies on nanostructured undoped ZnFe<sup>2</sup> O4, Zn1−xCax Fe2 O4 (x = 0.05 and 0.25), ZnFe1.9Gd0.1O4 , Zn0.95Sr0.05Fe2 O4 , and ZnFe1.9Al0.1O4 doped ferrites. The samples have been prepared by using the microwave-assisted combustion method, a rapid, green, and simple synthesis route, able to ensure good physical and chemical properties. We underline the relevance of Ca substitution due to the low toxicity of the substituent; on the other side, Gd doping is rarely reported in zinc ferrites. Raman experiments have been accompanied by detailed analyses on morphological, compositional, structural, and magnetic characterization. Thus, results from XRD, SEM-EDS are briefly presented as well as the magnetic behavior. A detailed analysis of A1g, E<sup>g</sup> , 3F2g Raman modes has been performed and Raman band parameters have been derived from best-fitting procedures and carefully compared to literature data. The results have been discussed in relation with grain size, inversion degree and doping of zinc ferrite spinel. An optimum purity level and homogeneity and crystallite sizes lower than 11 nm were determined for the doped samples. The inversion degree of the different samples was derived by both Raman and XRD data and a well agreement has been observed. The superparamagnetism seems to be favored in Ca doped samples, while in Gd doped one it is almost negligible. This is clearly associated to the inversion induced by the doping in the tetrahedral site. In addition by Raman studies we evaluated the stability of pure and doped ZnFe<sup>2</sup> O4 with laser heating just to reveal the presence of maghemite in the samples. This set of data gives evidence to the ability of Raman spectroscopy in a typical problem of solid-state material science. Finally, the present results will be the bases of further works aimed to exploit SERS effect using non-metallic nanostructured zinc ferrites eventually functionalized for specific biomedical applications.
