Acoustic vibrations of a silica-embedded gold nanoparticle: elastic anisotropy

Spherical gold nanoclusters are grown by 1.8 MeV Au++ ion implantation into an amorphous silica matrix and subsequent air annealing at 873 K. Ultraviolet and visible light absorption confirms the presence of a dipolar surface plasmon peak at the expected location for a gold sphere in silica. Grazing incidence X-ray diffraction shows peaks corresponding to fcc gold with lattice spacing close to that of bulk gold, as well as allowing estimation of cluster size using Scherrer's formula. Low frequency Raman scattering reveals a relatively narrow peak, suggesting a narrow distribution of nanocrystal diameters. Acoustic phonon frequencies corresponding to the spheroidal quadrupolar vibrations of a continuum sphere with the anisotropic elasticity of gold are calculated using a novel method of molecular dynamics and extrapolation to the continuum limit using relatively small numbers of point masses. The study confirms high energy ion implantation as a method capable of producing gold nanocrystals with spherical shape and well controlled size.