High-order vibration modes of bimetallic Ag-Au nanoparticles embedded in glass

High-resolution low-frequency Raman scattering measurements from embedded AgAu nanoparticles unveil efficient scattering by harmonics of both the quadrupolar and the spherical modes. Comparing the experimental data with theoretical calculations that account for both the embedding medium and the resonant Raman process enables a very complete description of the observed multiple components in terms of harmonics of both the quadrupolar and spherical modes, with a dominating Raman response from the former ones. It is found that only selected harmonics of the quadrupolar mode contribute significantly to the Raman spectra in agreement with earlier theoretical predictions.

Figure 1

(Color online) Absorption spectra of glasses containing ${\text{Ag}}_x{\text{Au}}_{1-x}$ nanoparticles for different annealing times. The inset shows the SPR peak position as a function of the Ag composition; the dashed line is a guide to the eyes.

Figure 2

(Color online) Polarized $\left(VV\right)$ low-frequency Raman spectra (in log-linear scale) of AgAu nanoparticles embedded in glasses annealed over different times. Inset: enlargement of the lower $\left(E_g,T_{2g}\right)$ doublet for the 64 H sample.

Figure 3

(Color online) Experimental (symbols) and calculated (solid line) depolarized $\left(VH\right)$ Raman spectra of the ${\text{Ag}}_{0.66}{\text{Au}}_{0.34}$ sample with a diameter of 23.5 nm (the spectra were normalized with respect to the $\left(\ell =2,n=1\right)$ peak); high-frequency enlargement in the inset. The thick bars are the Raman intensities calculated for the same nanoparticle considered as free, labeled according to their $n$ index (only harmonics with non-negligible intensities are shown). Top scale arrows: CFM predictions for the $\ell =2$ mode with the same embedded nanoparticle.

Figure 4

(Color online) Left: experimental (upper set) and calculated (lower set) of the $VV$ (solid) and $VH$ (dashed) Raman profiles of the ${\text{Ag}}_{0.66}{\text{Au}}_{0.34}$ sample (intensities were normalized with respect to the $\left(\ell =2,n=1\right)$ peak). Right: Raman difference spectrum $\left(I_{VV}-I_{VH}\right)$, experimental (thick line) and RSI calculated (thin line). Top scales: CFM predictions for the $\ell =0$ mode.