Effects of intrinsic Fano interference on surface enhanced Raman spectroscopy: Comparison between platinum and gold

Using pyridine as a probe molecule, we performed surface enhanced Raman spectroscopy (SERS) studies on platinum and gold nanodisk arrays at both plasmon resonant and off-plasmon resonant excitation wavelengths. A large Raman cross-section enhancement factor (EF) of ∼10⁶ was obtained with plasmon resonant excitation on the Au array, and the EF decreases with off-resonant excitations. However, for Pt nanodisks the experimental EF is much smaller (∼10²) and not sensitive to excitation wavelength. Electric field intensities calculated in Au and Pt nanoparticles using the discrete dipole approximation (DDA) with a dielectric function including or excluding interband transitions allowed us to explain the SERS EF differences at different excitation wavelengths. The observed SERS insensitivity to excitation wavelength in Pt was explained using Fano interference between the free plasmon electrons and continuum interband transitions. The importance of Fano interference was explored analytically in the electrostatic limit by varying the contribution from the interband transitions.