Sensitivity of Plasmonic Nanostructures Coated with Thin Oxide Films for Refractive Index Sensing: Experimental and Theoretical Investigations

This paper reports on the use of the Lorentz-Drude model to investigate the refractive index sensitivity, S (change of nanometer per refractive index unit, in nm RIU-1) of multilayered localized surface plasmon resonance (LSPR) interfaces. The investigated interface consists of an array of gold nanostructures (Au NSs) on glass (n(1) = 1.51) coated with dielectric overlayers of different refractive index (n(2) = 1.45-2.63) and varying thickness (0-300 nm). These interfaces are in contact with solvents of different refractive index (n(3) = 1.000-1.630). The refractive index sensitivity for architectures with n(1) <= n(2) is found to be proportional to the amplitude of the oscillation of the wavelength at maximal absorption (lambda(max)). It reaches its maximal value when the amplitude of the oscillation is maximal and its minimal value when the amplitude is null. The amplitude of the oscillations increases with an increase in refractive index of the dielectric. For n(1) > n(2), all the curves oscillate in phase leading quickly (d(dielectric) > 40 nm) to a low value of the sensitivity. Quantitatively, a LSPR architecture of glass/Au NSs/nanocrystalline diamond (n(2) = 2.4) with a thickness of 10-20 nm shows the best sensitivity. However, it has to be noted that the sensitivity is mainly affected by the thickness of the dielectric and not by the refractive index of the overcoating