Unique gap-related SERS behaviors of p-aminothiophenol molecules absorbed on TiO₂ surface in periodic TiO₂/Ni nanopillar arrays

We observed a unique interpillar gap-related surface-enhanced Raman scattering (SERS) behavior of p-aminothiophenol (PATP) molecules from periodic TiO2 nanopillar arrays with three gap sizes of 191, 297 and 401 nm, which is completely different from that on Ag and Ni nanopillar arrays. Especially, the gap-size-dependent charge-transfer (CT) resonance enhancement from TiO2/Ni has been indicated through comparisons of variation trend of SERS intensities with inter-pillar gap size between TiO2/Ni and Ag/TiO2/Ni as well as Ni nanoarrays, and been confirmed by spectra of ultraviolet-visible absorption and photoluminescence. Results demonstrate that the CT resonance enhancement is more susceptible to the change of the gap size compared with the surface plasmon resonance (SPR) enhancement in TiO2/Ni nanoarrays. Hence, SPR and CT enhancement showing different variation trend and rate with the gap size that leads to a different relative contribution of CT resonance to the overall SERS enhancement as gap size changes, and consequently results in a unique gap-related SERS behavior for TiO2/Ni nanoarrays. The present study is not only helpful for investigating SERS mechanism for semiconductors but also providing a method to design and optimize periodic metal/semiconductor SERS substrates in a controllable way.