## 1 Introduction  
In recent years the advantageous optical, electrical and magnetic properties of nanoparticle-based materials have made them the focus of an intensive research effort in a range of scientific disciplines. In particular, the drive for further miniaturization of electronic, magnetic and photonic devices has led to numerous experimental and theoretical efforts to understand the physical properties of various metal and semiconductor nanoparticles (NPs) with emphasis on the effects of size, shape and host matrix (seefor example [1; 2; 3; 4; 5; 6; 7]). Furthermore, metallic NPs embedded in a dielectric host matrix (such as SiO\({}_{2}\)) are of particular interest as their unique nonlinear opto-electrical properties have direct applications in nano-electronic devices (e.g., in optical memories, nonlinear waveguide devices and optical switches) [8]. The size-dependent structural, and vibrational properties of metallic NPs differ considerably compared to those of their bulk counterparts. Such differences stem from a large fraction of surface atoms compared to the NP volume, or large surface-to-bulk ratio (SBR). The large SBR typically results in a depression in the melting temperature of metallic NPs [9; 10; 11] and structural deviations including reduced coordination number and increased structural disorder [12]. Of the many different metallic NPs studied, Co NPs have attracted particular attention due to their size, shape, and phase-dependent magnetic properties [13; 14] leading to potential applications in biomedicine [15] and magnetic storage technology [16]. Co NPs can exist in either the hexagonal closed packed (HCP), face-centered cubic (FCC), body-centered cubic (BCC), or epsilon \(\epsilon\) (similar to the \(\beta\) phase of Mn [13]) phases, the relative stability of which depends on the route of formation [17; 18; 19] host matrix [20; 21; 22] and thermal history [23; 24]. Of these four phases, HCP has the highest coercive field [13; 25] and is thus the most obvious candidate for ultrahigh-density recording applications. Co nanorods and nanowires also exhibit unique perpendicular magnetic anisotropy, coercivity, remanence, and activation volumes that are heavily dependent on the nanorod length (or equivalently, major diameter) [26]. Novel methods to tailor and control the structure \(and\) shape of Co NP ensembles are thus of paramount importance if they are to be successfully implemented into future magnetic devices. An emerging technique that is particularly useful in modifying the various properties of NPs is the combination of ion implantation and ion irradiation. Ion implantation is in fact a well established technique in the semiconductor integrated-circuit manufacturing industry and is now routinely used to introduce dopants, modify electrical properties and promote compound formation (to name but a few applications). As an ion passes through a solid, energy is transferred to the host lattice via interactions with target nuclei (nuclear energy loss (\(S_{n}\))) and target electrons (electronic energy loss (\(S_{e}\))). The \(S_{n}\) and  
Figure 1: Electronic and nuclear energy loss, \(S_{e}\) and \(S_{n}\), respectively, as a function of ion energy for Au in a Co matrix. components of the stopping power of Au ions in Co as a function of ion energy are shown Figure 1. The energy dependence of the two contributions are evident with \(S_{n}\) dominating in the low energy ion irradiation (LEII) region (keV to low MeV), while above \(\sim\)30 MeV in the swift heavy ion irradiation (SHII) region the energy loss is predominantly \(S_{e}\). In the LEII regime, collisions with target nuclei may result in the formation of amorphous zones/clusters [27] and the accumulation of such zones may produce the amorphous phase. The open nature of the diamond structure and poor damage regeneration due to directional bonding in bulk semiconductors enhances the irradiation-induced disorder leading to the formation of the amorphous phase.