Vibrational density of states and thermodynamics at the nanoscale: the 3D-2D transition in gold nanostructures
Résumé
Surface enhanced Raman scattering (SERS) is generally and widely used to enhance the vibrational
fingerprint of molecules located at the vicinity of noble metal nanoparticles. In this work, SERS is
originally used to enhance the own vibrational density of states (VDOS) of nude and isolated gold
nanoparticles. This offers the opportunity of analyzing finite size effects on the lattice dynamics which
remains unattainable with conventional techniques based on neutron or x-ray inelastic scattering. By
reducing the size down to few nanometers, the role of surface atoms versus volume atoms become
dominant, and the “text-book” 3D-2D transition on the dynamical behavior is experimentally
emphasized. “Anomalies” that have been predicted by a large panel of simulations at the atomic scale,
are really observed, like the enhancement of the VDOS at low frequencies or the occurrence of localized
modes at frequencies beyond the cut-off in bulk. Consequences on the thermodynamic properties at
the nanoscale, like the reduction of the Debye temperature or the excess of the specific heat, have been
evaluated. Finally the high sensitivity of reminiscent bulk-like phonons on the arrangements at the
atomic scale is used to access the morphology and internal disorder of the nanoparticles.
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