Spaced Hybrid TiO<sub>2</sub>/Au Nanotube Arrays with\nTailored Optical Properties for Surface-Enhanced Raman Scattering

Controlling the overall geometry of plasmonic materials\nallows\nfor tailoring their optical response and the effects that can be exploited\nto enhance the performance of a wide range of devices. This study\ndemonstrates a simple method to control the size and distribution\nof gold (Au) nanoparticles grown on the surface of spaced titanium\ndioxide (TiO<sub>2</sub>) nanotubes by varying the deposition time\nof magnetron sputtering. While shorter depositions led to small and\nwell-separated Au nanoparticles, longer depositions promoted the formation\nof quasi-continuous layers with small interparticle gaps. The optical\nspectra of Au/TiO<sub>2</sub> nanotubes showed a region of strong\nabsorption (200–550 nm) for all samples and a region of decreasing\nabsorption with an increase of effective Au thickness (550–1100\nnm). This behavior led to distinct trends in the Raman signal enhancement\nof the underlying TiO<sub>2</sub> nanotubes depending on the excitation\nlaser wavelength. Furthermore, the quasi-continuous layers formed\nat higher effective Au thicknesses promoted an amplification of the\nsignal and an improvement in the detection limit of target molecules\nin surface-enhanced Raman scattering (SERS) experiments. These findings\nsuggest a simple method for designing efficient devices with tailored\nlight absorption and potential applications in detectors and other\noptical devices.