## 4 Ion Irradiation-Induced Amorphisation of Cobalt Nanoparticles
### Results  
Figures 7(a) and (b) show XTEM images for unirradiated (c-) and irradiated (a-) Co NPs, respectively, confirming the spherical shape of the NPs before and after irradiation. Figure 7: XTEM micrographs for (**a**) 3.7 nm c-Co NPs and (**b**) 3.6 nm a-Co NPs and (**c**) shows the radially integrated ED patterns for both samples. The NP size is effectively unchanged from the unirradiated state (3.7 nm). The ED patterns collected for the c-NPs and a-NPs were radially averaged and the resulting intensity profiles are shown in Figure 7(c). Indexing the ED patterns (peaks labeled A-E) yields the results listed in Table 4 and shows that the unirradiated c-Co NPs are comprised of a mixture of HCP and FCC phases. The well-defined peaks typical of randomly-oriented crystalline NPs vanish upon irradiation, consistent with (though not necessarily indicative of) the formation of an amorphous phase. Figure 8(a) shows background-corrected SAXS spectra for the c- and a-Co NPs with the fitted \(Q\) range overlaid as a solid line while Figure 8(b) demonstrates that the normalized size distributions are clearly Gaussian in shape. The average NP diameter before and after irradiation was 3.7 \(\pm\) 1.0 nm and 3.6 \(\pm\) 1.0 nm, respectively, and thus irradiation did not produce a change in NP size. These results agree well with the TRIDYN [62] simulations of Figure 9 for irradiation-induced atomic inter-mixing of a Co layer (3.7 nm in thickness) embedded in SiO\({}_{2}\) and irradiated with Au ions to a fluence of 2 \(\times\) 10\({}^{13}\) /cm\({}^{2}\) at an energy of 5.3 MeV. The Au ion energy for the TRIDYN simulation was that of the Au ions in SiO\({}_{2}\) at the peak Co concentration (1 \(\mu\)m) in the NP sample. Recoiled Co, Si and O atoms slightly alter the Co  
\begin{table}
\begin{tabular}{l c c c c c} \hline \hline
**HCP** & \(\mathbf{d}\) & **FCC** & \(\mathbf{d}\) & **Peak** & \(\mathbf{d}\) \\ (**hkl**) & **(Å)** & **(hkl)** & **(Å)** & & **(Å)** \\ \hline \((100)\) & \(2.17\) & \((111)\) & \(2.05\) & A & \(2.15(0.02)\) \\ \((002)\) & \(2  
atomic depth distribution leading to a less well defined Co/SiO\({}_{2}\) interface after irradiation. Inter-mixing of the matrix constituents and Co layer is clearly minimal and well below that required to stabilize the amorphous phase in bulk Co through the addition of Si and O atoms [63]. As such, it is argued that the irradiation-induced structural changes described below do not result from the presence of recoiled matrix constituents. As above, the nuclear and electronic energy losses are comparable. To identify if one or both energy loss regimes is/are responsible for the observed amorphisation would necessitate experiments as a function of Au ion energy. XANES spectra for two bulk standards plus c- and a-Co NPs are shown in Figure 10. The sensitivity of the XANES technique to valence state, crystallographic phase, multiple scattering effects (and more) makes for an effective means of \"fingerprinting\" the crystallographic phase of a material under investigation [45, 64]. The prominent features of the bulk HCP Co and c-Co NP spectra in Figure 10 result from multiple scattering resonances of the \(1s\) photoelectron [20]. The a-Co NP spectrum (and those of disordered or amorphous materials in general) exhibits less fine structure due to the loss of such multiple scattering contributions [45, 65]. This spectrum is also similar to that reported previously for thin-film amorphous Co [66]. The changes in the a-Co NP XANES spectra (compared to the c-Co NPs and bulk HCP Co) are consistent with those observed for other transition metals including Cu NPs [67] and bulk amorphous Fe prepared sonochemically [65]. The observed changes are attributed to the disappearance of multiple scattering (MS) features and shape resonances associated with these MS paths in the amorphous phase. Figure 9: TRIDYN simulation of ion beam mixing in a 3.7 nm Co layer embedded in SiO\({}_{2}\) induced by 9 MeV Au irradiation.