Precise control of hydrogen response of semicontinuous palladium film using piezoelectric resonance method

Abstract

During deposition of metallic material on a substrate, a semicontinuous film composed of isolated and connected clusters is formed transiently at a few nanometers thickness. The surface electrical conductivity at this moment is governed by the tunneling conduction, and slight subsequent evolution of the film morphology changes the conductivity markedly because of the island connection. When the semicontinuous palladium film is exposed to hydrogen, its morphology changes because of absorption of hydrogen, changing the surface conductivity drastically. This phenomenon is applicable for hydrogen sensing. However, it has been significantly difficult to fabricate an optimum semicontinuous structure because it appears in a very short time during deposition. In the present study, we precisely control the palladium film morphology using the piezoelectric resonance method. In this method, an electric field is excited around the substrate surface using the resonant vibration of the piezoelectric material. The electric field generates the electrical current in the deposited material, and the vibrational energy of the piezoelectric material is spent on it. Because the energy loss depends on the electrical conductivity (morphology) of the deposited material, by measuring the change in the attenuation of the resonant vibration, evolution of the morphology is detectable. Using this method, palladium films with several morphologies were fabricated, and the conductivity change in hydrogen was evaluated. The change ratio of the conductivity significantly depended on the morphology, and the conductive sensitivity to the hydrogen detection with the optimum structure was larger than that reported in the previous study by a factor of 12.