## 4 Ion Irradiation-Induced Amorphisation of Cobalt Nanoparticles
### Results
Nonetheless, the ability to recrystallise the material further supports the assertion that the presence of recoiled impurities is not responsible for amorphous phase formation. The result described above for annealing at 100 \({}^{\circ}\)C is of technological importance, demonstrating embedded a-Co NPs are stable at and above ambient conditions, in contrast to thin-film a-Co [66] that recrystallizes at \(-\)52 \({}^{\circ}\)C. As such, there is potential to exploit the possibly novel optical and/or magnetic properties associated with the amorphous phase in devices operating at room temperature. For the NP samples studied herein, the surrounding amorphous SiO\({}_{2}\) matrix may well help to stabilize the amorphous phase. Furthermore, recrystallisation of the entire NP ensemble necessitates a nucleation event in each NP in contrast to a continuous thin film, where conceivably only a single nucleation event is required. Indeed, comparing the a-Co NP results here with those of thin-film a-Co suggests that the recrystallisation process of a-Co NPs is potentially nucleation limited. Temperature-dependent FT EXAFS spectra are shown in Figure 15 for samples of bulk HCP Co, c-Co NPs and a-Co NPs. As discussed above, the EXAFS amplitude is influenced by \(\sigma^{2}\) which is comprised of static and thermal components. As the measurement temperature increases, so does the latter and as a consequence a decrease in amplitude is apparent in Figure 15. Thermally-induced damping of the amplitude is more pronounced in the bulk HCP Co and c-Co NPs compared to that of the a-Co NPs. As shown below, this trend is characteristic of a greater Einstein temperature for the a-Co NPs. Figure 15: FT EXAFS spectra for (**a**) bulk HCP Co, (**b**) 3.7 nm c-Co NPs and (**c**) 3.6 nm a-Co NPs as a function of measurement temperature. Figure 16(a) shows the temperature dependence of the \(\sigma^{2}\), obtained from individual fits of the first NN shell for all samples. Values for both the c- and a-Co NPs are offset from those of bulk HCP Co due to greater static disorder (\(\sigma^{2}_{s}\)), assumed to be temperature-independent, while, for any given sample, the general increase in \(\sigma^{2}\) as a function of measurement temperature is due to an increase in thermal disorder. Also included in Figure 16(a) are fits to the Einstein model defined in the previous section (Equation (1)). Calculated parameters are listed in Table 6. Comparing the bulk HCP Co and c-Co NPs, the decrease in Einstein temperature (\(\Theta_{E}\)) for the latter indicates the mean vibrational frequency has shifted to lower values. Such behaviour was attributed to the dominant influence of loosely-bound surface/interfacial atoms as discussed in Section 3, consistent with theoretical predictions [55]. Surprisingly, a significant increase in \(\Theta_{E}\) was measurable for the a-Co NPs. Though similar observations for a-Cu NPs have been reported [74], intuitively one might expect the disorder inherent in the amorphous phase to produce a decrease in \(\Theta_{E}\). (Compare, for example, bulk crystalline Ge (\(\Theta_{E}\) = 351 K) and amorphous Ge (\(\Theta_{E}\) = 343 K) [45]). An increase in \(\Theta_{E}\) implies bond stiffening, where the latter can result from an enhanced compressive state [75] or inhomogeneous internal strain [76]. The results presented herein may again reflect the influence of the embedding matrix. With the measurable decrease in \(CN\) and increase in \(BL\), a decrease in atomic density and increase in NP volume should accompany the crystalline-to-amorphous phase transformation for a NP with a fixed number of atoms. While the loss of atoms via recoils can lessen this constraint, the results presented herein suggest that the rigid embedding SiO\({}_{2}\) matrix may well impede NP volume expansion and as a consequence drive \(\Theta_{E}\) upward [77]. Figure 15: _Cont._  
**Figure 16**. : Thermal evolution of the first NN (**a**) \(\sigma^{2}\) and (**b**) \(C_{3}\) for bulk HCP Co, 3.7 nm c-Co NPs and 3.6 nm a-Co NPs.