Optical properties and refractive index sensitivity of reactive sputtered oxide coatings with embedded Au clusters

Abstract

In the present study, nanocomposite coatings of Au clusters embedded in two different oxides, TiO 2 and Al 2 O 3 , were synthesized using pulsed DC magnetron sputtering. The depositions were carried out in three steps, by depositing the oxide, the Au clusters, and again the oxide. The deposition time of the Au clusters was varied in order to achieve different cluster sizes, morphologies, and nanocomposite topographies. The structure, microstructure, morphology, and the optical properties of the coatings were studied. With the increase in Au content, red-shifted surface plasmon resonance (SPR) peaks with higher intensity and increased widths were observed due to changes in the metal clusters sizes and morphology and due to interparticle effects. In order to relate the peculiar SPR extinction bands with the different clusters shapes and distributions, a simulation of the optical properties of the nanocomposites was performed making use of the Renormalized Maxwell-Garnett approach. A theoretical study concerning the refractive index sensitivity was made in order to predict the optimal coatings parameters for sensing experiments. The increased surface area and the strong SPR extinction bands make these coatings suitable for gas sensing and also catalysis, albeit many other application fields can be envisaged. V C 2014 AIP Publishing LLC.

Structural and functional properties of nanocomposite Au–WO3 coatings

“…For small interparticle separation distances (distances inferior to the diameter of the nanoparticles) the intercoupling of the nanoparticles electromagnetic fields starts to take place provoking a red-shift of the LSPR signal as well [9,32]. The nanoparticle size does not influence the LSPR position but only its intensity through the volume of the sphere [13].…”

“…The nanoparticle size does not influence the LSPR position but only its intensity through the volume of the sphere [13]. The shape of the nanoparticles is more complex to analyse, however considering the case of spheroidal nanoparticles (both oblate and prolate shapes), when decreasing the nanoparticles aspect ratios the plasmon resonance splits into a strongly red-shifted long-axis mode and a slightly blue-shifted short axis mode [9]. A similar behaviour is also observed in nano-ligaments [33].…”

“…Three main parameters are responsible for the change in the surface plasmon resonance peak [9,13,32]: the refractive index of the host matrix, the shape of the nanoparticles and the nanoparticles size. For small interparticle separation distances (distances inferior to the diameter of the nanoparticles) the intercoupling of the nanoparticles electromagnetic fields starts to take place provoking a red-shift of the LSPR signal as well [9,32].…”

“…LSPR sensing shows several advantages against conventional SPR sensors based on metal thin films. LSPR sensors are much cheaper to produce, possess smaller footprint, have the potential to be more sensitive to shallow changes of refractive index and make even singlemolecule detection possible [9][10][11][12][13]. This new approach has been recently suggested and tested for the Au-WO 3 [14] and Ag-WO 3 [1] systems.…”

Synthesis of tunable plasmonic metal-ceramic nanocomposite thin films by temporally modulated sputtered fluxes

“…The LSPR gives rise to a strong absorption around the resonance energy 1 and can lead to an intense coloration of the material 2 . It also results in a strong localized enhancement of the electric field around the nanoparticles making these materials interesting for a number of applications, such as sensors 3 , absorbers [4][5][6] , substrates for surface enhanced Raman scattering 7 , optically active materials 8,9 and materials exhibiting an enhanced magnetooptical coupling 10 . The energy position and width of the absorption, as well as the plasmonic properties in such a nanocomposite, depend on the dispersion of the electromagnetic radiation in the dielectric matrix in combination with the bulk optical properties of the metal nanoparticle, the nanoparticle size and shape, and the inter-particle separation 11,12 .…”