Mechanical Coupling in Gold Nanoparticles Supermolecules Revealed by Plasmon-Enhanced Ultralow Frequency Raman Spectroscopy

Abstract

Acoustic vibrations of assemblies of gold nanoparticles were investigated using ultralow frequency micro-Raman scattering and finite element simulations. When exciting the assemblies resonantly with the surface plasmon resonance of electromagnetically coupled nanoparticles, Raman spectra present an ultralow frequency band whose frequency lies below the lowest Raman active Lamb mode of single nanoparticles that was observed. This feature was ascribed to a Raman vibration mode of gold nanoparticle "supermolecules", that is, nanoparticles mechanically coupled by surrounding polymer molecules. Its measured frequency is inversely proportional to the nanoparticle diameter and sensitive to the elastic properties of the interstitial polymer. The latter dependence as well as finite element simulations suggest that this mode corresponds to the out-of-phase semirigid translation (l = 1 Lamb mode) of each nanoparticle of a dimer inside the matrix, activated by the mechanical coupling between the nanoparticles. These observations were permitted only thanks to the resonant excitation with the coupling plasmon excitation, leading to an enhancement up to 10(4) of the scattering by these vibrations. This enhanced ultralow frequency Raman scattering thus opens a new route to probe the local elastic properties of the surrounding medium.

Time-domain imaging of gigahertz surface waves on an acoustic metamaterial

“…The values of the stiffnesses depend on the exact nature of the contacts in the sample, which are sensitive to the details of the preparation conditions. Raman and Brillouin spectroscopy studies have also revealed the existence of different types of vibrational modes involving particle contact [45,46]. A microscopic analysis of the origin of our observed stiffnesses is beyond the scope of this paper.…”

Plasmon‐enhanced inelastic scattering by 2D and 3D superlattices made of silver nanocrystals

“…On one hand, considering vibrations propagating in a solid with a sound velocity of few 10 3 m/s, confined acoustic‐like atomic vibrations in nm‐sized metallic NCs must belong to the THz range, as observed experimentally . On the other hand, oscillations of uncompressible NCs, composed of few 10 3 atoms and weakly bounded by ligand chains, are expected in the GHz range . Therefore, the dynamical coupling between the two types of oscillators is expected to be weak.…”

“…[3,4] On the other hand, oscillations of uncompressible NCs, composed of few 10 3 atoms and weakly bounded by ligand chains, are expected in the GHz range. [5,6] Therefore, the dynamical coupling between the two types of oscillators is expected to be weak.…”

Microcavity enhancement of low‐frequency Raman scattering from a CsPbI3 thin film

“…The vibrations detected in LFR region are generated by harmonic oscillations such as lattice phonons and localized intermolecular interactions such as Van der Waals interactions, π–π stacking, and hydrogen bonds . The LFR signal is sensitive to the vibrational modes associated with the material nanostructure and is therefore being used to characterize chiral purity of organic crystals and formulations, biomolecular assemblies, metal–organic frameworks, semiconductor superlattices, and halide perovskites . However, because of the intense Rayleigh scattering and instrumental limitations associated with blocking the exciting beam, observing LFR signatures has been considered difficult.…”

“…[11] The LFR signal is sensitive to the vibrational † Tal Ben Uliel and Laxman Gouda have contributed equally to this work. modes associated with the material nanostructure [12] and is therefore being used to characterize chiral purity of organic crystals and formulations, [13] biomolecular assemblies, [14] metal-organic frameworks, [15] semiconductor superlattices, [16] and halide perovskites. [17] However, because of the intense Rayleigh scattering and instrumental limitations associated with blocking the exciting beam, observing LFR signatures has been considered difficult.…”