In situ study of structural and optical properties of Pd thin film during hydrogen exposure

When Palladium film is exposed to hydrogen, it becomes palladium hydride. A change in the complex permittivity of the metal film results in a change of the optical properties that depends on hydrogen concentration. Ellipsometry is the technique of choice to measure the optical constants prior and during hydrogenation. Sensors are then usually designed and optimized to measure changes in transmittance or reflectance of the palladium films. Films of different thicknesses have been realized and tested to verify potential applications in hydrogen sensing by studying the optical response prior, during and after hydrogenation, to assess in particular the reversibility of the process. Within this work a deep analysis carried out by x-ray reflectance (XRR) shows that during hydrogenation the films change also their thickness, and the amount has been assessed for a specific hydrogen concentration. Ellipsometric measurements have been therefore corrected taking into account such variation to determine the optical constants. Such structural property of the palladium hydride may be exploited in surface plasmon resonance transducers, which are sensitive also to the change of the sensing film thickness during detection.