Vibrational density of states and thermodynamics at the nanoscale: the 3D-2D transition in gold nanostructures

International audienceSurface enhanced Raman scattering (SERS) is generally and widely used to enhance the vibrationalfingerprint of molecules located at the vicinity of noble metal nanoparticles. In this work, SERS isoriginally used to enhance the own vibrational density of states (VDOS) of nude and isolated goldnanoparticles. This offers the opportunity of analyzing finite size effects on the lattice dynamics whichremains unattainable with conventional techniques based on neutron or x-ray inelastic scattering. Byreducing the size down to few nanometers, the role of surface atoms versus volume atoms becomedominant, and the “text-book” 3D-2D transition on the dynamical behavior is experimentallyemphasized. “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 localizedmodes at frequencies beyond the cut-off in bulk. Consequences on the thermodynamic properties atthe nanoscale, like the reduction of the Debye temperature or the excess of the specific heat, have beenevaluated. Finally the high sensitivity of reminiscent bulk-like phonons on the arrangements at theatomic scale is used to access the morphology and internal disorder of the nanoparticles