## 4 Ion Irradiation-Induced Amorphisation of Cobalt Nanoparticles
### Results
\begin{table}
\begin{tabular}{l c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & **BL** & \(\mathbf{\sigma^{2}}\) & **C\({}_{3}\)** & **CN** & **Size** \\  & **(Å)** & **(10\({}^{-3}\) Å\({}^{2}\))** & **(10\({}^{-5}\) Å\({}^{3}\))** & **(atoms)** & **(nm)** \\ \hline Bulk HCP & \(2.481(0.002)\) & \(2.63(0.13)\) & \(-3.2(3.9)\) & \(12\) & – \\ c-Co NPs & \(2.469(0.003)\) & \(4.08(0.16)\) & \(-13.3(4.8)\) & \(10.0(0.2)\) & \(3.7(1.0)\) \\ a-Co NPs & \(2.491(0.004)\) & \(12.45(0.72)\) & \(78.9(13.3)\) & \(7.8(0.9)\) & \(3.6(1.0)\) \\ a-Co NPs/100 \({}^{\circ}\)C & \(2.486(0.007)\) & \(12.24(0.82)\) & \(82.0(13.4)\) & \(7.7(0.6)\) & \(3.6(1.0)\) \\ a-Co NPs/400 \({}^{\circ}\)C & \(2.466(0.004)\) & \(7.41(0.23)\) & \(-22.1(7.8)\) & \(9.0(0.7)\) & \(3.6(1.0)\) \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Refined EXAFS structural parameters and SAXS mean NP sizes for bulk HCP Co, 3.7 nm c-Co NPs, 3.6 nm a-Co NPs and 3.6 nm a-Co NPs annealed at 100 and 400 \({}^{\circ}\)C.  
The interatomic distance distributions shown in Figure 13 were reconstructed from the EXAFS-determined structural parameters. This figure best illustrates the irradiation-induced changes and also enables direct comparison with MD simulations. Results are shown for the latter with a quench rate of 1.0 K/fs, typical for the relaxation of a collision cascade [27]. A slower quench rate of 0.1 K/fs yielded a nominally higher amplitude consistent with a slightly less disordered, more crystalline-like  
Figure 12: FT EXAFS spectra for bulk HCP Co, 13.2 FCC and 5.2 nm HCP Co NPs prior to and after (labeled irr.) 9 MeV Au ion irradiation to a fluence of 2 \(\times\) 10\({}^{13}\) /cm\({}^{2}\). Open circles represent fits to experimental data. Figure 13: Reconstructed interatomic distance distributions for the first NN shell of bulk HCP Co, 3.7 nm c-Co NPs and 3.6 nm a-Co NPs compared to the interatomic distance distribution for amorphous Co from MD simulations. structure. Clearly the MD simulation agrees exceedingly well with the experiment, reproducing the amplitude, width and positive asymmetry of the distribution. The simulations and experimental data are also consistent with the experimental results shown in reference [66]. As anticipated, Au ion irradiation of bulk HCP and FCC Co standards yielded no change in the structural parameters of these crystalline metals, consistent with the general inability to form an irradiation-induced amorphous phase in an elemental metal. Finite-size effects in the form of inherent structural disorder clearly play a significant role in enhancing the sensitivity of embedded NPs to ion irradiation. Though the surrounding matrix may aid in stabilizing the amorphous NP structure, measurements with large c-Co NPs discussed above demonstrate that the presence of the matrix alone does not yield amorphous NPs for the given irradiation fluence. The thermal stability of the a-Co NPs was investigated via \(exsitu\) annealing of the as-irradiated EXAFS sample at temperatures of 100-400 \({}^{\circ}\)C for 1 h. Annealing at 100 \({}^{\circ}\)C yielded no observable changes in the FT EXAFS spectra (or in the calculated structural parameters), as shown in Figure 14 and Table 5. Scattering contributions from beyond the first NN shell, as consistent with the onset of recrystallisation, were apparent at annealing temperatures of 200 \({}^{\circ}\)C and above. The spectrum following annealing at 400 \({}^{\circ}\)C is included in Figure 14 and structural parameters are included in Table 5. As expected, the thermally-induced amorphous-to-crystalline transformation yields an increase in \(CN\), a decrease in \(BL\) and \(\sigma^{2}\) and a positive-to-negative change in value of \(C_{3}\). A component of residual disorder does however remain and thus higher annealing temperatures would be required to completely recover a structural state akin to unirradiated c-Co NPs (annealing of samples above 400 \({}^{\circ}\)C was not performed to maintain the integrity of the Kapton windows).