In this work, a versatile method is proposed to increase the sensitivity of optical sensors based on the localized surface plasmon resonance (LSPR) phenomenon. It combines a physical deposition method with the oblique angle deposition technique, allowing the preparation of plasmonic thin films with tailored porosity. Thin films of Au-TiO 2 were deposited by reactive magnetron sputtering in a 3D nanostructure (zigzag growth), at different incidence angles (0°α80°), followed by in-air thermal annealing at 400 °C to induce the growth of the Au nanoparticles. The roughness and surface porosity suffered a gradual increment by increasing the incidence angle. The resulting porous zigzag nanostructures that were obtained also decreased the principal refractive indexes (RIs) of the matrix and favoured the diffusion of Au through grain boundaries, originating broader nanoparticle size distributions. The transmittance minimum of the LSPR band appeared at around 600 nm, leading to a red-shift to about 626 nm for the highest incidence angle α=80°, due to the presence of larger (scattering) nanoparticles. It is demonstrated that zigzag nanostructures can enhance adsorption sites for LSPR sensing by tailoring the porosity of the thin films. Atmosphere controlled transmittance-LSPR measurements showed that the RI sensitivity of the films is improved for higher incidence angles.
Gas
sensing based on bulk refractive index (RI) changes, has been
a challenging task for localized surface plasmon resonance (LSPR)
spectroscopy, presenting only a limited number of reports in this
field. In this work, it is demonstrated that a plasmonic thin film
composed of Au nanoparticles embedded in a CuO matrix can be used
to detect small changes (as low as 6 × 10–5 RIU) in bulk RI of gases at room temperature, using a high-resolution
LSPR spectroscopy system. To optimize the film’s surface, a
simple Ar plasma treatment revealed to be enough to remove the top
layers of the film and to partially expose the embedded nanoparticles,
and thus enhance the film’s gas sensing capabilities. The treated
sample exhibits high sensitivity to inert gases (Ar, N2), presenting a refractive index sensitivity (RIS) to bulk RI changes
of 425 nm/RIU. Furthermore, a 2-fold signal increase is observed for
O2, showing that the film is clearly more sensitive to
this gas due to its oxidizing nature. The results showed that the
Au:CuO thin film system is a RI sensitive platform able to detect
inert gases, which can be more sensitive to detect noninert gases
as O2 or even other reactive species.
Thin films containing monometallic (Ag,Au) and bimetallic (Ag-Au) noble nanoparticles were dispersed in TiO2, using reactive magnetron sputtering and post-deposition thermal annealing. The influence of metal concentration and thermal annealing in the (micro)structural evolution of the films was studied, and its correlation with the Localized Surface Plasmon Resonance (LSPR) and Surface Enhanced Raman Spectroscopy (SERS) behaviours was evaluated. The Ag/TiO2 films presented columnar to granular microstructures, developing Ag clusters at the surface for higher annealing temperatures. In some cases, the films presented dendrite-type fractal geometry, which led to an almost flat broadband optical response. The Au/TiO2 system revealed denser microstructures, with Au nanoparticles dispersed in the matrix, whose size increased with annealing temperature. This microstructure led to the appearance of LSPR bands, although some Au segregation to the surface hindered this effect for higher concentrations. The structural results of the Ag-Au/TiO2 system suggested the formation of bimetallic Ag-Au nanoparticles, which presence was supported by the appearance of a single narrow LSPR band. In addition, the Raman spectra of Rhodamine-6G demonstrated the viability of these systems for SERS applications, with some indication that the Ag/TiO2 system might be preferential, contrasting to the notorious behaviour of the bimetallic system in terms of LSPR response.
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