Molecules in the mirror: how SERS backgrounds arise from the quantum method of images

Abstract

The Raman coupling of light to molecular vibrations is strongly modified when they are placed near a plasmonic metal surface, with the appearance of a strong broad continuum background in addition to the normal surface-enhanced Raman scattering (SERS) peaks. Using a quantum method of images approach, we produce a simple but quantitative explanation of the inevitable presence of the background, due to the resistive damping of the image molecule. This model thus suggests new strategies for enhancing the SERS peak to background ratio.

Fig. 1. Representation of method of images approach with (a) real molecule interacting with a plasmonic metal surface replaced by (b) a real-molecule–image-molecule interaction. (c) Dominant molecular vibrating bond of real and image molecules now represented by dipoles. Real and image dipoles separated by distance, 2d, and ω is the exciting laser field.

Fig. 2. (a) Stokes emission when the emitted Raman photon has a lower frequency than the pump laser ω, and (b) anti-Stokes emission when the emitted Raman photon has a higher frequency than the pump laser. (c) Experimental SERS spectrum from a monolayer of benzenethiol on Klarite^TM showing typical SERS peaks and background for Stokes and antiStokes scattering.

Fig. 3. Modeled spectrum Stokes spectrum of a broad background peak under a weaker and narrow Raman line, from (5) with , γ _ν′ = 1 cm^–1 and Γ _ν′ = 100 cm^–1.

Fig. 4. Modeled Stokes spectra from eqn (5) with varying values of γ _ν′ and Γ _ν′ using held constant, for (a–c) Γ _ν′ = 100 cm^–1 and (a) γ _ν′ = 0.01 cm^–1, (b) γ _ν′ = 1.5 cm^–1, (c) γ _ν′ = 10 cm^–1. (d and e) γ _ν′ = 1 cm^–1 and (d) Γ _ν′ = 1 cm^–1, (e) Γ _ν′ = 20 cm^–1, (f) Γ _ν′ = 40 cm^–1.

Fig. 5. Magnified image dipole for negative curvature roughness.