Role of localized surface plasmons in surface-enhanced Raman scattering of shape-controlled metallic particles in regular arrays

We studied the influence of localized surface plasmon resonance (LSPR) sustained by arrays of metallic nanoparticles on the efficiency of surface-enhanced Raman scattering (SERS). We found that the Raman enhancement that originates from the excitation of LSPR does depend on the particle shape. Two different shapes were investigated: cylindrical particles, for which the maximum of Raman enhancement occurred for a LSPR located around 650 nm, and ellipsoidal particles, which exhibit the strongest SERS signal at 700 nm. These results are compared to an analytical model in the quasistatic approximation. This model, though being quite simple, can give an explanation of the observed differences in terms of the lightning rod effect.

Figure 1

(a) Experimental setup. Extinction spectroscopy: the sample is illuminated in transmission by a collimated light source and the notch filters are removed. Raman spectrum: classical backscattering configuration, using the 632.8 nm output of a He-Ne laser. (b) Variation of the LSPR wavelength with the diameter of cylinders (open circle) or the length of ellipsoids major axis (full circle). Inset: SEM images of typical arrays of ellipsoidal (minor axis: 50 nm, major axis: 120 nm, height: 50 nm) and cylindrical (diameter: 100 nm, height: 50 nm) particles achieved by EBL. The gap between two nanoparticles is kept constant at 200 nm.

Figure 2

(a) Extinction spectra of an ellipsoidal particle array (major axis: 120 nm, minor axis: 50 nm, height: 50 nm) for two polarization directions of the incident light, parallel and perpendicular to the major axis. (b) Raman spectra of the BPE recorded for two polarization directions of the incident laser, parallel and perpendicular to the major axis. (c) Raman (open circle) and LSPR (mode along the major axis located at 750 nm, full circle) intensity plotted against the polarization angle (0° is for a polarization parallel to the major axis); the solid line is the ${\cos }^2$ fit.

Figure 3

Raman intensity of the $1200{\mathrm{cm}}^{-1}$ BPE line vs LSPR: (a) for cylinders, (b) for ellipsoids. Solid lines are the binned average values of the LSPR and the Raman intensity. The dashed and dotted lines are the calculated curves (dashed line: $g$ factor, dotted line: $G$ factor).