Structural properties of embedded Ge nanoparticles modified by swift heavy-ion irradiation

Silica-embedded Ge nanoparticles (NPs) of different sizes irradiated with swift heavy ions (SHIs) at a given energy may reportedly elongate along the incident ion direction, perpendicular to it, or not at all. Here, for a given NP size distribution, we have investigated the SHI energy dependence of the elongation process. Higher-energy irradiation generally yielded elongation along the ion track (as previously observed), but for lower-energy irradiation, elongation both parallel and perpendicular to the ion direction was observed. We demonstrate that NP size and electronic energy loss together govern the elongation process, reinforcing the proposed model where elongation perpendicular to the ion direction is only expected for Ge NPs bigger than the mean ion track diameter in silica. Here, a wide fluence range is also probed, enabling us to follow in more detail the transition from spherical unirradiated Ge NPs to Ge NPs elongated either parallel or perpendicular to the ion beam. X-ray absorption spectroscopy (XAS) measurements are utilized for the quantification of crystalline, amorphous, and oxidized environments around Ge atoms. Combining such results with transmission electron microscopy (TEM) observations shows the Ge NPs are rendered amorphous prior to elongation, potentially via a melt-and-quench process. Thereafter, stronger electron-phonon coupling in amorphous Ge compared to crystalline Ge may potentially influence the elongation process. The Ge NP amorphization occurs at lower fluences for higher irradiation energies, indicating electronic energy loss-and not ballistic effects-governs the amorphization. Subsequent to amorphization and elongation, TEM and XAS results also show the NPs gradually intermix with SiO2 and dissolve within the matrix as the irradiation fluence increases. We discuss the impact of such results in the ion beam tailoring of Ge NPs for technological applications.