## 6 Swift Heavy Ion Irradiation-Induced Shape and Structural Changes in Cobalt Nanoparticles
### Results
**Figure 25.****a**) Background corrected SAXS scattering intensity as a function of scattering vector \(Q\) and (**b**) the corresponding minor dimension distributions for Co NPs irradiated with 89 MeV Au ions at different fluences. The inset compares XTEM and SAXS distributions. The SHII-induced shape transformation of Co NPs was first observed by D'Orl\(\acute{e}\)ans _et al._[78]. They attributed the elongation to NP melting and creep deformation induced by an overpressure due to differences in volume expansion and compressibility of the metallic NP and insulating host matrix. A stress-driven deformation of the SiO\({}_{2}\) matrix due to SHII-induced in-plane strain perpendicular to the ion beam direction has also been suggested as a potential elongation mechanism [107]. The latter leads to the well known \"ion hammering\" effect [108, 109] causing the anisotropic expansion of both bulk SiO\({}_{2}\)[95, 110] and colloidal SiO\({}_{2}\)[111, 112] perpendicular to the incident beam. The lack of NP elongation in the absence of a sufficiently thick surrounding matrix, as shown by Penninkhof _et al._[113], supports this theory. Calculations using a viscoelastic model [114], however, show that the strain generated by the incident ions in the SiO\({}_{2}\) matrix alone is insufficient to deform the Co NPs. Furthermore, the strain mechanism cannot explain the observed \(S_{e}\)-dependent saturation of the NP width. As discussed above, the correlation between the minor dimension and the molten ion track diameter shown in Figure 26 suggest that the elongation is confined by the ion track. We have recently reviewed the material-dependent saturation in the minor dimension for various metallic NPs and have shown that the energy deposited in the NPs also explicitly plays an active role in the elongation process (_i.e._, SHII-induced melting) [115]. Recently, thermal spike calculations have shown that Au NPs of certain sizes do in fact melt under SHII [116, 117]. The \(S_{e}\)-dependent elongation of Co NPs presented here are hence potentially the result of a combination of NP melting and in-plane strain where the elongation is potentially a means of relaxing such strain [118]. Figure 26: Saturated minor dimension for Co NPs and SiO\({}_{2}\) ion track diameter as a function of electronic energy loss in SiO\({}_{2}\). The atomic environment of the Co NPs, as a function of SHII energy and fluence, was determined from XANES analysis, an example of which is shown in Figure 27(a). Prior to SHII the NPs were FCC in phase. At low fluences, before any significant change in NP shape, the FCC-to-HCP phase transformation was observed, as presented in Section 5. Figure 27(a) shows that with increasing fluence the NPs undergo further changes relative to the unirradiated and bulk spectra, including a decrease in intensity of the edge feature at 7,710 eV, a \"smearing\" of the features at 7,726, 7,730 and 7,760 eV and a shift of the absorption-edge to higher energy. All such changes are characteristic of oxide  
Figure 27: (**a**) XANES spectra for bulk standards and NP samples irradiated at 185 MeV as a function of fluence (/cm\({}^{2}\)). Open circles in (**a**) represent linear combination fits to experimental data with oxide and metal standards; (**b**) The metallic and oxide fractions determined from the XANES analysis for the irradiated NP samples as a function of SHII energy and fluence. formation [119]. A linear combination fit of the XANES region was performed to quantify the fraction of Co in metallic and oxidised phases. Examples of the fitting are included in Figure 27(a). The oxide phase is non-crystalline and has characteristic features of a \"molecular oxide\". In fact, the oxide structure observed from the XANES is similar to the as-implanted, NP-free state (labelled \"oxide\" in Figure 27(a)). The metallic and oxide fractions are reported in Figure 27(b) as functions of SHII fluence and energy.