The role of morphology in all-dielectric SERS: A comparison between conformal (T-rex) and non conformal TiO2 shells

All-dielectric optical nanoantennas and resonators are attracting an ever increasing interest for enhanced Raman spectroscopy. These systems can take advantage of many factors that concur to determine their Raman response, including the type of material, refractive index contrast between interfaces, size, shape, roughness and geometrical configuration. However, the relative impact of each one of those terms on the Raman enhancement is still unclear. Here we report the Raman characterization of SiO2@TiO2 semishell microspheres obtained by rf-magnetron sputtering deposition of a TiO2 thin layer on SiO2 cores. The samples were able to significantly amplify Raman scattering in comparison to planar counterparts, however the overall enhancement was very limited with reference to that observed in conformal, smooth and compact TiO2 shell layers obtained by atomic layer deposition. These results demonstrated that geometry is a key factor to maximize the Raman response of these systems and, in general, it is more important than other parameters, such as the surface roughness or even refractive index of the shell layer, which are usually considered crucial for achieving Raman enhancement.