The effect of excitation wavelength and metallic nanostructure on SERS spectra of C₆₀

Understanding the mechanisms of surface-enhanced Raman scattering (SERS) phenomena is essential for advancing next-generation Raman-based chemical and biological sensors. SERS studies typically examine the excitation wavelength dependence for a given nanostructure morphology. However, more comprehension of the SERS mechanisms can be gained by investigating the combined excitation wavelength and metal morphology dependencies. Here, using a thin Au film with tapered boundaries as the Raman enhancing surface, we investigate the SERS mechanisms exhibited along the sloped region of the sample. The tapered boundaries provide controllably different Au morphologies from densified thin film to nano-island to nanoparticle structures. A layer of Fullerene C₆₀ was thermally evaporated uniformly across the Au film. The excitation wavelength of 532 nm generated similar SERS spectra across the Au surface, which closely matched the regular Raman scattering spectrum, thus indicating enhancement of the normal Raman modes of C₆₀. In contrast, an excitation wavelength of 784 nm generated different SERS spectra across the boundary whose intensity from the tapered Au film surface was 10 times higher than the SERS signal from the Au thin film region. The spectral differences observed with 784-nm excitation are indicative of an electromagnetic mechanism controlled by near field interactions of local surface plasmon resonances. Using the configuration of tapered Au films can enable fast determination of the most enhancing Au nanostructure for a given laser excitation and provide an in-depth insight into the enhancement mechanisms associated with a particular analyte molecule, as shown here for C₆₀.