Acoustic Vibrations in Bimetallic Au@Pd Core–Shell Nanorods

Abstract

The acoustic vibrations of gold nanorods coated with palladium were investigated as a function of Pd amount using ultrafast pump-probe spectroscopy. Both the extensional and breathing vibrational modes of the nanorods were coherently excited and detected. This permits precise determination of their periods, which were found to decrease and increase with Pd deposition, for the extensional and vibrational modes, respectively. These opposite behaviors reflect changes of the nanoparticle size and mechanical properties, in agreement with numerical simulations. Comparison of experimental and computed periods yields information on the amount of deposited Pd, providing a novel tool to characterize bicomponent nano-objects for small fractions of one of the components (Pd/Au atomic fraction down to 5%).

Acoustic vibrations of metal nano-objects: Time-domain investigations

“…However, for the relevant growth directions of crystalline gold nanorods (i.e., long axis along [100] or [110] crystalline direction), though the breathing mode displacement is altered by loss of cylindrical symmetry (with, similarly to the sphere breathing mode, non uniform transverse surface movement, Fig. 6), its frequency is only weakly modified [63,64,110], with a maximum of 2% as compared to the polycrystalline case (Fig. 6a).…”

“…These approaches can be straightforwardly extended to nano-size objects, considered as mechanical oscillators. FEM has thus been largely used to compute the vibrational modes of nanoparticles formed by either isotropic or crystalline materials [79] and of different shapes, such as rods [63,107], columns [54], prisms [108], bipyramids [98,109] and cubes [58].…”

“…Shape anisotropy importantly modifies the acoustic vibration of an object, an effect that has been extensively investigated in the case of the model shapes formed by rods or long cylinders [39,63,107,110]. The former systems are particularly interesting, as they are experimentally relevant and permit to follow the evolution of the modes of a sphere to that of a rod by varying their aspect ratio [79].…”

“…Its frequency is then mostly set by Young's modulus along the growth direction of the rod (Eq. (18)), which for a cubic crystal is given by [63,74,110]:…”

“…. ) [39,45,[53][54][55][56][57][58][59], composition (mono-, bimetallic and hybrid nano-objects) [60][61][62][63], structure (crystalline or polycrystalline, alloyed or segregated multimaterial particles, . .…”