Elongation of gold nanoparticles by swift heavy ion irradiation: Surface plasmon resonance shift dependence on the electronic stopping power

Abstract

Gold nanoparticles embedded in a silica matrix were irradiated with 2 to 40MeV Cu or Si ions at fluences ranging from 1×1013 to 4×1015 ions/cm2, and their deformation from spheres to prolate ellipsoids with major axis parallel to the ion beam was studied using P and S polarized light. For fixed ion energy, the longitudinal surface plasmon resonance (SPR) at 520nm is red-shifted with an increase of the ion fluence up to a certain value where it reaches a plateau indicating that a maximum aspect ratio is obtained. This saturation in the wavelength shift was found to depend on the ion energy and reaches a maximum of 40nm. The SPR shift was also used to measure the electronic stopping power dependent deformation rate and to deduce the electronic stopping power threshold of (1.9±1.3) keV/nm required for shape transformation of the embedded gold nanoparticles. Ion track diameters of 0.18 to 1.4nm were inferred from the fluence dependence of the SPR shift. Analysis by transmission electron microscopy shows that large (d>10nm) particles are more elongated than smaller ones. Our data are consistent with a mechanism of gold nanoparticle elongation requiring both the silica matrix and the nanoparticles to melt following the passage of the swift heavy ion and with elongation being due to the relief of stress in the gold nanoparticle which had built up as a consequence of the deformation of the surrounding silica matrix.