Inelastic Light Scattering by Multiple Vibrational Modes in Individual Gold Nanodimers

Abstract

To be Raman-active (or, more generally, detectable using optical spectroscopy techniques), a vibrational mode of a nanosystem has to modulate its optical response. For small, isolated nanospheres, this is the case for only two categories of vibrational modes, namely quadrupolar and radial ones. However, assembling nanospheres as dimers makes additional modes Raman-active, as previously demonstrated by the detection in the ultralow frequency range of a hybridized quasi-translation mode in previous measurements on single and ensembles of gold nanosphere dimers. In the present work, we use our recently developed single-particle Raman spectroscopy setup to compare inelastic light scattering by single isolated and dimerized gold nanospheres in an extended frequency range (0-40 GHz). The Raman spectra acquired on isolated nanoparticles present a single peak associated to their fundamental quadrupolar mode, consistently with previous ensemble measurements. In contrast, the spectra measured on dimers are richer and display a number of peaks increasing with decreasing interparticle distance, with all = 2 − 8 Lamb modes detected in the quasicontact case. These observations are rationalized using a recently developed classical model of inelastic light scattering by nanospheres. Importantly, our modeling approach takes into account the real electric field within the nanoparticles DID contribution and the local changes of the intrinsic microscopic polarizabilities induced by the vibrations, and second the multiple scattering processes affecting the Stokes and anti-Stokes photons. Moreover, building a full opto-mechanical model of Raman scattering by nanoparticle dimers (or oligomers) simultaneously taking into account electromagnetic (as here) and acoustic (as in our previous works 24,25) interactions between close nanoparticles would be useful to address in more detail the inelastic scattering by =1 Lamb modes, which are highly affected by acoustic hybridization. ASSOCIATED CONTENT Supporting Information Determination of the input parameters; analysis of the extinction cross-sections; analysis of the Low Frequency Raman Spectra (LFRS) of the monomer, dimer A and dimer B; dependence of the LFSR on the inter-particle distance in dimers; comments about the dipole-induced-dipole mechanism; phenomenological modeling of the multiple scattering of the Stokes and anti-Stokes photons. This material is available free of charge via the Internet at http://pubs.acs.org.

Signatures of Small Morphological Anisotropies in the Plasmonic and Vibrational Responses of Individual Nano-objects

“…[22][23][24][25][26][27][28][29] Investigation of the vibrational modes of single nano-objects has been demonstrated using time-resolved spectroscopy 8,[30][31][32][33] and, more recently, inelastic light scattering. 34,35 Noticeably, the use of such single-particle approaches allows to shed light onto processes which can hardly be investigated in ensemble measurements, such as the polarization dependence of the optical properties of nano-objects, sensitive to their shape and orientation [36][37][38][39] and the quality factors of their plasmonic and vibrational modes. 8,[40][41][42][43][44][45][46] Quantitative polarization-resolved extinction measurements using spatial modulation spectroscopy enable optical characterization of the dimensions and orientation of simple nano-objects (nanorods and nanospheres) placed in a known environment and, vice versa, to measure the refractive index of the local environment if the morphology of the nanoobject has been previously determined.…”

Inelastic Light Scattering by Long Narrow Gold Nanocrystals: When Size, Shape, Crystallinity, and Assembly Matter

“…Finally, we note that the breaking down of the Raman selection rules which we report is quite dramatic. In the case of gold nanodimers, 19 the intensities of the additional peaks assigned to vibrations with > 2 was about one order of magnitude lower than the one for = 2. In the present work, the intensity of the peaks are about the same (see Figure 1).…”

“…During the last twenty years the mechanical properties of gold nanocrystals, their vibrational properties, have been shown to also strongly depend on these parameters and to provide valuable information about the nanostructures themselves (size, shape, crystallinity) and their environment. [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] This correlated dependency of the optical and mechanical properties make gold nanoparticles the simplest opto-mechanical transducers where they act both as source and detector. 22 Such optical and mechanical properties can be tuned by controlling their structural properties and environment.…”

“…The features observed for the NRs are mainly diameter-dependent and are therefore not the lowest frequency modes. To assign the peaks, we use selection rules when possible to restrict the number of modes to consider.Recent works have also shown that the plasmonic coupling between neighbor nanoparticles can break the usual Raman selection rules19 resulting in a larger number of peaks in the spectra. In that case, the additional experimental features observed for spheres have been successfully assigned to the fundamental spheroidal vibrations with = 3, 4, .…”

Vibrations of Dimers of Mechanically Coupled Nanostructures: Analytical and Numerical Modeling

“…[28][29][30][31][32] This effect offers the possibility to selectively probe dimers in optical spectroscopy experiments involving ensembles of nanoparticles, by tuning the light wavelength with a plasmonic resonance generated by plasmonic interactions. 24,25,33 The high sensitivity of the optical response of dimers to interparticle distance also makes their inelastic light scattering spectra much richer than those of isolated nano-objects. 33 The acoustic interactions between close supported nano-objects only coupled by their underlying substrate are typically weak, and initial experiments performed on nanodimers formed by two close non-touching nano-objects (pairs of nanoprisms 34 and nanocubes 35 lithographed on a substrate) showed no clear signature of coupling effects.…”