  The shape of metal nanoparticles embedded in dielectric matrices influences
the optical properties of the composite material. Swift heavy ion irradiation
can induce a controllable shape transformation in gold and other metals
embedded in amorphous silicon dioxide, where the particles elongate along the
direction of the ion beam. The details of this transformation are not fully
understood, but it is presumably related to nanometer-scale phase transitions
induced by individual ion impacts. The phenomenon has been reproduced using
atomistic simulations, although the time scale limitations and the lack of
accurate interatomic models within the metal-silica interface lead unavoidably
to severe simplifications. We improve the realism in the simulations with an
accurate model for surface adhesion between gold and silica and by simulating
the processes in the matrix between impacts. The simulations with correct
adhesion show that the nanoparticles can grow in aspect ratio in the molten
state even after silicon dioxide solidifies. Moreover, we demonstrate the
active role of the matrix: without explicitly modeling processes in the matrix
between impacts, the elongation is limited and does not reach significant
aspect ratios seen in experiments. These results significantly improve the
theoretical understanding of processes developing in embedded nanoparticles
under swift heavy ion irradiation. The knowledge brings forward the ion beam
technology as a precise tool for shaping of embedded nanostructures for various
optical applications.
