## 4 Ion Irradiation-Induced Amorphisation of Cobalt Nanoparticles
### Results
Figure 11 displays (a) \(k^{3}\)-weighted and (b) FT EXAFS spectra, respectively, for the HCP standard and NP samples. The isolated EXAFS spectra were analysed following the methods described in Section 2, with the exception that the spectra were FT over a \(k\) range of 3.3-14.5 A\({}^{-1}\). Comparing spectra for HCP Co and c-Co NPs, the reduction in amplitude of the latter is due to a decrease in \(CN\) and increase in \(\sigma^{2}\), as listed in Table 5. Such changes stem from finite-size effects as discussed in Section 3. A further, more pronounced reduction in amplitude is evident in the a-Co NP spectrum of Figure 11. Again a decrease in \(CN\) (due to less efficient atomic packing) and, to a greater extent, an increase in the \(\sigma^{2}\) (due to greater structural disorder) drive this change (see Table 5). Furthermore, an increase in \(BL\) and a change of sign in \(C_{3}\) are also observed. Similar structural changes have been observed in amorphous semiconductors [68, 69, 70] and attributed to, respectively, the increase in disorder intrinsic to the amorphous phase and the sampling of more anharmonicity in the interatomic potential. Comparing c- and a-Co NPs, the changes in all four structural parameters (\(CN\), \(\sigma^{2}\), \(BL\) and \(C_{3}\)) are consistent with theoretical predictions for the amorphous phase of an elemental metal [71]. The trends in the structural parameters, including a \(BL\) expansion and a \(BL\) distribution skewed to longer \(BL\)s are in excellent agreement with recent investigations of ion irradiated Cu and Au NPs [67, 72]. The aforementioned studies also report a decrease in the \(CN\) and a significant increase in the \(\sigma^{2}\) (\(\sim\)3 times larger than the bulk) in the disordered phase. Note also the lack of scattering beyond the first NN shell in the a-Co NP spectrum is in stark contrast to the c-Co NP counterpart where contributions from the first four NN shells are readily apparent. Such a trend is typical of the crystalline-to-amorphous phase transformation [69, 73] and thus further supports the assertion of amorphous phase formation upon irradiation of c-Co NPs. Figure 10: XANES spectra at the Co \(K\)-edge for bulk HCP Co, 3.7 nm c-Co NPs, 3.6 nm a-Co NPs and bulk crystalline Co\({}_{3}\)O\({}_{4}\) with spectra offset for comparison. Note that the \(CN\) for a-Co NPs listed in Table 5 is necessarily influenced by finite-size effects. Given the nil change in NP diameter upon irradiation and assuming (\(CN_{bulk\;a-Co}/CN_{bulk\;c-Co}\)) = (\(CN_{a-Co\;NPs}/CN_{c-Co\;NPs}\)), we estimate the \(CN\) of bulk a-Co should thus be 9.4 atoms, in excellent agreement with that previously determined for thin-film a-Co formed by vapour deposition and stable only at low temperatures [66]. The amorphous phase is also observable for 9 MeV Au ion irradiated 5.2 nm HCP Co NPs as demonstrated in Figure 12 with the loss of structure beyond the first NN shell readily apparent. In contrast, large 13.2 nm FCC Co NPs do not transform after irradiation and higher NN peaks are still observable, indicating that the amorphisation process is size dependent. The large 13.2 nm NPs have structural parameters similar to the bulk and as discussed in Section 1, the amorphous phase formation by ion irradiation has not been achievable in bulk elemental metals. Figure 12 also indicates that the amorphisation process in Co NPs is potentially insensitive to the initial crystal phase, as both mixed HCP/FCC and pure HCP phase NPs both transform with irradiation. Further experiments on small FCC Co NPs would need to be performed to confirm this however. Figure 11: (**a**) \(k^{3}\)-weighted EXAFS spectra and (**b**) corresponding phase corrected FT EXAFS spectra for bulk HCP Co, 3.7 nm c-Co NPs and 3.6 nm a-Co NPs (irradiated with 9 MeV Au ions to a fluence of 2 \(\times\) 10\({}^{13}\) /cm\({}^{2}\)) measured at 18 K. Open circles represent fits to experimental data.