Spaced Hybrid TiO₂/Au Nanotube Arrays with Tailored Optical Properties for Surface-Enhanced Raman Scattering

Controlling the overall geometry of plasmonic materials allows for tailoring their optical response and the effects that can be exploited to enhance the performance of a wide range of devices. This study demonstrates a simple method to control the size and distribution of gold (Au) nanoparticles grown on the surface of spaced titanium dioxide (TiO₂) nanotubes by varying the deposition time of magnetron sputtering. While shorter depositions led to small and well-separated Au nanoparticles, longer depositions promoted the formation of quasi-continuous layers with small interparticle gaps. The optical spectra of Au/TiO₂ nanotubes showed a region of strong absorption (200-550 nm) for all samples and a region of decreasing absorption with an increase of effective Au thickness (550-1100 nm). This behavior led to distinct trends in the Raman signal enhancement of the underlying TiO₂ nanotubes depending on the excitation laser wavelength. Furthermore, the quasi-continuous layers formed at higher effective Au thicknesses promoted an amplification of the signal and an improvement in the detection limit of target molecules in surface-enhanced Raman scattering (SERS) experiments. These findings suggest a simple method for designing efficient devices with tailored light absorption and potential applications in detectors and other optical devices.