  The acoustic vibrations of single monomers and dimers of gold nanoparticles
were investigated by measuring for the first time their ultra-low frequency
micro-Raman scattering. This experiment provides access not only to the
frequency of the detected vibrational modes, but also to their damping rate,
which is obscured by inhomogeneous effects in measurements on ensembles of
nano-objects. This allows a detailed analysis of the mechanical coupling
occurring between two close nanoparticles (mediated by the polymer surrounding
them) in the dimer case. Such coupling induces the hybridization of the
vibrational modes of each nanoparticle, leading to the appearance in the Raman
spectra of two ultra-low frequency modes corresponding to the out-of-phase
longitudinal and transverse (with respect to the dimer axis) quasi-translations
of the nanoparticles. Additionally, it is also shown to shift the frequency of
the quadrupolar modes of the nanoparticles. Experimental results are
interpreted using finite-element simulations, which enable the unambiguous
identification of the detected modes and, despite the simplifications made,
lead to a reasonable reproduction of their measured frequencies and quality
factors. The demonstrated feasibility of low frequency Raman scattering
experiments on single nano-objects opens up new possibilities to improve the
understanding of nanoscale vibrations, this technique being complementary with
single nano-object time-resolved spectroscopy as it gives access to different
vibrational modes.
