## 6 Swift Heavy Ion Irradiation-Induced Shape and Structural Changes in Cobalt Nanoparticles
### Results  
Figure 23 shows bright-field XTEM images and corresponding minor and major dimension distributions for the Co NPs before and after 89 MeV SHII. The figure also demonstrates the Co NPs transform into nanorods along the incident ion beam direction, consistent with previous studies [78; 103]. The elongation process is clearly cumulative with samples requiring fluences of the order 10\({}^{13}\) /cm\({}^{2}\) (or \(\sim\)200 overlapping impacts for an ion track of dimension 9.8 nm). Included in Figure 23(f) is a high resolution XTEM micrograph that shows a number of smaller spherical \"satellite\" NPs surrounding an elongated NP. Figure 23(d,f,h) also show that the minor dimension distributions determined from the XTEM analysis are narrow and Gaussian in shape. The major dimension distributions, however, are considerably broader and Log-Normal in form. The evolution of the nanorod aspect ratio calculated from the XTEM analysis for the 89 MeV sample set is shown in Figure 24. Each point in Figure 24 represents the mean value of the minor dimension for a given major dimension (\(\pm\)1 nm) and was determined by counting \(\sim\)600 NPs for each fluence. This figure demonstrates that there is a threshold (minimum) size for elongation of \(\sim\)6.5 nm for 89 MeV Au ion irradiation. NPs larger than this threshold elongate with ion fluence and the minor dimension saturates at approximately the same value as the threshold for shape transformation. The greatest aspect ratios were observed at intermediate fluences and with increasingfluence (\(>\)6 \(\times\) 10\({}^{13}\) /cm\({}^{2}\)) the aspect ratio decreased due to NP dissolution indicated with an arrow in Figure 24.  
Figure 23: Bright field XTEM micrographs (**a**), (**c**), (**e**), (**g**) and corresponding size distributions (**b**), (**d**), (**f**), (**h**) as a function of fluence for Co NPs under 89 MeV Au irradiation (/cm\({}^{2}\)). Red arrows indicate the direction of the ion beam. Complementary determinations of the nanorod minor dimensions and also the evolution of the small NPs were achieved via SAXS. The isotropic scattering from samples was analysed to determine the minor dimension. Figure 25(a) shows the background-corrected scattering intensity for the Co NPs as a function of \(Q\) following 89 MeV Au irradiation [106]. The oscillations in the scattering intensity at high fluences are indicative of a narrowing of the distribution. The reconstructed minor dimension distributions are shown in Figure 25(b). The size distributions determined from both XTEM and SAXS for the unirradiated sample are superimposed in the inset of Figure 25(b) for comparison and agreement between the two different techniques is apparent. A decrease in intensity is observed for the 2 \(\times\) 10\({}^{14}\) /cm\({}^{2}\) sample and is indicative of NP dissolution as discussed in the following section. Similar trends were observed for all irradiation energies. The narrowing of the minor dimension distributions  
Figure 23: \(Cont\). Figure 24: NP minor dimension verses major dimension as a function of fluence (/cm\({}^{2}\)) for 89 MeV Au irradiation. with increasing ion fluence is in agreement with the XTEM results presented above. The population of NPs with diameters \(<\)5 nm is attributed to the small spherical \"satellite\" NPs, apparent in the XTEM images at high resolution (Figure 23(f)). Although the XTEM size distributions include satellite NPs, the bimodal shape, like that in Figure 25(b), is not apparent. Statistical limitations apply to the XTEM analysis with distributions generated by counting \(\sim\)600 NPs. SAXS, on the other hand, probes a much larger volume of NPs and is thus more reliable from a statistical viewpoint. The saturation in the minor dimension extracted from the XTEM analysis is plotted in Figure 26 as a function of \(S_{e}\) in SiO\({}_{2}\). The Au ion track widths determined for SiO\({}_{2}\) from SAXS experiments [100] are also included for comparison and indicate that the elongation is confined by the molten ion track.