## 3 Formation of Cobalt Nanoparticles by Ion Implantation
### Results  
Representative XTEM images and corresponding volume-weighted size distributions determined from the SAXS analysis for three samples are shown in Figure 2. The NPs shown in Figure 2 were formed by implanting (a) 3.0 at.% Co and annealing at 800 \({}^{\circ}\)C for 1 h; (c) implanting 4.5 at.% Co at 400 \({}^{\circ}\)C; and (e) 3.0 at.% Co and annealing at 1100 \({}^{\circ}\)C for 1 h. The NPs increase in size with concentration and annealing temperature. The NP sizes are listed in Table 2 (these samples will be used in later sections to demonstrate the various size-dependent responses of Co NPs to ion irradiation). In this section we only focus on samples that have been annealed after implantation, since the as-implanted samples have a considerable fraction of Co atoms in an oxidised state. The fraction of oxidised Co atoms were progressively converted to the metallic HCP and FCC phases with an increase in temperature. The relative fraction of HCP and FCC phases was governed by both concentration and annealing temperature [41]. **Figure 2.** Bright field XTEM micrographs (**a**), (**c**), (**e**) and corresponding size distributions (**b**), (**d**), (**f**) determined from SAXS. The NPs were formed by implanting (**a**) 3.0 at.% Co and annealing at 800 \({}^{\circ}\mathrm{C}\) for 1 h; (**c**) implanting 4.5 at.% Co at 400 \({}^{\circ}\mathrm{C}\); and (**e**) 3.0 at.% Co and annealing at 1,100 \({}^{\circ}\mathrm{C}\) for 1 h. \begin{tabular}{l c c c c c c} \hline
**Sample** & **Annealing** & \(\bigtriangleup\)\(BL\) & \(\sigma^{2}\) & \(CN\) & \(C_{3}\) & **size** \\  & **temp**(\({}^{\circ}\)C) & **(Å)** & **(10\({}^{-3}\)Å)** & **(atoms)** & **(10\({}^{-5}\)Å)** & **(nm)** \\ \hline Bulk FCC & \(-\) & \(2.491(0.003)\) & \(2.98(0.12)\) & \(12(\text{fixed})\) & \(-3.9(3.3)\) & \(-\) \\ Bulk HCP & \(-\) & \(2.481(0.002)\) & \(2.73(0.09)\) & \(12(\text{fixed})\) & \(-2.2(2.4)\) & \(-\) \\ \(9.0\) at.\% & \(1,100\) & \(2.488(0.004)\) & \(2.64(0.14)\) & \(11.2(0.3)\) & \(-4.0(3.7)\) & \(17.4\) \\  & \(800\) & \(2.469(0.005)\) & \(3.74(0.17)\) & \(9.7(0.3)\) & \(-9.3(2.3)\) & \(4.0(0.9)\) \\ \(4.5\) at.\% & \(400^{a}\) & \(2.479(0.004)\) & \(3.20(0.14)\) & \(11.0(0.3)\) & \(-5.2(1.2)\) & \(5.2(1.2)\) \\ \(4.0\) at.\% & \(400^{a}\) & \(2.473(0.006)\) & \(3.61(0.20)\) & \(10.6(0.3)\) & \(-5.8(1.9)\) & \(4.9(1.0)\) \\ \(3.0\) at.\% & \(1,100\) & \(2.485(0.004)\) & \(2.75(0.12)\) & \(11.7(0.3)\) & \(-6.0(1.3)\) & \(13.2(3.7)\) \\  & \(800\) & \(2.464(0.006)\) & \(4.43(0.21)\) & \(9.2(0.3)\) & \(-11.6(2.8)\) & \(3.2(0.7)\) \\ \(0.9\) at.\% & \(1,100\) & \(2.483(0.004)\) & \(2.96(0.13)\) & \(11.7(0.3)\) & \(-7.7(1.7)\) & \(11.5(2.8)\) \\  & \(800\) & \(2.456(0.009)\) & \(5.59(0.34)\) & \(8.1(0.4)\) & \(-15.8(4.6)\) & \(2.8(0.5)\) \\ \hline \end{tabular}   
**Figure 3.** The \(\sigma^{Figure 3(a-e) shows representative \(k^{3}\)-weighted EXAFS spectra as a function of annealing temperature and implanted Co concentration. The amplitude increases with an increase in either parameter, consistent with the previously demonstrated increase in NP size. From a comparison with the two standards, the largest NPs are clearly bulk-like. Figure 3(b-f) shows the corresponding phase-corrected FT spectra with refined structural parameters listed in Table 2. As above, the FT amplitude increases with an increase in either temperature or concentration, due to an increase in average \(CN\) and decrease in structural disorder. Figure 3: (**a**), (**c**), and (**e**) \(k^{3}\)-weighted EXAFS spectra (offset for comparison) and corresponding phase-corrected FT spectra (**b**), (**d**), and (**f**) for bulk standards, 1,100 \({}^{\circ}\)C, and 800 \({}^{\circ}\)C samples measured at 18 K. Open circles (**b**), (**d**) and (**f**) represent fits to experimental data.