Hydrogen sensors based on plasmonic nanostructures present on palladium films

A highly sensitive and easy-to-fabricate hydrogen sensor based on a plasmonic 'gold nanowire array on a palladium layer deposited on a metallic substrate' is proposed. Plasmonic waveguide modes are excited in the gaps between the nanowires in this 'gold nanowire array on a palladium spacer layer deposited on a metallic substrate' system. As incident light is coupled into the plasmonic modes, a dip in the reflectance spectra is observed at the resonant wavelength, i.e., the wavelength at which the incident light is coupled into plasmonic modes. On exposure to hydrogen, the palladium spacer layer transforms to palladium hydride (PdH_x), where x, the atomic ratio of H:Pd, increases as the hydrogen concentration increases. This transformation changes the optical properties of the Pd layer, and hence the position of the resonance wavelengths (λ_res), i.e., the position of the reflection dips in the reflectance spectra of the Au-Pd-Au system, for various concentrations of hydrogen. The difference between the positions of the resonant wavelengths of PdHx and Pd, (λ_res(PdHx)−λ_res(Pd)), is used as a measure of the sensitivity of the proposed hydrogen sensor. Analysis of this shift in the plasmon resonance wavelength is done numerically, using Rigorous Coupled Wave Analysis (RCWA) for various values of d, the side length of the nanowires; t, the thickness of the Pd spacer; g, the gap between the adjacent nanowires and θ, the angle of the incident radiation. It is found that, in the presence of hydrogen, the maximum shift in the resonance wavelength for the proposed sensor is ~41 nm as compared to the case when hydrogen is absent. This shift in the resonance wavelength is higher than many currently employed plasmonic Pd-based hydrogen sensors. Thus, the proposed 'gold nanowire array on a palladium spacer layer deposited on a metallic substrate' is an easy-to-fabricate, selective and sensitive hydrogen sensor.