## 6 Swift Heavy Ion Irradiation-Induced Shape and Structural Changes in Cobalt Nanoparticles
### Results
After high fluence irradiation the fraction of Co atoms in the metallic phase decreases with a concomitant increase in the fraction of Co atoms in an oxidised state. The increase in the fraction of oxidised Co atoms is attributed to the dissolution of the Co NPs. Figure 27(b) also shows that the dissolution rate is enhanced with an increase in \(S_{e}\) and thus the process is not ballistic. The XTEM, SAXS and XANES results presented herein clearly demonstrate that the volume fraction of NPs embedded in the SiO\({}_{2}\) is not conserved upon irradiation in contrast to previous reports [78, 103, 104, 105]. Furthermore, SHII of thin 3\(d\) transition metal oxides and composite layers results in interface modification [120, 121], phase formation [122] / transformation [123] and atomic mixing. In these studies, the interdiffusion / modification of the thin layers was found to be enhanced with an increase in \(S_{e}\) and attributed to the interdiffusion of material within the molten ion track. The dissolution of the Co NPs quantified via the XANES analysis is also attributed to the molten state reactions within and around the molten ion track. The \(S_{e}\)-dependent changes, specifically the increase in the dissolution rate for higher \(S_{e}\), are attributed to the \(S_{e}\)-dependent temperature profile and molten ion track diameter leading to an increase in the dissolved fraction of Co in SiO\({}_{2}\). Structural parameters for the irradiated Co NPs were determined from EXAFS. Figure 28(a) displays the \(k^{3}\)-weighted EXAFS for the bulk standards, unirradiated NPs, NPs irradiated at 185 MeV as a function of fluence and oxide standard (Co implanted, NP free SiO\({}_{2}\)). The amplitude of the EXAFS signal decreases as the ion fluence increases. Figure 28(b) shows the corresponding phase-corrected FT spectra. Prior to SHII, the NP spectrum is comparable to the FCC standard. Upon irradiation, changes in the amplitude are readily observable. No Co\(-\)O contribution at \(\sim\)1.8 A is apparent in the spectra even for samples with \(\sim\)50% oxide fraction since O is a weak scatterer and the Co\(-\)O bonds are highly disordered. Note that different \(k\)-weights and different FT ranges did not enable a reliable fit to the experimental data. As such, no results have been reported for Co\(-\)O shell. The evolution of the \(\sigma^{2}\), \(BL\), \(C_{3}\) and \(CN\) determined from the EXAFS analysis for the different SHII energy and fluence combinations is shown in Figure 29(a-d), respectively. Prior to SHII, the NP structural parameters are similar to bulk FCC Co as anticipated for these large NPs. Figure 29 readily shows \(S_{e}\)-dependent structural modifications with higher \(S_{e}\) values resulting in the rapid increase (\(\sigma^{2}\)), or decrease (\(CN\), \(BL\), \(C_{3}\)) in the structural properties of the Co NPs with increasing ion fluence. **Figure 28.** (**a**) EXAFS spectra and (**b**) corresponding FTs for the bulk standards and NP samples irradiated at 185 MeV as a function of fluence (/cm\({}^{2}\)). Spectra are offset vertically for clarity. Open circles represent fits to experimental data. XAS, like SAXS, is sensitive to the volume fraction of the different absorber-scatter environments present in a sample. For the irradiated NP samples, two different Co atomic environments have been identified with XAS: (i) metallic Co atoms from the NPs (made up of both large elongated NPs and small spherical NPs) and (ii) Co atoms dissolved in the matrix and bonding at the NP surface that increase in fraction with ion fluence. As discussed above, (i) is the result of the SHII-induced shaping while (ii) is attributed to the dissolution of the NPs with ion fluence. As discussed in Section 3, the \(\sigma^{2}\), \(BL\), \(CN\) and \(C_{3}\) are influenced by the Co NP size. The SHII-induced changes in these structural parameters with ion fluence are attributed to (i) and the increasing SBR for both the elongated and small NPs.